Ageostrophic motions in the Brasil-Malvinas confluence region

2017 edition

Marta Masdeu Navarro

Any ocean perturbation tends towards an equilibrium situation where the vertical balance is hydrostatic and the dominant horizontal forces are pressure gradients and Coriolis force, the so called geostrophic balance. Any motion that represents a desviation from this balance is named ageostrophic. At spatial and temporal scales of order 10-100 km (mesoscale) and longer, these desviations are small except within boundary layers related to the sea-surface wind stress and the sea-bottom. friction. At shorter scales (submesoscale), however, they may represent a major part of the motion: these ageostrophic movements correspond to high Rossby values (the ratio of the inertial period, of order 1 day, and the advective time scale) and are usually accompanied by significant vertical motions.

The Brazil-Malvinas Confluence (BMC) region is located approximately 200-400 km offshore from Rio de la Plata, where the Brazil Current (BC) and Malvinas Current (MC) collide. This clash leads to the generation of very energetic mesoscale and submesoscale features, as well as to different kinds of propagating waves. During March 2015, the R/V Hesp├ęrides sampled the BMC with several high resolution transects of the BMC frontal region, including velocity, temperature and salinity measurements, among others. These field data is complemented with data from satellites, the Argo program and the Copernicus operational model. Here we present the results of the data analysis, including the determination of vertical velocities as obtained solving the Omega equation. The BMC turns out to be a region with very energetic ageostrophic motions, characterized by a surface filament and subsurface thermohaline intrusions, as well as substantial vertical velocities.