P

Note that the model's only surface forcing is the freshwater flux through the upper surface. The total meridional mass flux (indicated by the dot-dashed line) associated with E - P is about 0.3 Sv; this is really very small compared with the wind-driven circulation.

The vertical mixing in the model is set to a constant value of 0.3 x 10-4 m2/s. After 5,000 years of integration, the model reaches a state of aperiodic oscillation. The following discussion is based on the mean circulation averaged over several hundred years; details of the time evolution will be discussed in Section 5.4. Owing to precipitation at high latitudes, sea level there is increased. In fact, a center of sea-level high exists in the middle of the northern half of the basin, and a sea-level low exists at low latitudes near the eastern boundary (Fig. 5.106).

Fig. 5.106 Sea surface elevation, in cm, associated with the haline circulation.

The sea-level high is closely related to the barotropic anticyclonic circulation in the northern half of the basin (Fig. 5.107). There is a rather weak barotropic cyclonic circulation in the southern half of the basin. In addition, there are clearly western boundary currents required for closing the circulation in the basin (Fig. 5.107). This shows the barotropic meridional transport function obtained by zonally integrating the meridional volume flux over the whole depth. Readily seen are the southward flow driven by precipitation in the subpolar basin, the poleward flow driven by evaporation in the subtropical basin, and the western boundary currents required for closing the circulation, with the volume flux discussed above. The barotropic circulation is predicted by the Goldsbrough-Stommel theory. As discussed above, the barotropic circulation exists, independently of whether there is salt in the ocean or whether there is enough external mechanical energy to sustain the vertical mixing in the ocean.

The major new feature different from the Goldsbrough-Stommel theory is the existence of strong three-dimensional baroclinic circulation. The most important component is the meridional overturning cell, with an overturning rate more than 50 Sv, as shown in Figure 5.108. Water in the southern half of the basin is much saltier due to evaporation, and it sinks to the sea floor. Away from the southern boundary, the salty water gradually upwells

Fig. 5.107 Barotropic meridional transport function, in Sv.

and mixes with the relatively fresh water above. As discussed in Chapter 3, mixing in such a stratified fluid requires an external source of mechanical energy, which comes from both tidal dissipation and surface wind stress. Without such an energy source, the freshwater flux through the air-sea interface cannot drive strong circulation, as shown in Figure 5.108.

Precipitation tends to be very irregular in nature. Thus, in the mind's eye, there seems to be no clear connection between the tiny raindrops of precipitation and the large-scale organized haline circulation in the oceans. However, precipitation and evaporation are responsible for creating the salinity difference in the oceans, and thus regulating the haline component of the oceanic general circulation. In this model, precipitation at high latitudes reduces the salinity in the north, and evaporation at low latitudes increases salinity in the south; this is what really drives the circulation (Fig. 5.109). The salinity maximum is located in the southeastern corner, roughly at the location of the sinking branch of the haline circulation.

Under the freshwater forcing, there is a complicated three-dimensional haline circulation; thus, one should not take the barotropic circulation as the circulation pattern in the ocean. In fact, the strong subsurface upwelling in the middle of the basin works like an "Ekman compressor." This "compressor" moves upward and squeezes the water column in the upper

Fig. 5.108 Time-mean meridional streamfunction in Sv.

a h = 7.5 m b h = 317.6 m a h = 7.5 m b h = 317.6 m

Fig. 5.109 Salinity (deviation from the mean) distribution at a 7.5 m and b 317.6 m.

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