Thermohaline forcing of the Indian Ocean by the Pacific Ocean

Abstract

An idealized box model of the Indian Ocean forced by steady winds and Haney-type surface heat fluxes is used to examine the importance of the warm, fresh throughflow from the equatorial Pacific on the climate of the Indian Ocean. In particular, the hypothesis proposed by Godfrey and Weaver (1991, Progress in Oceanography, 27, 225–272), that the anomalous poleward buoyancy-forced Leeuwin current off the west coast of Australia is a manifestation of a basinwide thermohaline circulation driven by the Indonesian throughflow, is examined. The stronger Sverdrup circulation dominates the thermohaline circulation in most of the model ocean except near the eastern boundary. The effects of the throughflow, however, can be determined by comparing two runs forced by a Pacific Ocean with either the warm, fresh profile of the western equatorial Pacific or a cooler, more saline profile more typical of the eastern equatorial Pacific (where a more usual equatorward wind-driven boundary current is found). It is found that heat imported from the Pacific is transported zonally across the Indian Ocean to the western boundary by the South Equatorial Current. The enhanced meridional steric height gradient south of the SEC drives an eastward return flow back to the eastern boundary, where it turns south to form the poleward Leeuwin Current. The reverse path is traced out by the waters immediately below the thermocline. None of these features are observed when the model Pacific has the cooler profile typical of the eastern boundaries of other oceans. Indonesian throughflow significantly affects the surface heat fluxes and the meridional heat transport in the Indian Ocean. The importance of the throughflow in maintaining the very warm climate of the Indian Ocean (a net exporter of heat) is noted. In the model, the poleward western boundary current along the coast of Africa south of about 27°S appears to play only a very minor role in the basinwide thermohaline circulation described above. This differs from the “warm water route” proposed by Gordon (1986, Journal of Geophysical Research, 91, 5037–5046) where heat is returned to the South Atlantic past the Agulhas Retroflection. Large-scale, fairly long period (>100 days) barotropic eddies are found in the western portion of the basin for some solutions. The generation mechanism for these eddies appears to be barotropic instability in the model South Equatorial Current.