Source of the baroclinic waves in the southeast Indian Ocean

Abstract

The dynamics of the seasonal and interannual sea level variability in the southeast Indian Ocean are investigated using a simple model of low‐frequency quasi‐geostrophic thermocline variability in order to determine whether the observed variability responds primarily to local or remote forcing. This region is important for climate studies, in relation to the interannual variations of the tropical oceans and atmosphere. 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° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 100 and 200 days. There is also strong interannual variability. To investigate the origin of the observed long baroclinic waves, we use a simple reduced gravity model allowing the radiation of long waves due to Ekman pumping and the radiation of long waves from the eastern boundary. Eastern boundary conditions are given by expendable bathythermograph data. In the band 10°–15°S the thermocline depth oscillation corresponds mainly to waves radiating from wind forcing in the east. Their amplitude is strongly damped west of 90°E. In the southeastern tropical Indian Ocean (STIO) the influence of free waves emanating from the eastern boundary is small but significant. In addition, a strong interannual signal appears to originate from Lombok Strait to the north and propagates southwestward into the STIO. In the band 20°–35°S the observed waves appear to be free waves generated by eastern boundary processes.