Hydrographic section along 55° E in the Indian and Southern oceans

Abstract. A hydrographic section along 55° E, south of 30° S, was visited from December 2018 to January 2019 as the first occupation under the Global Ocean Ship-based Hydrographic Investigation Program. The water column was measured from the sea surface to 10 dbar above the bottom with eddy-resolving station spacings and the state-of-the-art accuracy. The upper profile was characterised by a conspicuous front between 42.5° and 43° S and a cold-core eddy at 39° S. The front was identified as the confluence of Subtropical and Subantarctic fronts. The Agulhas Return Current front was found at 41.6° S. When combined with the section north of 30° S observed in 2018, another subsurface front was found in dissolved oxygen around 28° S at depths of 1500 to 3000 dbar. In the eastern Weddell-Enderby Abyssal Plain, no obvious mean flow was observed at depths greater than 3000 dbar. We used transient tracers to estimate isopycnal diffusivity there to be 72±16 m² s^-1. Antarctic Bottom Water in the basin consisted of water masses originating from the Cape Darnley region (0–35 %) and Weddell Sea Deep Water (5–75 %), diluted by Lower Circumpolar Deep Water above. These snapshot observations not only confirm hydrographic features reported earlier in the Madagascar and Crozet Basins, but also describe the diffusive nature of the deep to bottom circulation in the Weddell-Enderby Abyssal Plain. Some of the stations in the Crozet Basin were sampled in the 1980s and 1990s. Changes in the temperature-salinity relationship since then indicate warming of Upper Circumpolar Deep Water, volume reduction of Antarctic Bottom Water, and slight freshening which is stronger southward.