A combination of observational data and modelling reveals the potential significance of the north and tropical Atlantic Ocean in driving change in Antarctic winds and sea ice on decadal timescales and longer. See Letter p.538
In recent years, the polar regions have provided some striking examples of rapid environmental change. Perhaps the most notable of these has been a reduction of more than 30% in the summer extent of Arctic Ocean sea ice since the late 1970s1. In the Antarctic, the pattern of change has been more complex. Although the extent of Antarctic sea ice has fallen significantly in some regions, it has increased in others, leading to a slight rise in overall winter-ice extent2(Fig. 1). Establishing the drivers of these changes has proved challenging, and the observed increase in Antarctic sea-ice extent — seemingly paradoxical in a warming climate — has frequently been used to question the widely accepted view that recent climate change is primarily anthropogenic in origin. A study by Li and colleagues3, reported on page 538 of this issue, suggests that long-term warming of the north and tropical Atlantic may be the ultimate cause of the observed changes in the Antarctic. The authors' findings imply that growing Antarctic sea ice may be consistent with a generally warming Earth.
In contrast to ice in the Arctic Ocean, which is confined by the surrounding continents, Antarctic sea ice is largely free to drift with the wind and ocean currents. Its extent is therefore strongly influenced by the pattern of surface winds around the continent. Much of the year-to-year variability in Antarctic sea ice is captured by a pattern known as the Antarctic dipole4, which is characterized by anomalies in ice extent of opposing signs in the Bellingshausen Sea and the western Ross Sea. The ice anomalies are a result of wind variations associated with changes in atmospheric-pressure patterns around the Antarctic. It is well established that these changes are connected to anomalies in sea surface temperature (SST) in the tropical Pacific Ocean4,5 through the generation of large-scale atmospheric waves by deep convection in the tropical atmosphere. These waves, known as Rossby waves, can propagate to polar latitudes and influence the atmospheric circulation there. Much of the year-to-year variability in Antarctic sea ice can thus be attributed to variability in tropical Pacific SSTs.
The observed long-term trends in ice extent — retreat in the Bellingshausen Sea and compensating advance in the western Ross Sea — strongly resemble the Antarctic-dipole pattern and closely match long-term trends in winds over the Southern Ocean6. It would therefore be natural to look first to the Pacific as the driver of this change. However, long-term trends in tropical Pacific SSTs are small and cannot explain the observed trends in the Antarctic. In their study, Li and co-authors highlight instead the potential importance of the Atlantic in driving change in the Antarctic. This result is motivated by observations7 showing that, in contrast to the tropical Pacific, north Atlantic SSTs have warmed significantly since 1979. The authors demonstrate that warmer Atlantic SSTs drive anomalous Southern Ocean winds that are consistent with the observed regional trends in Antarctic ice extent. Although forcing from the tropical Pacific dominates the variability of Antarctic winds and sea ice on interannual timescales, Atlantic forcing becomes important on decadal and longer timescales, in which Pacific SST variability is smaller.
By establishing a chain of attribution linking warming of the tropical and north Atlantic with trends in Antarctic atmospheric circulation and sea ice, Li and colleagues' work helps to resolve the paradox of growing Antarctic sea-ice extent over a period when global mean temperature has increased. The researchers have also demonstrated that global climate models can simulate the connection between Atlantic SSTs and Antarctic winds. Why, then, have climate models such as those used in last year's fifth assessment report by the Intergovernmental Panel on Climate Change been unable to reproduce the observed regional pattern of change in Antarctic sea ice8?
Two reasons suggest themselves. First, the recent warming of the Atlantic is the result of a combination of anthropogenic forcing and natural internal variability of the climate system7. Only the effects of the former can be predicted in a deterministic way by climate models, with natural variability appearing as 'noise' in the climate-model simulations. Second, sea ice is one of the most challenging elements of the Earth system to model. The rate at which it forms or melts is controlled by the small difference between large fluxes of heat from the atmosphere and the ocean, and its distribution is strongly influenced by winds and ocean currents. Small biases in the models' representation of the atmosphere or ocean can thus translate into large errors in modelled sea ice.
Although accurate modelling of Antarctic sea-ice trends will require a realistic representation of the processes connecting Atlantic SSTs and Antarctic winds, this might not be sufficient. Given the importance of Antarctic sea ice to the Southern Ocean marine ecosystem, and its role in driving global ocean circulation by the production of ocean bottom water, understanding its behaviour and improving its representation in climate models must remain a high priority for climate scientists.
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The Southern Ocean ecosystem under multiple climate change stresses - an integrated circumpolar assessment
Global Change Biology (2015)