Coexistence of Multiscale Processes in the Brazil-Malvinas Confluence Region

Abstract

The Brazil-Malvinas Confluence (BMC), the encountering site of waters of subtropical and subantarctic origin, is one of the most energetic regions in the world ocean. During March 2015, the R/V Hespérides carried out the TIC-MOC cruise, with 14 days of measurements in the BMC region. Field and remote-sensing data collected during the TIC-MOC cruise clearly illustrate the presence of mesoscale and submesoscale structures of different sort on top of the very energetic large-scale Brazil and Malvinas Currents. These two meridional currents, each of the order of 100 km wide, collide frontally before diverting east, leaving behind vortices of opposite sign near the collision region, but also much farther away, as well as multiple submesoscale structures. Here we characterize the spatial structure of the two colliding currents and of three (one cyclone and two anti-cyclones) vortices found in the near field (at distances less than 500 km from the collision point). We use the satellite images to identify several submesoscalar structures at the frontal system and in the margins of both the main currents and the mesoscalar structures. We also employ high resolution field data to study two different types of submesoscale structures observed in the frontal system. These are a very shallow (5-20 m), thin (5-50 km) and intense (velocities close to 2 m s-1) filament that flows east over the frontal system, carrying brackish waters from Rio de la Plata, and a number of relatively small thermohaline intrusions (thickness 10-100 m and width of the order of 10 km) that intrude several tens of km into both sides of the frontal system. An energy analysis of the frontal system shows that the flux of kinetic energy into the confluence region,associated with the two large scale currents,is of the order of 10 GW while the mesoscalar structures in the near field contain some 107-108 GJ of kinetic energy, giving a residence time of the order of 10-100 days. While some of these mesoscalar structures may flow away from the BMC region during these time intervals, others remain in the area for longer periods, hence requiring substantial energy transfer from the meso- to the submesoscale