The Role of the Southern Annular Mode and the El Niño-Southern Oscillation on Extreme and Unprecedented Antarctic Heat

It was recently demonstrated using an ensemble of seasonal hindcasts with the “UNprecedented Simulated Extremes using ENsembles” (UNSEEN) technique that most of the Antarctic continent could experience record-breaking heat in both January and August. Here this analysis is continued, through the investigation of the role of large-scale modes of variability with known teleconnections to Antarctica, namely the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO), towards bringing relative warmth to Antarctica and its ice shelves during these months. The relationship between 2-metre temperature (T2m) and the SAM in the UNSEEN ensemble is consistent with the observed correlations: predominantly negative in both January and August. This negative correlation is strongest in magnitude along the coast of East Antarctica, while in the extreme north of the Peninsula a weaker positive correlation emerges. January correlations between T2m and ENSO are mostly positive in both observations and the UNSEEN ensemble, but spatial disparity between the two arises in August and perhaps suggests that the phase of ENSO could have a more varied influence on heatwave occurrence on different parts of the continent.

The polarity of the SAM dominates the Antarctic-wide mid-level circulation, and the teleconnection of ENSO is superimposed on top of this through modulation of the Amundsen Sea Low. This behaviour is identified in both observations and the UNSEEN ensemble. Therefore, for much of the continent heatwave days are dominated by negative SAM (SAM-) and are often combined with El Niño (EN) conditions. For example, SAM- patterns are more than twice as common during Antarctic-wide heatwave days than during all other days, and the combination of SAM- and EN is the most prevalent pattern that leads to heatwave days in the UNSEEN ensemble. However, in some locations (notably on ice shelves along the Peninsula) the relative occurrence of SAM- is no different between all days and heatwave days, and heatwaves occur with approximately equal probability across all combinations of SAM and ENSO phases. Strikingly, unprecedented T2m in Antarctica does not result from unprecedented SAM or ENSO values, suggesting either a deficiency in the UNSEEN ensemble, or that other processes not considered in this work are responsible for the most exceptional heatwaves in Antarctica. Further investigation into the large-scale drivers of unprecedented heat days in Antarctica is therefore required.