A Theory of the Indian Ocean Dipole Mode

A conceptual coupled atmosphere-ocean-land model is developed to understand the origin of the Indian Ocean Dipole Mode (IDM). The model contains six boxes that represent the equatorial western and eastern Indian Ocean, the western Pacific/maritime continent, the equatorial eastern Pacific, the South Asian monsoon and the western North Pacific monsoon regions. The key physical processes involved in the model include the radiation-cloud-SST feedback, the wind-evaporation-SST feedback, the monsoon induced large-scale east-west circulation, local Hadley and Walker Cells, the wind-thermocline-SST feedback, and the effect of equatorial ocean waves.

In the absence of external forcing of the intraseasonal oscillation (ISO) and ENSO, the model has a decayed oscillatory solution with an interannual frequency, suggesting that the IDM is a natual decayed coupled air-sea mode. The strongest negative feedback comes from the cloud-shortwave radiation-SST feedback. The monsoon may also nagatively feed back into the IDM through its remote and local impacts. The remote impact involves the large-scale east-west circulation that changes the SSTA in the western Pacific/maritime continent - the latter of which may further alter the strength of the Walker circulation over the Indian Ocean. The local impact involves the monsoon-SST local feedback mechanism discussed by Meehl (1987).

The role of the external forcing of the ENSO and ISO on the IDM is investigated by specifying the observed Nino3 SSTA and ISO winds from 1950-98. In the model, the ENSO influences the IDM through the following three processes. First, it influences the intensity of the South Asian monsoon that further changes the wind over the Indian Ocean. Second, it alters the western Pacific SSTA through induced anomalous winds over the central Pacific. Third, it induces anomalous cyclonic/anticyclonic flows over the north western Pacific that alters the strength of the monsoon heating and cross-equatorial (and along-coastal) winds. The ISO, on the other hand, acts as a stochastic forcing to reinvigorate the IDM through anomalous winds over the Indian Ocean. An irregular, sustainable oscillation of the IDM is simulated by the model in the presence of the ISO and/or ENSO forcing. The mechanism that causes the phase locking of the IDM to the annual cycle is also discussed.