Fig. 4.81 a Idealized wind stress (in 0.1 N/m2) and b Ekman pumping (in 10 6 m/s) profiles used in the analytical model.

lower layer thickness along the eastern boundary. In this section, we will simply assume that the upper layer outcrops along 18° N, and the reduced gravity for the upper and lower interfaces is 0.02 m/s2. The only free parameter in this section is the lower layer thickness along the eastern boundary, h1e.

For the case with h1e = H0 = 150 m, the solution is shown in Figure 4.83. The upper layer covers the area south of the outcrop line and west of the cyclonic gyre defined by the zero streamline, as shown in Figure 4.83a. Within the cyclonic gyre, there is the lower layer only, and its thickness declines toward the center of the cyclonic gyre (Fig. 4.83b). The lower interfacial depth clearly shows the bow-shaped structure of the anticyclonic gyre in the subtropics and the dome-shaped cyclonic gyre near the eastern boundary (Fig. 4.83c). The total equatorward volume flux in the upper layer going through the choking latitude, which is 9.5° N as dictated by the Sverdrup function, is more than 3 Sv (Fig. 4.83d).

The meridional volume transport in the lower layer depends on its thickness along the eastern boundary (Fig. 4.84). Note that south of the outcrop line, the lower layer is shielded from Ekman pumping, so the meridional volume transport function is also the streamfunc-tion, except within the cyclonic gyre where the lower layer is subject to Ekman pumping directly. Three circulation regimes exist south of the outcrop line: the ventilation zone, where both the upper and lower layers are in motion; the shadow zone, where the lower layer is stagnant; and the cyclonic gyre, where the upper layer vanishes and the lower layer is in motion (indicated by dashed contours). The shadow zone is located between the two zero streamlines depicted by the heavy lines in this figure. For example, there is no shadow zone for the case with H0 = 100 m, but the shadow zone is prominent in Figure 4.84c, d. As the lower layer thickness along the eastern boundary increases, the a C b we

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 lj 1 1 _

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