Distinguishing the Influence of Heat, Freshwater, and Momentum Fluxes on Ocean Circulation and Climate

Abstract

The influence of surface fluxes of heat, freshwater, and momentum on ocean circulation and transports are analyzed using a coupled atmosphere?ocean climate model. A control simulation is compared with experiments in which freshwater and wind stress forcing are suppressed separately and together. Thermal forcing is the dominant driving force of the Atlantic meridional overturning circulation, with freshwater and wind stress forcing both having a smaller positive influence. In the Pacific, on the other hand, freshwater forcing has a negative influence; eliminating it intensifies the meridional overturning, leading to the formation of a deep western boundary current. This difference in sign is consistent with the Atlantic being a net evaporative basin and the Pacific one with net precipitation. Deep outflow from the Atlantic at 30°S is more strongly dependent on freshwater than on wind stress forcing. The sensitivity to freshwater forcing is increased when the model is modified to include the eddy mixing parameterization of Gent and McWilliams. Suppressing the wind stress forcing reduces the flow through Drake Passage and eliminates the subtropical barotropic gyres. Even without the gyres, however, there is still a so-called gyre component of ocean meridional freshwater and heat transport. Hence, the ?gyre? and ?overturning? components of transport cannot be identified with wind-driven and thermohaline circulations, respectively. The model results suggest that the Atlantic thermohaline circulation may compensate for the net evaporation from the basin by transporting freshwater northward at 30°S.