The features of near bottom nepheloid layer at the steep ocean slope near Western Africa

Abstract

Processes in the near bottom boundary layer of littoral ocean regions with intensive upwelling circulation are important for investigation of suspended matter transport and currents of biogen elements carried out from the bottom to the surface. Vertical profiles of turbidity from the ocean surface to the bottom with the Sea Tech Mark nephelometer were obtained during the research vessel Academic Mstislav Keldysh cruise in winter 1991-1992. Also vertical profiles of temperature, salinity and potential density were obtained with a Neil Brown probe. Measurements were performed at the shelf and continental slope near western Africa. More than a hundred profilings in the continuous mode at the steep ocean slope during vessel drift were made. At most vertical profiles the near bottom nepheloid layer (BNL) consisted of an upper thicker part with less turbidity and a thinner mixed part with greater turbidity. Structural parameters of the BNL were found to correlate with ocean depth. At the slope region near bottom layers were mainly powerful. A benthic turbid event characterized by strong mixing in the 125 m bottom layer was observed at an ocean depth of about 1000 m. The eddy with horizontal dimension of about 3 km that caused the event was found to occupy practically all the water column. The isopycnal bend at the eddy position was about ten meters; the eddy's lifetime was more than 2.5 days. A high turbidity BNL was also found at a depth of about 560 m. This benthic event was evidently caused by upwelling influence on the near bottom layer and followed the approach of the characteristic inclination value for the semi-diurnal internal tide M2 to the value of the bottom slope.