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

Abstract

An inverse variational model is applied to four exceptionally cloud‐free sea surface temperature image sequences of the Brazil‐Malvinas Confluence area for the purpose of estimating sea surface velocity fields. The velocity fields are constructed by minimizing a mixed layer integrated form of the heat balance equation while the horizontal divergence and vorticity of the flow field are controlled. The source terms involved in the heat balance are determined by fitting a polynomial to diurnal temperature variations as a function of temperature and longitude. A sensitivity study of the solution to perturbations of the weights imposed to the constraints on divergence and vorticity as well as of the representation of the source terms yields error fields associated with the velocity. A comparison of the estimated velocity fields with available concomitant in situ velocity measurements shows that the fields are realistic. Errors are of the order of 15–20% in magnitude and 20°–25° in direction. These errors fall within the error bars of 25–30% in magnitude and of 30°–35° in direction, except 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 region, 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 hypotheses on the vertical shear of the horizontal flow the transport of the Malvinas Current is estimated to be 25±5 Sv between 41° and 40°S.