Hydrographical observations west of the Banc d'Arguin, Mauritania, in May 1988

Abstract

The present paper discusses the distribution of hydrographical parameters in Mauritanian waters between 19° 20′ N and 20° 40′ N. Data were collected during the first leg of the Dutch Mauritania Expedition in May 1988 with the R/V ‘TYRO’ and comprise measurements of water temperature (T), salinity (S) and dissolved oxygen contents (O₂) in the area off the Banc d'Arguin. The time of investigation fell in the transitional period between the cold and warm season, which in this area is characterised by a peak in the trade wind speed and progressive heating of waters. Wind speed and sea surface temperature distribution in May 1988 were close to the seasonal averages which suggests that the observed hydrographical conditions were normal for this period as well. The highest spatial variability of T, S and O₂ was found near the southern end of the Banc d'Arguin, where the combined effect of coastal upwelling and seasonal heating made the thermocline upper limit rise towards the surface and upwelling waters reach the shallowest parts of the shelf. In the northern part of the study area upwelling phenomena were observed in the subsurface layers below 50 m depth; upwelling water did not reach beyond the outer edge of the shelf. The analysis of the TS diagram showed that water masses in the upper 300 m layer were mainly composed of South Atlantic Central Water with an admixture of warm, low-salinity surface water of Guinean origin. Traces of North Atlantic Central Water were found in the subsurface layer and at the deepest levels of measurements. Almost no traces of warm, saline water originating from the shallow Banc d'Arguin were registered during the survey.