Earlier this year, I wrote a post about how subtleties in the convergence and divergence of large-scale surface winds can explain many features of local climates. However, as I thought about it more, I realized that I still wasn't satisfied with my understanding of why the seasonal variation in strength of a subtropical ridge over an ocean depends on the ocean in consideration. Specifically, the subtropical ridges over the Pacific, Atlantic, and Indian Oceans in the southern hemisphere as well as over the Atlantic Ocean in the northern hemisphere are stronger (higher pressure) during those respective hemispheres' winter halves of the year, but the subtropical ridge over the Pacific Ocean in the northern hemisphere is stronger during the northern hemisphere's summer half of the year.
Having looked more at the Columbia University interactive maps of mean monthly wind velocities, I am reasonably more confident that I can explain these & related phenomena mostly through surface wind dynamics, though the explanations aren't complete. 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. Follow the jump to see these explanations.
Impinging Continents Opposite the Equator from the Ocean
If there is a significant landmass across the equator from an ocean in a given hemisphere, especially at tropical or subtropical latitudes, then the following happens; for simplicity, I will refer to the hemisphere of the ocean & subtropical ridge in question as hemisphere X and of the land opposite the equator from the ocean as hemisphere Y. The winter half of the year for hemisphere X is the summer half of the year for hemisphere Y. This means that air rising over hot land (likely to be in the ITCZ) in hemisphere Y is less likely (though not completely forbidden) to fall over land or oceans in hemisphere Y, which is warmer than hemisphere X. Instead, it will travel a longer distance across the equator to hemisphere X, and more air falling will strengthen the subtropical ridge over the ocean there. By contrast, during the summer half of the year for hemisphere X, air is much less likely to rise over land in hemisphere Y (which is cold during its winter half of the year), and although air is more likely to rise over adjacent land in hemisphere X, it is also less likely to fall within hemisphere X due to the whole hemisphere being warmer during its summer half of the year. Essentially, the half of the Hadley cell that ranges from the ITCZ in hemisphere Y during its summer half of the year across the equator and to the subtropical ridge over the ocean in hemisphere X during its winter half of the year is stronger, with more cycling of air, than the half of the Hadley cell that remains within hemisphere Y during its summer half of the year.
This applies to:
- The Indian Ocean, which basically only exists in the southern hemisphere and has all of Asia across the equator from it (as the Indian Ocean is not big enough in the northern hemisphere to support subtropical ridges or associated oceanic gyres),
- The Atlantic Ocean in the northern hemisphere, which is across the equator in its western half from the equatorial/tropical landmass of South America, and
- The Atlantic Ocean in the southern hemisphere, which is across the equator in its eastern half from the tropical landmass of northern & western Africa (especially the Sahara Desert & Sahel).
I think that this also applies to the Pacific Ocean in the southern hemisphere, which is across the equator in its western half from the tropical landmass of North America (essentially the highlands of Mexico, which support the ITCZ well due to heating of its elevated plateaus by forcing ordinary easterly & reversed westerly tradewinds to further converge by going up mountains in order to reach even lower pressures). However, the much bigger size of the Pacific Ocean compared to the Atlantic & Indian Oceans may explain why the relative seasonal difference in the subtropical ridge strengths over it in the southern hemisphere is much less than is observed for the subtropical ridges over the Atlantic & Indian Oceans.
Pacific Ocean in the Northern Hemisphere
The Pacific Ocean in the northern hemisphere is an exception, because there is no significant landmass across the equator from it. (The eastern portion of the mainland of Australia, which is the only significant landmass in the southern hemisphere at longitudes east of Japan, is not big enough or close enough to tropical latitudes to support the ITCZ enough to modify the subtropical ridge over the Pacific Ocean in the northern hemisphere.) This might suggest that the strength of the subtropical ridge over it should not change much with season. On average, that is the case, but that averaging glosses over two competing effects.
