THE WiNDDRivEN ciRcuLATioN And The Great Gyres

To better understand how the processes described above produce an ocean circulation like that described in chapter 2, let us consider an idealized ocean, with much simplified geometry, and see if we can first understand how that works. Our idealized view of the gyres is illustrated in figure 4.1, which the reader will appreciate is an abstraction

Figure 4.1. An idealized gyre circulation in a rectangular ocean basin in the Northern Hemisphere, showing the subtropical gyre (lower, typically extending from about 15°N to 45°N), the subpolar gyre (upper), and the intense western boundary currents on the left.

of the real circulation of the world's ocean. The main questions we wish to answer are relatively simple:

1. Why do the gyres exist in the first place? What determines the way they go around and how strong they are?

2. Why are they more intense on the western sides of the oceans?

The gyres exist because the mean winds provide a mechanical forcing, a stress, on the oceans, and this stress causes the water to accelerate. For the oceans to be in mechanical balance, the imposed forces must be counteracted by frictional forces where the water rubs against the ocean bottom or side. Frictional forces only arise when the water is in motion, so that if there is a wind blowing, then the ocean must be in motion, and an overall balance between the wind and the frictional forces ultimately comes about. However, there are important effects caused by Earth's rotation that determine the structure of the gyres, as we will see.

For the sake of definiteness, we consider the subtropical gyre in a rectangular ocean—the lower gyre of figure 4.1. The winds blow eastward on the poleward side of the gyre (these are the midlatitude westerly winds) and westward at low latitude (the tropical trade winds), and it seems entirely reasonable that the ocean should respond by circulating in the manner shown. However, in the last chapter we noted that Earth's rotation plays a significant role in large-scale circulation and that flows are generally in geo-strophic balance, except for the Ekman layer in the upper ocean, where the flow is at right angles to the wind. How does this description square with the notion of a gyre that seems to go around in the same direction as the wind?

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