Wednesday 24 August 2005

P1
1400-1520 hours

290
Modeling the effect of Indian Ocean SST on Asian monsoons, El Nino and
northern hemispheric circulation
Annamalai, H¹
1 IPRC/SOEST, University Of Hawaii, USA
Author email: hanna@hawaii.edu
Sensitivity experiments with Atmospheric General Circulation Models (AGCMs) are performed to elucidate the impact of Indian Ocean SST anomalies, particularly off Sumatra in boreal summer and fall, and over Southwest Indian Ocean (SWIO) in boreal winter and into the following spring, on local and remote climate variability. During boreal summer, AGCMs results demonstrate that inclusion of Sumatra SST anomalies leads to changes in local wave-response: the low-level anti-cyclone that develops as a Rossby-wave response to convective heating anomalies off Sumatra enhances the southwesterly flow over Northern Indian Ocean, thereby leading to a stronger monsoon over India. During boreal fall of El Nino years, if basin-wide warm SST anomalies are observed atmospheric Kelvin waves associated with easterly flow over the equatorial west-central Pacific are generated, thereby weakening the westerly anomalies associated with the developing El Nino. In contrast, for an east-west SST pattern a significant Kelvin wave response is not generated and there is little effect on the El Nino-induced westerlies. In the former case, the on-going El Nino is weakened while in the latter El Nino strengthens. During boreal winter of El Nino years, inspection of the solutions reveals that SWIO precipitation anomalies force a wave train arching northeastwards into and through middle latitudes. Analysis of 500 hPa height anomalies show that over the Pacific-North American (PNA) region, the height anomalies forced by Indian Ocean SST anomalies "oppose" and "constructively interfere" with those forced by tropical Pacific SST anomalies. The persistence of SST anomalies over SWIO into the following spring prevents the north-northwestward migration of the ITCZ and the associated deep moist layer, causing a significant delay in the Indian summer monsoon onset in June by 6-7 days. The large sensitivities to Indian Ocean SST anomalies indicate a need to predict the SST in the Indian Ocean accurately.

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