Fig. 4.56 Density stratification (thin lines) and horizontal velocity (heavy lines, in 10-2/m/s) at two sections; the dashed lines indicate the base of the wind-driven gyre with stagnant water below, with the heavy dashed lines indicating the base of moving water in the wind-driven gyre: a zonal section taken along 55° N; b meridional section taken along the outer edge of the western boundary.

a penetrates to a great depth in the subpolar basin. Since the wind-driven circulation is so deep, it seems rather difficult to separate the wind-driven circulation from the thermohaline circulation there; thus, the purely wind-driven circulation discussed in this section is only an idealization, and care should be taken when applying it to the oceanic situation.

Note that in this section the base of the wind-driven gyre reaches the maximal depth at latitude 46° N; thus, water belonging to this deepest part does not come from the southern boundary of the subpolar basin; instead, it comes from the western boundary at latitudes higher than 45° N. Therefore, potential vorticity for this part of the thermocline may not be homogenized toward the southern boundary of the subpolar basin. As a working assumption, we assume that potential vorticity on such an isopycnal surface is homogenized toward the planetary vorticity at the outer edge of the western boundary; thus, it is not -fopf: instead it is -fbpa, where fb is the Coriolis parameter for this specific water mass to enter the wind-driven gyre from the western boundary.

The sea surface elevation map clearly shows the cyclonic gyre, with a maximum sea-level depression of 45 cm in the middle of the western boundary (Fig. 4.57a). The sea surface density map clearly demonstrates the outcropping of isopycnal surfaces, with the heaviest density surface outcrops in the middle of the western boundary (Fig. 4.57b). The a = 27 kg/m3 surface has a dome shape, and the shadow zone along the eastern edge is too narrow to be shown in this panel (Fig. 4.57c).

The wind-driven gyre penetrates to a maximum depth of more than 3.5 km in the southern part of the western boundary region (Fig. 4.57d). Since most high-latitude oceans are not very deep, the bottom topography is likely to interact with the wind-driven gyre; thus, the a Surface elevation (cm) b Surface density (a)

a Surface elevation (cm) b Surface density (a)

Fig. 4.57 Basic structure of the wind-driven circulation in a subpolar basin interior: a surface elevation (cm); b surface density, in a units (kg/m3); c depth of a = 27.0 isopycnal surface (in 100 m); d depth of the wind-driven gyre (km).

simple Sverdrup relation for a purely wind-driven circulation may need some modification. On the other hand, any current below 2 km is rather weak, so the solution obtained from the model may still give useful information about the structure of the subpolar gyre.

Water mass formation and erosion

The subpolar gyre is dominated by Ekman upwelling and the associated water mass transformation from the thermocline to the surface mixed layer. The water mass transformation

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