Roles of the Indian Ocean in the Australian summer monsoon-ENSO relationship

A negative correlation is observed between interannual variations of the Australian summer monsoon (ASM) and El Nino-Southern Oscillation (ENSO). This negative relationship is well simulated in the Center for Ocean-Land-Atmosphere (COLA) interactive ensemble coupled general circulation model (CGCM). The present study investigates roles of the Indian Ocean in the ASM-ENSO relationship through controlled numerical experiments with the COLA CGCM. It is found that air-sea coupling in the Indian Ocean plays an important role in maintaining the negative ASM-ENSO relationship. When the Indian Ocean is decoupled from the atmosphere, the ASM-ENSO relationship is significantly weakened or even masked by the internal atmospheric variability. This change in the ASM-ENSO relationship is related to complementary roles of Indian Ocean sea surface temperature (SST) anomalies in the ASM variability and feedbacks from the Indian Ocean on ENSO. Without a coupled Indian Ocean, the ENSO amplitude is reduced, leading to a decrease in the ENSO-induced ASM variability, and the constructive impacts of the Indian Ocean SST anomalies on the ASM variability are substantially reduced. This reduces the ASM variability related to ENSO. Consistent with the change in the ASM-ENSO relationship, the local air-sea relationship in the ASM region displays important differences with and without a coupled Indian Ocean. The long-term change in the ASM-ENSO relationship is related to that in ENSO amplitude in the interactive ensemble coupled model. A relatively higher ( lower) negative correlation occurs in periods of larger (smaller) ENSO amplitude. This relationship, however, is not clear in the anomaly coupled model with only one atmospheric realization. This difference is attributed to changes in the signal-to-noise ratio in the ASM variability. A comparison is made with observations and the long-term change in the Indian summer monsoon (ISM)-ENSO relationship in the model.