Mixing and energetics of the oceanic thermohaline circulation

Using an idealized tube model and scaling analysis, the physics supporting the oceanic thermohaline circulation is examined. Thermal circulation in th e tube model can be classified into two categories. When the cooling source is at a level higher than that of the heating source, the thermal circulat ion is friction-controlled; thus, mixing is not important in determining th e circulation rate. When the cooling source is at a level lower than that o f the heating source, the circulation is mixing controlled; thus, weak (str ong) mixing will lead to weak (strong) thermal circulation. Within realistic parameter regimes the thermohaline circulation requires ex ternal sources of mechanical energy to support mixing in order to maintain the basic stratification. Thus, the oceanic circulation is only a heat conv eyor belt, not a heat engine. Simple scaling shows that the meridional mass and heat fluxes are linearly proportional to the energy supplied to mixing . The rate of tidal dissipation in the open oceans (excluding the shallow mar ginal seas) is about 0.9-1.3 (X10(12) W); the rate of potential energy gene rated by geothermal heating is estimated to be 0.5 X 10(12) W. Accordingly, the global-mean rate of mixing inferred from oceanic climatological data i s about 0.22 X 10(-4) m(2) s(-1). Using a primitive equation model, numerical experiments based on a fixed en ergy source for mixing have been carried out in order to test the scaling l aw. In comparison with models under fixed rate of mixing, a model under a f ixed energy for mixing is less sensitive to changes in the forcing conditio ns due to climatic changes. Under a surface relaxation condition for temper ature and standard parameters, the model is well within the region of Hopf bifurcation, so decadal variability is expected.