Variability of the Indian Ocean: Relation to monsoon and ENSO

Recent years have brought renewed interest in Indian Ocean variability and its relationship with both Australasian monsoon variability and the El Nino-Southern Oscillation (ENSO). There is an ongoing debate as to whether the Indian Ocean is entirely a 'slave' to ENSO or whether it has its own modes of coupled variability that are independent of ENSO. The objective of this observational study is to document the variability of the Indian Ocean, to examine its relationship to variability in the tropical Pacific, and to investigate the potential influence of the Australasian monsoon on the variability in both basins. Indian Ocean variability is shown to be strongly seasonal and related to ENSO variability. In fact, when seasonality is carefully considered, the link between ENSO and the Indian Ocean becomes quite robust. This does not imply that the Indian Ocean does not have any variability on seasonal-to-interannual time-scales that is independent of ENSO. It does, however, suggest that a preponderance of this variability is related to ENSO variability. Cause and effect is not be determined here. A significant fraction of the variability in the tropical Indian and Pacific Oceans is related to variations with three centres of action: cold (warm) sea surface temperature anomalies (SSTA) in the western Indian Ocean, warm (cold) anomalies in the eastern Indian and western Pacific Oceans, and cold (warm) anomalies in the eastern Pacific. These three centres of action have a distinct annual march so that the SSTA in the Indian Ocean is of one sign during December to May, and a so-called dipole structure develops during June to November. The Indian monsoon is shown to be strongly connected to the annual march of this Indo-Pacific SST variability.