Search

The expansion of Antarctic sea ice since 1979 in the presence of increasing greenhouse gases remains one of the most puzzling features of current climate change. Some studies have proposed that the formation of the ozone hole, via the Southern Annular Mode, might explain that expansion, and a recent paper highlighted a robust causal link between summertime Southern Annular Mode (SAM) anomalies and sea ice anomalies in the subsequent autumn. Here we show that many models are able to capture this relationship between the SAM and sea ice, but also emphasize that the SAM only explains a small fraction of the year‐to‐year variability. Finally, examining multidecadal trends, in models and in observations, we confirm the findings of several previous studies and conclude that the SAM–and thus the ozone hole–are not the primary drivers of the sea ice expansion around Antarctica in recent decades. Plain Language Summary: Unlike its Arctic counterpart, sea ice around Antarctica has been growing since 1979, even as the levels of carbon dioxide in the atmosphere have increased. Given that the ozone hole formed over the South Pole around the same time, one is led to ask whether the ozone hole may be responsible for the growth of Antarctic sea ice (recall that there is no ozone hole over the North Pole). In this study, looking at both models and observations, we show that the ozone hole is capable of affecting the surface winds and these, in turn, can make sea ice expand. However, the magnitude of this effect is small. Also since the ozone hole started healing after the year 2000, while Antarctic sea ice kept expanding, we conclude that ozone depletion is not the main reason for the expansion of Antarctic sea ice in recent decades. Key Points: Many CMIP5 models are able to capture the observed seasonal correlation between summertime Southern Annular Mode (SAM) and Antarctic sea ice extentThe SAM, however, only explains 15 of the year‐to‐year sea ice extent variability in the fall, in both models and observationsSAM trends, and ozone depletion, are not the primary drivers of the observed Antarctic sea ice expansion in the last four decades [ABSTRACT FROM AUTHOR]

We analyze two preindustrial experiments from the Community Earth System Model version 2 to characterize the impact of sea ice physics on differences in coastal sea ice production around Antarctica and the resulting impact on the ocean and atmosphere. The experiment in which sea ice is a more realistic "mushy" mixture of solid ice and brine has a substantial increase in coastal sea ice frazil and snow ice production that is accompanied by decreasing bottom ice growth and increasing bottom melt. The more realistic "mushy" physics leads to an increase in water mass formation at denser water classes due primarily to surface ice processes. As a result, the subsurface ocean is denser, saltier, and there is an increase in Antarctic Bottom Water formation of ∼0.5 Sv. For the atmosphere, "mushy" ice physics leads to decreased turbulent heat flux and low level cloud cover near the Antarctic coast. Plain Language Summary: We analyze experiments from the Community Earth System Model to better understand the impacts of representing sea ice as a mixture of salty water and solid ice rather than just solid ice. We focus on sea ice produced around the Antarctic coasts and find that the ways in which the sea ice grow and melt change with the two representations of sea ice, but the differences compensate so that the average sea ice state is minimally changed. However, the near surface ocean water is denser in the experiment with sea ice represented by a mix of solid ice and salty water, mainly because the ocean is saltier due to surface sea ice processes. This leads to increased formation of dense Antarctic Bottom Water. In addition, there is less energy input into the atmosphere and less low level cloud cover around the Antarctic coasts in the experiment with the sea ice represented as a mix of salty water and solid ice. Thus, there are important impacts on the Earth system based solely on the way sea ice is represented. Key Points: Choice of sea ice thermodynamics does not lead to large differences in sea ice state due to compensating thermodynamic changesAntarctic Bottom Water production increases by 0.5 Sv and upper ocean becomes denser due to increasing salinity with mushy thermodynamicsWintertime air‐sea fluxes, atmospheric low‐level mixing, and low cloud cover all decrease with mushy thermodynamics [ABSTRACT FROM AUTHOR]