Two-Way Interactions Between Antarctic Sea Ice and the Atmospheric Circulation: Mechanisms for and Implications of Recent Sea-Ice Loss
September 10, 2025
Chloe Boehm
Committee: David Thompson (Advisor); David Randall; James Hurrell; Daniel McGrath (Geosciences)
Abstract
In contrast to the Arctic, Antarctic sea ice has experienced both long-term positive and negative trends across the observational record. From the 1980s until 2015, Antarctic sea ice experienced a slight positive trend, while Arctic sea ice experienced a consistent negative trend. These weak but steady increases ended abruptly during the austral summer of 2015, when Antarctic sea-ice extent dropped by roughly 2 million km2, setting record negative anomalies in November 2016. This low sea-ice state has persisted since, with new record negative anomalies set in July 2023, which were more than four standard deviations below the 1979-2022 mean. The reasons behind these unprecedented changes and the impacts of this new low sea-ice state on the climate system remain open to debate. This dissertation focuses on providing novel observational insights into the effects of the atmospheric circulation on recent sea-ice variability and vice versa.In Part 1, we provide insights into the role of the Southern Annular Mode (SAM) in driving variations in Antarctic sea-ice area (SIA). Utilizing a mixture of reanalysis and satellite data, we show that the influence of the SAM on Antarctic sea ice exhibits more pronounced seasonality than that indicated in previous work. At the time of the sea-ice minimum, positive anomalies in the SAM - and thus enhanced westerlies over the high latitude Southern Ocean - lead to increased SIA over the following four months. In contrast, at the time of the sea ice maximum, positive anomalies in the SAM lead to decreased SIA over the following twelve months. Differences in the persistence of the SAM-SIA relationship from one season to the next lead to an outsized impact of variations in the SAM during the sea-ice maximum on annual-mean SIA. The persistence of SAM-induced sea-ice anomalies following the sea-ice maximum is consistent with both anomalous shortwave radiative feedbacks and heat storage in the upper ocean. It is argued that a notable fraction of the dramatic losses in annual-mean SIA over the past decade can be traced to unprecedented variations in the SAM during the time of the sea-ice maximum.
In Part 2, we provide insights into the impacts of Antarctic sea-ice decline on regional changes in the atmospheric circulation. We perform a gridpoint-by-gridpoint regression technique on reanalysis and satellite data to create an observational estimate of the impacts of Antarctic sea-ice loss. The results highlight a strong relationship between sea-ice loss and increases in turbulent heat flux, precipitation, cloud fraction, and near-surface temperature and wind speed. The strongest relationships occur during the austral winter season, with upwards of a 50% increase in precipitation and near-surface temperature from one standard deviation of sea-ice loss. We then compare these observational results to two atmosphere-only simulations and a fully-coupled model. The atmosphere-only simulations help establish causality and provide the opportunity to isolate the pathways by which sea-ice loss impacts precipitation. The results indicate that a large portion of the predicted Southern Ocean precipitation changes can be attributed to the effects of sea-ice changes on local variations in the surface heat fluxes. The changes in the heat fluxes influence not only local vertical motion, but also vertical motion downwind of the sea-ice anomalies due to changes in the vertical mixing of free tropospheric winds and thus the horizontal convergence of the surface flow.