Advection of upwelled waters in the form of plumes off Oman during the Southwest Monsoon

Advanced Very High Resolution Radiometer (AVHRR) imagery of sea-surface-temperature, TOPEX/POSEIDON measurements of sea-level-anomaly (SLA), and modeled surface winds and wind-stress fields are used in concert with other ancillary data to describe the influence of the 1995 Southwest Monsoon on the distribution of upwelled waters off the coast of Oman. The Oman upwelling zone is characterized by the entrainment of cold upwelled waters into plumes extending from the coast into the deep ocean unaffected by the steep bottom gradients. The most prominent of these plumes is found offshore of Ras al Madraka. A mechanism for the entrainment of upwelled water into plumes is hypothesized, and validated by observational data. It is proposed that the location of the plume is primarily governed by the sea level structure away from the coast and that coastally upwelled water is passively advected offshore through regions of low sea level. Analysis of the surface wind-stress fields show significant spatial variability associated with the predominantly cyclonic mean wind-stress curl, with relatively weak curl observed in the region south of Ras al Madraka and north of Ras Marbat. Decomposition of the surface wind-stress fields through Principal Component Analysis shows that, at certain periods, the development of strong along-shore winds and cyclonic wind-stress curl in the region north of Ras al Madraka. This information, combined with concurrent observations of TOPEX/POSEIDON sea-level-anomalies (SLAs), satellite derived sea-surface-temperatures (SST), and surface current measurements, shows that the combined effects of a strong along-shore wind field and positive wind-stress curl forces a depression in sea level in the region north of Ras al Madraka. The sea level gradient, caused by the presence of a sustained high sea level to the south of Ras al Madraka, causes geostrophic advection of coastally upwelled waters away from the shelf. Acoustic Doppler Current Profiler (ADCP) velocity measurements along with SST maps further prove that the upwelled water is geostrophically advected offshore as opposed to being an offshore deflection of a wind-driven coastal current. Comparison of interannual features in the TOPEX/POSEIDON SLAs suggest that the plumes coming off coast in the Oman upwelling zone may not be directly linked to the coastal topography or bathymetry but are a result of interaction between mesoscale variations in the wind held and the underlying ocean. The strong along-shore winds and cyclonic wind-stress curl to the north of Ras al Madraka becomes enhanced when the Findlater Jet moves closer to the Oman coast than its mean position. (C) 1998 Elsevier Science Ltd. All rights reserved.