West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing

Abstract

Recent ice loss from the West Antarctic Ice Sheet has been caused by ocean melting of ice 9 shelves in the Amundsen Sea. Eastward wind anomalies at the shelf break enhance the 10 import of warm Circumpolar Deep Water onto the Amundsen Sea continental shelf, which 11 creates transient melting anomalies with an approximately decadal period. No 12 anthropogenic influence on this process has been established. Here, we combine 13 observations and climate model simulations to suggest that increased greenhouse-gas 14 forcing caused shelf-break winds to transition from mean easterlies in the 1920s to the 15 near-zero mean zonal winds of the present day. Strong internal climate variability, primarily 16 linked to the tropical Pacific, is superimposed upon this forced trend. We infer that the 17 Amundsen Sea experienced decadal ocean variability throughout the 20 th century, with 18 warm anomalies gradually becoming more prevalent, offering a credible explanation for 19 the ongoing ice loss. Existing climate model projections show that strong future 20 greenhouse-gas forcing creates persistent mean westerly shelf-break winds by 2100, 21 suggesting a further enhancement of warm ocean anomalies. These wind changes are 22 weaker under a scenario in which greenhouse gases are stabilised.23

Interannual variability of summer surface mass balance and surface melting in the Amundsen sector, West Antarctica

“…This promotes a southward flow on the eastern flank of the ASL, towards the glaciers, with resulting increased moisture convergence, precipitation and therefore SMB. Our study hence provides further support for the connection between Antarctic precipitation and the ASL longitudinal position that was previously described by Hosking et al (2013) based on the ERA-Interim reanalysis. In terms of climate indices, this corresponds to an anticorrelation between SMB and the ASL longitudinal position.…”

“…Summers tend to be associated with high SMB when the ASL migrates westward and southward because this places the northerly flow (ASL eastern flank) over the Amundsen Sea, thereby increasing the southward humidity transport and snowfall. This corresponds to the large-scale features described by Hosking et al (2013) but is here described for the SMB of individual drainage basins. By contrast, longitudinal migrations of the ASL are not the main driver of surface melting variability, as previously noted by Deb et al (2018).…”

“…Numerous previous studies used the absolute rather than relative central pressure to characterize the ASL, but this index is strongly correlated to the SAM index and cannot be considered independently (Table 1). As proposed by Hosking et al (2013), the ASL relative central pressure (i.e. actual central pressure minus pressure over the AS sector) allows for a better understanding of West Antarctic climate as it removes the influence of large-scale variability such as ENSO and SAM.…”

“…3). A westward shift of the ASL induces northerly flow anomalies over the Amundsen Sea, leading to warmer conditions and increased moisture transport over the ice sheet (Hosking et al, 2013Thomas et al, 2015;Raphael et al, 2016;Fyke et al, 2017). Variations in the ASL central pressure also largely impact the West Antarctic climate: anticyclonic anomalies near 120 • W lead to marine air intrusion over the ice sheet, thereby increasing cloud cover, longwave downward radiations and surface air temperature over the West Antarctic Ice Sheet (WAIS; .…”

Effect of Subshelf Melt Variability on Sea Level Rise Contribution From Thwaites Glacier, Antarctica

“…Available Amundsen Sea oceanographic observations have clearly demonstrated that annual-to-decadal variability in the depth of mCDW on the continental shelf is a dominant ocean driver of ice-shelf melting in the region, obscuring any long-term trend that might exist there (Christianson et al, 2016;Dutrieux et al, 2014;Jacobs et al, 2013;Jenkins et al, 2016Jenkins et al, , 2018. Variability in the depth of the thermocline (the sharp gradient in temperature separating warmer, deep water and cooler, shallow water) is closely regulated by atmospheric conditions, particularly wind stress magnitude, which effects downwelling and sea-ice driven ocean stratification (Jenkins et al, 2016;Holland et al, 2019). This variability in the vertical ocean temperature profile and the associated ice-shelf melt response has been directly linked to the El Niño/Southern Oscillation (ENSO) (Jenkins et al, 2016;Holland et al, 2019;Steig et al, 2012;Paolo et al, 2018).…”