Variations of Coastal Monsoon Climates with Latitude

I have learned about different aspects of the Earth's climate and shared what I've learned over 5 posts in 2022, including basic intuitions for why different climates occur in different regions [LINK], my assessment of the deficiencies of the Trewartha climate classification system [LINK], what I would change about the Trewartha climate classification system [LINK], how my proposed changes to the Trewartha climate classification system can be applied to understand what climates occur where in middle latitudes [LINK], and why popular understanding of the effects of the Gulf Stream over the Atlantic Ocean on the climate of Europe is incorrect in many ways [LINK]. Ultimately, my learning about different aspects of the climates of the world was done with the personal aim of understanding why cities on opposite coasts of the US at the same latitudes have such different climates, with those on the west coast having characteristically mild to hot arid rainless summers & cool (but not cold) rainy winters and those on the east coast having typically warm or hot humid rainy summers & cool or cold slightly drier but still rainy or snowy winters. I did learn about that to a great extent, but as I learned more, I started to question whether my previous intuitions (from when I started learning about different climates) were correct. Follow the jump to see more and the resolution to this problem. Again, I am not a trained climatologist or meteorologist; I can't guarantee that this information is accurate, and I can only say that my intuitions seem through my limited understanding to align with superficial aspects of more detailed explanations.

Original motivation

Original intuition about winters after learning

My intuitive answer to this question after learning what I did was as follows. The coast matters a bit more for the differences in summer weather, whereas differences in winter weather depend a bit more on topography and the presence, sizes, and arrangement of landmasses. I mean these points in the following senses.

In the middle latitudes, most coastal areas (i.e. not counting inland areas far from any ocean or major inland lake or sea) of continents have humid winters with a lot of precipitation regardless of which coast is considered, though the specific coast matters for determining whether the precipitation is more steady versus sporadic. Water has a higher specific heat capacity than land, so in the winter hemisphere, oceans tend to be warmer than land, so any air blowing over the warmer oceans will pick up moisture and then precipitate it upon the colder land; this precipitation will likely be either a steady rain & fog from the prevailing westerlies going from the cool eastern edge of an ocean to the west coast of an adjacent continent, or more sporadic, intense storms going from the warmer western edge of an ocean (possibly but not necessarily from the subtropical ridge closer to the eastern edge of that ocean, though the subtropical ridge would be closer to the equator in the winter hemisphere) to the east coast of the continent adjacent to the western edge of that ocean. I could find only two east coast regions with truly dry winters in the middle latitudes. One is in South America, somewhat closer to the pole, due to cooler water from the Antarctic Ocean displacing warmer water from the Atlantic Ocean especially in the winter half of the year along with the Andes Mountains drying but not fully blocking the prevailing westerlies. The other is East Asia due to the formation in the large landmass of Asia in the winter half of the year of a large-area system of intensely high pressure (from the settling of cold dry air) such that cold dry winds blow westerly from the continent to the Pacific Ocean; a similar phenomenon occurs in North America for similar reasons, but the much smaller size & more tapered shape of North America compared to Asia means that the system of high pressure is much smaller and therefore has a much smaller drying effect on the east coast of North America.

On any coast in the middle latitudes in the winter, except along the windward sides of mountainous west coasts closer to poles (like in North America & South America), precipitation will be somewhat low in an absolute sense because of the lower equilibrium partial pressure of water vapor at lower temperatures. Fog typically forms in the winter in the respective Central Valleys of California in the US (in the northern hemisphere) and of Chile (in the southern hemisphere) after a winter rain because of a temperature inversion arising from the contrast between cool moist air near the surface & warm dry air falling from the surrounding mountains above; it also sometimes forms in east coasts in the winter under specific conditions but is otherwise rare. In any case, because the land temperature near a west coast is moderated enough by the prevailing westerlies in the middle latitudes carrying cool moist air, the conditions needed for cold air to settle into a system of high pressure don't really occur that close to a west coast, though it is hard to test this with the arrangement of landmass on Earth because the landmasses of the southern hemisphere are too small, Europe is too fractal-shaped & surrounded by oceans, and North America has too many mountains near the west coast. I suspect that if North America, South America, or Europe had no mountains along lines of latitude (blocking air flow to & from the corresponding pole), even if they had mountains along lines of longitude, the west coasts of those continents would experience somewhat colder temperatures & more intense sporadic storms with possibly less precipitation overall due to incursions of colder dry air from the corresponding pole (which would lead to quicker precipitation and be less able to support as much moisture overall), though the west coasts of North America & Eurasia would probably still have slightly warmer winters with somewhat more steady precipitation in the middle latitudes than those continents' respective east coasts at similar latitudes.

Original intuition about summers after learning

It was easy enough for me to understand in isolation why middle latitude east coast summers tend to be humid with a lot of precipitation. The humidity comes from winds coming over a nearby warm ocean current, and the high amount of precipitation comes from that warm moist air rising quickly over land (so the moisture condenses & precipitates as it cools higher in the air). Farther from the equator, warm moist air might not rise quickly enough over hot dry land to immediately lead to precipitation, but this is compensated by incursions of cold dry air from the pole.

It was also easy enough for me to understand in isolation why middle latitude west coast summers tend to have little precipitation & little humidity. This is because at those latitudes, the subtropical ridge would lie closer to the pole and usually send easterly trade winds over hot dry land toward the west coast. Even the subtropical ridge on a given day happens to be closer than usual to the equator, the cool prevailing westerlies carry less moisture & move quicker over land due to the steadiness of wind coming off of the subtropical ridge (compared to sporadic easterly sea breezes onto an east coast), leaving less opportunity for that moisture to collect, condense, and fall; instead, especially but not exclusively in proximity to mountains that promote temperature inversions (as in North America & South America), that moisture may condense near the ground as fog at night when the land has cooled a lot more and then evaporate the next day as the land has heated.

North American monsoon leading to questioning that intuition

The existence of the North American monsoon in the Southwest in the US made me question the intuitions about coastal summers in the middle latitudes. In particular, in the Southwest in the US, heating of dry land creates a strong enough system of low pressure to draw warm moist air from the southwest over the Gulf of California that rises fast enough to quickly condense & precipitate in intense thunderstorms even in the absence of incursions of colder air. This is the same thing that happens in & near the South in the US, though the North American monsoon brings much less & more sporadic precipitation to the Southwest in the US compared to the wet summers of the South in the US. Many west coasts in middle latitudes, irrespective of the presence of mountain ranges parallel (though those will have an effect in cutting off moisture from the adjacent ocean), have landmasses that heat up a lot during the day in the summer. This should in principle draw enough moisture from the adjacent ocean to lead to that moisture quickly rising over the hot dry land and then precipitating, or to at least lead to more humidity, so the question is why that doesn't happen.

Resolving the questions about that intuition with arguments about the subtropical ridge and atmospheric stability

The answer seems to be that if the subtropical ridge is closer to the pole, then the dominant winds over the west coast at middle latitudes will be the trade winds blowing hot dry air from the east & from the direction of the pole over the continent, so there won't be a prevailing westerly wind blowing in cool moist air from over the ocean until one reaches points closer to the pole anyway; the low pressure from air rising over the hot dry land near the west coast at these latitudes is not strong enough in pulling moisture from over the ocean to overcome the trade winds coming off of the subtropical ridge and pushing toward the ocean. Even if the subtropical ridge happens to be a bit closer to the equator such that the prevailing westerlies can temporarily bring cool moist air into the systems of low pressure over hot dry land in California, the low temperature of the prevailing westerlies & of the moisture in those winds means that not enough moisture can accumulate at an intermediate altitude to form a storm. That moisture will instead condense at night near the surface as fog (especially as part of a broader temperature inversion) or during the day only at a very high altitude as a small cloud that is then quickly blow further to the east by the prevailing westerlies.

To make this argument clearer, it is worth noting that the equilibrium water vapor pressure is 0.9 kPa (kilopascals) at 5 degrees Celsius, 1.2 kPa at 10 degrees Celsius, 1.7 kPa at 15 degrees Celsius, 2.3 kPa at 20 degrees Celsius, 3.2 kPa at 25 degrees Celsius, and 4.2 kPa at 30 degrees Celsius. Additionally, at the end of 2023 July (in the middle of the summer in the northern hemisphere), the water along the west coast of California in the US varied from 9-22 degrees Celsius from north to south (with most of the coast having water closer to 9 degrees Celsius), while the water along the east coast of the US in the same latitude range as California varied from 22-28 degrees Celsius from north to south. Thus, air saturated with water vapor in 2023 July would have 2-3 times more water vapor over the east coast of the US than over the west coast of the US. Moreover, typical formulas for the variation of temperature with elevation suggest that if hot dry land has a temperature of 30 degrees Celsius at sea level, then the air at an altitude of 3,000 meters above it will have a temperature of 10 degrees Celsius, so air going from over the Pacific Ocean to the west coast of California can rise to an altitude of 3,000 meters before condensing (even as a cloud) at equilibrium, whereas that will not be possible on the east coast of the US near the Atlantic Ocean.

Any discussion of air rising or falling must invoke the notion of [gravitational] atmospheric stability. Essentially, if one considers imaginary parcels of air, the atmosphere in a given place is stable with respect to introducing a parcel of air if that introduction leads to the parcel of air falling toward the surface of the Earth or staying roughly at the same altitude, whereas instability implies that parcel of air rising a lot. A system of low pressure from the adiabatic expansion of air over hot dry land (or over a warm ocean) is unstable by definition of the expansion (as warm air becomes less dense and therefore rises in an adiabatic process), while a thermal inversion is stable because cold air is trapped & settles below warm air (as the land or ocean may happen to be cool enough to support this sort of variation of air temperature with altitude), even if the thermal inversion is otherwise relatively easy to disrupt.

On a middle latitude west coast in the summer, regardless of whether cool moist air from over the ocean can easily be pushed by the prevailing westerlies from closer to the equator toward the pole or must fight against the easterly trade winds over hot dry land coming from the subtropical ridge closer to the pole, when that cool moist air from the ocean enters columns of hot dry air over land, the minimal cool moist air may provide a cooling breeze that dissipates further inland as the moisture itself may rise to a high altitude that can be driven away (either to the east by the prevailing westerlies or to the west by the trade winds, depending on the position of the subtropical ridge) and the remaining cool dry air stays near the ground in the configuration of a temperature inversion (more likely near mountains) and gradually reaches thermal equilibrium with the hot dry air over land. The position & strength of the subtropical ridge on a given day determines how far this daytime cooling effect penetrates & whether nighttime fog may arise in a mountainous region, with more daytime cooling & more nighttime fog (in a mountainous region) arising when the subtropical ridge happens to be closer to the equator or otherwise weaker. This effect can be seen in the northern-to-central part of California in the US, in which the Sacramento-San Joaquin River delta breeze flows from the Pacific Ocean through the Golden Gate and up the Sacramento-San Joaquin River delta, leading to nighttime fog in San Francisco & an afternoon cooling effect in Sacramento that both strengthen when the subtropical ridge is closer to the equator or otherwise weaker. A similar effect occurs in Santa Monica in California in the US, though the warmer temperatures there (due to being closer to the equator than San Francisco is) mean that moisture at night is more likely to condense at a higher altitude as a low cloud rather than as fog (and dissipate during the daytime).

