Intraseasonal variability in the southwestern Arabian Sea and its relation to the seasonal circulation

An analysis of TOPEX/POSEIDON altimeter data and in situ current and temperature data obtained between April 1995 and October 1996 from a moored array shows strong intraseasonal fluctuations in the southwestern Arabian Sea, an oceanic region where the Great Whirl (GW), a predominantly wind-generated, very energetic anticyclone, is present during the Southwest Monsoon. Fluctuation periods between 30 and 50 days, up to 100 days during some years, are observed in the 8-year altimetric dataset, mostly during late summer and fall. These fluctuations are largest in a 1000 km-wide region off the Somali, Omani and Yemeni coasts north of 5degreesN, suggesting a local generation mechanism. The in situ data at different moorings show strong and coherent fluctuations that are characterized by southwestward phase propagation and northward energy propagation. Their periods range from 30 to 60 days and increase steadily from July 1995 to January 1996. In the first stage, these periods are at and below the cut-off period of freely propagating, first baroclinic mode Rossby waves, but approach this theoretical limit later in the year. Instabilities of the flow in the transition region between the Southern Gyre and the GW are likely sources of these fluctuations. (C) 2003 Elsevier Science Ltd. All rights reserved.