On the seasonal variability and eddies in the North Brazil Current: insights from model intercomparison experiments

The time dependent circulation of the North Brazil Current is studied with three numerical ocean circulation models, which differ by the vertical coor dinate used to formulate the primitive equations. The models are driven wit h the same surface boundary conditions and their horizontal grid-resolution (isotropic, 1/3 degrees at the equator) is in principle fine enough to per mit the generation of mesoscale eddies. Our analysis of the mean seasonal c urrents concludes that the volume transport of the North Brazil Current (NB C) at the equator is principally determined by the strength of the meridion al overturning, and suggests that the return path of the global thermohalin e circulation is concentrated in the NBC. Models which simulate a realistic overturning at 24 degreesN of the order of 16-18 Sv also simulate a realis tic NBC transport of nearly 35 Sv comparable to estimates deduced from the most recent observations. In all models, the major part of this inflow of w arm. waters from the South Atlantic recirculates in the zonal equatorial cu rrent system, but the models also agree on the existence of a permanent coa stal mean flow to the north-west, from the equator into the Carribean Sea, in the form of a continuous current or a succession of eddies. Important di fferences are found between models in their representation of the eddy fiel d. The reasons invoked are the use of different subgrid-scale parameterisat ions, and differences in stability of the NBC retroflection loop because of differences in the representation of the effect of bottom friction accordi ng to the vertical coordinate that is used. Finally, even if differences no ticed between models in the details of the seasonal mean circulation and wa ter mass properties could be explained by differences in the eddy field, no netheless the major characteristics (mean seasonal currents, Volume and hea t transports) appears to be at first order driven by the strength of the th ermohaline circulation, (C) 2001 Elsevier Science Ltd. All rights reserved.