By contrast, on a middle latitude east coast in the summer especially closer to the equator, when warm moist air from over the ocean encounters hot dry air rising over land, there is much more moisture in the warm moist air than in cool moist air, and the warm moist air is much less dense than the hot dry air due to the much higher moisture content than hot dry air despite the lower temperature, so the warm moist air rises, implying an instability. As the warm moist air quickly rises, it slowly cools due to the high specific heat capacity & high latent heat of condensation of water, and this clash of temperatures with a lot of moisture at various altitudes will lead to precipitation. In the South in the US on a typical summer day as well as in the Southwest in the US during the North American monsoon, this process does not require any incursions of cold dry air from the pole to force precipitation, though precipitation is much less frequent & much smaller in amounts in the North American monsoon in the Southwest in the US compared to typical summer rains in the South in the US. However, such cold dry air is more often needed in the Midwest, Mid-Atlantic, and Northeast in the US, as those places are too close to the pole for the land to become hot enough to by itself promote such a strong instability. (As a reminder, the mountains around the west coast of the US block incursions of cold dry air from the pole, which is another reason why the west coast of the US has especially little precipitation & humidity in the US compared to the west coasts of Europe, Africa, and Australia at similar latitudes.) In particular, if cold air is already present over land, then incursions of warm moist air from over the ocean will lead to moisture rising to condense at a lower altitude, which in turn leads to the temperature inversion, low clouds, and prolonged precipitation characteristic of a warm front, while if cold dry air comes from the pole into a region where hot dry air over land & warm moist air from over the ocean are mixing, then the cold dry air will force moisture to rise & precipitate quickly & intensely with the towering clouds characteristic of a cold front.

Although I used examples from North America when building these arguments, these arguments generally hold on other continents too, with modifications to account for different configurations of landmasses & mountains. There are two exceptions of sorts. One is the fact that cities along the respective western edges of the Adriatic Sea & the Black Sea have climates similar to those along western edges of oceans (east coasts of continents) in middle latitudes (despite Europe as a whole being on the western side of Eurasia and therefore having more places with climates typical of or similar to those of west coasts in middle latitudes), while those along the eastern edges of the Adriatic Sea & the Black Sea have climates similar to those along eastern edges of oceans (west coasts of continents) in middle latitudes. The other is the fact that East Asia has genuinely dry winters & very wet summers for somewhat different reasons than most east coasts.

Peculiarities of climates around the Adriatic Sea and the Black Sea

Essentially, the Adriatic Sea & the Black Sea, like the Gulf of Mexico, are shallow enough to increase more in temperature (compared to the Mediterranean Sea or most oceans) in the summer (though in decreasing more in temperature in the winter, especially given the greater distances from the equator). Thus, along the western edges of the Adriatic Sea & the Black Sea, the same dynamics of warm moist air over the sea sporadically drifting over hot dry land are at play as along the western edge of a major ocean. Along the eastern edge of the Adriatic Sea in the winter, the prevailing westerlies are dried out but not stopped by the Apennine Mountains in Italy, so they pick up moisture from the warmer Adriatic Sea and dump it upon cities along the eastern edge of the Adriatic Sea (with this effect amplified by proximity to mountains to the east). In the summer, the prevailing westerlies blow too far to the north (closer to the pole), so cities along the Adriatic Sea get similarly low levels of precipitation, though higher levels of humidity due to the greater warmth of the Adriatic Sea compared to the cold currents along the eastern edges of oceans, compared to cities along the west coasts in middle latitudes of major continents. The situation is similar along the eastern edge of the Black Sea in the summer & winter as in the Adriatic Sea, though the bigger size of the Black Sea & continued (albeit weaker, but nonzero) presence of the prevailing westerlies over the Black Sea through the summer (as the Black Sea is closer to the pole than the Adriatic Sea) together lead to more overall precipitation through the year (though more in the winter half of the year than in the summer half of the year) over the eastern edge of the Black Sea.

Differences between the climates of East Asia versus other east coasts

In the winter, unlike other east coasts in middle latitudes which get moderate though perhaps slightly less precipitation than in the summer, East Asia is truly dry; as a reminder, this is because of the formation in the large landmass of Asia in the winter half of the year of a large-area system of intensely high pressure (from the settling of cold dry air) such that cold dry winds blow westerly from the continent to the Pacific Ocean, whereas no other continents have enough landmass at those latitudes (along with the absence of significant mountain ranges blocking or oceans mitigating the flow of cold dry air from the pole) to support a similarly intense or persistent system of high pressure in the winter. In the summer, East Asia gets much more precipitation than most other east coasts in middle latitudes for the following reason. Most other east coasts in middle latitudes get sporadic intense storms from random incursions of warm moist air onto land, not driven by any predictable wind. By contrast, in the summer, East Asia gets the tropical monsoon despite being in middle latitudes. This suggests the need for an explanation of the tropical monsoon.

Tropical monsoon

The intertropical convergence zone (ITCZ) is where the trade winds from the northern & southern hemispheres converge; it is a band of low pressure where less-dense air above the consistently warmest points on Earth (over land or oceans) rises. In a naïve view (like I had in my very first post in this series of posts about climates of the Earth), the trade winds are easterly in both hemispheres and converge very close to the equator. However, in fact, the ITCZ moves a lot seasonally over the course of a year, and because the northern hemisphere has a lot more landmass, the ITCZ is much farther into the northern hemisphere from the equator during the northern hemisphere summer, whereas it stays close to the equator and is not even consistently within the southern hemisphere during the southern hemisphere summer. This seasonal motion of the ITCZ means that places in the tropics can experience significant seasonal rotations in the trade winds over the course of a year. Depending on the configuration of landmasses and mountains as well as which coast is considered, this means that a place may get significant precipitation throughout the year, little precipitation throughout the year, or significant precipitation only in the summer half of the year.

Many tropical places, dry or wet, experience a seasonal full reversal (180 degree rotation) in the trade winds. This happens because a place in a given summer hemisphere may lie between the ITCZ (closer to the pole in the same hemisphere) and the subtropical ridge in the winter hemisphere, so the easterly trade winds from the winter hemisphere, upon crossing the equator, experience a reversal in the sign of the Coriolis force and continue toward the ITCZ, so their easterly components weaken or even reverse to become westerly components (while still having longitude components corresponding to coming from the winter hemisphere). This seasonal full reversal of trade winds occurs more in the northern hemisphere than in the southern hemisphere because, as discussed above, the ITCZ is farther north in the northern hemisphere summer than it is south in the southern hemisphere summer. However, not all places in the tropics that experience seasonal rotations in the trade winds experience full reversals in the trade winds, yet those places can still experience different wet & dry seasons over the course of a year.

In particular, at the peak of summer in the northern hemisphere, the greater amount of landmass in the northern hemisphere means that the ITCZ migrates as far north as 30 degrees in latitude over Afroeurasia, covering Northern Africa, most of Southwest Asia, most of Pakistan, the northernmost parts of India, most of Nepal, the southern half of China, all of Taiwan, and the southern tip of Japan. From there, over the Pacific Ocean, the ITCZ gradually turns further south toward the Tropic of Cancer, and upon reaching America, it covers the southern 2/3 of Mexico, all of Central America, and the northernmost countries of South America before turning again to the north over the Atlantic Ocean. By contrast, at the peak of summer in the southern hemisphere, the ITCZ is relatively close to the equator over the Atlantic Ocean and then covers Western Africa (south of the Sahara Desert), Central Africa, the southern half of Tanzania along with the northern 1/4 of Mozambique, the northern 1/3 of Madagascar, the Indian Ocean close to the equator, most of the islands of Southeast Asia, the northern tips of Australia, the Pacific Ocean in a wavy pattern, the southern half of Colombia, and the northern 1/3 of Brazil.

Tropical places that have climates with at least one wet season (with the first letter A in the Köppen categorization, Trewartha categorization, or my modification of the Trewartha categorization, as opposed to arid or semi-arid climates with the first letter B in the Köppen categorization, Trewartha categorization, or my modification of the Trewartha categorization) generally do not experience large swings in temperature through the year; this is partly because tropical places do not experience large swings in the maximum amount of daylight per day over the course of a year and partly because these places get enough moisture to moderate the temperatures overall. As I will discuss in later subsections, this means that for any tropical place with at least one dry season & at least one wet season, the hottest time of the year occurs during the driest season, even if that occurs in the winter half of the year. Tropical places with distinct wet & dry seasons can support enough vegetation growth from the wet seasons to then support wildfires from the dry seasons. This means that wildfires can occur in some tropical places in the winter half of the year (if the summer half of the year is the wet season and the winter half of the year is the dry season) and in other tropical places in the summer half of the year (if the summer half of the year is the dry season and the winter half of the year is the wet season), and the difference in the half of the year that corresponds to the dry season does not by itself typically lead to a significant change in vegetation. This contrasts with places in the middle latitudes, in which only those with dry summers & wet winters reliably support wildfires because the summer half of the year is much hotter & with much more potential daylight than the winter half of the year; places in the middle latitudes with wet summers & dry winters typically do not experience wildfires in the winter half of the year because of the much colder temperatures & much lower levels of potential daylight, so in places in the middle latitudes which have distinct wet & dry seasons, the time of the year of the dry season matters a lot more for determining wildfire risk & associated differences in vegetation, and the mere existence of a dry season is not enough. (In places in the subpolar latitudes & inside the polar circle, such as those in the interiors of Canada & Russia, precipitation levels are relatively low through the year and potential daylight hours are much higher in the summer half of the year, so even though the actual amount of solar power falling may be somewhat less due to the oblique angle of the sun close to the pole during the summer half of the year, the compensatory effect of the much larger amount of potential daylight during the summer half of the year along with lower levels of precipitation due to lower overall temperatures creates conditions ripe for wildfires. I won't focus on those locations further because they are not close to any coast.)

Tropical places that get a full seasonal reversal of trade winds and a wet summer

Tropical places in the northern hemisphere that get a full seasonal reversal of trade winds and a wet summer include Southeast Asia, South Asia (except for most of Pakistan because of there being more landmass to the southwest), Western Africa, the southern & western parts of Mexico & Central America (south & west of the chain of mountain ranges going from south to north), and the northernmost parts of Colombia & Venezuela. The wet monsoon winds in the summer half of the year are the reversed westerly trade winds coming from the southern hemisphere over the Indian Ocean toward Southeast Asia & South Asia, over the Atlantic Ocean toward Western Africa, and over the Pacific Ocean toward the southern & western parts of Mexico & Central America as well as the northernmost parts of Colombia & Venezuela. The dry monsoon winds are the ordinary easterly trade winds coming from the north (during the northern hemisphere winter, when it is colder) over big landmasses or blocked by mountain ranges.

Tropical places in the southern hemisphere that get a full seasonal reversal of trade winds and a wet summer include the west coast of Ecuador, the west coasts of Cameroon, Equatorial Guinea, Gabon, and the Republic of Congo, and the islands of Java & Sulawesi in Indonesia as well as the northernmost peninsulas of Australia. These groups of places get wet monsoon winds in the summer half of the year that are the reversed westerly trade winds coming from the northern hemisphere respectively over the Pacific Ocean, Atlantic Ocean, and Indian Ocean. The dry monsoon winds are the ordinary easterly trade winds coming from the south (during the southern hemisphere winter, when it is colder) over big landmasses or blocked by mountain ranges (more often the latter due to the lesser presence of landmass in the southern hemisphere).

The cold currents along the eastern edges of the Pacific Ocean & Atlantic Ocean in the southern hemisphere are consistently much colder than their counterparts in the northern hemisphere because of the influences of Antarctica, the Antarctic Ocean, and the shapes of the west coasts of South America & Africa guiding those currents directly to the equator along the coasts instead of into the bulk of the ocean (as in the northern hemisphere) where those currents can warm further. This means that the cold currents along the eastern edges of the Pacific Ocean & Atlantic Ocean in the southern hemisphere winter come much closer to the equator in the southern hemisphere than the cold currents along the eastern edges of the Pacific Ocean & Atlantic Ocean in the northern hemisphere winter do in the northern hemisphere. This means that deserts in the tropics closer to the middle latitudes (which get hot dry air over land from the easterly trade winds and do not get much moisture from the adjacent cold ocean currents) along the west coasts of South America & Africa come much closer to the equator than do the deserts in the tropics closer to the middle latitudes along the west coasts of North America & Afroeurasia. This in turn means that climates that get wet summer monsoons closer to the equator skew more to the northern hemisphere in America & Africa, and even those close to or marginally south of the equator especially along the west coast of Africa see a marked drop in precipitation for 1-2 months around the peak of the southern hemisphere winter. (This also has to do with the migration of the ITCZ further to the north at the peak of the southern hemisphere winter, which is the northern hemisphere summer.)

