Dynamics of near-surface eastward flows in the South Indian Ocean

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Objectives and intellectual Merit: A remarkable aspect of the upper-ocean circulation in the tropical and subtropical South Indian Ocean (SIO) is the presence of near-surface, eastward flow across the basin, which often appears as a set of distinct currents and jets. These currents have been detected in observations and simulated in numerical models. At the same time, their underlying dynamics remain unclear. In this regard, they flow counter to the westward direction predicted by both Ekman and Sverdrup theory. Furthermore, they appear to be linked to the Leeuwin Current (LC) along the west coast of Australia and to larger-scale circulations , namely, the Indonesian Throughflow (ITF) and the Southern Ocean. Thus, the eastward flow represents a fundamental gap in our understanding of the large-scale mean flow in the SIO. This project will use analyses of in situ observations and numerical models to fill that gap. The observational analysis will rely on three main datasets. Each dataset combines Argo data with historical data, the different mapping techniques resulting in products with characteristics suited to different aspects of the analysis: the CARS dataset developed at CSIRO is a high-quality monthly-mean dataset of T , S , and other water-mass properties, which allows for tracing watermasses; the IPRC dataset incorporates the additional product of absolute dynamic topography, allowing an estimation of absolute geostrophic velocity; and the UTas dataset maps Argo and historical data on density surfaces, which will be useful in resolving finer frontal features and characterizing downwelling/upwelling regions. Satellite winds will be used to compute Ekman and Sverdrup flows and to force the models. To test specific hypotheses about the processes that control the surface eastward currents, an ocean circulation model will be used.The research team combines the talents of scientists at the IPRC, CSIRO, and the University of Tasmania, with scientists at the latter institutions contributing their extensive knowledge of in situ observations in the SIO to the project Broader Impact: Understanding the dynamics of these first-order features is necessary to validate and improve climate models. Moreover, these currents interact with the LC and the ITF, and are likely part of a basin-scale overturning circulation closed by downwelling within the LC and near the Antarctic Circumpolar Current. The dynamics of the surface eastward currents therefore should play an important role in SIO decadal variability. The results will likely apply to similar features in the other oceans.