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SST relaxation SST relaxation latitudes higher than 30° N (Fig. 5.95). Thus, in the case without a hydrological cycle, the oceanic current will transport more heat poleward and release it to higher latitudes, partially compensating for the decline in poleward heat flux caused by the lack of latent heat flux associated with the hydrological cycle.

Other experiments for the global oceans were also run with a low horizontal resolution of 4° x 4° and 15 layers. The Indonesian Throughflow is included in these low-resolution simulations. The model's temperature and salinity were initialized with the Levitus and Boyer (1994) data in each experiment. The model was run for 1,000 years to reach a quasi-equilibrium.

In the standard experiment, both the surface temperature and salinity were relaxed toward the monthly mean climatology, using the Hellermann and Rosenstein (1983) monthly mean wind stress data. In the second experiment, salinity was set to a constant value of 35, but everything else remained the same as in the standard experiment. The poleward heat flux was calculated, using the definition discussed in the Appendix, so the heat flux in both the South Pacific and Indian Oceans was uniquely defined.

It is clear that by turning off the salinity effect, the total northward heat flux in the Northern Hemisphere increases, while it declines in the Southern Hemisphere (Fig. 5.96). Most interestingly, the northward heat flux in the North Pacific Ocean increases substantially. This increase in heat flux at mid latitudes is probably associated with the newly created thermal mode of the meridional circulation in the North Pacific Ocean, caused by the lack of the salinity effect which opposes the thermal mode. The northward heat flux in the Atlantic Ocean declines due to the diminishing effect of the global conveyor belt that is driven, at least partially, by the salinity difference between the Pacific and Atlantic Oceans. As a result of the decline of the global conveyor belt, the poleward heat flux in the South Indian Ocean declines. Note that the heat flux in the South Atlantic Ocean is now southward, as required by the thermal mode in this basin.

A major assumption that we made use of is that the vertical mixing coefficient and wind stress remain the same for all these experiments. This is a highly idealized assumption. Since thermohaline circulation is so closely related to climate, changes in the circulation

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Fig. 5.95 Poleward heat flux (a) and its divergence (b) (due to advection) diagnosed from the standard model with both temperature and salinity relaxed (thin line) and the model with uniform salinity (S = 35, heavy line) (Huang, 2005b).

must affect both the wind stress and mixing coefficient; however, a more comprehensive study is difficult, and this is left for readers interested in pursuing this line of thought.

5.3.2 Surface boundary conditions for salinity

Salinity balance is one of the most critical components of the oceanic circulation. The suitable upper boundary conditions for salinity balance have evolved gradually over the past several decades. Early in its development, two types of salinity boundary conditions were used, i.e., the relaxation condition and the virtual salt flux condition. As our understanding of oceanic circulation physics deepens, the natural boundary condition is now used in more and more model studies. This section is devoted to the examination of salinity boundary conditions in the upper ocean.

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