and so the balancing boundary current transport, Ywest_bdy, can be read off Fig. 10.21. It is the value of Y at the western margin of the Pacific. However, it is instructive to obtain an explicit expression for the transport as follows. Let us assume that the wind stress only has an east-west component (a useful approximation to reality, cf. Fig. 10.2) so Twind = (Twindx,0)
Ly = ywest - ytrade is a measure of the meridional scale over which the zonal wind changes from the westerly wind belt to the trades, as marked in Fig. 10.21 (left most panel). Then, since z ■ Vx rwind = - dT°wyydx,
Eq. 10-21 yields:
where Lx is the east-west extent of the basin, assumed constant. Inserting numbers into the above expression typical of the Pacific—tx = 0.1Nm-2, fi = 1.98 x 10-11 m-1 s-1, Ly = 3000 km; Lx = 8000 km— we find a maximum transport of the subtropical gyre of 44 Sv, roughly in accord with the detailed calculation given in Fig. 10.21. The transport of the subtropical gyre of the Atlantic Ocean is somewhat smaller, about 30 Sv, largely on account of the much reduced east-west scale of the basin.
The interior Sverdrup transport of the gyre is thus returned meridionally in narrow western boundary currents as marked in Fig. 10.21 in the Pacific. In the Atlantic, it is clear from Fig. 10.13 that the horizontal extent of the Gulf Stream is about 100 km. The hydrographic section of Fig. 9.21 (top) shows that the vertical extent of the region of strong lateral temperature gradients in the Gulf Stream is about 1 km. If a current of these dimensions is to have a transport of 30 Sv, then it must have a mean speed of some 30 cm s-1, roughly in accord with, but somewhat smaller than, direct measurement.8
Before going on, it should be mentioned that there is one major current system in Fig. 9.13 that cannot be addressed in the context of Sverdrup theory—the Antarctic
Circumpolar Current. The ACC is not in Sverdrup balance because there are no meridional barriers that allow water to be ''propped up'' between them, hence supporting a zonal pressure gradient and meridional motion. Dynamically, the ACC is thought to have much in common with the atmospheric jet stream discussed in Chapter 8; eddy processes are central to its dynamics.
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