Sea surface velocities from sea surface temperature image sequences 2. Application to the Brazil-Malvinas Confluence area

An inverse variational model is applied to four exceptionally cloud-free se a surface temperature image sequences of the Brazil-Malvinas Confluence are a for the purpose of estimating sea surface velocity fields. The velocity f ields are constructed by minimizing a mixed layer integrated form of the he at balance equation while the horizontal divergence and vorticity of the fl ow field are controlled. The source terms involved in the heat balance are determined by fitting a polynomial to diurnal temperature Variations as a f unction of temperature and longitude. A sensitivity study of the solution t o perturbations of the weights imposed to the constraints on divergence and vorticity as well as of the representation of the source terms yields erro r fields associated with the velocity. A comparison of the estimated veloci ty fields with available concomitant in situ velocity measurements shows th at the fields are realistic. Errors are of the order of 15-20% in magnitude and 20 degrees-25 degrees in direction. These errors fall within the error bars of 25-30% in magnitude and of 30 degrees-35 degrees in direction, exc ept for the summer image series. Surface velocities of 0.5-0.6 m s(-1) are obtained across the Malvinas Current. They are > 1 m s(-1) in the frontal r egion, in the Brazil Current overshoot, and in warm and cold core rings and vary between 0.3 and 0.8 m s(-1) in the Brazil Current. With some hypothes es on the vertical shear of the horizontal flow the transport of the Malvin as Current is estimated to be 25 +/- 5 Sv between 41 degrees and 40 degrees S.