5 - Currents and water masses over the continental shelf

Abstract

The analysis of data collected in several projects, mainly Habitats and DEPROAS, together with an enquiry into previous works, made possible to describe the hydrothermodynamic characteristics of the Campos Basin Continental Shelf (CBCS), including its spatio-temporal variability and main forcings. Two regions with distinct oceanographic characteristics were identified in the CBCS: the northern part (CBCS-NP), located north of Cape São Tomé and the southern part (CBCS-SP), comprised between Cape São Tomé and Cabo Frio. The main oceanographic difference between these two regions is the degree of intrusion towards the coast of the two oceanic water masses transported by the Brazil Current (BC), the Tropical Water (TW) and the South Atlantic Central Water (SACW). The volume of SACW in the CBCS-SP is relatively large, both in the summer and in the winter, since this water mass occupies wide areas of the bottom and deep layers, mainly at the far south, close to Cabo Frio. In the CBCS-NP the relative volume of the SACW is lower than at the south, with TW generally prevailing. The coastal upwelling that takes place in the CBCS-SP is intermittent and controlled by seasonal and synoptic winds. In periods of coastal upwelling, the SACW presents even higher volumes in the CBCS-SP, especially around Cabo Frio. At any time of the year, the most frequent currents in the CBCS have SW direction, aligned with the isobaths and leaving the coast right of the motion, from the continental shelf break to depths of 20-30m, in response to the shear stress of the winds that blow from E-NE, prevailing in the region, and to the average flow of the BC. This circulation towards SW is associated in general with currents converging to the coast in the bottom layer, contributing for SACW intrusions towards the coast. Inversions in the direction of the currents, to NE and leaving the coast to the left, are associated mainly to shear stresses of the S-SW winds that occur during the passage of cold fronts. These fronts are more frequent and intense in the winter and in the spring. The currents that are almost parallel to the isobaths present small vertical sheer, being essentially barotropic. In the direction normal to the isobaths, the first baroclinic mode must be considered in order to explain most part of the sub-inertial variance of the currents. The tidal currents are smaller than the sub-inertial currents in the direction almost parallel to the coast.