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Satellite observations have shown that the largest and most prolonged Maud Rise open-ocean polynya since the 1970s appeared on 14 September 2017 (similar to 9.3 x 10(3) km(2)) within the seasonal sea-ice cover which expanded maximum on 1 December 2017 (similar to 298.1 x 10(3) km(2)) and existed for 79 days. Record negative anomalies of sea-ice concentration were observed in and around the polynya. The occurrence of the polynya was associated with a large cyclonic eddy and negative wind stress curl that facilitated melting of sea-ice. Concurrently, a region of positive sea level pressure anomalies extended over the entire northern Weddell Sea accompanied by record low negative anomalies (deep depressions) over the southwest Weddell Sea and the Maud Rise. The atmospheric circulation anomalies advected moist-warm air from the midlatitudes, resulted a record atmospheric warming (similar to 11.5 degrees C) in the Maud Rise that favored this rare event as one of the largest open-ocean polynyas.

Plain Language Summary The polynya plays an important role in the Earth's climate system by modulating the albedo, air-sea exchange of heat, fresh water, carbon, and ocean-atmospheric circulation. The occurrence of such feature is critical for assessing the role of high latitude ocean-atmospheric dynamics in the global climate and also for the Antarctic marine ecosystem. Satellite observations show that a large and most prolonged Maud Rise polynya (Lazarev Sea) reappeared on 14 September 2017 for the first time since its frequent appearance during the 1970s. On 14 September 2017, the areal extent of the polynya was similar to 9.3 x 10(3) km(2) which expanded maximum on 1 December 2017 up to similar to 298.1 x 10(3) km(2), lasting for 79 days. The formation of the polynya was due to the combined influence of the (i) existence of the geological feature such as a seamount (leads to local upliftment of thermocline), (ii) upwelling of warm water into the upper ocean from the thermocline (induced by a large cyclonic ocean eddy and negative wind stress curl), and (iii) the large-scale anomalous atmospheric warming.