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.