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-tem perature, TOPEX/POSEIDON measurements of sea-level-anomaly (SLA), and model ed surface winds and wind-stress fields are used in concert with other anci llary data to describe the influence of the 1995 Southwest Monsoon on the d istribution of upwelled waters off the coast of Oman. The Oman upwelling zo ne 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 h ypothesized, and validated by observational data. It is proposed that the l ocation of the plume is primarily governed by the sea level structure away from the coast and that coastally upwelled water is passively advected offs hore through regions of low sea level. Analysis of the surface wind-stress fields show significant spatial variability associated with the predominant ly cyclonic mean wind-stress curl, with relatively weak curl observed in th e region south of Ras al Madraka and north of Ras Marbat. Decomposition of the surface wind-stress fields through Principal Component Analysis shows t hat, at certain periods, the development of strong along-shore winds and cy clonic wind-stress curl in the region north of Ras al Madraka. This informa tion, combined with concurrent observations of TOPEX/POSEIDON sea-level-ano malies (SLAs), satellite derived sea-surface-temperatures (SST), and surfac e current measurements, shows that the combined effects of a strong along-s hore wind field and positive wind-stress curl forces a depression in sea le vel in the region north of Ras al Madraka. The sea level gradient, caused b y 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 s helf. Acoustic Doppler Current Profiler (ADCP) velocity measurements along with SST maps further prove that the upwelled water is geostrophically adve cted offshore as opposed to being an offshore deflection of a wind-driven c oastal current. Comparison of interannual features in the TOPEX/POSEIDON SL As 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 re sult of interaction between mesoscale variations in the wind held and the u nderlying ocean. The strong along-shore winds and cyclonic wind-stress curl to the north of Ras al Madraka becomes enhanced when the Findlater Jet mov es closer to the Oman coast than its mean position. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.