An Inverse Model for Seasonal Circulation over the Southern Brazilian Shelf: Near-Surface Velocity from the Heat Budget

Abstract

An inverse model was developed of the seasonal upper-layer circulation in the region adjacent to the coast of southern Brazil between 30° and 35°S, 50° and 55°W near the boundary of the Brazil?Malvinas confluence. In this region there are no in situ data on circulation, while a large array of data on sea surface temperature and surface heat fluxes acquired through the last 30 years is available. The model is based on the joint analysis of observed seasonal changes of SST along with observed fields of insolation and surface heat fluxes. From the equation of heat conservation integrated over the variable depth of the mixed layer the ?residual? flux fields required to explain the observed temperature changes between seasons were computed. Generally, this residual flux accounts for advection, diffusion, and entrainment. Diffusion has been neglected based on scaling estimates of term sizes, while entrainment was parameterized through observed changes of the mixed layer depth, with the assumption that the temperature in the seasonal thermocline was linear in depth. The remaining ?residual flux? must be associated with advection. In this way, the seasonal fields of surface advection were constructed. The fields of advection together with the fields of SST determine the cross-isotherm components of velocity fields, while the along-isotherm components are in the ?null space? of the inversion. However, given the cross-isotherm components, along-isotherm components can be determined from the equation of continuity, with the condition of no motion at the rigid coastal boundary. Following this scheme, total near-surface velocity fields were computed for each season on a regular grid (20? ? 20?) in the chosen region. Results suggested that in all seasons the coastal part of the region is dominated by northward flow, probably related to the alongshore spreading of waters discharged from the Plata estuary. The offshore part of the region is involved in the southward motion associated with the Brazil Current. The impact of some specific dynamical features of the region, such as bottom topography and water discharge from the Lagoa dos Patos, is also evident in the model results.