A thin ice cover, a strong summer cyclone, and the record minimum Arctic sea ice extent in 2012

The Arctic sea ice cover has been subject to years of shrinking and thinning in a warming environment. On 26 August 2012 Arctic sea ice extent broke the previous record low of 4.17×106 km2 set on 18 September 2007, according to the National Snow and Ice Data Center (NSIDC). Remarkably, the record was broken in the middle of the melt season, more than three weeks ahead of the usual time when the Arctic sea ice extent reaches a minimum. By 16 September 2012 the NSIDC reported that the Arctic sea ice extent had dropped to its minimum for the year of 3.41×106 km2, a new record low. The 26 August 2012 record followed the passage on 6–8 August of a large cyclone over most of the Pacific sector of the Arctic Ocean. The storm was unprecedented in extent, intensity, and depth (Simmonds and Rudeva, 2012). To quntify the role of the cyclone in the record minimum Arctic sea ice extent in 2012, a numerical investigation was conducted using the Pan-arctic Ice-Ocean Modeling and Assimilation System (PIOMAS). Model results show that the Arctic sea ice cover was already thin before the cyclone passage, with the simulated Arctic ice volume ~40% below the 2007–2011 mean. The thin sea ice cover and the presence of ocean heat in the near surface temperature maximum (NSTM) layer created conditions that made the ice particularly vulnerable to storms. During the storm, ice volume decreased about twice as fast as usual, owing largely to a quadrupling in bottom melt caused by increased upward ocean heat transport. This increased ocean heat flux was due to enhanced mixing in the oceanic boundary layer, driven by strong winds and rapid ice movement. The mixing transferred heat from the NSTM layer to the surface mixed layer, where it was available to melt ice. The cyclone-enhanced bottom melt reduces ice extent significantly for about two weeks, with a declining effect afterwards. Because of the declining effect, the simulated Arctic sea ice extent minimum in September 2012 is reduced by the cyclone, but only by 0.15×106 km2 (4.4%) compared to a simulation without the storm. Thus, even without the August cyclone, a new record minimum ice extent in 2012 would likely still have occurred.