Unlike other tropical eastern oceans which absorb heat from the atmosphere, the south-east Indian Ocean is a region with large poleward and westward heat fluxes and significant heat loss to the atmosphere. Variations in the ocean dynamics of this region will therefore have important implications on the ocean-atmosphere heat budget. In this presentation, we look at the ocean dynamics responsible for the intraseasonal, seasonal and interannual sea level variability observed in the south - east Indian Ocean. The eastern Indian ocean is directly forced by the strong seasonal monsoons as well as by a remote ocean forcing from the tropical Indian Ocean and by the western Pacific via the Indonesian Throughflow. As a result, the dynamics of the southeast Indian Ocean are unique, with unusually large variability and bands of energetic Rossby waves. The annual wave signal around 10｡S is clearly marked and a band of propagating mesoscale variability between 20｡S and 35｡S extends across the entire Indian Ocean, with characteristic timescales between 100 and 200 days. Along the eastern boundary there is also strong interannual variability, related to low-frequency wind forcing in the Indian and variations in the Throughflow from the Pacific. Finally, intraseasonal variations are evident in the South Tropical Indian Ocean (STIO).
To investigate the origin of the observed Rossby waves, we use a simple reduced-gravity model to calculate the sea level response to Ekman pumping. The model allows the radiation of long baroclinic waves due to such pumping and the radiation of long waves from the eastern boundary. Eastern boundary conditions are given by XBT data. In the band 10｡S-15｡S the thermocline depth oscillation corresponds mainly to waves radiating from wind forcing in the east. In some years, their amplitude is strongly damped by opposing winds west of 90｡E. There is also a small but significant influence of free waves emanating from the eastern boundary in the STIO. In addition, a significant signal appears to originate near Lombok Strait to the north and propagates southwestward into the STIO. Anomalies forming around Java occur at intraseasonal time-scales, with a strong semi-annual modulation by the seasonal reversal of the South Java Current, and annual and interannual modulations influenced by the local winds and the flow through Lombok Strait.
In the band 20｡S-35｡S, the observed near-semiannual waves appear to be free waves generated by eastern boundary processes. Local wind-forced upwelling is not responsible for the eastern boundary thermocline variations. Rather, altimetric sea level and tide gauge data show evidence of poleward propagating coastal Kelvin waves at annual and semi-annual frequencies which are important in redistributing anomalies from the Indonesian region further south. Most of the semi-annual signal appears to radiate westward between 20-35｡S, and a triggering mechanism may be the change in bathymetry and their interaction with the coastal Leeuwin Current.
