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This study investigates the variability of the Southern Hemisphere super gyre (SHSG), using remotely sensed altimeter measurements, in situ Argo observations, and results from an ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean. Analyses of altimeter data show large trends of sea surface height, and their positive-negative contrast suggests a strengthening of subtropical gyres in all the three Southern Hemisphere oceans since 1993. Analyses of Argo data and the Estimating the Circulation and Climate of the Ocean estimate indicate that these dynamic signals of southern subtropical gyres extend to at least 2,000m. The three southern subtropical gyres are interconnected through the Tasman and Agulhas leakages and vary consistently during the period 1993-2016. The Tasman and Agulhas leakages also show an increasing trend of inter-ocean water exchange with a typical increase of similar to 2 Sv (1 Sv=10(6)m(3)/s) per decade, indicative of a two-decade-long spin-up of the SHSG. The strengthening and poleward shift of westerly winds are associated with an increasing southern annular mode, which affect the midlatitude and high-latitude Southern Hemisphere oceans and contribute to the spin-up of the SHSG.

Plain Language Summary The Southern Hemisphere super gyre (SHSG) is an interconnected circulation system of the southern subtropical gyres. It sweeps out of the Tasman Sea, transports water westward, and feeds the upper limb of the Atlantic thermohaline circulation through the Tasman and Agulhas leakages. The SHSG absorbs substantial quantities of atmospheric gases, such as oxygen and CO2, thus directly contributing to climate change. In this study, we analyze remotely sensed altimeter measurements, in situ observations, and outputs of an ocean general circulation model. The results reveal a two-decade-long spin-up of the SHSG. The inter-ocean exchange through the Tasman and Agulhas leakages shows an increasing trend during the period 1993-2016. This long-term trend may be attributed to changes in westerly winds associated with an increasing southern annular mode.