The water mass variability and southward shift of the Southern Hemisphere mid-depth supergyre

The Southern Hemisphere subtropical supergyre at intermediate depths connects all three ocean basins and plays a significant role in responding and conveying the climate-change-related variations in the global ocean. On the basis of the Simple Ocean Data Assimilation (SODA) ocean reanalysis, the thermohaline variability and southward shift of the mid-depth supergyre are demonstrated. The steric height of the subsurface relative to 1 500 m (400-1 500 m) from the SODA depicts exactly the flow patterns and variability of the oceanic supergyre. During 1958-2007 the water masses in the gyre interiors become cooler/fresher, with the significant exceptions of the Agulhas Current system and Agulhas leakage. The results also exhibit a pronounced strengthening of the inter-basin connection of the supergyre, and the strongest southward shift, by about 2.5A degrees over the whole period, occurs in the central-south Pacific, which is associated with the changes in the basin-scale wind forcing.