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Towards understanding the unusual Indian monsoon in 2009

Francis, PA and Gadgil, Sulochana (2010) Towards understanding the unusual Indian monsoon in 2009. In: Proceedings of the Indian Academy of Sciences - Earth & Planetary Sciences, 119 (4). pp. 397-415.

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The Indian summer monsoon season of 2009 commenced with a massive deficit in all-India rainfall of 48% of the average rainfall in June. The all-India rainfall in July was close to the normal but that in August was deficit by 27%. In this paper, we first focus on June 2009, elucidating the special features and attempting to identify the factors that could have led to the large deficit in rainfall. In June 2009, the phase of the two important modes, viz., El Nino and Southern Oscillation (ENSO) and the equatorial Indian Ocean Oscillation (EQUINOO) was unfavourable. Also, the eastern equatorial Indian Ocean (EEIO) was warmer than in other years and much warmer than the Bay. In almost all the years, the opposite is true, i.e., the Bay is warmer than EEIO in June. It appears that this SST gradient gave an edge to the tropical convergence zone over the eastern equatorial Indian Ocean, in competition with the organized convection over the Bay. Thus, convection was not sustained for more than three or four days over the Bay and no northward propagations occurred. We suggest that the reversal of the sea surface temperature (SST) gradient between the Bay of Bengal and EEIO, played a critical role in the rainfall deficit over the Bay and hence the Indian region. We also suggest that suppression of convection over EEIO in association with the El Nino led to a positive phase of EQUINOO in July and hence revival of the monsoon despite the El Nino. It appears that the transition to a negative phase of EQUINOO in August and the associated large deficit in monsoon rainfall can also be attributed to the El Nino.

Item Type: Journal Article
Additional Information: Copyright of this article belongs to Indian Academy of Sciences.
Keywords: Indian summer monsoon; equatorial Indian Ocean Oscillation; El Nino; SST gradient; tropical convergence zone; drought; climate; coupled systems; monsoons.
Department/Centre: Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Depositing User: Id for Latest eprints
Date Deposited: 28 Sep 2010 06:57
Last Modified: 28 Sep 2010 06:57
URI: http://eprints.iisc.ac.in/id/eprint/32474

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