Rediscovering the second core of the Atlantic NECC

The North Equatorial Countercurrent (NECC) is investigated at 35°W through a combination of theory, high resolution Ocean General Circulation Model outputs, and observations. Transport from ADCP measurements during four WOCE cruises and from 5-year SeaWinds scatterometer wind data (QuikSCAT satellite) were used to support model results. It is found that the NECC in the annual mean is approximately in Sverdrup balance, but that seasonal changes in the wind stress curl lead the transport expected from the Sverdrup balance by one month, the propagation time of the seasonal Rossby waves from the African coast to 35°W. An investigation of the vertical structure of the NECC shows that there is an eastward core throughout the year, but in spring it is below the westward Ekman flow. Only 60% of the total transport are above the thermocline. The most interesting result of the present study is that model as well as observations describe a distinct second, northern core of the NECC which has not yet found much attention in the literature. It is shown here that the two cores of the NECC are the direct result of the finite width of the Inter-tropical Convergence Zone (ITCZ). This suggests that observational programs that try to determine the NECC transport have to cover the area up to 15°N.