An investigation of Ekman upwelling in the North Atlantic

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Abstract
Surface winds from the Fleet Numerical Oceanography Center (FNOC) for 1979–1986 were used to compute the monthly mean Ekman up welling fields 〈we〉m, the seasonal and annual climatologies of 〈we〉m (〈we〉s and 〈we〉a, respectively), and the basin wide monthly mean vertical Ekman velocities 〈we〉mb in the North Atlantic Ocean (the analyses excluded the zone within ±5° of the equator). The annual climatology compares very well with the climatology of Leetmaa and Bunker (1978). The analyses show a pronounced minimum in 〈we〉mb during 1981–1982. Additional analyses using 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts exhibit a similar pattern in 〈we〉mb. Further examination of the FNOC 〈we〉m fields shows that the primary source of the interannual signal was the region off NW Africa in the vicinity of the Guinea Dome. Other sectors of the basin experienced no significant interannual trends. Hydrographic data and sea surface temperature data from the NW Africa sector for 1981–1986 indicate a cooling trend beginning in late 1982, consistent with increased upwelling. Also, the fall and winter seasons' mixed layers at the center of the Guinea Dome were deeper in 1984 and 1985 than in previous years. Finally, the potential impact of large interannual variations in Ekman upwelling on basin wide primary productivity is discussed.