Post‐1990s Warming of Circumpolar Deep Water Off West Antarctica and Its Drivers

Ice loss off East Antarctica has been linked to the warming of Circumpolar Deep Water (CDW) associated with a poleward shift of the southern boundary of the Antarctic Circumpolar Current (ACC). In West Antarctica, where the fastest ice-shelf melting occurs, it has been proposed that off-shelf CDW warming could be additionally driven by enhanced lateral heat transport by a strengthened Ross Gyre and increased isopycnal heat transfer across the ACC. Although previous studies have proposed links between Ross Gyre variability, cross-ACC exchange, and off-shelf CDW warming around West Antarctica, their relative contributions have not yet been quantified. Here, we document an interdecadal (1990s–2010s) warming of off-shelf CDW around West Antarctica driven by two processes: (a) ∼90% results from enhanced lateral heat transport along isopycnals at the ACC's southern boundary, linked to an intensification of the Ross Gyre and cross-ACC isopycnal heat transfer; and (b) ∼10% arises from a poleward expansion of the ACC's hydrographic structure, analogous to that observed off East Antarctica. Both sets of processes were concurrent with a strengthening and poleward shift of westerly winds over the Southern Ocean, suggesting a causal link. As climate projections indicate that such wind trends will continue through the 21st century, the ongoing off-shelf CDW warming may also persist in future, bolstering the shelf break source of oceanic heat supply to the West Antarctic Ice Sheet.