Tropical places that get a full seasonal reversal of trade winds and a wet summer sometimes have lower temperatures in the summer half of the year despite more sunlight technically falling during the summer half of the year. This is because the clouds & precipitation (given the high specific heat capacity of water) during the wet monsoon season create a cooling effect that more than compensates for the higher amount of sunlight; plus, such tropical places don't have such big differences in sunlight between the summer versus winter halves of the year anyway. That said, this depends a lot on the arrangement of surrounding land masses & bodies of water as well as local topography.

Tropical places that get a full seasonal or year-round reversal of trade winds and a lot of precipitation through the year

The west coast of Colombia near the Pacific Ocean gets a full seasonal reversal of trade winds, but it gets a lot of precipitation through the year because the reversed westerly trade winds over the Pacific Ocean in the summer and the ordinary easterly trade winds over the Atlantic Ocean/Caribbean Sea in the winter bring a lot of moisture. In both cases, the shape of the continent of South America and the narrow & low landmass at certain points in the isthmus of Panama mean that the trade winds are not dried out. Some parts of the west coast of Colombia are close to the Andes Mountains, which lie to the east, which can amplify precipitation. In some cases, the west coast of Colombia may get reversed westerly trade winds even in the winter, which will still bring a lot of precipitation then. Many of the islands of Southeast Asia near the Indian Ocean get a full seasonal reversal of trade winds and a lot of precipitation through the year for similar reasons as the west coast of Colombia. Additionally, all of those places lie within the ITCZ during the peak of the southern hemisphere summer, although many of those places are technically in the northern hemisphere.

Arguably, some parts of Western Africa near the Atlantic Ocean get a lot of precipitation through the year too. Those places are close to the ITCZ throughout the year, so in some places, the trade winds are westerly through the year and can bring high levels of precipitation from moisture from the Atlantic Ocean through the year. That said, the effect of the cold ocean current along the eastern edge of the Atlantic Ocean is more relevant here (markedly decreasing precipitation in the peak of the southern hemisphere winter).

Tropical places that get a full seasonal reversal of trade winds but little precipitation through the year

The Horn of Africa and a large part of Southwest Asia are tropical and experience a full seasonal reversal of trade winds but get little precipitation through the year. This is because there is too much land mass to the northeast & southwest (along the direction of the trade winds, whether ordinary or reversed). This is especially notable for the Horn of Africa, which is on the east coast of Africa and would therefore otherwise be expected to get a lot of precipitation from the ordinary trade winds; essentially, over the Arabian Sea, during the summer half of the year, the trade winds reverse to go toward South Asia and completely miss the Horn of Africa. That said, although the southeast coast of the Horn of Africa is oriented parallel to the trade winds in the peak of the northern hemisphere summer (when they are reversed southwesterly trade winds) & the peak of the northern hemisphere winter (when they are ordinary northeasterly trade winds), cities along that coast do get some precipitation around the equinoxes, when the trade winds are southeasterly due to being midway through the seasonal reversal; additionally, although little precipitation falls along that coast during the peaks of the northern hemisphere summer & winter due to the prevailing trade winds being parallel to the coast in both cases, the air can be quite humid due to localized sea breezes bringing mild moist air from over the Indian Ocean (as the general warmth of the Indian Ocean at that latitude is mitigated to some degree by specific cold currents in that area).

There are other places, like southeastern India, the south coast of Western Africa between the cities of Abidjan in the Ivory Coast & Lagos in Nigeria, and the north coast of South America between the cities of Barranquilla in Colombia & Caracas in Venezuela, that get somewhat less precipitation than would be expected given their tropical locations & proximity to the ITCZ. Southeastern India only gets moisture from the ordinary easterly trade winds over the Bay of Bengal as the ITCZ migrates toward the southern hemisphere in the northern hemisphere fall. The south coast of Western Africa between the cities of Abidjan in the Ivory Coast & Lagos in Nigeria is parallel to the direction of the trade winds, whether ordinary or reversed, so those winds can't actually bring much moisture upon those places. Finally, the north coast of South America between the cities of Barranquilla in Colombia & Caracas in Venezuela is in a rain shadow of mountains on islands to the north, so the ordinary northeasterly trade winds in the winter dry out over those mountains and the reversed southwesterly trade winds in the summer dry out over the landmass of South America.

Tropical places that get a full seasonal reversal of trade winds and a wet winter

I could only find a full seasonal reversal of trade winds and a wet winter in a few places. I could find this climate in the islands of Aruba, Bonaire, and Curaçao north of South America (creating the rain shadow over the north coast of South America). Here, the ordinary easterly trade winds in the winter bring a lot of moisture from over the Caribbean Sea, but the reversed westerly trade winds in the summer dry out over the landmass of South America. I could also find it around Baracoa in Cuba, along the north coasts of Haiti & the Dominican Republic, along the north coast of Puerto Rico in the US, around Trincomalee in Sri Lanka, along the east coast of the peninsula of Thailand & mainland Malaysia, along the east coast of the island of Sumatra, around Sandakan in island Malaysia, and along the central part of the east coast of Vietnam. These places similarly get warm moist air over oceans in the winter from the ordinary easterly trade winds in the winter especially being on the windward side of mountains with respect to the ordinary easterly trade winds, while in the summer, these places are on the leeward side (in the rain shadow) of the reversed westerly trade winds. I had listed some of these places above as getting consistent precipitation through the year; I also list them here because in a relative sense, their winters are significantly wetter than their summers even as their summers are wet in an absolute sense.

Hypothetically, places like the Horn of Africa would have wet winters if the large landmass of Southwest Asia were replaced by ocean. Alternatively, if the main continent of Oceania (mainland Australia) were extended to the north to cross the equator and had the northern part of the east coast oriented from southwest to northeast, the northeastern part of that continent along the east coast would have wet winters. All of these examples should make clear how sensitive monsoon rain patterns in different places are to the arrangements of landmasses & mountains.

Tropical places that do not get a full reversal of trade winds

Part of the northeast coast of Brazil gets high levels of precipitation through the year from easterly trade winds bringing warm moist air from the Atlantic Ocean. This is helped by the fact that the northeast coast of Brazil is in the northern hemisphere and along or north of the ITCZ through the year.

Part of the southeast coast of Brazil, most of Madagascar, the east coast of Mozambique, part of the northeast coast of Australia, and the north & east coasts of many islands in the Caribbean Sea are tropical (not middle latitude) locations that lie on the same side of the ITCZ through the year, so the ordinary trade winds remain northeasterly in the northern hemisphere & southeasterly in the southern hemisphere; in the summer hemisphere, there may be a relatively greater component of the wind coming from the pole of that hemisphere. Precipitation patterns depend on topography, especially the presence of mountains whose windward sides get consistent high levels of precipitation and leeward sides get much less precipitation. For example, the east coast of Madagascar is on the windward side of mountains with respect to the easterly trade winds through the year. By contrast, the west coast of Madagascar as well as the east coast of Mozambique are on the leeward side of mountains near the east coast of Madagascar with respect to the southeasterly trade winds, so those winds only carry enough warm moist air during the summer half of the year to overcome the rain shadow, leading to wet summers & dry winters. It is worth noting that in the peak of the northern hemisphere summer, the east coast of southern Mexico as well as Central America is technically within the ITCZ; in any case, those locations get consistently high levels of precipitation through the year.

This discussion should make clear that unlike what I naïvely thought about the trade winds supposedly always being easterly (and therefore supposedly consistently bringing precipitation to any tropical place on an east coast), being on an east coast in the tropics is a neither necessary nor sufficient condition for getting a lot of precipitation through the year. It is not sufficient as is demonstrated in the case of the Horn of Africa. It is also not necessary given how many west coasts in the tropics get consistent precipitation through the year due to both the reversed westerly trade winds & the ordinary easterly trade winds bringing warm moist air through the year or due to being close to the ITCZ through the year. Additionally, places in the Amazon Basin or Congolian Basin are quite far from any ocean but get consistent precipitation because the tropical rainforests there store enough moisture to create their own climates. Only some parts of the east coast of Brazil, the east coasts of southern Mexico & Central America, the east coast of Madagascar, and some of the islands of Southeast Asia get a lot of precipitation through the year with little to no seasonal rotation in the trade winds, and in many of these places, precipitation is higher more because those places are on the windward sides of mountains with respect to the trade winds.

East Asia

The high levels of precipitation in the summer in East Asia comes from the same tropical monsoon, as the ITCZ covers the southern half of China, Taiwan, and the southern tip of Japan during the summer; it is unusual that the ITCZ covers the middle latitudes and therefore that middle latitudes can get a tropical monsoon, and this is very different from the reason for wet summers in other middle latitude east coasts. However, the dynamics of the tropical monsoon in East Asia are slightly different compared to the tropical monsoon in many other places. In particular, warm moist air comes into the ITCZ from the subtropical ridge to the southeast over the Pacific Ocean (near North America) at that time of year, so the wet monsoon winds are the ordinary northeasterly trade winds instead of reversed westerly trade winds. (The reversed westerly trade winds would of course go in the opposite direction, but the Pacific Ocean is big enough that this doesn't seem to have as much of an effect on the middle latitude summer climate of the western part of North America.) In the winter, a system of intense persistent high pressure from cold air settles over Asia, so over the east coast of East Asia, the trade winds first come out from the west (over the continent) toward the ocean; thus, the dry monsoon winds are ordinary trade winds that happen to be westerly or northwesterly at that point (with that system of high pressure functioning like a subtropical ridge but not being the subtropical ridge per se, which is instead caused by sinking air from the upper atmosphere which heats as it falls adiabatically) but curve clockwise (to become northerly and then northeasterly) upon going further into the Pacific Ocean. Thus, although East Asia does technically experience a full seasonal reversal of the trade winds, the wet monsoon winds & dry monsoon winds are respectively in opposite directions from what are found with the tropical monsoon in other places. Additionally, most places that experience the tropical monsoon with full seasonal reversal of the trade winds are between the ITCZ & the winter hemisphere subtropical ridge in each half of the year, but this is not really the case in East Asia.

It is worth noting that quirks of geography allow some places in the middle latitudes of East Asia to get a lot of precipitation through the year. For example, Niigata on the west coast of Japan has high levels of precipitation through the year, with peaks during the peak of summer and the peak of winter. The peak of precipitation in the peak of summer is because the ordinary easterly trade winds bring air from over the Pacific Ocean that is warm enough & moist enough to overcome the rain shadow effect of the mountains of Japan. The peak of precipitation in the peak of winter is because the westerly winds blown away from the system of high pressure over Asia pick up mild moist air from over the Sea of Japan and dump it over Niigata. By contrast, the east coast of Japan only has high levels of precipitation during the summer, because in the winter, the mild moist air over the Sea of Japan that dumps precipitation then over Niigata dries out too much over those mountains before reaching the east coast of Japan.  

