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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 _

0°E 10°E 20°E 30°E 40°E 50°E 60°E 0°E 10°E 20°E 30°E 40°E 50°E 60°E

Fig. 4.82 Circulation driven by a slightly modified Ekman pumping field, with a small positive Ekman upwelling rate near the eastern boundary of the model ocean: a barotropic Sverdrup transport (Sv); b Ekman pumping rate (in 10-6/m/s); c upper layer thickness (in 100 m); d lower layer thickness (in 100 m).

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I " ' I " ' I " ' I " ' I " ' I " ' I " ' I " ' I " ' I " ' I ' "

0°E 10°E 20°E 30°E 40°E 50°E 60°E 0°E 10°E 20°E 30°E 40°E 50°E 60°E

Fig. 4.82 Circulation driven by a slightly modified Ekman pumping field, with a small positive Ekman upwelling rate near the eastern boundary of the model ocean: a barotropic Sverdrup transport (Sv); b Ekman pumping rate (in 10-6/m/s); c upper layer thickness (in 100 m); d lower layer thickness (in 100 m).

shadow zone expands westward, and the volume flux going through the choking latitude in the lower layer gradually declines from 2 Sv (H0 = 100 m) to 0.5 Sv (H0 = 200 m), and finally becomes zero for the case with H0 = 250 m. Since this 2/2-layer model satisfies the Sverdrup constraint exactly, the net volume flux in the upper layer is the difference between the Sverdrup flux and the volume flux in the lower layer.

The volume flux going through the choking latitude is, thus, sensitively dependent on the choice of the lower layer thickness along the eastern boundary, as shown in Figure 4.85. When h1e is larger than 240 m, the communication pathway in the lower layer is entirely blocked.

Wind-driven circulation b h1

Wind-driven circulation b h1

d M2

d M2

150°E 180° 150°W 120°W 90°W

Fig. 4.83 Circulation of a 2l/2-layer model ocean mimicking the North Pacific with realistic Ekman pumping; the lower layer thickness along the eastern boundary is h\e = Ho = 150 m, and the outcrop line is at 18° N. Panels a, b, and c for layer thickness (in 100 m); panel d is the meridional volume flux in the upper layer (Sv).

Fig. 4.83 Circulation of a 2l/2-layer model ocean mimicking the North Pacific with realistic Ekman pumping; the lower layer thickness along the eastern boundary is h\e = Ho = 150 m, and the outcrop line is at 18° N. Panels a, b, and c for layer thickness (in 100 m); panel d is the meridional volume flux in the upper layer (Sv).

4.7.3 Communication windows in the world's oceans

It is obvious that using a simple two-layer model to decide whether there is an interior communication between the subtropics and tropics in the lower layer is rather subjective. It is desirable to calculate the interior communication rate independently of the choice of the layer thickness along the eastern boundary.

Meridional transport in the world's oceans

We begin with the barotropic flow in the world's oceans driven by the annual Ekman pumping rate obtained from the annual mean wind stress data of Hellerman and Rosenstein a H0 = 100m b H0= 150m

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