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

Observations from four towed profiler surveys undertaken between December 1 994 and October 1995 examine the seasonal and spatial variability of the up per ocean response to the Monsoon cycle in the Arabian Sea. Although observ ed atmospheric forcing agrees well with modern climatologies? cross-basin p atterns of mixed-layer depth and water properties observed in 1994-1995 are not entirely consistent with an upper-ocean response dominated by Ekman pu mping. 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 expla in observed wintertime mixed-layer depths. Except for waters associated wit h a filament extending offshore from the Omani coast, mixed-layer depths an d water properties show only modest cross-basin contrasts during the Southw est Monsoon. Filament waters differ from surrounding mid-basin waters, havi ng shallow mixed-layers and water propel ties similar to those of waters up welled near the Omani coast. In September, following the Southwest Monsoon, waters within 1000 km of the Omani coast have cooled and freshened, with m arked changes in stratification extending well into the pycnocline. Estimat es of Ekman pumping and wind-driven entrainment made using the Southampton Oceanographic Center 1980-1995 surface flux and the Levitus mixed-layer cli matologies indicate that during the Southwest Monsoon wind-driven entrainme nt is considerably stronger than Ekman pumping. Inshore of the windstress m aximum, Ekman pumping partially counters wind-driven entrainment, while off shore the two processes act together to deepen the mixed-layer. As Ekman pu mping is too weak to counter wind-driven mixed-layer deepening inshore of t he windstress maximum, another mechanism must act to maintain theshallow mi xed-layers seen in our observations and in climatologies. Offshore advectio n of coastally upwelled water offers a mechanism for maintaining upper ocea n stratification that is consistent with observed changes in upper ocean wa ter properties. Ekman upwelling will modulate wind-driven entrainment, but these results indicate that the primary mechanisms acting inshore of the wi ndstress maximum are wind-driven mixing and horizontal advection.;(C) 2000 Published by Elsevier Science Ltd. All rights reserved.