Observations from four towed profiler surveys undertaken between December 1994 and October 1995 examine the seasonal and spatial variability of the upper ocean response to the Monsoon cycle in the Arabian Sea. Although observed atmospheric forcing agrees well with modern climatologies, cross-basin patterns of mixed-layer depth and water properties observed in 1994–1995 are not entirely consistent with an upper-ocean response dominated by Ekman pumping. During the winter monsoon, the mixed-layer deepens dramatically with distance offshore. Surface cooling intensifies with offshore distance, and a one-dimensional response dominated by convective overturning could explain observed wintertime mixed-layer depths. Except for waters associated with a filament extending offshore from the Omani coast, mixed-layer depths and water properties show only modest cross-basin contrasts during the Southwest Monsoon. Filament waters differ from surrounding mid-basin waters, having shallow mixed-layers and water properties similar to those of waters upwelled near the Omani coast. In September, following the Southwest Monsoon, waters within 1000 km of the Omani coast have cooled and freshened, with marked changes in stratification extending well into the pycnocline. Estimates of Ekman pumping and wind-driven entrainment made using the Southampton Oceanographic Center 1980–1995 surface flux and the Levitus mixed-layer climatologies indicate that during the Southwest Monsoon wind-driven entrainment is considerably stronger than Ekman pumping. Inshore of the windstress maximum, Ekman pumping partially counters wind-driven entrainment, while offshore the two processes act together to deepen the mixed-layer. As Ekman pumping is too weak to counter wind-driven mixed-layer deepening inshore of the windstress maximum, another mechanism must act to maintain the shallow mixed-layers seen in our observations and in climatologies. Offshore advection of coastally upwelled water offers a mechanism for maintaining upper ocean stratification that is consistent with observed changes in upper ocean water properties. Ekman upwelling will modulate wind-driven entrainment, but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind-driven mixing and horizontal advection.

The upper-ocean response to monsoonal forcing in the Arabian Sea: seasonal and spatial variability

The existence of a surface barotropic front-jet system at the confluence region off the eastern tip of Oman (Ras Al Hadd or RAH) is documented for 1994–1995 through advanced very high resolution radiometer (AVHRR) and acoustic Doppler current profiler (ADCP) observations. The thermal signature of this confluence is visible in 1995 between early May and the end of October, i.e., throughout the SW Monsoon and into the transition period between SW and NE Monsoons. The thermal characteristics are those of a NE-oriented front between cooler water of southern (upwelled) origin and warmer waters of northern Gulf of Oman origin. During the period when the thermal front is absent, ADCP data suggest that the confluence takes a more southward direction with Gulf of Oman waters passing RAH into the southeastern Oman coastal region. The thermal gradient is initially small (June–July) but later increases (August–October) into a front that exhibits small-scale instabilities. Surface current velocities within the jet, estimated by tracking these features in consecutive satellite images, are 0.5–0.7 m s−1 and in remarkable agreement with concurrent ADCP retrievals in which the seasonal maximum in velocity is 1 m s−1. ADCP observations collected during several US JGOFS cruises reveal a weakly baroclinic current in the confluence region that drives the waters into the offshore system. The fully developed jet describes a large meander that demarcates two counter-rotating eddies (cyclonic to the north and anticyclonic to the south of the jet) of approximately 150–200 km diameter. The southern eddy of this pair is resolved by the seasonally averaged, sea-level anomaly derived from TOPEX/Poseidon observations. During the SW Monsoon, the RAH Jet advects primarily cold waters along its path, but as soon as the wind system reverses with the transition to the intermonsoonal period, a warm current is rapidly established that advects the surface coastal waters of the Gulf of Oman offshore. In accordance with the interannual variation of the wind forcing phase, the reversal of the currents from NE to SW occurred earlier in 1994 than in 1995, confirming that the RAH Jet is integral part of the East Arabian Current. The transport of the Jet, estimated by combining SST information on the width with ADCP data on the velocity's vertical structure, is found to fluctuate between 2–8×106 m3 s−1 and its thickness between 150–400 m. These significant fluctuations are due to the time-variable partition of horizontal transport between eddies and the RAH Jet and are potentially important to the nutrient and phytoplankton budgets of the Arabian Sea.

The Ras al Hadd Jet: Remotely sensed and acoustic Doppler current profiler observations in 1994–1995

Acoustic Doppler current profiler (ADCP) data from the R/V T.G. THOMPSON is part of the core data for the US JGOFS Arabian Sea project along with hydrographic and nutrient data. Seventeen cruises on the THOMPSON are scheduled to take place between September 1994 and January 1996. This is the second in a series of data reports covering the ADCP data from the Arabian Sea JGOFS cruises TNO43 through TNO46. ADCP data are being collected on all the JGOFS Arabian Sea cruises using an autonomous data acquisition system developed for ship-of-opportunity cruises. This system, referred to as the AutoADCP, makes it possible to collect the ADCP data without the constant monitoring usually necessary and assures constant data coverage and uniform data quality. This data report presents ADCP results from the second group of four JGOFS cruises, TNO43 through TNO46, concentrating on the data collection and processing methods. The ADCP data itself reside in a CODAS data base at Brookhaven National Laboratory and is generally available to JGOFS investigators through contact with the authors. The CODAS data base and associated ADCP processing software were developed over a number of years by Eric Firing and his group at the University of Hawaii. The CODAS software is shareware available for PC`s or Unix computers and is the single most widely used ADCP processing program for ship mounted units.

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Acoustic Doppler current profiling from the JGOFS Arabian Sea cruises aboard the RV T.G. THOMPSON: TN043, January 8, 1995--February 4, 1995; TN044, February 8, 1995--February 25, 1995; TN045, March 14, 1995--April 10, 1995; TN046, April 14, 1995--April 29, 1995

C.N. Flagg

Papers

Acoustic Doppler current profiling from the JGOFS Arabian Sea cruises aboard the RV T.G. THOMPSON: TN043, January 8, 1995--February 4, 1995; TN044, February 8, 1995--February 25, 1995; TN045, March 14, 1995--April 10, 1995; TN046, April 14, 1995--April 29, 1995