An Interannual Study of the Atlantic North Equatorial Countercurrent

Abstract

Beginning in 1983, the dynamic height difference at the sea surface defining the countercurrent trough at 38°W was continuously observed by Inverted Echo Sounders at 3°N and 9°N for six years and ten months. The amplitude of the average annual signal for that time period was 36 cm, and the range of the interannual variability was 25 cm. Thus, the annual signal dominates as expected, but the interannual variability is of comparable magnitude. Two years, 1983 and 1987, are noted as years of sharp increase in the interannual signal (periods immediately following El Niño events in the Pacific). From a derived relationship for geostrophic volume transport, it is estimated that every year the North Equatorial Countercurrent attained a transport of 20 Sv (≡ 106 m3 s?1) for at least one month, while in the cited years it sustained this value for over half the year. The output of an unbounded reduced-gravity model, forced by monthly averaged wind stress, is compared to the observations. The model can be tuned so that the observed annual variation is approximated but none of the interannual variability is captured by the analysis.