Return of the Maud Rise Polynya: climate litmus or sea ice anomaly? [in “State of the Climate in 2017”]

The Maud Rise polynya is a persistent area of open water within the sea ice cover of the Southern Ocean, which overlies an area of elevated topography called Maud Rise (66°S, 3°E) located in the eastern sector of the Weddell Sea (Fig. SB6.1a). It is termed a "Weddell polynya" if it grows and migrates westward into the central Weddell Sea. This larger sized polynya was first observed in satellite data in 1974 and recurred for each of the two subsequent austral winters (Zwally and Gloersen 1977; Carsey 1980). Its large size, ∼300 000 km 2 , meant that it could contribute strongly to the transfer of heat from the ocean to the atmosphere in winter and, hence, instigate dense water production and the renewal of deep ocean waters in the Weddell Sea (Gordon 1978). The amount of deep water formed via this route was estimated at 1–3 Sverdrups (Martinson et al. 1981). The 1974–76 polynya may have been responsible for up to 34% of observed warming of the deep Southern Ocean (Zanowski et al. 2015). Smaller features, perhaps associated with topographically driven upwelling of warm waters, have been observed subsequently (Comiso and Gordon 1987), but a large polynya had not re-appeared until recently and unexpectedly during austral winters 2016 and 2017.