A more accurate view of coastal climates at different tropical, middle, and polar latitudes

With these ideas in mind, I am now more comfortable explaining trends in coastal climates with changing latitude in a more accurate way than I did in the first post in this series. These explanations will assume coastal locations at sea level; as always, with respect to a given wind, locations on the windward side of a mountain will get much more precipitation and locations on the leeward side of a mountain will get much less precipitation than might be expected for a location at sea level in the absence of mountains there. Additionally, the location & orientation of landmasses & other mountains as well as other oceans or inland seas is critical. In particular, in the northern hemisphere, the ordinary trade winds are northeasterly and the reversed trade winds as well as the prevailing westerlies are southwesterly, so for the purpose of having a high level of precipitation in at least one season, I will define in the northern hemisphere northeast & southwest coasts respectively as "favorable" east or west coasts, southeast & northwest coasts respectively as "unfavorable" east or west coasts, and east or west coasts (that lie along lines of longitude) respectively as "neutral" east or west coasts; north coasts are moderately favorable for the [northeasterly] ordinary trade winds, while south coasts are moderately favorable for the [southwesterly] reversed trade winds & prevailing westerlies. Similarly, in the southern hemisphere, the ordinary trade winds are southeasterly and the reversed trade winds as well as the prevailing westerlies are northwesterly, so for the purpose of having a high level of precipitation in at least one season, I will define in the southern hemisphere southeast & northwest coasts respectively as "favorable" east or west coasts, northeast & southwest coasts respectively as "unfavorable" east or west coasts, and east or west coasts (that lie along lines of longitude) respectively as "neutral" east or west coasts; south coasts are moderately favorable for the [southeasterly] ordinary trade winds, while north coasts are moderately favorable for the [northwesterly] reversed trade winds & prevailing westerlies. Finally, I should note that all statements about east coasts in the middle latitudes have the exception of East Asia for the reasons discussed above.

The upshot is that I now understand how important it is to account for monsoon climates around the world. I also understand how the mechanisms of monsoon climates can be analogous to the mechanisms of climates in middle latitudes and how there can be continuity of climates (in a broad sense) when moving away from the equator along any east or west coast.

It is worth noting that when the ITCZ is directly on the equator, the ordinary trade winds from each hemisphere are easterly, and there is no net rotation in the winds, which is consistent with the Coriolis force vanishing at the equator. When the ITCZ, which, as a reminder, is a belt of systems of low pressure, is in the northern hemisphere, the ordinary northeasterly trade winds converge with the reversed southwesterly trade winds in a counterclockwise way, which is consistent with systems of low pressure being counterclockwise in the northern hemisphere; in the southern hemisphere, the ordinary southeasterly trade winds converge with the reversed northwesterly trade winds in a clockwise way, which is consistent with systems of low pressure being clockwise in the northern hemisphere. Additionally, in the northern hemisphere, the ordinary northeasterly trade winds, the reversed southwesterly trade winds, and the southwesterly prevailing westerlies all turn clockwise like ocean currents (though not in a full circle like ocean currents), while in the southern hemisphere, the ordinary southeasterly trade winds, the reversed northwesterly trade winds, and the northwesterly prevailing westerlies all turn counterclockwise like ocean currents (though not in a full circle like ocean currents).

Equator to 15 degrees in latitude

On west or east coasts, dry climates will occur if there is a lot of landmass toward the equator & west and toward the pole & east; this can occur on an unfavorable east or west coast. This explains the dryness of the Horn of Africa (which forms an effective unfavorable southeast coast along with the Arabian peninsula connecting the continents of Asia & Africa), the northern part of the west coast of Peru which is an unfavorable southwest coast (the latter in addition to the rain shadow effect of the Andes Mountains toward the pole & east), and parts of the north coast of South America (which are in a rain shadow toward the pole and have a lot of landmass toward the equator).

Consistently wet climates will occur in places on either coast in three different configurations. The first possible configuration is being close to the ITCZ through the year, as in the west coast of Colombia, parts of the east coast of Brazil, and parts of the west coast of Western Africa, as the trade winds will either be ordinary easterly trade winds consistently bringing warm moist air to the east coast or reversed westerly trade winds consistently bringing warm moist air to the west coast. The second possible configuration is if a place is close to ocean both toward the pole & east and toward the equator & west, meaning that both the east & west coasts are favorable & close to each other; this usually happens only in small tropical islands far from big continents, like in Southeast Asia, because the only continental place where the land is narrow enough & oriented in the correct direction for this to occur, namely the isthmus of Panama, has a mountain range that blocks the flow of ordinary easterly trade winds to the west coast in the winter and reversed westerly trade winds to the east coast in the summer (though the latter does not lead to dry summers for the east coast of Panama because the east coast of Panama still gets ordinary easterly trade winds too, as that is where that part of the ITCZ sits in the peak of the northern hemisphere summer). The third possible configuration is that which is a bit further from the equator (in this general latitude range) but has a favorable east coast (as the trade winds would not fully seasonally rotate at all and would partially seasonally rotate less at those latitudes compared to closer to the equator); this is the only configuration of landmasses in the tropics that guarantees consistent precipitation for an east coast but not a west coast, and this happens in the east coast of Madagascar (helped by being on the windward side of mountains) and the east coast of the northern peninsula of Queensland in Australia.

Climates with wet summers will usually occur on west coasts where the ocean lies toward the equator & west and land lies toward the pole & east, which is a favorable west coast. This occurs along the west coasts of southern Mexico, central America, Ecuador, the westernmost part of Western Africa (including the coasts of Guinea-Bissau, Guinea, Sierra Leone, and Liberia), southwestern India, Southeast Asia, and the western parts of the respective peninsulas of the Northern Territory & Queensland both in Australia. This list encompasses most of the places that exhibit a full seasonal reversal of the trade winds and distinct wet & dry seasons, in which the wet monsoon winds are the reversed westerly trade winds over the ocean in the summer and the dry monsoon winds are the ordinary easterly trade winds over land in the winter.

Climates with wet winters will usually occur on east coasts where the ocean lies toward the pole & east and land lies toward the equator & west, which is a favorable east coast. These places would exhibit a full seasonal reversal of the trade winds and distinct wet & dry seasons, in which the wet monsoon winds are the ordinary easterly trade winds over the ocean in the winter and the dry monsoon winds are the reversed westerly trade winds over land in the summer. As a reminder, the only places where I could find this are a few islands immediately to the north of South America, the east coasts of islands like Sri Lanka, Malaysia, and Indonesia, the east coast of the peninsula of Thailand & mainland Malaysia, and the central part of the east coast of Vietnam; also, hypothetically, places like the Horn of Africa would have wet winters if the large landmass of Southwest Asia were replaced by ocean.

Thus, it is interesting to note that in the tropics, in contrast to the middle latitudes, favorable east coasts would have wet winters while favorable west coasts would have wet summers.

Subtropical ridge changes

Beyond 15 degrees in latitude, the subtropical ridge becomes very important to the climate, mostly to the west coast but also (to lesser extents) to the interior & east coast of a continent. The subtropical ridge forms from the air from the Hadley cell at high altitude cooling as it goes further from the equator and then falling as that cold air becomes more dense. That air adiabatically increases in temperature under compression as it falls and then diverges on the surface toward the ITCZ as the ordinary easterly trade winds (within the same hemisphere) and away from the ITCZ (within the same hemisphere) as the prevailing westerlies.

In the northern hemisphere, which has a lot of landmass, around the peak of summer, the greater temperature overall in that hemisphere pushes the subtropical ridge farther from the equator because the land & ocean are warm enough to pump more air [rising] into high altitudes before that air at high altitude cools & collects enough to fall; this is also what leads to the subtropical ridge being stronger (higher pressure at the surface) in the summer in the northern hemisphere. Because the land is a lot hotter than the ocean and there is a lot more land, air over the ocean cools faster than over land when moving toward the equator, so the subtropical ridge is more prominent over the ocean, though it is still farther from the equator in the peak of summer than in the peak of winter. Additionally, around the peak of summer, although the subtropical ridge sometimes stretches over an ocean between continents in a given latitude range, it is often more concentrated around the eastern edge of the ocean in that latitude range near the west coast of an adjacent continent because the warm ocean currents at the western edges of oceans create zones of lower pressure that more often effectively cut off the subtropical ridge there. The strong subtropical ridge near a west coast in the peak of summer in the northern hemisphere sends cool moist air in the prevailing westerlies further to the north while blocking the development of sporadic systems of low pressure that would bring more moisture to the immediate surroundings (on either side with respect to latitude) of the subtropical ridge; a weaker subtropical ridge allows more systems of low pressure to form and bring more moisture to a west coast.

In the winter, the cooling of the ocean & land means that less air is pumped to higher altitudes and the air at high altitude cools closer to the equator (than in summer) before falling. This explains why the subtropical ridge is typically weaker (lower pressure at the surface) & closer to the equator in the winter than in the summer. Additionally, at those latitudes closer to the equator, the land may be a little hotter than the ocean, so the subtropical ridge may still be in the eastern edge of an ocean at those latitudes albeit at a weaker pressure, but farther from the equator, high pressure forms more readily over land from cold continental air settling at the surface. The weakening of the subtropical ridge typically corresponds to a stronger polar front (persistent system of low pressure near the polar circle) that brings more moisture to some locations near the polar circle in the winter.

In the southern hemisphere, the explanations about the seasonal position (with respect to latitude) of the subtropical ridge and about the position of the subtropical ridge over the eastern edge of an ocean still hold for the same reasons. However, from what I have read, because the southern hemisphere has so much less landmass than the northern hemisphere, the southern hemisphere is generally less hot in the summer and less cold in the winter than the northern hemisphere, so pressures vary less over space & time in the southern hemisphere until one approaches the latitudes corresponding to the coast & continent of Antarctica. In particular, the subtropical ridges over the eastern edges of oceans in the southern hemisphere are counterintuitively stronger in the peak of the winter than in the peak of the summer, perhaps because the ITCZ draws enough hot air from the northern hemisphere during its summer to send more of it to the southern hemisphere even during its winter.

15 to 30 degrees in latitude

If a west coast is favorable, as in Mexico, it can continue to get reversed westerly trade winds bringing wet summers. Otherwise, the subtropical ridge tends to settle over a west coast in this latitude range through the year, leading to desert climates in these places, as can be seen in Peru & Chile, Northern Africa, Southern Africa, and Australia. I am not aware of any west coasts of continents (as opposed to small islands far from any continent) in this latitude range that are capable of getting a lot of precipitation through the year, because any precipitation would have to come from the reversed westerly trade winds, which only arise in the summer half of the year.

If an east coast is favorable, as in the southern tip of Florida in the US, the southern part of the southeast coast of Brazil, and the central part of the east coast of Australia, trade winds will send warm moist air bringing wet summers. Apart from the middle latitudes of East Asia which feature a different wind circulation pattern in the summer anyway, the southern part of the east coast of Mexico which is more influenced by the warmer Gulf of Mexico, and the northeastern peninsula of Australia, almost all continents' east coasts in this latitude range have favorable orientations. Differences arise when moving away from the equator, because the subtropical ridge producing trade winds from the eastern edge of an ocean whose western edge is adjacent to a given continent's east coast will shift toward the equator in the winter half of the year in that hemisphere. As one goes along an east coast toward the pole, the trade winds have less of an effect in the winter (leading to relatively dryer winters) and then through the year, so only at points closer to the equator in a favorable orientation will there be consistently high precipitation through the year; once the points farthest from the equator in this latitude range are reached, the east coast has a climate like that of an east coast in middle latitudes, depending more on sporadic sea breezes, localized atmospheric instability, and incursions of cold dry air from the pole, as opposed to dominant trade winds. It may be possible for an east coast with an unfavorable orientation to be somewhat more dry, but I don't think a desert climate would be possible along an east coast in this latitude range because of the consistency of the easterly trade winds. Additionally, except for East Asia, although an east coast location that gets consistent precipitation in the summer from the ordinary easterly trade in the summer but not the winter (due to the subtropical ridge having moved too close to the equator in the winter) may get relatively much more precipitation in the summer than in the winter, it is rare for such a location to have a truly dry winter because the localized interactions in the winter of warm moist air over the ocean with cold dry air over land promote precipitation even in the winter; the localized nature of these interactions means that this point holds exactly on the coast irrespective of nearby mountains which may affect global wind patterns like the trade winds or the prevailing westerlies. For example, along the northeast & east coasts of the state of Queensland in Australia, which are somewhat unfavorably oriented with respect to the trade winds for precipitation, there is still a lot of precipitation in the summer half of the year, and precipitation in the winter half of the year still occurs due to sea breezes from over the warm ocean current; that said, just 50 miles away from the coast, the climate transitions to a desert because of the combination of the unfavorable orientation of the coast with respect to precipitation from the ordinary easterly trade winds and in part because of a slight rain shadow effect from mountains closer to the coast.

