VELOCITY STRUCTURE AND TRANSPORT IN THE WINDWARD-ISLANDS PASSAGES

During 1991-1994, repeated measurements of current structure and water mass properties were made in the major southern passages to the Carib bean Sea between Trinidad and Dominica. A total of 10 cruises were per formed in different seasons, consisting primarily of serial station oc cupations in the Grenada, St Vincent, St Lucia, and Dominica Passages. This data set is by far the most comprehensive available in these pas sages and better determines the mean flow and range of variability tha n do previous studies. The flow structure in these passages is charact erized by a strong and relatively stable inflow above the thermocline (approximately the upper 100 m), and a more highly variable flow regim e within and below the thermocline. Typical near-surface inflow veloci ties in Grenada and St Vincent passages were 40-60 cm s(-1), with maxi mum observed currents of 90 cm s(-1). Frequently, counterflows were ob served below this surface layer flowing out of the Caribbean, trapped to the southern side of the passages. This subsurface counterflow appe ared to be strongest and most prevalent in Grenada Passage, where outf low speeds as large as 30 cm s(-1) were observed. As a result, the ens emble mean sections for the three southern passages exhibit a deep inf low concentrated in the northern and central parts of the passages, an d weak outflow on the southern sides. The ensemble of measurements fro m this program indicate a mean transport of 9.5 +/- 3 x 10(6)m(3)s(-1) through the southern passages, with a range of variability from 3 to 17 x 10(6) m(3) s(-1). No clear annual cycle is apparent in the passag e transports. Mean transports calculated for the individual passages w ere 4.7, 3.4, 0.9, and 0.5 x 10(6) m(3) s(-1), respectively, for Grena da, St Vincent, St Lucia, and Dominica Passages, decreasing monotonica lly northward and indicating that Grenada and St Vincent Passages larg ely dominate the inflow to the southern Caribbean. Consideration of th is data set, historical data, Sverdrup theory and numerical model resu lts suggests that very little of the mean inflow to the Caribbean thro ugh the southern passages can be attributed to wind-driven circulation , and that most of the transport (6-8 x 10(6) m(3) s(-1)) is derived f rom South Atlantic waters crossing the equator in the upper limb of th e Atlantic thermohaline cell. The remainder of the upper ocean thermoh aline transport likely follows a different route into the subtropical gyre, possibly flowing northward in the interior and entering the Cari bbean through the more northern passages.