Subinertial and tidal currents on the Abrolhos Bank shelf

Abstract

Analysis of simultaneous current measurements in the middle (point PA1) and outer (point PA2) Abrolhos Bank shelf showed different characteristics and forcing mechanisms between the two shelf regions. The mean currents at both positions point generally southwestward but are weak due to the high time variability; at PA1, the mean along-shelf currents are almost one order of magnitude larger than the mean cross-shelf currents, while at PA2, both mean current components exhibit the same order of magnitude. At both points, the tidal currents are more important in the cross-shelf direction than they are in the along-shelf direction. The subinertial currents are mainly barotropic and along-shelf at PA1, while at PA2, the baroclinic first mode dominates the subinertial variability, especially in the cross-shelf direction, though the currents are also more energetic in the along-shelf direction. Correlations are significant between the along-shelf subinertial currents and synoptic winds at PA1, showing a local response (almost in phase) to the wind stress. At PA2, the correlations between winds and currents are generally not significant, showing that the subinertial current variability near the shelf break is forced by other mechanisms; the mesoscale variability of the Brazil Current is most likely important at this position. A simple frictional balance model allows a dynamical interpretation of the significant correlations between winds and currents at PA1.