As far as I am aware, apart from places in the rain shadow of a mountain or mountain range, the only coastal places that are guaranteed to be deserts (apart from polar tundra or ice cap regions), in the absence of continued monsoon effects (as in the west coast of Mexico in this latitude range due to the position of the ITCZ in the summer), are those on the western side of a continent (including the coast) in the latitude range of 15-30 degrees. This is because when the subtropical ridge settles closer to the equator in this latitude range near a west coast (as in the winter), it ensures that cool moist prevailing westerlies from the side of the subtropical ridge closer to the pole go toward the pole, so the land in the same latitude range as the subtropical ridge does not experience a strong prevailing wind, whereas when the subtropical ridge settles farther from the equator inside or even outside of this latitude range (as in the summer), it produces easterly trade winds going over land toward the ITCZ and thereby bringing hot dry air to these regions.

Furthermore, although desert climates in this latitude range are often associated with extreme heat and generally extreme variations in temperature, places in this latitude range on the west coast of a continent, even if in a desert, can experience mild temperatures overall with little diurnal (day versus night) or seasonal variation in temperature if they are directly on the coast and other specific conditions hold. For example, the cities in the Atacama Desert along the west coasts of the northern part of Chile & the southern part of Peru, in the latitude range of 7-30 degrees south (between the cities of Chiclayo in Peru & La Serena in Chile), have diurnal & seasonal average temperature ranges that span less than 8 degrees Celsius because the cold current of the eastern Pacific Ocean (influenced in part by the Antarctic Ocean) in the southern hemisphere produces sea breezes with cool moist air that ensure atmospheric stability (in turn ensuring little precipitation) & moderate temperatures; additionally, there is very little landmass from west to east in the Atacama Desert (before encountering the Andes Mountains when going from the Pacific Ocean in the west toward the east) to heat up excessively and support prevailing winds bringing hot dry air to other areas. Similarly, the cities in the Namib Desert along the west coasts of South Africa, Namibia, and Angola, again in the latitude range of 8-30 degrees south (between the cities of Luanda in Angola & Port Nolloth in South Africa), again have diurnal & seasonal average temperature ranges that span less than 8 degrees Celsius because the cold current of the eastern Atlantic Ocean (influenced in part by the Antarctic Ocean) in the southern hemisphere produces sea breezes with cool moist air that ensure atmospheric stability (in turn ensuring little precipitation) & moderate temperatures; in contrast to the Atacama Desert, there is somewhat more landmass from west to east in the Namib Desert (when going from the Atlantic Ocean in the west toward the east), but the cold ocean current is enough to keep the coast itself cool or moderate in temperature, though moving even a few miles inland into the Namib Desert can lead to much larger diurnal & seasonal temperature variations as the effects of the cool sea breezes quickly dissipate (with respect to distance from the coast). By contrast, the cities along the west coast of Australia (most of which is a desert anyway) experience much bigger diurnal & seasonal average temperature ranges, partly because the cold current in the eastern Indian Ocean in the southern hemisphere is not as cold as its eastern counterparts in the Pacific & Atlantic Oceans in the southern hemisphere and partly because the shape of the west coast of Australia allows the cold current in the eastern Indian Ocean in the southern hemisphere to curve away from the coast more easily (unlike the shapes of the west coasts of Africa & South America in the southern hemisphere), so there is less moderation & atmospheric stabilization by sea breezes coming over the cold current; additionally, there is much more landmass in Australia east & toward the pole from the west coast, so there is much more opportunity for the land to heat up a lot and for trade winds to blow hot dry air toward the west coast. Similar effects & explanations hold for the cities along the west coast of North America in the deserts of the Baja California Peninsula as well as the cities along the west coast of Africa in the Sahara Desert; in the latter case, a few cities do have smaller diurnal & seasonal variations in temperature, but these are all cities on parts of the coast oriented from the southwest to the northeast, such that the trade winds are not traveling over as much hot dry land before reaching those cities, and even nearby cities along the coast can get much hotter if the coast is oriented differently, showing how sensitive this effect is to the coastline orientation if the cold current is not cold enough.

30 to 45 degrees in latitude

A west coast will generally always have wet winters & dry summers in this latitude range. However, because of the fact that the subtropical ridge is closer to the equator in each hemisphere during that hemisphere's winter but is stronger in both hemispheres during the northern hemisphere summer & weaker in both hemispheres during the northern hemisphere winter (southern hemisphere summer), the reasons for having wet winters & dry summers differ by hemisphere. In either hemisphere, a favorable orientation of the west coast may make more precipitation more likely to fall in the winter.

In the northern hemisphere in the summer, the subtropical ridge is closer to the pole in this latitude range and stronger. This means that the prevailing westerlies in the summer only bring cool moist air (and in turn precipitation) toward the west coast at latitudes closer to the poles barely inside & mostly outside of this latitude range, while in most of this latitude range, the subtropical ridge brings easterly trade winds toward the equator over hot dry land. Effectively, the desert climate closer to the equator from the subtropical ridge in the latitude range of 15-30 degrees effectively extends further toward the pole. That said, if the subtropical ridge happens to be closer to the equator in the summer on a given day, there may be a little bit more precipitation than usual in the summer due to the influence of the prevailing westerlies bringing cool moist air, while if the subtropical ridge happens to be weaker (irrespective of position in the sense of latitude) in the summer, the west coast will have more variable temperature & precipitation patterns in the summer owing more to the sporadic formation of localized systems of low pressure; a favorable orientation of the west coast could make a difference in precipitation levels in the summer when the subtropical ridge is closer to the equator or weaker. In the winter, the subtropical ridge is closer to the equator in this latitude range & weaker, so the west coast gets both steady but light precipitation from the weaker prevailing westerlies coming from closer to the equator and more sporadic precipitation from localized systems of low pressure that are more likely to form with a weaker subtropical ridge. Going toward the pole, summers and winters will become a bit more wet; summers will become more wet because those locations are more likely to experience the cool moist prevailing westerlies & less likely to experience the hot dry trade winds on a given day, and winters will become more wet because of a greater contrast in temperature between the cool moist air of the prevailing westerlies over the ocean and the cold dry land (more because the land becomes colder closer to the pole, as the ocean temperature will not change as much over that latitude range along the eastern edge of an ocean).

In the southern hemisphere in the summer, the subtropical ridge is closer to the pole in this latitude range but weaker; additionally, it does not deviate as far from the equator in the summer in the southern hemisphere as in the northern hemisphere because the cold currents along the eastern edges of major oceans tend to be colder in the southern hemisphere than in the northern hemisphere through the year. Thus, the dryness of the summer can be explained on the side of the subtropical ridge closer to the equator by the production of easterly trade winds over hot dry land and on the side of the subtropical ridge closer to the pole by the weakness of the subtropical ridge in the summer compared to the winter. In the winter, the subtropical ridge is closer to the equator in this latitude range but stronger. Thus, the wetness of the winter can be explained on the side of the subtropical ridge closer to the pole by the greater strength, beyond the mere existence, of the prevailing westerlies in this latitude range in the southern hemisphere.

In the winter hemisphere, the subtropical ridge is not the only system of high pressure present, though it is mostly located over the oceans. Major continents also support systems of high pressure due to the settling of cold air. These systems of high pressure slow, divert, or otherwise block prevailing winds, so moisture from the prevailing westerlies that precipitates over a west coast does not usually penetrate much farther. Instead, the prevailing westerlies may go around the systems of high pressure over the continent, whether toward the pole or toward the equator, and continue to the next ocean and its subtropical ridge. In effect, the subtropical ridges over each ocean and the systems of high pressure over adjacent continents together constitute a nearly continuous belt of high pressure in the middle latitudes, in which cool moist air from the prevailing westerlies over an ocean to the west of a continent can be steered around the continent and toward the next ocean and continent to the east. The only caveat to this explanation is that the system of high pressure over Asia in the winter half of the year is strong enough to completely divert the prevailing westerlies toward the equator or pole and effectively create its own prevailing westerlies moving toward North America.

An east coast in either hemisphere in this latitude range will almost always have moderate precipitation through the year because of sporadic interactions between sea breezes & hot or cold air over land, regardless of nearby mountains. The orientation of the east coast may have a small effect, because a favorable orientation implies the existence of an ocean separating the land mass in question from places closer to the pole, so that ocean can mitigate the effects of the incursions of cold dry air from the pole (though this can also reduce precipitation by reducing the possibility of cold dry air coming in anew and forcing precipitation from existing warm moist air, so the net effect may be close to zero); for example, in Australia, the east coast of the state of New South Wales, which is closer to the pole, is favorable, so moisture & precipitation can penetrate further inland and one must go farther inland to find desert climates, whereas the east coast of the state of Queensland, which is closer to the equator (in the latitude range of 15-30 degrees), is unfavorable, so moisture & precipitation do not penetrate as far inland and one does not have to go as far inland to find desert climates. In this latitude range, the prevailing westerlies are typically too weak & dry to have much of an effect (beyond perhaps introducing more air from over hot dry land in the summer or cold dry land in the winter), so most precipitation comes from sporadic & more local clashes of air masses. That said, even though the prevailing westerlies either do not reach locations closer to the equator in this latitude range and instead are more likely to reach locations closer to the pole in this latitude range by which point they are essentially dry, the localized interactions of warm moist air over the ocean with either cold dry air over land in the winter or some combination of hot dry air over land along with cold dry air from the pole in the summer essentially prevent the formation of deserts along east coasts in this latitude range. The only exception to this is the east coast of Argentina in the latitude range of 40-45 degrees (and also in the latitude range of 45-50 degrees for similar reasons), where the warm current of the western Atlantic Ocean closer to the pole is replaced by the cold current of the Antarctic Ocean extending toward the equator along that coast, so there is not much chance for enough moisture to accumulate in the air to then precipitate above land; this does not happen along the east coasts of North America or Russia in those latitude ranges because the east coast of North America in those latitude ranges is still affected by the Gulf Stream over the Atlantic Ocean to enough of an extent to ensure high levels of precipitation in the summer half of the year and the east coast of Russia in those latitude ranges gets moist easterly winds going into the ITCZ in the summer half of the year (that goes as far as 35 degrees north in latitude in Asia during the peak of the northern hemisphere summer), and those east coasts during the northern hemisphere winter are close enough to the influence of the polar front to get high levels of precipitation during the winter half of the year.

I should note that none of this contradicts what I've previously said about the effects of the Gulf of Mexico on the climate of North America east of the Rocky Mountains through the year. In particular, the subtropical ridge off of the west coast of Afroeurasia promotes trade winds & ocean currents that sweep clockwise into the Gulf of Mexico and bring precipitation from warm moist air into North America east of the Rocky Mountains (though more into the US than into Canada) through the year.

It is also worth noting that because the subtropical ridge sends cool moist air to west coasts toward the pole & warm moist air to east coasts toward the equator, the range of latitudes with distinct wet & dry seasons on the west coast (30-45 degrees) is closer to the pole than on the east coast (15-30 degrees). By contrast, in the latitude range of 15-30 degrees, the west coast is a desert, while in the latitude range of 30-45 degrees, the east coast is consistently moderately wet. Additionally, locations in the Trewartha categorization with climate types with the first letter C (subtropical) tend to be in this latitude range on west & east coasts but also tend to extend closer to the pole on a west coast than on an east coast; this may seem counterintuitive given that east coasts are adjacent to warm ocean currents while west coasts are adjacent to cold ocean currents, but the simple explanation is that the condition ensuring that all months have mean temperatures of at least 10 degrees Celsius is most easily achieved with the prevailing westerlies consistently bringing cool moist air to moderate the temperatures in the coldest part of the year as opposed to depending on local sea breezes in east coast locations where the prevailing westerlies would bring cold dry air over land, and the action of the Coriolis force on the subtropical ridge & on global ocean currents necessitates that the prevailing westerlies in the middle latitudes go over cold ocean currents before reaching land. As a reminder, the discussion about dry & wet seasons in this latitude range is specifically about coastal locations; things can change even relatively short distances inland from either coast.

