A new index used to characterise the extent of Antarctic marine coastal winds in climate projections

Antarctic marine coastal near-surface winds play a key role in Southern Ocean circulation. Using the ERA5 reanalysis dataset, this paper develops directional wind constancy as a tool for identifying key features in these winds and their relationship with the mid-latitude westerly jet. In particular, the Antarctic coastal wind boundary (ACWB), defined as the minimum offshore directional constancy boundary, is shown to be a useful way to define the marine nearcoastal region where the Antarctic topography plays an important role in influencing the wind direction. We show that, while the ACWB is linked to large-scale modes of atmospheric circulation through its close association with variability in the mid-latitude westerly jet, it also highlights key regions where topographically-influenced, meridional flows are dominant. These meridional flows are not identified in current regional climate indices. Future changes in the ACWB are examined using CMIP6 projections for a high emissions scenario. This indicates that by the end of this century the ACWB is projected to shift poleward by about 60 km, less than the 130 km shift in the mid-latitude westerly jet, indicating a reduction in the extent of the circumpolar trough.