DEEP CROSS-EQUATORIAL FLOW IN THE ATLANTIC MEASURED WITH SOFAR FLOATS

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 330 0 m were tracked for 21 months in the vicinity of tropical boundary cu rrents in the Atlantic near 6-degrees-N and at several sites near 11-d egrees-N as well as along the equator. Trajectories at 1800 m show a s wift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7-degrees-N to the equator. The average transport per unit depth in the DWBC was estimated to be 13.8 x 10(3) m2/S. Coupling this value with mean velocities measured in th e DWBC by current meters gave a volume transport of 15 x 10(6) m3/s be tween depths of 900 m and 2800 m. Approximately 6 x 10(6) m3/s recircu lated northward between the DWBC and the Mid-Atlantic Ridge, leaving 9 x 10(6) m3/s as cross-equatorial transport. No obvious DWBC nor swift equatorial current was observed by the 3300-m floats; a low mean velo city at this depth lay between F-11 and higher velocity cores above an d 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 n ear the equator, calculated by grouping floats in a box along the equa tor, 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 finked to low-fre quency variability of the structure of the equatorial current system. Floats in Antarctic Intermediate Water at 800 m revealed a northwestwa rd 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-degrees- S and 6-degrees-N at a mean velocity of 11 cm/s; one reached 5-degrees -W in the Gulf of Guinea, suggesting that the equatorial currents at t his depth extended at least 35-degrees-40-degrees along the equator. T hree of these floats reversed direction near the end of the tracking p eriod.