Velocity structure and transport in the Windward Islands Passages

Abstract

During 1991–1994, repeated measurements of current structure and water mass properties were made in the major southern passages to the Caribbean Sea between Trinidad and Dominica. A total of 10 cruises were performed in different seasons, consisting primarily of serial station occupations in the Grenada, St Vincent, St Lucia, and Dominica Passages. This data set is by far the most comprehensive available in these passages and better determines the mean flow and range of variability than do previous studies. The flow structure in these passages is characterized by a strong and relatively stable inflow above the thermocline (approximately the upper 100 m), and a more highly variable flow regime within and below the thermocline. Typical near-surface inflow velocities in Grenada and St Vincent passages were 40–60 cm s −1, with maximum observed currents of 90 cm s −1. Frequently, counterflows were observed below this surface layer flowing out of the Caribbean, trapped to the southern side of the passages. This subsurface counterflow appeared to be strongest and most prevalent in Grenada Passage, where outflow speeds as large as 30 cm s −1 were observed. As a result, the ensemble mean sections for the three southern passages exhibit a deep inflow concentrated in the northern and central parts of the passages, and weak outflow on the southern sides. The ensemble of measurements from this program indicate a mean transport of 9.5±3 × 10 6 m 3 s −1 through the southern passages, with a range of variability from 3 to 17 × 10 6 m 3 s −1. No clear annual cycle is apparent in the passage transports. Mean transports calculated for the individual passages were 4.7, 3.4, 0.9, and 0.5 × 10 6 m 3 s −1, respectively, for Grenada, St Vincent, St Lucia, and Dominica Passages, decreasing monotonically northward and indicating that Grenada and St Vincent Passages largely dominate the inflow to the southern Caribbean. Consideration of this data set, historical data, Sverdrup theory and numerical model results suggests that very little of the mean inflow to the Caribbean through the southern passages can be attributed to wind-driven circulation, and that most of the transport (6–8 × 10 6 m 3 s −1) is derived from South Atlantic waters crossing the equator in the upper limb of the Atlantic thermohaline cell. The remainder of the upper ocean thermohaline transport likely follows a different route into the subtropical gyre, possibly flowing northward in the interior and entering the Caribbean through the more northern passages.