Preliminary results onmid-depth circulation features on Rio Grande Rise

Rio Grande Rise (RGR), a large positive topographic anomaly off Southern Brazil coast, has become an object of increasing scientific interest in the last decade, that being centered about the possibility of future mining for ferromanganese crusts (and the needs to acknowledge the environmental risks associated with mining activities) on RGR as well as its peculiar geological history, yet to be clarified. Several expeditions carried by the Geological Survey of Brazil (CPRM) and supported by local and foreign academic institutions and by the Brazilian Navy, were made in the last years to survey the area, including its geological, geophysical, and oceanographic features, revealing a complex and unique environment. Flow measurements carried by a vessel-mounted ADCP during a pilot oceanographic cruise in 2011 were analyzed in order to deduce local mean flow and its variability at greater depths (500-700m). Despite located away from strong western boundary jets, that region seems to be influenced by rather intense and variable currents peaking at 50 cm.s-1. Spectral analysis showed that most variability of the measured currents was due to tidal forcing, expressed as internal wave packets generated by barotropic tide dissipation. After low-pass filtered (T > 40h), the residual flow revealed to have been mild during both legs, with velocities seldom reaching 10 cm.s-1 and usually decreasing with depth. Low frequency variability was detected, possibly due to flow interaction with bottom topography. On the main plateau, several sub-mesoscale meanders were spotted at 650-700 m water depth as the near-bottom current crossed the main fracture zone. The possibility of a topographic Rossby wave and its persistence are discussed. The signature of a mesoscale anticyclone was registered over one of the shallowest flanks of a secondary rise. The topography configuration below suggests a persistent, trapped circulation cell referred to as Taylor cap, although other possibilities of its formation are considered. Due to irregular spatial sampling (except at the site where the anticyclone was detected), definite circulation patterns are yet to be described over most of the surveyed area, although some sites with persistent (throughout the entire cruise) and stronger flows were noted on the main plateau, one of those sites even hosting a curious assemblage of mega-ripples (dunes) and deep canyons, which further support the idea of a locally prevailing intense currents.