Abstract :
Hydrographic data gathered during four cruises are used to evidence upwelling on the Yucatan Shelf, and describe its structure. Subsurface Caribbean water (characterized by 16 to 20°C, 36.1 to 36.5 S, and 8 to 14 μmol 1−1 nitrate concentrations) upwells along the eastern slope of the Yucatan Shelf. This water, which originates at depths of about 220–250 m in the Yucatan Channel, rises at about 10−2 cm s−1 to dome into the euphotic zone, but only occasionally breaks the ocean surface. This upwelling appears to follow a seasonal cycle. During spring and summer the upwelled water intrudes over the Yucatan Shelf to create a two-layered water column. Strong stratification between Caribbean surface water (CSW) and the Yucatan upwelling water (YUW) layer on the shelf is likely to prevent mixing between them, except during the winter periods of northern winds. The YUW layer moves westward across the shelf and contributes to the formation of a cyclonic gyre north of Cape Catoche. The curl of the prevailing wind stress is not likely to induce upwelling along the eastern Yucatan slope. The observations imply that the upwelling is probably caused by bottom friction or other topographical mechanisms. A subsurface countercurrent runs southward along the eastern slope of the Yucatan Shelf, and appears to be closely related to the upwelling process. The Salinity Maximum is ruptured on the western side of the Yucatan Channel as a consequence of the upwelling. The data presented support the hypothesis that Yucatan is one of the most important upwelling regions on a western oceanic margin.
