Montag, 15. Dezember 2014

More ice in a warming world

     Antarctic sea-ice coverage has increased over the past few decades. A new study explains why this is the case, why models do not capture the increase and what humans might have to do with the expanding ice cover.

In September 2014, Antarctic sea ice covered more than 20 million square kilometers for the first time since the beginning of continuous satellite measurements in 1979. This maximum extent continues an overall increase of Antarctic sea ice that has puzzled scientists and the general public alike, in particular given the ongoing overall warming of our planet that simultaneously causes Arctic sea ice to retreat rapidly.

In a new study published this week in Geophysical Research Letters, scientists from the Max Planck Institute for Meteorology (MPI-M) in Germany explain the underlying cause of the long-term Antarctic sea-ice increase and analyze why climate models fail to reproduce it. „There is primarily one thing you need to get right if you want to model Antarctic sea ice: The wind pattern“, explains Alexander Haumann, lead author of the new study. „We are primarily talking about regions with winter-time temperatures far below freezing here, and even with the ongoing warming, there is still a lot of ice growth in the ocean around Antarctica. Forced by strong winds, this sea ice is sometimes transported over more than a thousand kilometers. So if you want to know where it goes and which area it covers, you need to understand how it's being pushed around by the winds. “

And these winds have changed over the past decades. „We see a stronger off-shore component of wind patterns in particular in the Ross Sea, which is the sector of Antarctica that opens towards the Pacific. There, the winds have changed such that they blow the ice further away from the coast where new ice continuously forms“, explains Haumann's colleague Dirk Notz who leads the Sea-ice research group at MPI-M. While this was suspected also by earlier studies, the new study now shows this relationship to play the main role in ice coverage until today. „Where the winds blow more strongly away from the Antarctic continent, the ice cover increases because the ice is further blown to the north and the ocean refreezes in the south. It's as simple as that. “

Two questions then of course remain: Why have the winds changed? And why do models not capture these changes? The new study addresses both these questions. „The changing wind patterns are caused by lower surface pressure in some areas around Antarctica. In our model simulations, we get such lower surface pressure if we include both the effect of the ozone hole and the increase in greenhouse gas concentration. The ozone hole cools the high atmosphere over Antarctica, while the greenhouse gases warm the lower atmosphere. In combination, this can explain a change of the wind pattern, but we’re not sure yet whether this is also the main driver of the changes in the real world.“, says Hauke Schmidt, who leads the Middle and Upper Atmosphere research group at MPI-M.

But even though the model simulates a lowering of the surface pressure, it does not get the ice increase. Also this the scientists now understand: They suspect that the model does not capture the influence of the smaller scale topography around the continent and surface processes over ice and snow accurately enough. These processes influence the surface-pressure distribution and hence the direction of the wind. “In our model, the pressure changes such that the wind primarily blows stronger parallel to the coast line rather than away from it. Once this is better represented in the model, we should get better simulations of the sea-ice trend”, concludes Haumann.

So the scientists have understood why the sea ice increases and why their model does not capture it. Now they work on improving the atmospheric circulation around the continent, trusting that this will eventually allow them to reproduce the observed sea-ice increase in their model. Then the puzzle of Antarctic sea ice would fully be solved.

Original Publication
Haumann, F.A., D. Notz, and H. Schmidt (2014), Anthropogenic influence on recent circulation-driven Antarctic sea-ice changes, Geophys. Res. Lett., doi: 10.1002/2014GL061659

Dr. Annette Kirk Kommunikation
Max-Planck-Institut für Meteorologie

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