Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica

Abstract

The southern boundary (SB) of the Antarctic Circumpolar Current, the southernmost extent of the upper overturning circulation, regulates the Antarctic thermal conditions. The SB’s behavior remains unconstrained because it does not have a clear surface signature. Revisited hydrographic data from off East Antarctica indicate full-depth warming from 1996 to 2019, concurrent with an extensive poleward shift of the SB subsurface isotherms (>50 km), which is most prominent at 120°E off the Sabrina Coast. The SB shift is attributable to enhanced upper overturning circulation and a depth-independent frontal shift, generally accounting for 30 and 70%, respectively. Thirty years of oceanographic data corroborate the overall and localized poleward shifts that are likely controlled by continental slope topography. Numerical experiments successfully reproduce this locality and demonstrate its sensitivity to mesoscale processes and wind forcing. The poleward SB shift under intensified westerlies potentially induces multidecadal warming of Antarctic shelf water.

Poleward shift of Circumpolar Deep Water threatens the East Antarctic Ice Sheet

“…2a). This temperature change is broadly in line with those estimated previously in the same region 7,15,16 and, remarkably, it is concurrent to an even more pronounced warming of 0.8-2ºC (or 0.1-0.4ºC per decade) over the continental slope (Fig. 1).…”

“…Our focus on the Indian Ocean sector stems from this region's glaciers contributing the bulk of East Antarctic ice-mass loss in recent decades 9,17,18 . Our evaluation of long-term water-mass variability off East Antarctica presents two important advances relative to previous studies 15,16 , in that our dataset: (i) has a substantially improved spatial coverage of the seasonally ice-covered areas near the East Antarctic margins, particularly over the continental slope and shelves; and (ii) spans a longer (by several decades) period.…”

“…While investigations of the interdecadal evolution of CDW temperature reveal a widespread, persistent warming to the north of and within the ACC, equatorward of ~60ºS 7,13,14 , there has been little indication of robust long-term temperature changes further to the south. Only recently, two studies 15,16 have put forward the suggestion that a CDW warming might be underway poleward of ~60ºS, possibly linked to a southward shift of the ACC's southern boundary in the eastern Indian region (115-150ºE) 15 or a CDW shoaling south of Tasmania 16 . However, spatio-temporal sampling limitations prevented these works from assessing the generality of CDW warming across the Indian Ocean sector of East Antarctica and, crucially, whether such warming (i) extends all the way to the continental slope, the access point of CDW to the East Antarctic shelves, and (ii) reaches the East Antarctic Ice Sheet.…”

Seasonal Variations and Drivers of Surface Ocean pCO2 in the Seasonal Ice Zone of the Eastern Indian Sector, Southern Ocean

“…One of the factors that may cause variations of xCO 2 sea is the spatial distribution of front positions. The discovery by Yamazaki et al (2021) that the Southern Boundary (SB) of Antarctic Circumpolar Current (ACC) moved southward during the period between BROKE and this study indicates that the upwelling region of Circumpolar Deep Water (CDW) moved southward. As a result, the maximum concentration of nitrate near the TML increased, and the difference in nitrate concentrations between the ocean surface and the TML increased (Figure 13).…”

“…Significant changes have taken place in the marine environment of the Indian sector during the two decades since the BROKE study (Auger et al, 2021). For example, Yamazaki et al (2021) have shown that the Southern Boundary (SB) of the Antarctic Circumpolar Current (ACC) moved southward from 1996 to 2019. This caused changes in the distribution of nutrients and might influence the seasonal variability of pCO 2 .…”