A Lagrangian study of summer upwelling along the Uruguayan coast

Abstract

This study focuses on the surface transport in the Uruguayan coastal zone during summertime under favorable wind upwelling conditions. The surface transport in the area was investigated for the first time with the application of a Lagrangian technique computing the backward-in-time Finite Size Lyapunov Exponents (FSLE) using the current velocity field derived from a high resolution numerical model (CROCO, 1/36° horizontal grid). Intense summer upwellings were studied focusing on surface stirring characterized by FSLE temporal averages. Subsequently, the analysis of the attracting FSLE field maxima allowed the detection of Lagrangian Coherent Structures (LCS), providing information on the location of the upwelling frontal regions and on the transport patterns associated with the flow. The FSLE analysis during summertime revealed an intense mixing activity concentrated in the oceanic coastal area, concordant with the location of the summer climatological upwelling, while during the most intense upwelling dates an increase in the mixing activity was found in the estuarine coastal region. When concentrating on the FSLE maxima during intense upwelling dates, we identified attractive LCS in the estuarine region locating retention fronts. The identified structures' location coincided with the frontal regions observed from both, simulated SST and satellite Chlorophyll-a distribution, highlighting the Lagrangian approximation's importance in locating retention fronts with different origins. Finally, the LCS evolution analyzed for two particular upwelling events, together with the simulated dispersion of synthetic Lagrangian particles, evidenced the importance of this approximation to study the transport processes in the area.