Surface pressure and winds in the Amundsen Sea Embayment during the 20th century

Abstract:

Circumpolar Deep Water (CDW) is the primary driver of ice-shelf thinning in the Amundsen Sea Embayment. Abundant evidence shows that incursions of CDW onto the continental shelf is enhanced by westerly wind events occurring at the shelf break. It is virtually certain that melt rates have increased since the mid 20th century, which suggests that there has also been a shift in regional wind activity. However, since direct observations of climate in this region begin only in 1979, there has been no empirical evidence to document such shifts. We present annual zonal surface wind and surface pressure reconstructions during the 20th century at the continental shelf break of the Amundsen Sea Embayment, generated using a data assimilation technique that combines paleoclimate proxy data with physical constraints from climate models. We compare reconstructions generated using different climate models to evaluate the robustness of the results. All reconstructions, regardless of climate model used, show very good agreement with modern reanalysis products, such as ERA5, for both pressure and winds. The pressure reconstructions exhibit a robust low pressure trend in the region during the 20th century of approximately -3 hPa/century. Zonal surface winds show greater uncertainty, with trends ranging from approximately -1 to 0 m/s/century, suggesting an easterly or negligible trend. Numerous sensitivity experiments provide confidence that there is no long term westerly trend during the 20th century, contrary to the results of previous modeling studies. We suggest that instead, increased ice shelf melt in the Amundsen Sea region is associated with specific large-magnitude wind events, such as occur during El Nino events as earlier work had suggested. It is also possible that an increase in the variability of seasonal wind activity -- which would not be captured in our annual mean reconstructions -- plays an important role. Changes related to ocean heat or bathymetry that are unrelated to changes in wind forcing, cannot be ruled out.