Indian Ocean variability in the GFDL coupled climate model

The interannual variability of the Indian Ocean, with particular focus on the Indian Ocean dipole/ zonal mode ( IODZM), is investigated in a 250- yr simulation of the GFDL coupled global general circulation model ( CGCM). The CGCM successfully reproduces many fundamental characteristics of the climate system of the Indian Ocean. The character of the IODZM is explored, as are relationships between positive IODZM and El Nino events, through a composite analysis. The IODZM events in the CGCM grow through feedbacks between heat- content anomalies and SST- related atmospheric anomalies, particularly in the eastern tropical Indian Ocean. The composite IODZM events that co- occur with El Nino have stronger anomalies and a sharper east - west SSTA contrast than those that occur without El Nino. IODZM events, whether or not they occur with El Nino, are preceded by distinctive Indo- Pacific warm pool anomaly patterns in boreal spring: in the central Indian Ocean easterly surface winds, and in the western equatorial Pacific an eastward shift of deep convection, westerly surface winds, and warm sea surface temperature. However, delayed onsets of the anomaly patterns ( e. g., boreal summer) are often not followed by IODZM events. The same anomaly patterns often precede El Nino, suggesting that the warm pool conditions favorable for both IODZM and El Nino are similar. Given that IODZM events can occur without El Nino, it is proposed that the observed IODZM - El Nino relation arises because the IODZM and El Nino are both large- scale phenomena in which variations of the Indo- Pacific warm pool deep convection plays a central role. Yet each phenomenon has its own dynamics and life cycle, allowing each to develop without the other. The CGCM integration also shows substantial decadal modulation of the occurrence of IODZM events, which is found to be not in phase with that of El Nino events. There is a weak, though significant, negative correlation between the two.