Using an idealized tube model and scaling analysis, the physics supporting the oceanic thermohaline circulation is examined. Thermal circulation in the 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 circulation is friction-controlled; thus, mixing is not important in determining the circulation rate. When the cooling source is at a level lower than that of the heating source, the circulation is mixing controlled; thus, weak (strong) mixing will lead to weak (strong) thermal circulation. Within realistic parameter regimes the thermohaline circulation requires external sources of mechanical energy to support mixing in order to maintain the basic stratification. Thus, the oceanic circulation is only a heat conveyor 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 ... Abstract Using an idealized tube model and scaling analysis, the physics supporting the oceanic thermohaline circulation is examined. Thermal circulation in the 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 circulation is friction-controlled; thus, mixing is not important in determining the circulation rate. When the cooling source is at a level lower than that of the heating source, the circulation is mixing controlled; thus, weak (strong) mixing will lead to weak (strong) thermal circulation. Within realistic parameter regimes the thermohaline circulation requires external sources of mechanical energy to support mixing in order to maintain the basic stratification. Thus, the oceanic circulation is only a heat conveyor 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 ...