Float trajectories in the deep western boundary current and deep equatorial jets of the tropical Atlantic

Abstract

Fourteen neutrally buoyant SOFAR floats at a nominal depth of 1800 m were tracked acoustically for 3.7 yr in the vicinity of the western boundary and the equator of the Atlantic Ocean. The trajectories revealed a swift, narrow, southward-flowing deep western boundary current (DWBC) extending from 7N across the equator. Two floats crossed the equator in the DWBC and went to 10S. Two other floats left the DWBC and drifted eastward in the equatorial band (3S–3N). Three floats entered the DWBC from the equatorial current system and drifted southward. These results suggest that at times the DWBC flows directly southward across the equator with a mean velocity of 8–9 cm/s averaged over long distances (∼2800 km). At other times DWBC water is diverted eastward near the equator for long periods (2–3 yr), which can reduce the mean along-boundary velocity to 1–2 cm/s. This is much less than the instantaneous along-boundary velocities in the DWBC, which are often above 25 cm/s and occasionally exceed 50 cm/s. Mean eastward-flowing jets were observed near 2N and 2S bounding a mean westward jet centered on the equator (1S–1N). The southern jet at 2S coincides with a CFC-rich plume centered south of the equator. The CFC plume is inferred to have been advected by the southern jet across the Atlantic and into the Gulf of Guinea.