Research papers Subinertial and tidal currents on the Abrolhos Bank shelf Belmiro M. Castro a,n , Marcelo Dottori a , Adriene F. Pereira b a Oceanographic Institute, University of Sao Paulo, Praca do Oceanografico,191, 05508-120 Sao Paulo, SP, Brazil b Hidromares Environmental & Oceanographic Consultancy, Avenida Afonso Pena,167, 11020-001 Santos, SP, Brazil article info Available online 5 October 2013 Keywords: Abrolhos Currents Winds Subinertial variability Tidal currents 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. & 2013 Published by Elsevier Ltd. 1. Introduction The Abrolhos Bank shelf (AB) is located between 171S and 201S on the Eastern Brazilian Continental Shelf (Fig. 1). Reaching up to 245 km in width (Knoppers et al., 1999), AB is basically a Type A Platform, or a “Wide Shelf with a Western Boundary Current at the Shelf Edge”, according to the shelf classi fication of Loder et al. (1998). Circulation over shelves of this type can be forced by the western boundary current (the Brazil Current (BC) in the case of AB) especially in the outer shelf; by local or remote winds and tides occurring shelf-wide; or by buoyancy fluxes in the inner and outer shelf. AB is relatively short in the along-shelf direction ( E500 km; Ekau and Knoppers, 1999), so the “long shelf” assumption (e.g., Gill and Schumann, 1974) is not strictly valid, thereby hampering the development of Continental Shelf Waves and other long shelf trapped waves. AB isobaths are oriented mostly in the North–South direction, and the shelf break is relatively shallow, with depths between 60 and 100 m. The topography of the AB is smooth from the coast to the 20 m isobath, becoming more complex farther offshore. There are many shallow banks, fringing and bank reefs, paleochannels and features that outcrop the sea surface, such as the Abrolhos Archipelago. The northern part of the AB hosts the largest South Atlantic reef complex, approximately 6000 km 2 in area (Leao, 1999), which is experiencing an accelerated decline of reef corals (Francini-Filho et al., 2008). Two long channels run almost parallel to the coast of the AB: the Southeast Channel (SC), located near the coast and measuring approximately 10 m deep, and the Abrolhos Channel (AC), located approximately 40 km offshore from the coast and measuring approximately 20 m deep. Hydrodynamic processes are fundamental for controlling sea- water characteristics around and within coral reef ecosystems. Currents bring in the nutrients and organic matter necessary for coral maintenance and growth and take out a variety of chemical compounds and particulate matter that can harm the corals (Roberts et al., 1980). The spatial scales of coral reef hydrodynamic processes vary from the scale of coral branches (a few millimeters), those of boundary layer dynamics (1–10 m) and reef scale flows (100– 1000 m) and up to large scale circulation (100 to 1000 km) (Monismith, 2007). Mesoscale processes ( E100 km), such as coastal upwelling, can also be important for pumping nutrient rich oceanic waters into coral reef regions (Andrews and Gentien, 1982). Within the large-scale processes, King and Wolanski (1996) have shown that interactions between tidal currents and the complex topography of reef regions can lead to important residual currents and to highly spatially variable advection processes. Wolanski and Benett (1983) have stressed the importance of large scale Continental Shelf Waves for the circulation around the Great Barrier Reef region. The complex topography in coral reef regions plays an essential role in determin- ing the sea water response to the forcing mechanisms for the circulation – the wind stress, the tides, and the influence of the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.csr.2013.09.025 n Corresponding author. Tel.: þ55 11 30916613. E-mail address: bmcastro@usp.br (B.M. Castro). Continental Shelf Research 70 (2013) 3–12