The Indian Ocean zonal mode (IOZM) is one dominant pattern of interannual variability of the Indian Ocean. The generation mechanism of this mode is not well understood. Some hypotheses postulate that this mode is forced by ENSO, but others suggest that this mode results from the air-sea interactions of the Indian Ocean itself. These hypotheses suggest very different ENSO-IOZM relationship. This study perform experiments with a coupled atmosphere-ocean general circulation model (CGCM) to investigate this relationship. Two CGCM simulations are performed. One include both the Indian and Pacific Oceans in the ocean model component (i.e., the Indo-Pacific Run). The other simulation includes only the Indian Ocean in the ocean model component (i.e., The indian-Ocean Run). The Indo-Pacific Run is integrated for 100 years and the Indian-Ocean Run run is integrated for 50 years.

The Indo-Pacific Run simulate ENSO events realistically and produces strong decade-to-decade ENSO changes. Both this run and the Indian-Ocean Run simulate the Indian Ocean Zonal mode but with different dominant timescales. This result suggest that the coupled Indian Ocean-Monsoon system itself is capable of producing the Indian Ocean dipole mode without the forcing from ENSO. However, ENSO is capable of changing the dominanted timescales of the Indian Ocean dipole mode. In this talk, we will discuss how ENSO can influence the timescales of IOZM and how those influences change on decadal timesacles.