Observing the seasonal variability in the tropical Atlantic from altimetry


  • Yves Menard


The tropical Atlantic circulation is dominated by a strong wind-driven seasonal cycle which is here described by means of the sea level heights measured by GEOS 3 and Seasat altimeters. Seasat worked for just 3 months, and GEOS 3 yielded an inhomogeneous space-time coverage during its 3.5 years of operation. To remedy this coverage shortfall, all GEOS 3 and Seasat data were merged together on a monthly basis without regard to year, thereby ignoring the weak interannual perturbations of the tropical Atlantic. In this way, a complete time series was produced for 1 year. Along each profile, altirnetric heights were referenced to a global mean sea surface and then filtered to remove high- and low-frequency noise. The residual heights were objectively analyzed to produce monthly maps of sea level anomaly between 10°S and 20°N. These maps show a large annual signal in the western side of the basin, between the equator and 10°N, where the North Equatorial Countercurrent and South Equatorial Current are predominant. The signal has an amplitude of 8 cm from a minimum in April–May to a maximum in October–November. In the east the signal is weaker and depicts a semiannual cycle. These results have been compared to similar maps obtained from historical hydrographic data (Merle and Arnault, 1985). The two sets of maps are in good agreement, especially north of the equator where the largest seasonal variability occurs. The major difference comes from underestimation of the altimetric signal amplitude due to the strong filtering and uneven distribution of the data.