Intermediate water masses off south-southwest Portugal: Chemical tracers

Abstract

Biogeochemical tracers, as dissolved oxygen and nutrients, were measured during several surveys carried out between 1999 and 2001, off south-southwestern Portugal. The dense vertical and horizontal sampling allows the extensions of intermediate water masses present in the region to be resolved. A subsurface minimum salinity water layer, exhibiting values of apparent oxygen utilization (AOU) in the range 40-60 μmol/kg and intruding into the study area mainly from the south-southwest, is proposed to be a remnant of the Subarctic Intermediate Water (SAIW). A deeper salinity minimum displaying high concentrations of nutrients and AOU (∼100 μmol/kg) and identified as a branch of the Antarctic Intermediate Water (AAIW), is observed to derive from the western African coast and to penetrate the area predominantly from the south-southeastern side. The salty and warm Mediterranean Water (MW) present in the same density range as the fresher intermediate waters, reveals differentiated chemical properties at the various depths depending on the surrounding waters. Analysis of water masses gives an indication that the collapse of the fresher waters into a narrow range of salinity values (35.58-35.64) at intermediate levels is favored by the strong presence of the MW outflow, and that the admixture of the fresher waters with the MW outflow very likely induces the formation, splitting and spreading of several MW cores extending westwards along the Portuguese coast.