Time-Lapse Volumetric Seismic Imaging of Water Masses at a Major Oceanic Confluence in the South Atlantic Ocean

Water-mass interaction processes within the Southern Ocean strongly influence the global oceanic circulation system.  For example, the western side of the South Atlantic Ocean is dominated by the confluence between the Brazil Current (BC) and Falkland/Malvinas Current (MC). At this confluence, tropical/subtropical (i.e. warm and salty) waters are transported southward by the BC where they interact with subantarctic (i.e. cold and fresh) waters transported northward by the MC. This interaction creates a highly dynamic frontal system that is characterized by complex water mass interactions and intense diapycnal mixing. Here, we exploit time-lapse volumetric seismic imaging of the Brazil-Malvinas Confluence (BMC) in order to elucidate the detailed thermohaline structure of this critical region. Careful signal processing of a ~25 terabyte survey, acquired during February 2013, reveals a spectacular northeastward dipping oceanic front that extends as deep as ~1800 m. Significantly, a deep transient mesoscale eddy is embedded in this front. This eddy appears to grow and decay over ~11 day period and it has a maximum diameter of ~40 km. Time-lapsed imagery also reveals mesoscale to sub-mesoscale complexity at all depths. Long wavelength temperature fields extracted from our acoustic velocity measurements reveal a pattern of cool anomalies on the MC side together with a steep and fanning temperature gradient close to the front but above the eddy, indicative of heat transfer. Evolution of this prominent eddy embedded in the front can be independently investigated using velocity fields calculated from the GLORYS12v1 product for the period of interest. Tracked particles, which are released daily through the confluence area down to 1800 m, flow along the MC from 40° S  to 36° S and are deflected clockwise by the BMC. This flow suggests that the observed eddy is cyclonic and related to MC recirculation, as a result of the combination of the steep continental slope and geometry of the BMC. In this way, cooler water masses are juxtaposed against the front. A simple one-dimensional steady-state model is used to examine heat transfer across the front. Our results highlight the importance of combining high quality three-dimensional seismic imagery with hydrographic observations in order to elucidate the fluid dynamics of complex oceanic fronts.