The circulation of the subtropical South Pacific derived from hydrographic data

Authors

  • M. N. Tsimplis,

  • S. Bacon,

  • H. L. Bryden


Abstract

The circulation and distribution of water masses of the subtropical South Pacific are described, based on salinity and temperature data from two trans-Pacific zonal sections at 32°S and 17°S (World Ocean Circulation Experiment sections P6 and P21, respectively). A circulation scheme is found using an inverse model solved by singular value decomposition. Approximately 12 Sv (1 Sv = 106 m3 s−1) of Circumpolar Deep Water that is transported northward through the Southwest Pacific Basin, mainly in the deep western boundary current, is returned southward at intermediate depths. The zonally integrated flow near the surface is northward. The southward transport of the East Australian Current is estimated to be 26 Sv at 25°S and 43 Sv at 32°S. Assuming zero net mass transport, the derived southward heat fluxes are −0.17±0.25 PW and −0.04±0.25 PW at 17°S and 32°S, respectively. Thus there is an implied net loss of heat of 0.13 PW between the two sections, equivalent to an average heat loss to the atmosphere of 6.5±5.8 W m−2. When the net mass transport is assumed to be nonzero, in accordance with a westward Indonesian Throughflow from the Pacific to the Indian Ocean, the sign of the temperature transport changes but the meridional divergence remains essentially the same. The vertical distribution of the assumed net mass transport is crucial in determining the temperature flux. In particular, when the net mass transport is assumed to be 6 Sv above 250 dbar and 4 Sv between 250 and 1000 dbar, the temperature fluxes are directed northward with magnitudes of 0.33±0.25 PW and 0.44±0.25 PW for the 17°S and the 32°S section, respectively. The freshwater fluxes suggest that there is a net evaporation of 0.39 Sv over the region enclosed by the two sections, which is about twice as large as the climatological estimates.

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