Summary
The continent of Antarctica has experienced some of the largest rates of both surface warming and cooling found anywhere on the planet. Unlike in the Arctic, where sea ice has been declining, despite some regional decreases overall sea ice extent in the Southern Ocean has been increasing in recent decades. Stratospheric ozone depletion has generally been implicated in aspects of climate change in the Southern Hemisphere1. Often such inferences are based on climate model simulations that exclude ozone chemistry. Trends in Antarctic sea ice are generally not reproduced in contemporary climate model simulations2; consequently, at present, causes of sea ice trends in the Southern Ocean are poorly understood. Here we present results using simulations by the NIWA-UKCA global atmosphere-ocean chemistry-climate model3; these simulations form part of our submission to the Chemistry-Climate Modelling Initiative (CCMI). We use an ensemble of three simulations following the “REF-C2” specification with all anthropogenic forcings operating, as well as sensitivity simulations in which ozone depletion, or alternatively increases in greenhouse gases (GHGs), have been suppressed4. In these simulations, we study the processes linking atmospheric and sea ice change. We find that the patterns of sea ice trends in some of the simulations are remarkably similar to observations for the post-1980 period. In the REF-C2 simulations and in the simulation with suppressed GHG increases, total sea ice extent declines markedly until around 1980 but increases or remains unchanged for a period thereafter. We present evidence based on ice-core data, supporting the notion that there has indeed been such a turn-around. By contrast, the simulation with suppressed ozone depletion does not exhibit this turn-around. There are important differences in the geographic patterns of sea ice trends among the REF-C2 ensemble members. This suggests the presence of chaotic regional decadal-scale climate variations which may contribute towards the lack of agreement on patterns of sea ice change characterizing recent climate model simulations2. 1. Arblaster, J. M., and G. A. Meehl (2006), Contributions of external forcings to Southern Annular Mode trends, J. Climate, 19, 2896-2905. 2. Mahlstein, I., P. R. Gent, and S. Solomon (2013), Historical Antarctic mean sea ice area, sea ice trends, and winds in CMIP5 simulations, J. Geophys. Res. Atmos., 118, 5105–5110 3. Morgenstern, O., et al. (2009), Evaluation of the new UKCA climate-composition model – Part 1: The stratosphere, Geosci. Model Dev., 2, 43-57.
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