Due to evaporation at low latitudes and precipitation at high latitudes, there is a meridional salinity gradient at the sea surface. As shown in the previous section, the meridional density gradient due to the salinity difference is opposite to the meridional density gradient due to the thermal forcing in the upper oceans. Thus, in the present climate setting, the hydrological cycle is a brake for the poleward transport of heat transportation carried by the meridional thermally-forced circulation in the oceans. In other words, the poleward latent heat flux associated with the water vapor cycle has a negative feedback on the oceanic sensible heat flux.
If there were no hydrological cycle, the meridional overturning cell and the associated poleward heat flux in all five basins would be intensified. As an example, in the North Atlantic Ocean the surface density difference between the equator and high latitudes is about Aa = 3.39 kg/m3, based on the climatological data (Fig. 5.94a). If there were no evaporation and precipitation, there would be very little salinity difference in the oceans, so we set salinity to a constant value of 35. Assuming that the surface temperature distribution remains unchanged - a very bold assumption that is unlikely to be true in the real world - the corresponding surface density difference would be increased to Aa = 5.39 kg/m3 (Fig. 5.94b). Due to the increase in the north-south density difference, both the meridional overturning rate and poleward heat flux should increase.
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