I think this is a good point at which to further discuss & reiterate why almost the entire interior of Australia is a desert, in contrast to the contiguous US, despite the two countries having similar shapes & sizes. First, Australia is much closer to the equator than the contiguous US. This means that while the weather of the coasts in the contiguous US is mostly dominated by the positions of the subtropical ridges in the Pacific & Atlantic Oceans, Australia is covered in part by the ITCZ in the southern hemisphere summer (though the positions of the subtropical ridges in the Indian & Pacific Oceans through the year matter too). This in turn means that winds turning clockwise from the subtropical ridge over the eastern Atlantic Ocean can sweep westward & clockwise over the Gulf of Mexico and bring reliable, if sporadic, precipitation to the eastern half of the contiguous US, whereas Australia is too close to the equator & the ITCZ for winds from the subtropical ridge over the the eastern Pacific Ocean to sweep westward & counterclockwise and bring precipitation through the north coast of Australia & into its interior. Additionally, even if such hypothetical winds could reach the north coast of Australia, they would lose a lot of moisture on the windward sides of mountains in the islands of Southeast Asia, whereas this is less of an issue for winds sweeping west and then north over the Gulf of Mexico. Second, during the southern hemisphere winter, cold air at high pressure settles over most of Australia, though the west coast of Australia gets steady moderate precipitation from the prevailing westerlies originating from the subtropical ridge extending over the Indian Ocean in the southern hemisphere, and the subtropical ridge over the Pacific Ocean in the southern hemisphere is too far away to contribute much. By contrast, the Atlantic Ocean in the northern hemisphere is small enough compared to the Pacific Ocean in the southern hemisphere that even in the northern hemisphere winter, when cold air at high pressure settles over much of North America, the subtropical ridge over the eastern Atlantic Ocean still brings significant precipitation to the eastern half of the contiguous US as its winds sweep over the Gulf of Mexico & the western Atlantic Ocean. Third, desert climates are closer to the east coast of Australia than to the east coast of the contiguous US for the aforementioned reasons and also partly because of the different orientations of land masses & coastlines. Fourth, during the southern hemisphere summer, the subtropical ridge over the eastern Indian Ocean not only moves closer to the pole but extends further east into the Great Australian Bight directly to the south of the main continental landmass of Australia, and as this body of water has cold currents associated with the Antarctic Ocean, the ordinary easterly trade winds originating over this body of water bring cool moist air, in turn bringing atmospheric stability & little precipitation over most of Australia (including the west coast of Australia in the summer half of the year).

Polar front changes

The system of low pressure from air rising near the polar circle (where the Ferrel cell meets the polar cell) is known as the polar front. In the southern hemisphere, through the year, the prevailing westerlies bring cool moist air over the Antarctic Ocean and then hits a wall of cold dry air coming from the system of high pressure (the polar high) over the south pole in the interior of Antarctica. This relatively warmer & less dense air (due to the moisture continent) quickly rises and slowly warms the cold dense dry air below, leading to a persistent system of low pressure around the coast of Antarctica. The polar front in the southern hemisphere thus persists through the year and is characterized by violent storms. The southern hemisphere from 60 degrees in latitude to the pole is covered by the landmass of the continent of Antarctica, so while the temperature of the continent & the pressure in the polar front may change seasonally, the qualitative dynamics are the same through the year. Thus, any further discussion of climates from 60 degrees in latitude to the pole will exclusively be about the northern hemisphere.

In the northern hemisphere, the greater landmass, including north (toward the pole) of 60 degrees in latitude, means that systems of low pressure cannot persist through the year as they do in the southern hemisphere. In particular, these systems of low pressure typically form in the winter south or southwest of Alaska in the US, immediately to the south of Greenland, and immediately to the north of Scandinavia in Europe, and dissipate in the summer. In both hemispheres, the polar easterlies blow from the polar highs toward the polar fronts, but they are much more irregular; additionally, because systems of low pressure near the polar circle are rare throughout the northern hemisphere in the summer and are confined to more oceanic locations in the northern hemisphere in the winter, cold dry air from the poles can more easily blow directly into North America & the eastern half of Asia through the year.

45 to 60 degrees in latitude

A west coast in this latitude range will be wet through the year, though precipitation levels will be less through the year than in the latitude range of 30-45 degrees because the colder air & water temperatures lead to a lower equilibrium partial pressure of water. A favorable orientation can promote higher levels of precipitation through the year; there is no comparable effect for a favorable orientation of an east coast in this latitude range, just like how a favorable orientation only of an east coast (not a west coast) in the tropics can lead to consistently high levels of precipitation through the year. Summers may still be relatively slightly more dry than winters even though higher temperatures correspond to higher equilibrium partial pressures of water vapor. However, the reason for this differs by hemisphere.

In the northern hemisphere in the summer, the subtropical ridge is usually strong and close to the point in this latitude range closest to the equator along a west coast. This means that summer precipitation is moderately heavy & steady close to the subtropical ridge on the side closer to the pole. In the winter, the subtropical ridge weakens & moves closer to the equator, which means that the contribution of moisture from the prevailing westerlies is less as the equilibrium partial pressure of water vapor is lower with lower temperature (as in the winter). However, this is more than compensated for by the much greater strength of the polar front, which can steer cool moist air from closer to the pole (including initially cold dry air from the polar high that then gains temperature & moisture) toward the east & toward the equator in this latitude range.

By contrast, in the southern hemisphere, the only major landmass in this latitude range is South America, and the polar front is strong through the year (though relatively weaker in the summer than in the winter) & mostly confined to the coast of Antarctica far from any other continent. This means that the polar front has less of an effect on the west coast of South America in this latitude range except for the Falkland Islands & other islands in the Tierra del Fuego region. Instead, the wet winters can be explained by the greater strength of the subtropical ridge in the southern hemisphere in the winter (even as the subtropical ridge is slightly closer to the equator), leading to heavier steady precipitation in this latitude range, and the relatively dry but still absolutely somewhat wet summers in this latitude range can be explained by the weakening of the subtropical ridge (even as it moves slightly closer to the pole), allowing the formation of more localized systems of low pressure bringing precipitation to the west coast.

For a west coast in either hemisphere, if the subtropical ridge happens to be stronger & farther from the equator, summers in this latitude range will be cooler & wetter due to the prevailing westerlies being stronger & picking up more moisture over the ocean (as higher pressure corresponds to the compressed air being at higher temperature). If the subtropical ridge is stronger but closer to the equator, summers in this latitude range will be hotter & drier due to the prevailing westerlies being stronger & picking up more moisture over the ocean closer to the latitude range of 30-45 degrees. If the subtropical ridge is weak, irrespective of position, summers in this latitude range may have more variable temperature & precipitation patterns influenced by the more sporadic formation of localized systems of low pressure.

Just as in the latitude range of 30-45 degrees, in the winter hemisphere, the subtropical ridge is not the only system of high pressure present, though it is mostly located over the oceans. Major continents also support systems of high pressure due to the settling of cold air. These systems of high pressure slow, divert, or otherwise block prevailing winds, so moisture from the prevailing westerlies that precipitates over a west coast does not usually penetrate much farther. Instead, the prevailing westerlies may go around the systems of high pressure over the continent, whether toward the pole or toward the equator, and continue to the next ocean and its subtropical ridge. In effect, the subtropical ridges over each ocean and the systems of high pressure over adjacent continents together constitute a nearly continuous belt of high pressure in the middle latitudes, in which cool moist air from the prevailing westerlies over an ocean to the west of a continent can be steered around the continent and toward the next ocean and continent to the east. The only caveat to this explanation is that the system of high pressure over Asia in the winter half of the year is strong enough to completely divert the prevailing westerlies toward the equator or pole and effectively create its own prevailing westerlies moving toward North America.

An east coast in this latitude range will have a much less predictable climate, because in this latitude range, the warm ocean currents from the equator would have given way to cold ocean currents & winds from the poles. If an east coast happens to be near or otherwise influenced by the polar front, it may be quite wet at least during the winter; otherwise, it will be dry through the year, almost like a desert. That said, I could only find such dry climates in east coasts in this latitude range in South America. By contrast, east coasts in this latitude range in Canada & Russia are too greatly affected by nearby polar fronts especially in the winter despite in principle the simultaneous presence of drying effects of systems of high pressure from cold air settling in the continents of North America & Asia. In this latitude range, the orientation of the east coast will not matter much for precipitation.

It is therefore notable that the only latitude range in the northern hemisphere where climates on the west coast & east coast can look quite similar is 45-60 degrees (mostly directly to the south, closer to the equator, of the polar front in the winter); in this case, the climates have moderately wet winters & somewhat dry summers. This is because the west coast of Alaska in the US, the east coast of Kamchatka in Russia, and the east coast of Canada in this latitude range are all close to polar fronts in the winter due to the shapes of the continent. (Europe has a much less regular shape in this latitude range and is much more affected by the warmth of the Gulf Stream, as the polar front goes much further to the north, even further north than Scandinavia even in the winter.) Additionally, in this latitude range, the warm current of every major ocean turns away from the east coast of the continent to the west, flows to the east, and then turns toward the equator upon hitting the west coast of the continent to the east, so the west coast in this latitude range is adjacent to the major ocean current at its coldest, while the east coast in this latitude range is adjacent to cold minor ocean currents coming from the pole. In the southern hemisphere, the only major landmass in this latitude range is South America, and it is much smaller in this latitude range than comparable landmasses in the northern hemisphere, so this fact in conjunction with the persistence of the polar front as a continuous belt around Antarctica through the year mean that these arguments about climates in this latitude range in the northern hemisphere do not apply to the southern hemisphere.    

60 to 75 degrees in latitude

In the northern hemisphere, Scandinavia in Europe is south of the polar front in the winter and is therefore much more influenced by the prevailing westerlies through the year; it is therefore somewhat anomalous, and this happens because of the warmth of the Gulf Stream over the Atlantic Ocean (which is one of the few times that a popular lay explanation about the warmth of Europe involving the warmth of the Gulf Stream is correct). Additionally, the precipitation patterns of Iceland are much more influenced by the strength of the polar front in the winter (as opposed to the summer) in the immediate vicinity, leading to wetter winters throughout the country. Finally, the west coast of Alaska in the US & the east coast of Kamchatka in Russia are too close to each other to see a significant difference in climates between west & east coasts in that sense (as the Bering Strait is quite narrow).

Greenland is the only big island/continental landmass that is surrounded by ocean on most sides (though the landmass of North America is nearby to the southwest) in this latitude range and has the polar front to the south. Additionally, the warming effects of the Gulf Stream ensure that the oceans & seas around Greenland do not freeze for most of the year. Therefore, one can predict and indeed observe that the west coast of Greenland experiences relatively more wet summers due to the effects of the prevailing westerlies bringing cool moist air over the ocean & dry winters due to the polar easterlies blowing over cold dry land, while the east coast of Greenland experiences relatively more dry summers due to the effects of the prevailing westerlies blowing over cold dry land & wet winters due to the polar easterlies bringing cool moist air over the ocean.

