S 20s 0 20n 40n 60s 40s 20s 0 20n 40n

Fig. 5.91 Contribution of temperature and salinity to stratification along 169.5° W, inferred from climatology: a N2(heavy line), N2 due to temperature only (thin line), and N2 due to salinity only (dashedline), in 10-4/s2; b stratification contribution due to temperature; c stratification contribution due to salinity.

Thus, p is the commonly used pressure, pr is the pressure due to temperature distribution in the water column only, and pS is the pressure due to salinity distribution only. Since density is a nonlinear function of temperature, salinity, and pressure, the above definition does not provide an exact partition between temperature and salinity.

As shown in Figure 5.92a, at the middle level there is a southward pressure gradient force in the Atlantic Basin, which is responsible for maintaining the southward flow at the middle level as the return component of the present meridional overturning in the Atlantic Ocean. By examining the pressure distribution in the North Atlantic Ocean, it can readily be seen that temperature and salinity contributions have opposite signs at middle depth. In fact, the southward pressure force at the middle level is mostly provided by the salinity distribution (Fig. 5.92d), rather than the temperature (Fig. 5.92c). This suggests that meridional overturning in the Atlantic Ocean may be rather sensitive to changes in salinity, which is in turn related to the hydrological cycle. This connection will be explored shortly in the discussion about the thermohaline catastrophe associated with freshening of the subpolar North Atlantic Ocean.

On the other hand, the strong fronts associated with the ACC are primarily due to the temperature front, with a relatively small contribution from the salinity distribution.

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