The interannual variability in the tropical Indian Ocean as simulated by a CGCM

Abstract

The interannual variability in the tropical Indian Ocean, and in particular the Indian Ocean di- pole mode (IODM), is investigated using both obser- vations and a multi-decadal simulations performed by the coupled atmosphere–ocean general circulation model SINTEX. Overall, the characteristics of the simulated IODM are close to the features of the ob- served mode. Evidence of significant correlations be- tween sea level pressure anomalies in the southeastern Indian Ocean and sea surface temperature anomalies in the tropical Indian and Pacific Oceans have been found both in observations and a multi-decadal simulation. In particular, a positive SLP anomaly in the southeastern part of the basin seems to produce favorable conditions for the development of an IODM event. The role played by the ocean dynamics both in the developing and closing phases of the IODM events is also inves- tigated. Our results suggest that, during the developing phase, the heat content and SST variability associated with the IODM are influenced by a local response of the ocean to the winds, and a remote response with the excitation of Kelvin and Rossby waves. Ocean wave dynamics appear to be important also during the dying phase of the IODM, when equatorial downwelling Kelvin waves transport positive heat content anomalies from the western to the eastern part of the basin, suppressing the zonal heat content anomaly gradient. The results obtained from the model suggest a mechanism for the IODM. This mechanism is generally consistent with the characteristics of the observed IODM. Furthermore, it might give some clue in understanding the correlation between IODM and ENSOactivity found both in the model and in the observations.