Annual Cycle in Southern Tropical Indian Ocean Bottom Pressure

Abstract The seasonal monsoon drives a dynamic response in the southern tropical Indian Ocean, previously observed in baroclinic Rossby wave signatures in annual sea level and thermocline depth anomalies. In this paper, monthly mass grids based on Release-05 Gravity Recovery and Climate Experiment (GRACE) data are used to study the annual cycle in southern tropical Indian Ocean bottom pressure. To interpret the satellite data, a linear model of the ocean’s response to wind forcing—based on the theory of vertical normal modes and comprising baroclinic and barotropic components—is considered. The model is evaluated using stratification from an ocean atlas and winds from an atmospheric reanalysis. Good correspondence between model and data is found over the southern tropical Indian Ocean: the model explains 81% of the annual variance in the data on average between 10° and 25°S. Model solutions suggest that, while the annual baroclinic Rossby wave has a seafloor signature, the annual cycle in the deep sea generally involves important barotropic dynamics, in contrast to the response in the upper ocean, which is largely baroclinic.