Decadal Variation of the Indian Ocean Dipole

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The processes that affect the decadal variation of the Indian Ocean Dipole (IOD) are investigated.1) Using outputs from a high-resolution coupled general circulation model (CGCM), decadal modulations of El Nino/Southern Oscillation (ENSO) is examined. Changes in the phase-locking nature of the ENSO to the seasonal cycle are shown to play an important role in the decadal modulation of the ENSO. This, in turn, modulates the heat transport by the Indonesian Throughflow, and thus the occurrence of the IOD.2) Heat budget analysis using the above model outputs has revealed that the occurrence of IOD events is governed by variations in the southward Ekman heat transport across 15°S, which is strongly linked with the Mascarene high activities.3) To understand the variability of the Agulhas Current, which influences the heat budget of the Indian Ocean, the interannual variation in the west coast of Africa is studied. It is found that the Angola Dome located in the above region undergoes significant interannual variations associated with the Atlantic Nino, which is a major climate mode in the tropical Atlantic.4) Since understanding of the past IOD is useful in understanding the current and future variability of the IOD, atmospheric general circulation model (AGCM) experiments with the permanent IOD and E1 Nino condition seen in Pliocene are conducted. The heat absorption from the atmosphere to the ocean in the tropics and the heat release from the ocean to the atmosphere in the mid-latitude are significantly reduced under this condition.