lnterannual SST variability in the north Indian Ocean

INTERNATIONAL SYMPOSIUM ON INFORMATION TECHNOLOGY IN OCEANOGRAPHY (ITO-98)

GOA, INDIA 12-16 OCTOBER 1998

lnterannual SST variability in the north Indian Ocean

S. K. Bahera, P. S. Salvekar and T. Yamagata
Institute for Global Change Research, Tokyo 105, Japan
Indian Institute of Tropical Meteoroiogy, Pune-8, India
Department of Earth and Planetary Physics,
Graduate School of Science, The University of Tokyo, Tokyo, Japan

A 2½ layer, moderately complex numerical ocean model is used to simulate interannual variability in the thermodynamic characteristics of the north Indian Ocean (north of 30 S). The model is forced by the mean monthly surface heat flux and momentum flux derived from National Centre for Environment Prediction (NCEP) reanalysis data sets, for the period 1982-91. The model has well simulated the interannual SST in most part of the model domain. The phase and amplitude of the time series of SST anomalies compare well with that of the observed SST anomalies. Model SST anomalies did not show the high frequency sub-seasonal variability as seen in the observed SST anomalies. The model deficiency is due to the absence of high frequency variability (less than a month) in the surface forcing. Interannual variability in the surface heat forcing plays a key role in producing the realistic model SST anomajies. In the absence of interannual variability in the surface heat forcing, the variability in the interannual wind alone could not produce the desired observed SST anomalies, in most part of the model domain. The influence of interannual wind is only limited to regions near to the coast in the Arabian Sea during the Indian summer monsoon season and a small region in the central Indian Ocean, south of 5 S, from November through January. Warm anomalies of the order of 0.5 °C are found to cover the whole equatorial region during 1982-83, 1987-88 and l991, in both model results and observation. These periods coincide with the El-Nino events of the equatorial Pacific Ocean. Positive heat flux in excess of 10 Wm¯², west of 50 E, coincides with the start of warming in the western equatorial Indian Ocean.

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