Sea ice loss in association with Arctic cyclones

Very rapid sea ice loss events have recently increased during the summer. These events occur in regions of enhanced atmospheric pressure gradients where sea ice is relatively thin and more susceptible to forcings from ocean waves and atmospheric winds, and are preceded by tropopause polar vortices.
Published in Earth & Environment
Sea ice loss in association with Arctic cyclones
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In August of 2012, a powerful atmospheric cyclone developed over the Arctic Ocean, lasting for nearly two weeks.  In just a matter of days, around 500,000 square kilometers of sea ice vanished (or about 25% of the total seasonal sea ice loss), setting the stage for the lowest sea ice extent on record by the end of the summer.  For many researchers, this rang the alarm bell to the possibility that the Arctic Ocean could be ice-free even sooner than models predicted.   But how could a single cyclone lead to such a large amount of ice loss, and why was this just starting now?  Was there a feedback process between the atmosphere and sea ice that numerical models were missing?

 Our research team soon began examining past events of Arctic cyclones and noticed that the 2012 cyclone was not an isolated event and that there had indeed been numerous other cases of very rapid sea ice loss events associated with Arctic cyclones in the past. For example, Figure 1 shows a case of an event in August 2006 with an Arctic cyclone in close association with a tropopause polar vortex (TPV) and reduced sea ice near the same locations.  However, none of the previous sea ice loss events had occurred over as widespread of areas as observed in 2012.  What had changed?  Furthermore, not every cyclone was associated with rapid sea ice loss, and we soon learned that, in contrast, most cyclones did not lead to sea ice loss.  In fact, some cyclones could even lead to net gains in sea ice.  This motivated our team to seek out whether there could be a pattern to determining which cyclones could produce conditions for rapid sea ice loss.

 

To better understand, we went back through satellite records of sea ice and combined these data with state-of-the-art high-resolution atmospheric data.  However, we needed to filter out the already slower changes in sea ice that already have well-established connections to the atmosphere, such as changes from the Arctic Oscillation and other typical seasonal and intraseasonal oscillations.  To do this, we isolated sea ice variability that occurred at weekly time scales or less.  When we put this together with the atmospheric data, it soon became apparent that major sea ice loss events only occur when the cyclones moved over or near relatively thin and typically first-year sea ice.  This was consistent with how these events tended to happen over more widespread regions of the Arctic during the summer season and why these events weren’t as noticeable in the past when there was thicker, older sea ice.  But still, most summer cyclones didn’t necessarily lead to significant sea ice loss events.

Our research uncovered that that not only must a cyclone need to move over thinner ice but that the pressure gradient around the cyclone must be strong enough to cause surface winds with enough force to both move ice and produce large ocean waves.  In other words, if the cyclone was adjacent to a particularly strong high pressure, then certain situations could produce exceptionally strong pressure gradients.  This is particularly common on the Pacific sector of the Arctic Ocean in and around the Beaufort Sea because there is a climatological high pressure in this region (Figure 2).  Arctic cyclones that formed near the central and western Siberian coast and moved toward the Beaufort Sea were the cyclones most likely to produce these exceptionally strong pressure gradients. 

Questions remain on the exact physical mechanisms leading to such a quick loss of sea ice.  In-situ observations in these events are extremely rare, given the dangerous sea conditions during these events.  One hypothesis is that large ocean waves from storms can penetrate thin sea ice and accelerate its breakup.  Another is that the wind energy input from the cyclones induces upwelling, which quickly brings subsurface warmer water to the surface to melt the ice.  Another important factor that has been noted is that if there is high pressure present over the Arctic Ocean early in the summer, this pattern allows plenty of sunlight to reach the surface to help the ice become thin enough so that Arctic cyclones can eventually have an impact. 

 This research has helped uncover a positive feedback that can contribute to enhanced sea ice loss at very short time scales.  In order to accurately predict the conditions favorable for rapid sea ice loss events or to accurately project when exactly the Arctic Ocean may become ice-free, numerical models will need to be able to resolve the atmospheric pressure gradients and processes that lead to the rapid breakup of sea ice.  Regarding the atmosphere, this research uncovered that tropopause polar vortices, or TPVs, are a critical precursor to the formation of the Arctic cyclones that lead to rapid sea ice loss events.  While Arctic cyclones themselves can generally be predicted by up to one week in advance, the TPVs that produce Arctic Cyclones can be present for months beforehand.  This helps provide researchers clues to focus future research studies for making longer-term predictions of rapid sea ice loss events.

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