The connection between water vapor transport and the thermohaline circulation is examined with a simple global coupled ocean-atmosphere energy-salinity balance model (ESBM). Both latitudinal intrabasin and interbasin water vapor fluxes are considered. It is demonstrated that interbasin and intrabasin water vapor fluxes play interdependent and competitive roles in affecting the state of the thermohaline circulation. Increasing intrabasin water vapor flux in the North Atlantic, by decreasing water density in the high latitudes, decreases the North Atlantic deep water production and hence the thermohaline circulation, while increasing interbasin water vapor flux from the Atlantic to the Pacific, by increasing the mean density of the Atlantic and decreasing that of the Pacific, increases the strength of the global thermohaline circulation. The global thermohaline circulation and its asymmetry are sensitive to the latitudinal hydrological cycle in the North Atlantic because of the large water vapor flux from the Atlantic to Pacific Ocean. Global thermohaline circulation exhibits bimodal equilibria as a consequence of imbalances in rates of change of advective and eddy freshwater fluxes in the high-latitude North Atlantic. One equilibrium mode resembles the modern ocean circulation with a strong global asymmetric thermohaline circulation associated with dominant deep water production in the North Atlantic and an effective “heat pump” operating in the Atlantic Ocean. In the other equilibrium, deep water is produced primarily in the Southern Ocean; in particular, North Atlantic deep water is replaced by Southern Ocean deep water. Deep water is produced in the North Pacific in this mode, but is, for reasonably large interbasin water vapor transport, insufficient to reverse the direction of deep water flow into the South Pacific. Based on estimated water vapor fluxes for the present climate, our study suggests that the present thermohaline circulation is dynamically stable, i.e., far from the critical regions of rapid transition between two modes.
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