A similar effect to the west coast of Greenland, for the same reasons, arises in the south coasts of the mainlands of Alaska in the US & Kamchatka in Russia (while the Aleutian Islands get much more precipitation through the year, with more in the winter, by being far enough south to be influenced more by the faraway subtropical ridge in the summer bringing a little precipitation & the nearby polar front in the winter bringing a lot of precipitation): the prevailing westerlies dominate in the summer, bringing cool moist air over the ocean from the south, while the polar easterlies dominate in the winter, bringing cold dry air over land from the north. In principle, a similar effect to the east coast of Greenland, for the same reasons, should arise in the north coasts of the mainlands of Alaska in the US & Kamchatka in Russia, as the prevailing westerlies dominating in the summer should bring cold dry air from over land while the polar easterlies dominating in the winter should bring cool moist air from over the ocean, but instead, there is more precipitation in the summer than in the winter (and little precipitation overall) because large-scale freezing of the Arctic Ocean & nearby seas in the winter prevents the polar easterlies from picking up much moisture.

75 degrees in latitude to pole

In the northern hemisphere, this latitude range is mostly taken up by the Arctic Ocean. Very little land exists in the form of small islands, and in any case, the spherical shape of the Earth means that uniform ranges of latitude correspond to smaller surface areas closer to the poles. Thus, there is not much more that I can say about climates in this latitude range.

Trends in coastal climate types by latitude

It is worth noting the following. In progressing from the equator to the pole in each latitude range, the west coast climate progresses from very wet summers to desert to moderately wet winters to consistently somewhat wet to somewhat wet summers, while the east coast climate progresses from very wet winters (in principle) to moderately wet summers to consistently somewhat wet to either desert-like or very wet winters to somewhat wet winters. Thus, the wet & dry seasons being summer versus winter flips between coasts in the latitude range of the equator to 15 degrees and in the latitude range of 60-75 degrees, and then simultaneously between coasts & between latitude ranges for latitude ranges in between; other precipitation distributions also flip between simultaneously coasts & between latitude ranges. Additionally, the progressions of climate types in east versus west coasts are not merely cyclic permutations of each other, particularly because west coasts are more strongly influenced by the subtropical ridge in the latitude range of 15-75 degrees and by the polar front in the latitude range of 45-75 degrees, whereas east coasts are influenced by the subtropical ridge more in the latitude range of the equator to 30 degrees and by the polar front in the latitude range of 60-75 degrees; deserts occur along west coasts in the latitude range of 15-30 degrees because of the greater influence of the subtropical ridge but along east coasts in the latitude range of 45-60 degrees more because of the weaker prevailing westerlies bringing cold dry air over the continent and cold ocean currents replacing warm ocean currents, and consistently wet climates occur along west coasts in the latitude range of 45-60 degrees because of the interplay between the subtropical ridge in the summer half of the year & polar front in the winter half of the year but along east coasts in the latitude range of 30-45 degrees because of local interactions with warm ocean currents through the year in the absence of stronger prevailing winds.

It is particularly notable that the places which experience easterly winds in the winter & westerly winds in the summer, whether ordinary easterly & reversed westerly trade winds in the tropics or polar easterlies & prevailing westerlies in the polar/subpolar latitudes, have east coasts that can experience dry summers & west winters and west coasts that can experience wet summers & dry winters. In the middle latitudes, the situation flips for the west coast, which experiences dry summers & wet winters due to the flip between the ordinary easterly trade winds & prevailing westerlies. Additionally, apart from the conditions specific to East Asia, there is no east coast in the middle latitudes that experiences a truly distinct dry winter that contrasts with a wet summer, because precipitation that can occur because of localized interactions of air masses in the summer can have the same interactions produce precipitation in the winter, though there can be east coast locations that relatively much more precipitation in the summer than in the winter; for the same reason, the only latitude range where east coasts can be deserts is 45-60 degrees, where warm currents from a major ocean are replaced by cold currents from the pole.

Subtropical monsoon

The idea of a subtropical monsoon may be problematic or controversial. (This is separate from the tropical monsoon occurring in the middle latitudes of East Asia.) I don't claim to have completely convincing arguments in favor of the existence of subtropical monsoons, but these arguments could have some merit.

Subtropical wet-summer monsoon

The only example of a subtropical wet-summer monsoon that I could find is the North American monsoon. In this case, the system of low pressure from air rising over hot dry land is localized and is not part of the ITCZ, the source of warm moist air is the Gulf of California rather than a large ocean, and although the dry monsoon winds are the ordinary easterly trade winds, the wet monsoon winds are reversed westerly winds drawn more locally into the system of low pressure as opposed to a true reversal of the trade winds from the southern hemisphere.

Arguably, the east coast of any continent in the latitude range of 15-30 degrees, closer to the pole in this range, exhibits a subtropical wet-summer monsoon (as locations closer to the equator are more likely to be consistently wet through the year due to the consistent effect of the ordinary easterly trade winds through the year). There are somewhat clear differences in wet & dry seasons. However, the easterly trade winds in the summer do not typically reverse in the winter for east coasts in this latitude range, as it is unlikely for the prevailing westerlies in the winter from the subtropical ridge adjacent to the west coast of the same continent to reach the east coast of that continent in this latitude range. Additionally, the winter is not typically truly dry, because even if the subtropical ridge along the eastern edge of the ocean adjacent to the given east coast moves too close to the equator in the winter to produce trade winds hitting the given east coast as they do during the summer, localized interactions of air masses produce sporadic precipitation through the winter.

Subtropical wet-winter monsoon

Arguably, the west coast of any continent in the latitude range of 30-45 degrees exhibits a subtropical wet-winter monsoon. There are clear differences in wet & dry seasons, and these correspond to a reversal of winds between the easterly trade winds & the prevailing westerlies. However, unlike the tropical monsoon where wet & dry seasons arise from the ITCZ moving over a location such that a location is always between the ITCZ & the winter hemisphere subtropical ridge but the seasons flip which hemisphere winter occurs in, this supposed subtropical wet-winter monsoon has wet & dry seasons arising from the subtropical ridge moving past a location such that a given location is always between the subtropical ridge of its own hemisphere (regardless of season) and either the ITCZ (in the summer) or the polar front (in the winter). In particular, the reversal of winds has nothing to do with air rising over hot dry land per se or with the Coriolis force reversing trade winds from the winter hemisphere, unlike the case for the tropical monsoon.

It is hard for me to imagine a place that could support an analogue of the North American monsoon in middle latitudes but with a wet winter & dry summer, arising from factors that have nothing to do with the subtropical ridge. This would seem to require the formation over land of high pressure in the summer & low pressure in the winter far from any major continent. I'm not aware of any place that exhibits this, perhaps because land heats up enough in the summer to support low pressure (making high pressure over land harder to find in the summer) & cools down enough in the winter to support high pressure (making low pressure over land harder to find in the winter).

Subpolar monsoon

The idea of a subpolar monsoon may be even more problematic or controversial than the idea of a subtropical monsoon. I don't claim to have completely convincing arguments in favor of the existence of subpolar monsoons, but these arguments could have some merit.

The west coast of Greenland as well as the south coasts of mainland Alaska in the US & mainland Kamchatka in Russia have distinctly wet summers & dry winters. The east coast of Greenland has distinctly dry summers & wet winters. Both of these are caused by reversals of the winds from the prevailing westerlies in the summer to the polar easterlies in the winter, as the polar front shifts toward the pole in the summer & toward the equator in the winter. However, just as with the subtropical monsoon, the reversal of winds has nothing to do with a reversal of the Coriolis force; it is due to a location being between the polar front and either the subtropical ridge in the summer or the polar high in the winter.

Implications of global warming

Global warming could have significant effects on climates around the world, with respect to temperatures, humidity levels, precipitation levels, and precipitation patterns through the year. Changes to climates could arise directly from increases in ocean temperatures, directly from increases in land temperatures, indirectly from the effects of ocean warming on the subtropical ridge & polar front, and indirectly from the effects of land warming on the ITCZ. I will go through each change individually, all else being equal, and then discuss where these different effects may work either in tandem with or against each other.

Changes arising directly from land warming

Presumably, as the land warms, temperatures will warm overall. This much seems obvious, though it will only hold as a long-term average.

The effects of warming land on precipitation patterns, all else being equal (i.e. assuming no ocean warming and assuming no changes to the major seasonal global systems of high or low pressure), is somewhat more subtle. Because there can be no discussion of warming land in the tropics without accounting for changes in the ITCZ and because the polar circle is the Arctic Ocean in the northern hemisphere in contrast to the continent of Antarctica in the southern hemisphere (making direct comparisons between hemispheres difficult), I will focus on the effects of warming land in the middle latitudes except in Asia.

On any continent in the summer hemisphere, hotter land will lead to locally lower pressure over land. This will draw in moist air from both oceans. Air drawn over warm currents, especially along an east coast, will be warm & more moist and therefore more likely to contribute to atmospheric instability, leading to more intense precipitation events even in the absence of incursions of cold dry air from the pole (while such incursions could increase the number & intensity of precipitation events). Air drawn over cold currents, especially along a west coast, will be cool & only marginally moist and therefore more likely to contribute to atmospheric stability, so days will be hotter & drier regardless of incursions of cold dry air from the pole (which is rare along west coasts in the middle latitudes because of topography). Thus, in the middle latitudes, the eastern half of a continent will experience hotter summers with more precipitation, while the western half of a continent will experience hotter summers that are even more dry (with respect to precipitation as well as ambient humidity). Of course, different topographies will lead to different local or regional effects.

On any continent in the winter hemisphere, hotter land will exhibit a lower temperature contrast with oceans compared to now, where oceans tend to be warmer than land. This existing temperature contrast partially explains why winters in many parts of a continent tend to be wet, so a decrease in this contrast may lead to less precipitation in many parts of a continent in the winter hemisphere. Of course, different topographies will lead to different local or regional effects.

Changes arising directly from ocean warming

Warming oceans by themselves can be assumed to have little direct effect on land temperatures. The indirect effects of warming oceans on land temperatures & precipitation patterns, all else being equal (i.e. assuming no land warming and assuming no changes to the major seasonal global systems of high or low pressure), is subtle.

In the northern hemisphere, ocean warming could make the Arctic Ocean free of ice for more months (around the peak of summer) in the year. This means that the east coast of Greenland and the north coasts of Alaska in the US & of Kamchatka in Russia could become progressively more wet in the winter relative to the summer due to the flipping between the prevailing westerlies over dry land in the summer & the polar easterlies over the ocean in the winter. This corresponds to the polar front moving closer to the pole even in the winter as the Earth warms. That said, I'm not sure if the polar high will also weaken due to the warming and if this will in turn weaken the effect of more precipitation in the winter in these locations.

In both hemispheres, ocean warming may change tropical wet monsoon seasons in the following ways (assuming no change to the ITCZ). Places that get tropical wet monsoons already will likely get more precipitation during the same times of year that they already get precipitation (whether in the summer versus winter halves of the year), simply because warmer oceans can support wet monsoon winds with a higher partial pressure of water vapor due to the higher temperature. Many west coasts in the tropics get reversed westerly trade winds as wet monsoon winds, but especially in South America & Africa, the very cold currents in the eastern Pacific & Atlantic Oceans coming from the southern hemisphere reduce the amount of precipitation on land during the wet monsoon seasons. As these currents warm further over many years, more precipitation may fall on these west coasts, including in places that are currently the northwestern parts of the Atacama Desert in South America & Namib Desert in Africa. Initially, this increase in precipitation may devastate communities as the dry desert sand & soil cannot absorb so much water, but over even longer periods of time, these desert climates may shift toward supporting more vegetation, effectively shifting the points of these deserts closest to the equator farther from the equator than they are now. A similar effect may occur in the deserts of northwestern Mexico & the southwestern US, the Sahara Desert, and the western part of the Great Australian Desert, but to a lesser extent than for the Atacama & Namib Deserts because for the other deserts, the cold currents on the eastern edges of the corresponding oceans are already weaker & do not hug the west coasts of adjacent continents as tightly between the equator & 15 degrees in latitude (compared to the cold currents of the eastern Pacific & Atlantic Oceans in the southern hemisphere). Additionally, even apart from the question of whether precipitation levels will increase, humidity will increase through the year.

