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One of the essential ingredients for decadal/interdecadal variability is the mixed boundary conditions, i.e., a relaxation condition for temperature and a flux condition for salinity. As discussed in the previous section, haline circulation under the freshwater flux condition alone can develop oscillatory solutions. Thus, it is easy to understand why a model under the mixed boundary conditions tends to develop oscillatory solutions.

In the ocean, the decadal/interdecadal oscillations may be due to a combination of SST (sea surface temperature) and salinity. Ahigher-than-normal SST can induce a positive salinity anomaly at lower latitudes. Advection brings the salinity anomaly to high latitudes. Since the thermal expansion coefficient is small at high latitudes, a positive salinity anomaly gives rise to a higher density and lower sea level there. The resulting stronger-than-normal meridional pressure gradient induces an enhancement of the meridional overturning circulation, and the system develops oscillatory behavior similar to that discussed above (Fig. 5.165).

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