The oxygen minimum zone in the eastern North Atlantic south and east of the Cape Verde Islands

The open-ocean oxygen minimum zone (OMZ) south and east of the Cape Verde Islands is studied from CTD hydrography, ADCP velocities, Argo float trajectories, and historical data, with a focus on the zonal supply and drainage paths. The strongest oxygen minimum is located north of the North Equatorial Countercurrent (NECC) at about 400 to 500-m depth just above the boundary between Central Water and Antarctic Intermediate Water (AAIW). It is shown that the NECC, the North Equatorial Undercurrent at 4 to 6 degrees N, and a northern branch of the NECC at 8 to 10 degrees N are the sources for oxygen-rich water supplied to the OMZ in summer and fall. A weak eastward NECC at 200-m depth also exists in winter and spring as derived from Argo floats drifting at shallow levels. Historical oxygen data from 200-m depth confirm this seasonality showing high (low) oxygen content in summer and fall (spring) within the supply paths. Compared to the strong oxygen supply at 150 to 300-m depth, the ventilation of the OMZ at 300 to 600-m depth is weaker. Westward drainage of oxygen-poor water takes place north of the Guinea Dome, i.e., north of 10 degrees N, most pronounced at 400 to 600-m depth. In July 2006 the total eastward transport of both NECC bands above sigma(theta)= 27.1 kg m(-3) at 23 degrees W was about 13 Sv (1 Sv = 10(6) m(3) s(-1)). About half of this water volume circulates within the Guinea Dome or recirculates westward north of the Guinea Dome.