Thursday 25 August 2005

P2
1400-1520 hours

401
Decadal modulation of ENSO-monsoon relationship in the IPCC AR4 simulations
Annamalai, H¹
1 IPRC/SOEST, University Of Hawaii, Hawaii, USA
Author email: hanna@hawaii.edu
The waxing and waning of the ENSO-Indian summer monsoon relationship at decadal time scales are examined in the twentieth century simulations (20c3m) from 19 coupled ocean-atmosphere general circulation models. The hypothesis that the changes in local air-sea interactions, in particular off Sumatra in the eastern equatorial Indian Ocean are a candidate in the decadal changes of ENSO-monsoon relationship is examined. Diagnostic reveal that only six models have a reasonable representation of monsoon precipitation climatology during boreal summer. First, the fidelity of these six models in capturing the ENSO-monsoon relationship in interannual time scales, both in terms of spatial pattern and lead/lag correlations are examined. Results from the individual members of the ensemble simulations indicate that only two models are capable of capturing the observed ENSO-monsoon relationship. A systematic evaluation of the results suggests that a realistic representation of the relationship crucially depends on the models' ability to simulate the space-time evolution of ENSO-related SST anomalies in the equatorial Pacific and the associated changes in the Pacific Walker Circulation. Second, in these two-coupled models, the changes in the ENSO-monsoon association at decadal time scales are investigated. Consistent with our hypothesis, the simulations from the models indicate that decades when strong air-sea interactions occur off Sumatra the known ENSO-monsoon relationship weakens. The interpretation is that 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 and overcoming the suppressing effect due to El Nino. Finally, at decadal time scales it is seen that the Pacific Decadal Variability (PDV) is effective in preconditioning the thermocline off Java-Sumatra for the occurrence (or non-occurrence) of local air-sea interactions that subsequently influence the ENSO-monsoon relationship.

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