- The subtropical ridge over the Pacific Ocean in the northern hemisphere is wider in the winter half of the year, spreading almost between Japan & North America. I think that this is because in the summer half of the year, as the water in the western part warms more and the huge landmass of East Asia warms enough to support the ITCZ, the western edge of the subtropical ridge must recede eastward. However, unlike the seasonal system of cold air over the mainland of Australia being essentially continuous with the subtropical ridges over the Indian & Pacific Oceans during the winter half of the year in the southern hemisphere, the subtropical ridge over the Pacific Ocean in the northern hemisphere shows a clear spatial discontinuity with the seasonal system of cold air over East & North Asia during the winter half of the year. I think this difference is for the following two reasons.
- The mainland of Australia is still largely at subtropical latitudes, so although it is relatively cooler during the winter half of the year than the summer half of the year, the temperature contrast with oceans is not as strong. Thus, the seasonal system of cold air over the mainland of Australia during its winter half of the year has roughly similar temperatures, pressures, and other characteristics as the subtropical ridges over adjacent oceans.
- By contrast, the mainland of East & North Asia is at poleward middle & subpolar latitudes and is much bigger than the landmass of the mainland of Australia. This leads to much colder air at much higher pressure settling over the landmass than is found in the subtropical ridge over the Pacific Ocean in the northern hemisphere, such that air over land essentially cuts through what would otherwise be the western edge of the subtropical ridge over the Pacific Ocean in the northern hemisphere. This is why in the winter half of the year, Japan gets strong northwesterly winds (with sea-effect snow on the west coast & cool dry air on the leeward east coast) instead of calm dry air throughout.
- The highest pressure observed in the subtropical ridge over the Pacific Ocean in the northern hemisphere is higher in the summer half of the year (along the eastern edge of that ocean), contrary to what happens in the Pacific Ocean in the southern hemisphere or in the Atlantic & Indian Oceans. This is harder for me to understand & explain, but I suspect that it may be related to the landmass of East & North Asia being so big and the ITCZ being so strong there that air rising in the ITCZ over there then travels east within the same [northern] hemisphere and falls in the subtropical ridge over the Pacific Ocean in the northern hemisphere. It may be helped by convergence at high elevation with air rising from the ITCZ over North America and traveling to the northwest. This is unlike the usual picture of the Hadley cell in which air rising over the ITCZ travels mostly along lines of longitude, not lines of latitude. However, this seems to be consistent with the theory of the Rodwell-Hoskins mechanism (source: Wikipedia), which may also explain wind divergence & dry conditions persisting so far east of the subtropical ridge over the Atlantic Ocean in the northern hemisphere in the summer half of the year (into the eastern part of the Mediterranean Sea) as a result of air rising from the ITCZ over East & North Asia traveling west at high altitude & converging with air rising from the ITCZ elsewhere.
Other Notes
There were two other related points that confused me once I learned about the effects of wind divergence but seem clearer to me now, after having thought about these things a little more & looked more carefully at these maps of wind velocity.
First, Madagascar is close to the subtropical ridge even in the summer half of the year. Ordinarily, the accelerating & diverging ordinary southeasterly tradewinds would suggest a lack of precipitation. However, the ordinary northeasterly tradewinds generated from the settling of cold air over the Tibetan Plateau during its winter half of the year converge with the ordinary southeasterly tradewinds generated by the subtropical ridge over the Indian Ocean. That convergence happens essentially over Madagascar & the Mozambique Channel, resulting in significant precipitation (including along the west coast of Madagascar, which is leeward of the mountains of Madagascar with respect to the ordinary southeasterly tradewinds).
Second, Uruguay & northern Argentina are similarly close to the subtropical ridge even in the summer half of the year. Ordinarily, the accelerating & diverging ordinary southeasterly tradewinds would suggest a lack of precipitation. However, central Argentina during the summer half of the year is heated significantly to support something like the ITCZ, so that convergence overcomes the divergence from the subtropical ridge over the Atlantic Ocean (which is weaker during the summer half of the year) to yield precipitation in the summer half of the year.