In the southern hemisphere, warming of the Antarctic Ocean may lead to more precipitation through the year in the southern parts of South America as well as the southern parts of Australia & New Zealand. However, I'm only somewhat confident saying this about the southern part of Australia due to the potential for sea breezes coming upon the desert along the south coast of Australia leading to precipitation from destabilizing rather than stabilizing (as is currently the case) the atmosphere; with respect to South America & New Zealand, this is a rougher conjecture. I don't expect as much change along the south coast of South Africa from the warming of the Antarctic Ocean because South Africa already gets a warm current close to its south coast and it is too far from the Antarctic Ocean anyway.

In both hemispheres in the middle latitudes, when considering the winter half of the year, west & east coasts will get more precipitation (assuming no changes to land temperatures or to the major seasonal global systems of high or low pressure) due to a greater temperature contrast between even warmer oceans versus cold land. When considering the summer half of the year, the direct effects of ocean warming differ by coast. Along a west coast in the middle latitudes, just like in a west coast desert in the latitude range of 15-30 degrees, warming oceans will lead to greater humidity. Additionally, although the temperature contrast between land & water may decrease, sea breezes may start to destabilize rather than stabilize the atmosphere, leading to sporadic thunderstorms in places that currently have almost completely dry summers. Along an east coast in the latitude range of 40-50 degrees, cold currents from the Arctic or Antarctic Oceans will move farther from the equator through the year than is currently the case (though they will still come closer to the equator in the winter half of the year than in the summer half of the year), so those places will get more precipitation & humidity through the year akin to the present climates of places in the latitude range of 30-40 degrees. Along an east coast in the latitude range of 30-40 degrees in the summer hemisphere, humidity levels will increase further. As the temperature contrast between land & water may decrease, precipitation in the summer half of the year may become less frequent but more intense, whether due to atmospheric instability from sea breezes carrying warmer air with more moisture or due to collisions of such sea breezes with incursions of cold dry air from the pole.

Changes arising indirectly from changes to the subtropical ridge, polar front, and polar high

The polar high & polar front are directly tied to the polar cell. As the planet warms, I suspect that the polar high will weaken and that this will make the polar front weaken too. However, with more moisture from the tropical & middle latitudes, the polar front could instead strengthen. I'm not sure which intuition is correct, so I won't discuss this further.

There are 3 possible changes to the subtropical ridge from global warming. The effect of global warming on the strength of the subtropical ridge, all else being equal, is unclear to me, because more warm air rising in the ITCZ and traveling through the Hadley cell before falling could strengthen the subtropical ridge, but warming of the cold currents over which the subtropical ridge is concentrated could lead to localized low pressure that weakens the subtropical ridge. I can more clearly see that ocean warming, by pushing cold currents further from the equator through the year (on average), will on average lead to the subtropical ridge moving farther from the equator through the year compared to now (though even in the future, in any given hemisphere, it will still be farther from the equator during the summer half of the year than in the winter half of the year). I can also more clearly see that ocean warming, by confining colder parts more strongly to the eastern part of an ocean (when considering the Pacific, Atlantic, and Indian Oceans) than is currently the case and supporting lower pressure farther east of the western part of an ocean, will confine the subtropical ridge more strongly to the eastern edge of an ocean through the year (though the confinement to the east will still be relatively stronger in the summer hemisphere than in the winter hemisphere, as is the case now).

The subtropical ridge being farther from the equator during each month (on average) than is currently the case means that deserts that are currently present along west coasts in the latitude range of 15-30 degrees will extend even farther from the equator (while also being pushed away from the equator at the existing closest points due to the direct effects of the warming ocean). This also means that the latitude range of west coast locations that have dry summers & wet winters will shift farther from the equator. For example, climate scientists have already predicted that by 2100, Los Angeles & San Diego in the US will have desert climates like Los Cabos in Mexico does now, while San Francisco in the US will have a semi-arid climate like parts of the Greater Los Angeles area & San Diego metropolitan area in the US do now. I suspect that east coasts in the middle latitudes may get more precipitation (through the year closer to the equator or more in the summer closer to the pole) from the subtropical ridge along the eastern edge of the adjacent ocean moving closer to the pole as easterly trade winds sweep across the ocean in a circle toward and then away from the equator to the west (clockwise in the northern hemisphere & counterclockwise in the southern hemisphere, due to the Coriolis force), but I'm not totally sure about this.

I don't have a good intuition about how the subtropical ridge being more confined to the eastern edge of an ocean will affect precipitation levels along the west coast of a closer adjacent continent to the east or along the east coast of a farther adjacent continent to the west. Therefore, I won't discuss this issue further.

Changes arising indirectly from changes to systems of high pressure over continents in the winter hemisphere and to the ITCZ

As land warms further, the ITCZ will migrate farther from the equator into the summer hemisphere. For example, I intuitively expect that by 2100, Los Cabos in Mexico may actually get a tropical wet summer & dry winter monsoon climate as part of the ITCZ, being on the west coast of Mexico (on the southern tip of the Baja California Peninsula). Additionally, I intuitively expect that by 2200, if warming trends continue, the ITCZ will on average move farther north, such that Los Angeles & San Diego in the US will also experience a tropical wet summer & dry winter monsoon climate as part of the ITCZ; this would be a major reversal from the current climates of Los Angeles & San Diego in the US, which are subtropical and feature very dry summers (with frequent wildfires) & modestly wet winters. This migration of the ITCZ away from the equator would reinforce the direct effects of warming oceans in terms of the Atacama & Namib Deserts as well as some other west coast deserts (between the equator & 30 degrees in latitude) receding further from the equator. For example, based on the shifting of the ITCZ as well as warming of oceans, I expect only based on long-term changes in global weather patterns that the Great Australian Desert will recede from the north (moving farther from the equator) and east. That said, it is worth emphasizing that the expansion or recession of desert climates is not solely depend on long-term changes in global weather patterns; human activity & overgrazing by wild animals can change soil properties, leading to expansions of desert even in places where (all else being equal) one might predict recession of desert due to long-term changes in global weather patterns.

Systems of high pressure over continents in the winter hemisphere may weaken as the land warms. This may reduce the blocking effect by high pressure on prevailing westerlies carrying cool (but not cold) moist air and therefore may lead to more precipitation & higher temperatures even over the interior of a continent in the winter hemisphere. Already, climate scientists are finding a weakening of this system of high pressure over Asia in the winter half of the year and in turn more precipitation in the winter half of the year in Asia in the middle latitudes (which was previously very dry & cold at the peak of winter). That said, weakening of these systems of high pressure may weaken prevailing westerlies reaching continents to the east of a given continent and may weaken ordinary easterly trade winds bringing wet winter monsoons to some tropical locations; additionally, weakening of the system of high pressure specifically over Asia in the winter half of the year may mean less precipitation in the winter half of the year along the west coast of Japan.

Putting these factors together to predict precipitation changes

Of the aforementioned factors (ocean warming, land warming, shifts in the subtropical ridge, shifts in the ITCZ, and weakening of systems of high pressure over continents in the winter hemisphere), the most relevant to polar climates is ocean warming along with associated melting of ice. As discussed above, this will lead to more precipitation overall and a transition of east coasts in polar latitudes to having wet winters & dry summers.

Tropical west coast locations often get wet summers & dry winters. This means that shifts in the ITCZ into the summer hemisphere farther away from the equator, shifts in the subtropical ridge away from the equator, warming of the oceans, and weakening of systems of high pressure over major continents in the winter hemisphere won't change winter precipitation much. Shifts in the ITCZ into the summer hemisphere farther away from the equator & of the subtropical ridge farther away from the equator won't affect summer precipitation much, but warming oceans will probably make summer precipitation events more likely & more intense.

Tropical east coast locations often get wet winters & dry summers. This means that shifts in the ITCZ into the summer hemisphere farther away from the equator, shifts in the subtropical ridge away from the equator, and warming of the oceans won't change summer precipitation much. Shifts in the ITCZ into the summer hemisphere farther away from the equator & of the subtropical ridge farther away from the equator won't affect winter precipitation much, but warming oceans will probably make winter precipitation events more likely & more intense. This is most relevant for places like the east coast of Brazil in the southern hemisphere between the equator & 15 degrees in latitude as well as the northeast coast of Australia in the same latitude range, because toward the pole & east of these places, there are no large landmasses in the middle latitudes that support strong systems of high pressure over the continent to generate ordinary easterly trade winds whose weakening may affect precipitation in these places in the winter half of the year. In other tropical east coast locations, weakening of systems of high pressure over continents in the winter hemisphere toward the pole & east may lead to less precipitation in the winter half of the year. This seems to contradict the effect of warming oceans; perhaps the resolution is that such places in the winter half of the year may get more sporadic but intense precipitation.

Middle latitude west coast locations often get wet winters & dry summers. In the winter half of the year, warming land without any other changes may lead to less precipitation, but warming oceans without any other changes may lead to more precipitation. The net effect will depend on the latitude of the location relative to the new latitude of the subtropical ridge; places that are still on the side of the subtropical ridge toward the pole in the winter half of the year will get wet winters, though precipitation may come with less likelihood & more intensity, while places that are close enough to the equator that they would be on the side of the subtropical ridge toward the equator through the year (even if that is not the case now) would start to experience desert climates. Weakening of systems of high pressure over continents in the winter hemisphere won't affect the west coast per se but may allow precipitation to penetrate further inland, although weakening of those systems of high pressure over continents to the west in the winter hemisphere could weaken the prevailing westerlies overall and bring a little less precipitation. Thus, for middle latitude west coast places that in the winter half of the year are still on the side of the subtropical ridge toward the pole, it is not clear to me how winter precipitation patterns may change. In the summer half of the year, for places that are on the side of the subtropical ridge closer to the equator, there will likely be more humidity, a decrease in the probability of light precipitation events compared to now, and an increase in the probability of intense precipitation events compared to now (though that probability would still be much less than the current probability of light precipitation events), which explains how these locations can simultaneously be more dry & more vulnerable to intense summer precipitation events.

Middle latitude east coast locations often get precipitation through the year, though this is skewed more toward the summer. In the winter half of the year, for similar reasons as in middle latitude west coast locations, warming land without any other changes may lead to less precipitation, but warming oceans without any other changes may lead to more precipitation. Additionally, the shifting of the subtropical ridge on the eastern edge of the adjacent ocean toward the pole may lead to more frequent & intense precipitation from trade winds swinging west toward the equator and then the pole, but weakening of systems of high pressure over continents in the winter hemisphere may lead to further penetration of the prevailing westerlies from the ocean west of the continent into that continent, drying out but remaining mild in temperature, so the lack of moisture in the prevailing westerlies & the lesser temperature contrast with mild moist air over an adjacent warm ocean current may lead to less precipitation overall. Thus, for middle latitude east coast places, it is not clear to me how winter precipitation patterns may change. In the summer half of the year, precipitation events may be more likely due to warmer land (from atmospheric instability) & shifts in the subtropical ridge toward the pole but less likely due to warmer oceans (due to a lesser temperature contrast with land); the net effect may be more precipitation events, but that is hard for me to say for sure. However, it is clear that the intensity of precipitation events will increase due to warmer oceans (due to more warm moisture), warmer land (due to greater atmospheric instability near a warm ocean current), and shifts in the subtropical ridge toward the pole (as more moisture can come in).

This discussion should make clear that there are some clear but many unclear predictions from these simple intuitions about climates & their changes in a warming planet. This in turn should make clear that while these intuitions can help one understand climates as they presently exist, they have somewhat more limited value for predicting how those climates will change with global warming. Further predictive power comes from accounting for flows of air & water higher in the atmosphere as well as rigorous quantitative data analyses & simulations; these are the things that climate scientists do and then write about in peer-reviewed journal articles.