The Amundsen Sea Low: variability in present and future climates

Abstract

The Amundsen Sea Low (ASL) is a semi-permanent climatological low pressure system, located in the Southern Pacific Ocean, close to the West Antarctic Ice Sheet. It is an important regulator of West Antarctic and Antarctic Peninsula climate with the potential to accelerate future ice-loss, by driving atmosphere and ocean circulations. Using ERA5-reanalysis data, we develop an ASL climatology, investigating variability in both depth and location on different timescales. We find that the depth of the ASL is influenced by the two large-scale modes of atmospheric variability on the Southern Hemisphere: The Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO). Our results show that the combination of SAM+ (SAM-) and La Niña (El Niño) cause negative (positive) pressure anomalies in the Amundsen Sea, while other combinations show significantly reduced effects. The zonal location of the ASL is linked to the mid-tropospheric planetary waves, causing a 60◦ eastward shift during austral summer compared to other seasons. The synoptic scale atmospheric patterns in the Amundsen- Bellingshausen and Ross Seas are dominated by the ASL. The advection of both warm air (to the east) and cool air (to the west) around the ASL influences the climate of West Antarctica and the Antarctic Peninsula, creating large anomalies in temperature, precipitation and sea ice. Using the Community Earth System Model (CESM2), we are able to asses future ASL variability and its influence on the AIS, comparing a historical (1979-2014) and future period (2065-2100). The modelled ASL in CESM2 matches the observations from ERA5 well. We show that the future ASL will likely deepen as a consequence of anthropogenic forcing, further spatially extending its influence on near-surface wind patterns over parts of East Antarctica. While in present day climate an anomalously strong ASL has a net cooling effect on Antarctica, in the future the area of warming response increases, especially during summer and autumn.