The Santos Basin hydrodynamics: the interaction between the Brazil current and the Agulhas eddies

Abstract

The Brazil Current (BC) develops large meanders, which often turn into vortical rings as it flows off the Santos Basin shelf break. The most conspicuous and recurrent of these is the Cabo Frio Eddy (CFE), formed in the east area of the cape that gives it its name, and which marks a strong change in orientation on the south east continental margin. For this reason, and linked to the conservation of potential vorticity, the current scientific literature has always attributed to the BC's internal structure the ability to develop unstable meanders, such as the CFE during its trajectory towards the pole. However, studies from the end of the last decade suggest that there is an arrival of important features coming from the east, that is, from the interior of the South Atlantic basin. Among these, there are long vorticity waves with a probable non-linear nature. Other features are the anticyclonic rings formed in the Agulhas Retroflexion and, therefore, called Agulhas Eddies (AgE). According to such recent works, eight of these rings reach the western boundary layer per year in the latitude range between 20° and 35°S. At least 3 of them reach our continental margin north of 26°S, establish a direction parallel to BC and continuously interact with the current. However, there is no quantification of this interaction dynamically. It is not known whether there is a transformation in the vertical structure of the AgE and whether this external source of mass, energy and potential enstrophy is an equally or more relevant trigger for the formation of unstable meandering by BC. Additionally, the surface circulation mean field of the Santos Basin presents a persistent signal with 300 km diameter anticyclonic eddy adjacent to the BC jet base. This possible "docking" of what may be either a AgE or a crest of the long vorticity wave occurs exactly over the outermost portion of the São Paulo Plateau. It is in this location, under intense vortical influence, that the main wells of the Pre-Salt Pole are located, a region currently responsible for the largest extraction of oil and gas on the continental margin. Due to its economic, dynamic, and, perhaps, climatic relevance, this proposal aims to investigate the interaction processes between BC and oceanographic features coming from the east in the Santos Basin. For this purpose, datasets of synoptic observations, conducted by traditional oceanographic cruises and/or ARGO floats (profilers) will be used to describe and compare the vertical structure of both eastern and BC features and their unstable meanders (such as the CFEs and Santa Marta Eddy). Also, time series of multisatellite altimetry images, global (eddy-resolving), and regional (submesoscale resolving) numerical simulation outputs of high-resolution will also be employed. With these, we intend to understand the balance and energy exchanges between the various compartments, analyze instability processes and propose to explain the role of these features - if any - in the instability of the local western boundary currents system. The relevance of the proposed project is not limited, therefore, to academic issues, and as mentioned above, it can contribute to a better dimensioning of ocean engineering structures through better and more accurate project parameters. Moreover, in the end, the results of regional simulations - conducted domestically - can be used for coupling with numerical biogeochemical and sediment transport models. These can constitute important tools to answer questions regarding the monitoring of the oceanic environment. (AU)