Upper ocean currents in the northern Arabian Sea from shipboard ADCP measurements collected during the 1994–1996 U.S. JGOFS and ONR programs

Abstract

A large upper-ocean velocity data set was obtained using a shipboard acoustic Doppler current profiler (ADCP) on seventeen RV T.G. Thompson cruises during the JGOFS and ONR expedition to the northern Arabian Sea from September 1994 through January 1996. Seven of the cruises followed a large area survey track centered over the Arabian Basin, four cruises conducted SeaSoar surveys on either side of the Findlater jet axis, and six cruises were for the deployment and maintenance of moored instrumentation, together providing some 380 cruise-days and 96,000 track kilometers of coverage. The ADCP data extended over the upper 250–400 m of the water column depending upon the temporal/spatial distributions of acoustic scatterers. The velocity data revealed several items that differed significantly from the historical perspective. Maximum current magnitudes in this area varied from more than 1 m s -1 along the Arabian coast to 10–20 cm s -1 well offshore. Perhaps the most important result was the complete dominance of the velocity field by eddies that had offshore correlation length scales of roughly 100 km, a spectral peak at around 300 km, and kinetic energies that ranged from 70 to more than 90% of the total kinetic energy. The total kinetic energy was highest within about 300 km of the shore and decreased significantly in magnitude and vertical extent offshore. Within the coastal region, the temporal variability was such that currents of 50 cm s -1 or more could completely reverse within a two-week period, seemingly independent of any local atmospheric forcing. Mean and seasonal velocities also differed from historical results. There was a large anti-cyclonic feature located for most of the year south of Ras ash Shabatat (∼58°E) which intensified during the southwest monsoon. There was a strong jet-like current off Ras al Hadd (∼22°N), which also intensified during the southwest monsoon, although itflowed southwestward against the wind. In contrast to the historical ship drift data, which indicated that the surface currents followed the monsoonal winds, the ADCP data over the upper 200–400 m was highly variable, with overall seasonal means that were often directed against the wind. Except within 100–200 km of the Arabian coast, the mean currents were generally toward the southwest with a mean transport in the upper 150 m between the coast and 10°N of approximately 9×10 6 m 3 s -1.