ALBERT

Description: "Auftrieb 75" was a joint cruise of R.V. "Meteor" and R.R.S. "Discovery" performed in the area 21-26° N off the northwest African coast during January-March, 1975. Current meter observations from six moorings and CTD/STD observations from three repeatedly covered sections of the cruise are analysed in order to describe the distribution of the water masses and its variability with the changes in the current system. The Canary Current is seen to pass through the northwestern corner of the expedition area, carrying with it a geostrophic eddy with a period in excess of 30 days. The eddy is probably another example of the type generally found in the open ocean circulation. Further south, offshore currents display large variations in strength and direction with a typical time scale of 10-14 days which are believed to reflect largescale mixing processes along the boundary of the Canary Current and the poleward flow of the coastal region, involving North Atlantic Central Water (NACW) and South Atlantic Central Water (SACW). The coastal upwelling undercurrent has been observed along the shelf edge but undergoes large variations in response to the wind and to variations in the horizontal divergence of the offshore currents. On the shelf the circulation responds to the local longshore wind to some extent but is also strongly affected by variations of the currents at the shelf edge. The distribution of water masses at intermediate depth ( 100-800 m) is analysed quantitatively based on the assumption that mixing between NACW and SACW is effective along surfaces of constant density only. lt is shown that most intense mixing occurs at about 22-23° N and is still very important further south. At 25-26° N SACW has been absorbed into the surrounding NACW water mass to a high degree, even in the undercurrent. Isolated bodies of water are found in the southern part of the expedition area, some of which are carried north in the undercurrent. The large variability in the water mass distribution at the source level of the upwelling probably has important consequences for the variations in the efficiency of biological production of the upwelling region.