Formation of a basin-wide bottom nepheloid layer in the western Mediterranean after the winter 2005 dense shelf water cascading event

Abstract

The analysis of a compilation of deep CTD casts conducted in the western Mediterranean after winter 2005 have documented the role that dense shelf water cascading (DSWC) plays in transporting suspended particulate matter from the coastal regions down to the basin. Deep CTD casts revealed the formation of thick and persistent bottom nepheloid layer (BNL) associated with the winter 2005 DSWC event, which was not present in 2004. This BNL spread throughout the western Mediterranean basin and scaled in thickness with a thermo-haline anomaly generated by the mixture of dense waters formed by deep convection in the open sea and by cascading, being up to 1450 m thick at some locations. Excess of concentrations within the BNL in the central part of the basin were around 0.2 mg/l, but reached concentrations >2 mg/l close to the continental rise. This BNL could be observed to progress from the Gulf of Lions and Catalan margin towards the central part of the basin, reaching south of the Balearic Islands and west of Sardinia in May 2005, and covered the entire western Mediterranean basin in June 2006. BNL thickness and concentration diminished with distance away from its source and also with time, although its turbidity signal can still be detected at present (2010)