Recently, I happened to come across articles online [LINK] clarifying that there are some competing explanations for why the climate of Europe immediately to the east of the Atlantic Ocean is milder in the winter than the climate of North America at similar latitudes immediately to the west of the Atlantic Ocean but that the Gulf Stream in the Atlantic Ocean only plays a minimal role. It got me to think whether I have unwittingly repeated the myth of the importance of the Gulf Stream for the climate of Europe in recent blog posts like my most recent one about climate types [LINK]. Having gone through that blog post, I can say more confidently that I did not repeat that myth with respect to the big picture of Europe's climate, but there may have been certain aspects of Europe's climate (especially in eastern Europe) for which I overstated the effect of the Gulf Stream, so I want to set the record straight in an effort to not spread known misinformation or myths as if they were facts. Follow the jump to see more details. 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.
What I got right
From the linked article explaining why the Gulf Stream isn't the dominant factor in the climate of Europe in the winter, there are two factors that I got right. One is that even in the absence of oceanic circulation, because water has a much higher specific heat capacity than air or land, places near the ocean on any coast tend to have milder & wetter climates than places farther inland near the same coast (though wetness can increase a lot more on the windward side of a mountain, especially if those winds come directly over the ocean as in the mountains near the west coasts of North America & South America). The other is that in the middle latitudes, the prevailing westerlies bring mild moist air to the west coasts of continents, moderating their climates, whereas the east coasts of continents, if they get prevailing westerlies at all, would get them from more inland locations with less moisture & consequently less moderated temperatures. In any case, my linked blog post generally made clear that the effect of the Gulf Stream in the Atlantic Ocean in the northern hemisphere is to make climates in the landmasses immediately to the west & east of the Atlantic Ocean in the northern hemisphere respectively warmer than their respective counterparts at similar latitudes immediately to the west & east of the Atlantic Ocean in the southern hemisphere or the Pacific or Indian Oceans in both hemispheres. My linked blog post also made clear that it does not make sense to directly compare landmasses at similar latitudes on opposite sides of an ocean.
What I missed completely
From the linked article, I missed the point about how conservation of angular momentum ensures that the prevailing westerlies over North America from the Pacific Ocean, upon crossing the Rocky Mountains, turn toward the equator and then swing back over the Gulf of Mexico & the rest of the Atlantic Ocean toward Europe, which has the dual effect of exposing most of the eastern part of North America to colder air from the poles passing over colder inland areas and ensuring that Europe gets warmer oceanic air. Notably, this infusion of warmer air has nothing to do with the way that the Gulf Stream in the Atlantic Ocean in the northern hemisphere is anomalously warm & strong relative to similar currents in the Atlantic Ocean in the southern hemisphere or the Pacific Ocean or Indian Ocean in both hemispheres. Although I have a decent understanding (as a physicist) of conservation of angular momentum in rigid classical or quantum mechanical bodies, it is hard for me to intuitively imagine or visualize this process with respect to the atmosphere, so it may take a little more time for me to properly grasp this.
What I got wrong
Looking back at my own linked blog post (using labels from my modification of the Trewartha categorization), in the parts where I pointed out that the climate types Cfak & Dfak occur in Europe in cities around the Adriatic Sea & Black Sea significantly closer to the pole than cities with the same climate types in North America (mostly but not exclusively in the South, Mid-Atlantic, and Northeast regions of the US), I may have overstated the importance of the Gulf Stream regarding the greater closeness of these cities to the pole in Europe. It may be more accurate to say that these climate types occur in Europe closer to the pole than they do in North America just because of the combination of Europe being the western part of the northern part of the continent Afroeurasia (therefore getting mild moist air from the prevailing westerlies at those latitudes), the Adriatic Sea & Black Sea acting like miniature oceans (absorbing heat in the summer & releasing it in the winter) to the east of corresponding landmasses where these climate types occur in Europe, and the lack of mountains along lines of longitude in Europe (in contrast to North America) that would create rain shadows relative to the prevailing westerlies. In particular, I suspect that if North America did not have the Cascade Mountains, Sierra Nevada, or Rocky Mountains, the progression of climates from west to east at latitudes corresponding to the latitudes of Europe would be somewhat more similar to the progression of climates in Europe (though North America has fewer inland or quasi-inland seas that would moderate temperatures further like they do in Europe), at least up until longitudes in North America that hit the Gulf of Mexico; only at that point might the presence of the Gulf Stream in the Atlantic Ocean matter, such that climates in the western part of North America in this hypothetical situation might be marginally cooler and possibly drier than at corresponding latitudes in Europe.
Possible effects of the collapse of the Gulf Stream on the climate of Europe
There is a fair bit of evidence suggesting that the Gulf Stream in the Atlantic Ocean may collapse in the next few centuries. Popular misconceptions about the effects of the Gulf Stream on the climate of Europe have led to mistaken beliefs that such a collapse would imply a new ice age for Europe. However, the linked article makes clear that this is almost certainly not going to happen, that the overall decrease in temperature in the Atlantic Ocean in the northern hemisphere would be at most a couple of degrees (presumably Celsius), and that decrease would likely be overwhelmed by an increase in temperature from human-caused global warming over the same period.
What might happen? My intuition, ignoring human-caused global warming just for the sake of this hypothetical situation, would say that a collapse in the Gulf Stream in the Atlantic Ocean would make the water in the eastern part of the Atlantic Ocean in the northern hemisphere, along the west coast of Europe, slightly cooler and therefore more like the water in the eastern part of any other ocean in any hemisphere, along the west coast of a corresponding continent. This may allow Europe to support more of a subtropical ridge even a little closer to the pole, leading to warmer & drier summers, and have fewer months with mean temperatures of at least 10 degrees Celsius; essentially, the climate will become more like that of the west coast of North America at similar latitudes (which isn't that much of a change), though it might not be as dry due to the lack of mountains along lines of longitude creating a rain shadow.