We have now explained the underlying reason for gyres, but we have not explained one of their most important and indeed obvious aspects: the gyres are not symmetric in the east-west direction. Thus far, our explanation would lead to gyres that look like those in the left panel of figure 4.3, whereas in fact the gyres look more like those in the right-hand panel, with intense western boundary currents, of which the Gulf Stream in the western North Atlantic is the most famous example to Americans and Europeans, and the Kuroshio is the corresponding current off the coast of Japan. The presence of the Gulf Stream has been known for a long time—Benjamin Franklin was one of the first people to chart it. So our question is a simple one: why is the Gulf Stream in the west?
It turns out that the cause of the western intensification is that, as we discussed in chapter 3, the magnitude of the effective rate of rotation (specifically, the magnitude wind stress wind stress
of the Coriolis parameter) of Earth increases as we go poleward. This differential rotation causes the gyres to have a marked east-west asymmetry, with the flow in the west squished up against the coast. As the effect is both important and hard to grasp, we give a couple of explications. For definiteness we focus on the subtropical gyre in the Northern Hemisphere, but the same principles apply to the other gyres.
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