An inverse model of the eastern North Atlantic general circulation and thermocline ventilation

An inverse model is applied to a set of high-quality hydrographic measurements gathered between 1981 and 1993 in the eastern North Atlantic, between 24 degrees N and 54 degrees N and east of 35 degrees W. The method seeks a density field as close as possible to the hydrographic data and an absolute velocity field in exact geostrophic and hydrostatic balances, satisfying as well as possible the planetary vorticity balance, the Ekman dynamics at the surface and mass, salt and heat conservations in a top-to-bottom integrated form. A solution is found that departs only slightly from the hydrographic data and that presents reasonably small constraint residuals. The solution is discussed in terms of the eastern North Atlantic general circulation and thermocline ventilation during the observational period. The North Atlantic Current appears to be composed of several branches, and its influence extends down to more than 2500 m. Two-thirds of its total transport recirculates northward across 54 degrees N. The Azores Current appears as an almost zonal current around 33 degrees N that, while recirculating southward, is fed from the north by the southward recirculation of the North Atlantic Current. The total eastward transport of these two currents above the Mid-Atlantic Ridge amounts to 58 +/- 11 Sv, far more than predicted using the Sverdrup relation, and more than most of the previous estimates. It is shown that the interaction between the deep circulation and the bottom topography allows such a transport to be compatible with the planetary vorticity balance. Using the modeled circulation, thermocline ventilation is thoroughly studied. The subduction rate patterns show where the lightest varieties (27.0 < sigma(theta) <27.4) of Subpolar Mode Water (SPMW) are mainly formed and subducted. A total of 11.9 +/- 0.5 Sv of SPMW formation is estimated in the modeled area, while the total subduction transport amounts to 7.7 +/- 0.5 Sv, of which 2.5 +/- 0.4 Sv is SPMW subduction. Eddy diffusion along the isopycnals is estimated as playing no important role in SPMW subduction and in the first stages of its subsequent southward transport, but seems to be the dominant process by which that water crosses the Azores Front south of 36 degrees N. The Lagrangian, annual buoyancy budget of the winter mixed layer is finally diagnosed from the model fields, and it is confirmed that SPMW is formed where the mixed-layer loses buoyancy, and subducts as soon as the mixed layer gains buoyancy. (C) 1997 Elsevier Science Ltd.