Over the past forty years a small increasing trend in Antarctic sea ice extent has been observed. This is poorly understood, and currently not captured by global climate models which typically simulate a net decrease in Antarctic sea ice extent (Turner et al. 2013). The length of our observational time series in combination with our lack of confidence in global climate model results makes it difficult to assess whether the recent decline of Antarctic sea ice observed in 2016 and 2017 is the start of a new declining trend or just part of natural variability.
The net increase in Antarctic sea ice extent is the sum of stronger, but opposing, regional and highly seasonal trends as shown in Figure 2 (Holland, 2014). The trends grow throughout the spring resulting in the maximum trends in the summer, decaying away throughout the autumn to give negligible trends in the winter. This seasonality implies the role of feedbacks in modulating the observed trends.
We have used a highly simplified coupled sea ice—mixed layer model (a schematic is shown in Figure 3) as a tool to help quantify and compare the importance of different feedbacks in two contrasting regions of the Southern Ocean. The Amundsen Sea, which has warm shelf waters, atmospheric conditions that are relatively warm with a high snowfall rate and a diminishing sea ice cover. And the Weddell Sea, which has cold saline shelf waters, cold and dry atmospheric conditions and an expanding sea ice cover.
We have carried out simulations where we denied different feedbacks in combination with perturbing the surface air temperatures, and compared the results with simulations where the feedback is enabled, and can to respond to the surface air temperature perturbation. We found that in the Weddell Sea the feedback responses were generally smaller than the response of the ice cover to the surface air temperature. However in the Amundsen Sea, we found that the ice cover was very sensitive to the depth of the ocean mixed layer which determines the size of the ocean heat flux under the ice. Whenever the atmosphere warmed we found that the ocean heat flux to the ice decreased (due to a shallower mixed layer), and this acted against the atmospheric changes, buffering changes in the ice volume.
Using a simple model has made it easier to understand the different processes at play in the two regions. However, in order to try to better to understand how these feedbacks link back to the regional trends we will also need to consider spatial variability, which may act to change the importance of some of the feedbacks. Incorporating what we have learnt using the 1D model, we are now working on investigating some of the same processes using the CICE sea ice model, to explore the importance and impact of spatial variability on the feedbacks.
Turner et al. (2013), An initial assessment of Antarctic sea ice extent in the CMIP5 models, J. Climate, 26, 1473-1484, doi:10.1175/JCLI-D-12-00068.1
Holland, P. R. (2014), The seasonality of Antarctic sea ice trends, Geophys. Res. Lett., 41, 4230–4237, doi:10.1002/2014GL060172.
Petty et al. (2013), Impact of Atmospheric Forcing on Antarctic Continental Shelf Waters, J. Phys. Ocean., 43, 920-940, doi: 10.1175/JPO-D-12-0172.1