TRANSPORTS AND PATHWAYS OF THE UPPER-LAYER CIRCULATION IN THE WESTERNTROPICAL ATLANTIC

The mean warm water transfer toward the equator along the western boun dary of the South Atlantic is investigated, based on a number of ship surveys carried out during 1990-96 with CTD water mass observations an d current profiling by shipboard and lowered (with the CTD/rosette) ac oustic Doppler current profiler and with Pegasus current profiler. The bulk of the northward warm water flow follows the coast in the North Brazil Undercurrent (NBUC) from latitudes south of 10 degrees S, carry ing 23 Sv (Sv = 10(6) mf s(-1)) above 1000 m. Out of this, 16 Sv are w aters warmer than 7 degrees C that form the source waters of the Flori da Current. Zonal inflow from the east by the South Equatorial Current enters the western boundary system dominantly north of 5 degrees S, a dding transport northwest of Cape San Rogue, and transforming the NBUC along its way toward the equator into a surface-intensified current, the North Brazil Current (NBC). From the combination of moored arrays and shipboard sections just north of the equator along 44 degrees W, t he mean NBC transport was determined at 35 Sv with a small seasonal cy cle amplitude of only about 3 Sv. The reason for the much larger near- equatorial northward warm water boundary current than what would be re quired to carry the northward heat transport are recirculations by the zonal current system and the existence of the shallow South Atlantic tropical-subtropical cell (STC). The STC connects the subduction zones of the eastern subtropics of both hemispheres via equatorward boundar y undercurrents with the Equatorial Undercurrent (EUC), and the return flow is through upwelling and poleward Ekman transport. The persisten t existence of a set of eastward thermocline and intermediate counterc urrents on both sides of the equator was confirmed that recurred throu ghout the observations and carry ventilated waters from the boundary r egime into the tropical interior. A strong westward current underneath the EUC, the Equatorial Intermediate Current, returns low-oxygen wate r westward. Consistent evidence for the existence of a seasonal variat ion in the warm water flow south of the equator could not be establish ed, whereas significant seasonal variability of the boundary regime oc curs north of the equator: northwestward alongshore throughflow of abo ut 10 Sv of waters with properties from the Southern Hemisphere was fo und along the Guiana boundary in boreal spring when the North Equatori al Countercurrent is absent or even flowing westward, whereas during J une-January the upper NBC is known to connect with the eastward North Equatorial Countercurrent through a retroflection zone that seasonally migrates up and down the coast and spawns eddies. The equatorial zone thus acts as a buffer and transformation zone for cross-equatorial ex changes, but knowledge of the detailed pathways in the interior includ ing the involved diapycnal exchanges is still a problem.