Oceanic Fluxes on the WOCE A11 Section

Abstract

The most southerly WOCE one-time section in the South Atlantic, designated A11, was occupied in January 1993. The cruise track lay across the cool subantarctic zone of the Circumpolar Current in the west and the warm subtropical gyre in the east. In this paper estimates of the flux of heat, salt, oxygen, and other tracers across the section are presented. A brief description of the distribution of physical and chemical properties is followed by a determination of the flux in the surface Ekman layer. Direct measurements of current shear made from a shipborne ADCP and estimates from cruise wind data and climatological data all yield a flux of 5 ± 1 Sv (Sv = 106 m3 s?1) equatorward. When combined with geostrophic estimates relative to a level near 3500 dbar, or the bottom where shallower, an initial guess for the flow field is derived. This initial guess is combined with absolute currents derived from the ADCP when the ship is underway. Unrealistic aspects of the circulation are found, and we conclude that the ageostrophic ?noise? of the ADCP measurements is to blame. A second and preferred flow field is derived using an inverse analysis supplemented by a few estimates of total current transports. Climatological transports in the Falkland Current are estimated at about 45 Sv equatorward, while on the eastern margin a poleward transport of about 5 Sv is assumed. The equatorward flux of bottom water is taken from a recent determination to be 6 Sv. The heat flux estimate across the section is found to be robust and to have twice the value reported recently. The flux is equatorward with magnitude of 0.5 ± 0.1 PW and occurs entirely within the subtropical gyre. A net equatorward transport or 1 0 Sv of upper thermocline water, which takes place near the eastern margin, has also been inferred. This pair of estimates identifies the warm water path as the route for the upper-ocean replacement of NADW. The flux of salt, for zero volume transport across the section, is found to be 0 ± 5 Sv psu (1 Sv psu ≈ 106 kg s?1) and when interpreted as a freshwater flux is quite at variance with estimates based on the global freshwater balance. Oxygen and other nutrients have fluxes generally differing in sign from previous determinations. Sensitivity of the results to assumptions made is considered.