Hurricanes do not only occur in tropical and subtropical latitudes. Scientists also observe them in the Arctic, where they have been increasing in frequency and intensity over the past 50 years. These strong storms are always accompanied by an exceptionally large loss of sea ice. However, the reasons for this were not fully understood until now. Scientists aboard the South Korean research icebreaker Araon involuntarily witnessed a massive storm comparable to a Category 2 hurricane in August 2016 and used the opportunity to collect important data.
The ten-day storm certainly cost the researchers and crew some nerves, but for science it was a stroke of luck that the Araon was trapped in Arctic sea ice. For the first time ever, a research team was able to make “in situ” observations from inside the hurricane. Based on their measurements of air and ocean temperature, radiation, wind and ocean currents, the researchers from the University of Alaska Fairbanks and their international colleagues were able to disprove in a recently published study that sea ice melts solely from above due to atmospheric processes.
The study showed that sea ice receded 5.7 times faster than normal during the storm and that the rapid, storm-related sea ice loss was driven primarily by two physical processes in the ocean:
The surface water is transported away from the cyclone by the strong gusting winds. This results in the upwelling of warmer water from the depths, which thus reaches the surface. Nevertheless, a small layer of cold water remains directly beneath the sea ice. At the same time, the strong winds mix the surface water like a blender.
This interaction of upwelling of warmer water and surface mixing causes the entire upper ocean layer to warm, melting the sea ice from below.
According to Xiangdong Zhang, a scientist at the International Arctic Research Center at the University of Alaska Fairbanks and lead author of the study, there are also lasting consequences beyond the storm event: “It’s not just the storm itself. It has lasting impacts because of the enhanced ice-albedo feedback.”
Such strong storms create large open water areas that absorb more heat because of their dark surface, which melts more sea ice and creates even more open water. From August 13-22, 2016, the sea ice area in the entire Arctic Ocean decreased by 595,700 square kilometers, an area just over one and a half times the size of Germany and the third lowest sea ice extent ever recorded in the Arctic.
A new computer model that Xiangdong Zhang is working on for the U.S. Department of Energy will help assess whether climate change will increase Arctic cyclones. The largest Arctic cyclone, recorded in 2012, led to a record low in sea ice extent.
Julia Hager, PolarJournal
