ON MEAN AND SEASONAL CURRENTS AND TRANSPORTS AT THE WESTERN BOUNDARY OF THE EQUATORIAL ATLANTIC

Current measurements from two consecutive yearlong deployments of three moored stations at the western end of the equator in the Atlantic, along 44-degrees-W, are used to determine the northwestward flow of warm water in the upper several 100 m and of the southeastward counterflow of North Atlantic Deep Water (NADW). Measurements from three acoustic Doppler current profilers (ADCPs) looking upward from 300 m toward the surface allowed calculation of a time series of upper layer transports over 1 year. Mean transport through the array for the upper 300 m is 23.8 Sv with an annual cycle of only +/-3 Sv that has its maximum in June-August and minimum in northern spring. Estimated additional mean northwestward transport in the range 300-600 m is 6.7 Sv, based on moored data and shipboard Pegasus and lowered ADCP profiling. In the depth range 1400-3100 m a current core with maximum annual mean southeastward speed of 30 cm s-1 is found along the continental slope that carries an estimated upper NADW transport of 14.2-17.3 Sv, depending on the extrapolation used between the mooring in the core and the continental slope. This transport is higher than off-equatorial estimates and suggests near-equatorial recirculation at the upper NADW level, in agreement with northwestward mean flow found about 140 km offshore. Below 3100 m and above the 1.8-degrees-C isotherm, only a small core of lower NADW flow with speeds of 10-15 cm s-1 is found over the flat part of the basin near 1.5-degrees-N, clearly separated from the continental slope by a zone of near-zero mean speeds. Estimated transport of that small current core is about 4.5 Sv, which is significantly below other estimates of near-equatorial transport of lower NADW and suggests that a major fraction of lower NADW may cross the 44-degrees-W meridian north of the Ceara Rise. Intraseasonal variability is large, although smaller than observed at 8-degrees-N near the western boundary. It occurs at a period of about 1 month when it is dominant in the near-surface records and corresponds to earlier observations in the equatorial zones of all oceans and at a period of about 2 months when it is dominant at the NADW level and could be imported either from the north along the boundary or from the east along the equator. The existence of an annual cycle in the deep currents of a few centimeters per second amplitude, as suggested by high-resolution numerical model results, could neither be proven nor disproven because of the high amount of shorter-period variability.