Variability and Trends of the Amundsen Sea Low since the Early Twentieth Century from Seasonal-Station-Based Reconstructions

Abstract The Amundsen Sea is dominated by a quasi-stationary low-pressure region, the Amundsen Sea low (ASL). ASL variability impacts regional weather and the basal melting of ice shelves, an important contributor to sea level rise. To understand trends and variability of the ASL, it is important to have data for a long time period. However, the shortness of the satellite record starting in 1979 and the sparseness of Antarctic observational data prior to this make understanding variability on decadal scales challenging. Century-long reanalyses are available but have well-known pressure biases, meaning that trends cannot be reliably quantified. Other reconstructions are available at the annual resolution but mask important seasonal differences. Here, we reconstruct the ASL at seasonal resolution from 1905, using Southern Hemisphere weather station sea level pressure data, which takes advantage of well-known teleconnections between the tropics and Antarctica, although the strength of these is seasonally dependent and varies decadally. We compare our reconstructions with two centennial reanalyses and ERA5. Our reconstruction captures early twentieth-century variability associated with ice shelf melting and retreat and places recent ASL trends in a longer-term context. We find that the recent deepening of the ASL across all seasons is unprecedented since 1905, and there is increased uncertainty in the ASL index during the mid-twentieth century. We also assess the stationarity of the association between station data and the ASL by using data from a Pacific pacemaker climate model experiment. The stationarity assumption for our ASL reconstruction is valid, except for the austral autumn due to insufficient data availability. Significance Statement Our study provides a novel century-length seasonal reconstruction of pressure in the Amundsen Sea region [the Amundsen Sea low (ASL)]. On interannual time scales, a more positive (weaker) ASL can drive warm water intrusions under the ice shelves, leading to increased melt and significant contributions to sea level rise. The ASL also impacts sea ice and Antarctic temperatures. Reliable records, which start in 1979 in this data-poor region, are not long enough to separate the large natural variability, particularly at decadal scales, from human-induced changes. Our reconstruction compares favorably with those from centennial-scale reanalyses and is free from the high-pressure biases associated with these products. Our new record shows that the recent negative trend in the ASL is unprecedented since the early twentieth century.