INTEROCEAN EXCHANGE AND MERIDIONAL MASS AND HEAT FLUXES IN THE SOUTH-ATLANTIC

The meridional transport of mass and heat in the South Atlantic as wel l as the interocean exchange with the South Pacific and Indian Ocean i s examined using a model based on the adjoint method. Large amounts of historical temperature (theta) and salinity (s) data are assimilated and an optimal model circulation is searched to satisfy two main objec tives: (1) the vertical shear of the model hows is consistent with geo strophic shear estimates, and (2) the model flows correctly reproduce the measured distributions of theta and s. Information on baroclinic f lows contained in the historical theta/s distributions is incorporated in the model circulation via constraint 1, and the unknown reference velocities are chosen in a way such that the resulting absolute flow v elocities (horizontal and vertical) yield realistic theta and s fields (constraint 2). The model is mass, heat, and salt conserving and has realistic topography. It is shown that realistic theta and s model dis tributions can be obtained with hows that are consistent with geostrop hy. The model-derived reference velocities (2500 dbar) show southward flow of North Atlantic Deep Water (NADW) along the South American coas t and show eastward flow along the path of the Antarctic Circumpolar C urrent (ACC). Satisfactory theta/s simulations are obtained for a vari ety of model flows that differ considerably with respect to strength o f the meridional overturning cell, magnitude of meridional heat transp ort and inflow rate of Indian Ocean Central (thermocline) Water (IOCW) into the South Atlantic. A strong correlation is found between meridi onal heat transport, strength of the meridional cell and inflow from t he Indian Ocean. For all the model states that were found to be consis tent with hydrographic data, the northward transport above the NADW la yer is dominated by intermediate water with only relatively small cont ributions of warm surface water. The maximal acceptable meridional hea t transport across 30 degrees S amounts to 0.35 PW. Inflow (shallow an d intermediate) from the Indian Ocean into the South Atlantic in the r ange of 4-7 Sv can be accommodated by the model, but larger values hav e to be rejected. All acceptable model solutions show net heat gain of the South Atlantic from the atmosphere and a net export of warm water into the Indian Ocean.