Winds and Meltwater Together Lead to Southern Ocean Surface Cooling and Sea Ice Expansion

Abstract

Southern Ocean surface cooling and Antarctic sea ice expansion from 1979 through 2015 have been linked both to changing atmospheric circulation and melting of Antarctica's grounded ice and ice shelves. However, climate models have largely been unable to reproduce this behavior. Here we examine the contribution of observed wind variability and Antarctic meltwater to Southern Ocean sea surface temperature (SST) and Antarctic sea ice. The free‐running, CMIP6‐class GISS‐E2.1‐G climate model can simulate regional cooling and neutral sea ice trends due to internal variability, but they are unlikely. Constraining the model to observed winds and meltwater fluxes from 1990 through 2021 gives SST variability and trends consistent with observations. Meltwater and winds contribute a similar amount to the SST trend, and winds contribute more to the sea ice trend than meltwater. However, while the constrained model captures much of the observed sea ice variability, it only partially captures the post‐2015 sea ice reduction.

Anomalous Meltwater From Ice Sheets and Ice Shelves Is a Historical Forcing

“…The zonal average differences in the ensembles show clear and significant forced cooling in Southern Ocean sea surface temperatures and increases in sea ice concentrations (Figure 4) and warming in the ocean subsurface (Figure S1 in Supporting Information S1). Notably, the sign of the forced trends in temperature and sea ice concentration have changed and are better aligned with observations in the Southern Hemisphere, this is discussed further in Roach et al (2023). Subsurface temperatures around Antarctica also increase as the surface freshwater inhibits mixing though there is no detectable difference in net Antarctic Bottom Water production (Figures S1c and S1e in Supporting Information S1).…”

“…Finally, it is important to note that while this study covers the period through to the end of 2019 (including some extrapolation from 2016), Antarctic sea ice concentrations since 2015, and especially in 2022/2023, have plunged to record low levels for the satellite era (Roach et al, 2023), and since 2019, Antarctic grounded ice mass has increased slightly. It is as yet unclear what the proximate causes of these changes are and whether they are connected.…”

Fast and Slow Responses of Atlantic Meridional Overturning Circulation to Antarctic Meltwater Forcing

“…However, the time scale of each adjustment would not be differentiated, resulting in blurred atmospheric impacts. It's important to note that wind variability substantially influences the SO temperature (Roach et al, 2023), and constructive interference with the meltwater flux still holds the potential for non-monotonic AMOC responses. Overall, however, the non-monotonic evolution is less anticipated with gradual meltwater alone.…”

Fast and Slow Responses of Atlantic Meridional Overturning Circulation to Antarctic Meltwater Forcing

“…However, the time scale of each adjustment would not be differentiated, resulting in blurred atmospheric impacts. It's important to note that wind variability substantially influences the Southern Ocean temperature (Roach et al, 2023), and constructive interference with the meltwater flux still holds the potential for non-monotonic AMOC responses. Overall, however, the non-monotonic evolution is less anticipated with gradual meltwater alone.…”