The fundamental physical processes that give rise to El Nino-Southern Oscillation (ENSO) are believed to be within the tropical Pacific. However, climate features external to the tropical Pacific may be capable of affecting ENSO behaviors. In this study, we perform experiments with a coupled atmosphere-ocean GCM (CGCM) to examine the roles of the Indian Ocean-Monsoon system in the decadal modulation of ENSO.

In the control simulation, the oceanic component of the CGCM includes only the tropical Pacific Ocean (i.e., the Pacific Run). In the second CGCM simulation, both the Indian and Pacific Oceans are included in the ocean model component (i.e., the Indo-Pacific Run). In the third simulation, only the Indian Ocean is included in the ocean model component (i.e., The indian-Ocean Run). The Indo-Pacific Run has been integrated for 100 years, and the other two runs have been integrated for 50 years.Our CGCM experiments show that the Indian Ocean-Monsoon system can modulate the amplitude and frequency of ENSO and produce interdecadal ENSO variations. In this talk, we will examine the major differences between the strong and weak ENSO decades in their atmospheric and oceanic mean states. Focus will be placed on the thermocline depth, wind stress strength, Indonesian throughflow, and Asian Monsoon variation.

Our initial results indicate that the strong and weak ENSO decades are very different in their thermocline depths and Walker circulation strengths. We also found that ENSO-Monsoon relationship is less "typical" and more irregular when the role of Indian Ocean is considered. How the Monsoon circulation interacts with western Pacific trade wind system will be discussed.

This talk will also discuss the generation mechanisms of the Indian Ocean Zonal Mode (IOZM) by comparing the Indo-Pacific Run and the Indian-Ocean Run. 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. Our CGCM simulations show that this mode is produced in both the Indo-Pacific Run and the Indian-Ocean Run, but with different timescales. Our results 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.