The elusive sea ice edge
Last winter an almost forgotten sight presented itself to all those venturing the Barents Sea: sea ice as far south as Bjørnøya, equaling a sea-ice extent not seen since the eighties and nineties. Are you wondering how this is possible in times of global warming and a diminishing Arctic ice cap?
The answer lies in the strength of the North Atlantic current that transports large amounts of warm water from the south into the Barents Sea. The heat from these water masses melts sea ice drifting into the Barents Sea from the north, and prohibits local ice formation. Hence, years of strong Atlantic water inflow result in a largely ice-free Barents Sea, and vice versa. In a recent study, Nansen Legacy researcher Marius Årthun and colleagues from UiB studied in detail how natural variations in the water temperature and the strength of the North Atlantic current affect the size of the future ice cover in the Barents Sea. The study shows that in a warming world, years of sea ice expanses as seen this winter are becoming increasingly unlikely, although not entirely impossible. Importantly, it is possible to predict these natural variations in sea ice extent based on the strength of the Atlantic water inflow. Using this method, Årthun and colleagues were already in 2015 able to foresee sea ice pilling up in the Barents Sea this winter. The reason: A weaker and colder Atlantic current in recent years.
“Our results show that in the future it is still possible to have ten-year periods with increasing ice cover due to changes in the Atlantic flow.”
The variability in sea ice extent affects human activities. Short and long-term forecasting of the ice edge position is therefore required for safe operations in the Arctic. Seasonal sea ice forecasting is still in its infancy, but different algorithms already produce seasonal sea ice edge forecasts. Good methods to verify and improve the forecasting skills are still needed. Forecast evaluation requires accurate long-term satellite observations of various sea ice characteristics, such as ice concentration, thickness and ice edge position. In a recent study, Cyril Palerme and Nansen Legacy researchers at MET compared historical 25-member ensemble ice edge forecast to satellite ice observations. The comparison showed that different methods for verification for probabilities of meeting ice covered waters have different strengths and weaknesses, and choice of method will depend on the purpose of the seasonal forecast.
In addition to seasonal forecast, ship navigation in the Arctic requires shorter-term ice forecasts of high quality. For this purpose, there are many monitoring and forecast products available. Information on the expected accuracy of data or model results from these are traditionally available as a set of metrics that comes with the products. However, when comparing ice edge results from different products, some factors - such as large open water areas within the ice or local freezing along continents - can have a large impact on the quality assessment of the forecasts. To address this, Nansen Legacy researchers Arne Melsom and colleagues at MET examined a large number of metrics used, and concluded with a recommended best practice for the validation of sea ice edge forecasts.
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