The Ras al Hadd Jet: Remotely sensed and acoustic Doppler current profilerobservations in 1994-1995

The existence of a surface barotropic front-jet system at the confluence re gion off the eastern tip of Oman (Ras Al Hadd or RAH) is documented for 199 4-1995 through advanced very high resolution radiometer (AVHRR) and acousti c Doppler current profiler (ADCP) observations. The thermal signature of th is 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 a nd NE Monsoons. The thermal characteristics are those of a NE-oriented fron t between cooler water of southern (upwelled) origin and warmer waters of n orthern Gulf of Oman origin. During the period when the thermal front is ab sent, ADCP data suggest that the confluence takes a more southward directio n with Gulf of Oman waters passing RAH into the southeastern Oman coastal r egion. The thermal gradient is initially small (June-July) but later increa ses (August-October) into a front that exhibits small-scale instabilities. Surface current velocities within the jet, estimated by tracking these feat ures 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 d rives the waters into the offshore system. The fully developed jet describe s a large meander that demarcates two counter-rotating eddies (cyclonic to the north and anticyclonic to the south of the jet) of approximately 150-20 0 km diameter. The southern eddy of this pair is resolved by the seasonally averaged, sea-level anomaly derived from TOPEX/Poseidon observations. Duri ng 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 interm onsoonal period, a warm current is rapidly established that advects the sur face coastal waters of the Gulf of Oman offshore. In accordance with the in terannual 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 R AH Jet is integral part of the East Arabian Current. The transport of the J et, estimated by combining SST information on the width with ADCP data on t he velocity's vertical structure, is found to fluctuate between 2-8 x 10(6) m(3) s(-1) and its thickness between 150-400 m. These significant fluctuat ions are due to the time-variable partition of horizontal transport between eddies and the RAH Jet and are potentially important to the nutrient and p hytoplankton budgets of the Arabian Sea. (C) 1999 Elsevier Science Ltd. All rights reserved.