Deep cross‐equatorial flow in the Atlantic measured with SOFAR floats

Abstract

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the Atlantic near 6°N and at several sites near 11°N as well as along the equator. Trajectories at 1800 m show a swift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7°N to the equator. The average transport per unit depth in the DWBC was estimated to be 13.8 × 103 m2/s. Coupling this value with mean velocities measured in the DWBC by current meters gave a volume transport of 15 × 106 m3/s between depths of 900 m and 2800 m. Approximately 6 × 106 m3/s recirculated northward between the DWBC and the Mid‐Atlantic Ridge, leaving 9 × 106 m3/s as cross‐equatorial transport. No obvious DWBC nor swift equatorial current was observed by the 3300‐m floats; a low mean velocity at this depth lay between F‐11 and higher velocity cores above and below. The 1800‐m trajectories also suggest that at times (February‐March 1989) the North Atlantic Deep Water in the DWBC turned eastward and flowed along the equator and at other times (August‐September 1990) the DWBC crossed the equator and continued southward. The velocity near the equator, calculated by grouping floats in a box along the equator, was eastward at 4.1 cm/s from February 1989 to February 1990 and westward at 4.6 cm/s from March 1990 to November 1990. Thus the amount of cross‐equatorial flow in the DWBC appeared to be linked to low‐frequency variability of the structure of the equatorial current system. Floats in Antarctic Intermediate Water at 800 m revealed a northwestward western boundary current, although flow patterns were complicated. Three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm/s. Six 800‐m floats drifted eastward along the equator between 5°S and 6°N at a mean velocity of 11 cm/s; one reached 5°W in the Gulf of Guinea, suggesting that the equatorial currents at this depth extended at least 35°–40° along the equator. Three of these floats reversed direction near the end of the tracking period.