Transport variability of the Deep Western Boundary Current and the Antilles Current off Abaco Island, Bahamas

Abstract

Hydrography is combined with 1-year-long Inverted Echo Sounder (IES) travel-time records and bottom pressure observations to estimate the Deep Western Boundary Current (DWBC) transport east of Abaco Island, the Bahamas (near 26.5°N); comparison of the results to a more traditional line of current meter moorings demonstrates that the IESs and pressure gauges, combined with hydrography, can accurately monitor the DWBC transport to within the accuracy of the current meter array estimate at this location. Between 800 and 4800dbar, bounded by two IES moorings 82km apart, the enclosed portion of the DWBC is shown to have a mean southward transport of about 25Sv (1Sv=106m3s-1) and a standard deviation of 23Sv. The DWBC transport is primarily barotropic (where barotropic is defined as the near-bottom velocity rather than the vertical average velocity); geostrophic transports relative to an assumed level of no motion do not accurately reflect the actual absolute transport variability (correlation coefficient is 0.30). The IES-pressure gauge absolute transport within 1200–4800dbar agrees well with the current meter absolute transports (upper integration limit based on shallowest current meter level); the standard deviation of the difference is 12Sv and the mean difference is 0.2Sv. The correlation coefficient between these two time series is 0.76. The northward flowing Antilles Current (AC) east of Abaco Island has a mean baroclinic transport of 6Sv as estimated by the IESs and a standard deviation of 3Sv. The AC variations observed during 1996–1997 are uncorrelated with the Florida Current transport variations west of Abaco Island in the Florida Straits, however, the AC transport variations bear some resemblance to the historical estimates of the AC annual cycle.