The North Indian Ocean circulation and its variability as seen in a numerical hindcast of the years 1993–2004

A 12-year hindcast of the physical state of the North Indian Ocean has been carried out for the period 1993–2004, using a data-assimilative, primitive equation, multi-level circulation model that assimilated altimetric sea surface height anomalies and weekly MCSST, and was driven by 6-h ECMWF winds. This period encompasses the anomalous wind events in 1994 and 1997–1998 that led to anomalous oceanic state including anomalous sea surface heights and sea surface temperatures, especially in the equatorial regions. Since the in situ database in the Indian Ocean is rather sparse, the hindcast provides an alternative means of examining the state of the ocean, including its interior, during these anomalous years, as well as normal years during this period. By comparison with observations available during this period, it is shown that the model possesses reasonable skill to be useful in the description of various events in the North Indian Ocean. In this paper, we examine the circulation and its variability over the 12 years of the hindcast. We discuss equatorial events, as well as events in the Bay of Bengal and the Arabian Sea, including the Somali Current system. The heat and mass fluxes are examined. Finally, the hindcast is repeated with QuikSCAT wind stress fields available from July 1999 onwards. Comparison of the 2000–2004 hindcasts forced by ECMWF winds and QS wind stresses shows that in the former, the currents and fluxes are underestimated by 20–30%, but the circulation patterns are roughly similar.