Revisiting circulation and water masses over the East Antarctic margin (80–150°E)

Abstract

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m3) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.