Arctic Sea Ice Much More Resilient Than Thought
In recent years the Arctic sea ice has shown great resiliency and is currently at higher levels for this time of year when compared to all but two years going back to 2005.
Sea ice covers about 7% of the Earth’s surface and about 12% of the world’s oceans and forms mainly in the Earth’s polar regions. Specifically, much of the world’s sea ice is found within the Arctic ice pack of the Arctic Ocean in the Northern Hemisphere and the Antarctic ice pack of the Southern Ocean in the Southern Hemisphere. While the Antarctic sea ice extent is currently running at levels very close-to-normal, the Arctic sea ice extent is below-normal and has been running generally at below-normal levels since the middle 1990’s at which time there was a long-term phase shift from cold-to-warm in the North Atlantic Ocean sea surface temperature pattern. In recent years, however, the Arctic sea ice has actually shown great resiliency and is currently at higher levels for this time of year when compared to all but two years going back to 2005.
Atlantic Multidecadal Oscillation (AMO)
The Atlantic Multidecadal Oscillation (AMO) is a climate cycle that affects the sea surface temperature pattern of the North Atlantic Ocean on multidecadal timescales. The northern Atlantic Ocean switched sea surface temperature phases from cold-to-warm back in the middle 1990’s and this shift was directly correlated with the flipping of Arctic sea ice extent from above-normal levels at that time to generally below-normal thereafter until present. The sea surface temperature phase in the northern Atlantic Ocean is tracked by meteorologists with the Atlantic Multidecadal Oscillation (AMO) index and it has generally been in a positive (warm) phase since the middle 1990’s. In those initial years after the AMO phase shift, the Arctic sea ice extent dropped rather steadily. Over the past several years, however, the Arctic sea ice extent has stabilized with much more of a sideways trend as compared with the clear downward trend of the late 1990’s and early 2000’s.
This winter season has been rather favorable for the build-up of ice in the Arctic region with sustained cold and sea ice extent actually currently exceeds all but two years (2008, 2009) going back to 2005 at this stage of the winter season. This winter season has been somewhat typical for the Arctic region with overall temperatures for December and January not far from normal. Part of this can be attributed to the fact that the polar vortex has been stationed over the polar region more often than not this winter season. A polar vortex is a low-pressure system of cold polar air – a normal weather phenomenon – but in some recent winter seasons such as 2013-2014, this feature was occasionally displaced to lower latitudes or even broken up into multiple pieces. The result of this change to the more typical positioning and/or magnitude of the polar vortex in the winter of 2013-2014 was for abnormally cold air to frequently push into North America and, often times in that particular winter, there was less sustained harsh cold for the Arctic region compared to normal. Temperature anomalies for December and January of this winter season of 2019-2020 show overall fairly close-to-normal temperatures over the Arctic region with pockets of above-normal (e.g., near the North Pole) and below-normal (e.g., Alaska, Greenland) whereas the winter season of 2013-2014 featured generally above-normal temperatures throughout the polar region. One thing to note is that even above-normal temperatures during the Arctic winter season will be well below the freezing mark allowing for some buildup of ice in the region. The Arctic sea ice extent will typically reach a yearly peak during the latter part of their winter season which ends in late March.
Sideways trend in Arctic sea ice volume since 2010
In addition to sea ice extent, an important climate indicator to monitor is sea ice volume as it depends on both ice thickness and extent. Arctic sea ice volume cannot currently be observed on a continuous basis as observations from satellites, submarines and field measurements are all limited in space and time. As a result, one of the best ways to estimate sea ice volume is through the usage of numerical models which utilize all available observations.
One such computer model from the University of Washington is called the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS, Zhang and Rothrock, 2003) and it is showing a sideways trend in Arctic sea ice volume since around 2010 which followed a downward trend since the AMO phase shift in the middle 1990’s.