A Conceptual Model for the Indian Ocean Dipole mode

A conceptual model is proposed for the dipole mode recently discovered in the sea surface temperature (SST) anomalies of the tropical Indian Ocean. The model describes the basin-wide ocean-atmosphere interaction through feedbacks among the variations of thermocline depth, SST and surface wind stress, based on the following two hypotheses. Firstly, the wind-driven thermocline variability in the tropical Indian ocean can effectively influence the SST anomalies in the regions of upwelling near the Sumatra coast and west tropical Indian ocean. Secondly, through the wind-evaporation feedback under south-easterly wind background, the coastal SST anomalies near Sumatra can be sustained and spread westward into the ocean interior. Within a realistic parameter regime, this simple coupled model has a damped oscillatory coupled mode which shares similarity to the ENSO mode in the Pacific. Under stochastic forcing, the damped coupled mode can be excited and its basic characteristics are in agreement with the observed dipole-mode variability.