Extreme warm events in the South Orkney Islands, Southern Ocean: Compounding influence of atmospheric rivers and föhn conditions

Extreme warm events in the South Orkney Islands (SOIs) are investigated using synoptic observations from Signy and Orcadas stations for 1947-1994 and 1956-2019, respectively. Defining the extremes as temperatures exceeding the 95th percentile of the temperature distribution, we reveal the characteristics and associated drivers of the warm events, especially the top ten events in both summer and winter. At both stations, extreme warm events often involve a combined effect of atmospheric rivers (ARs) and localised föhn warming, with distinct characteristics due to the station locations relative to Coronation Island, the largest and highest island of the SOIs. For example, warm events at Signy are warmer (by an average of around 3°C) than the corresponding concurrent temperatures at Orcadas. The number of warm events per year has significantly increased over the record periods at both stations, which could potentially impact ecosystems by increasing melting of snow and ice. Extreme warm events at Signy are dominated by föhn warming in combination with ARs originating from the Southern Atlantic Ocean, where warm, moisture-rich air is rapidly advected towards the islands by enhanced northerly winds. By contrast, the Orcadas warm extremes involve both warm air advection and föhn warming associated with enhanced north-westerlies / westerlies with ARs originating in the Pacific Ocean that travel across the Drake Passage. Simulation of one of the top ten warm events for Signy station using a 1 km grid-spacing configuration of the atmosphere-only MetUM model is used to disentangle the role of local versus large-scale forcing. We find that the majority of the warming can be attributed to föhn effects for the case study. These results demonstrate the complexity of Antarctic temperature extremes.

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