Sea ice cover peaks in March and is at its minimum in September. Accurate modern data, from a microwave radiometer and a defence satellite, have accurately created a time series of sea ice extent since 1979 (Serreze et al, 2007). The series shows a decline in sea ice every month since this date, on average decreasing 100,000km per year which averages 9.1% per decade (Stroeve et al, 2007). Most notable every year since 2001 the September minima have become increasingly pronounced.
The observed ice loss can be attributed to both thermodynamic and dynamic processes (Serreze et al 2007). Thermodynamic processes include changes in surface air temperature, radiative fluxes and oceanic conditions. While dynamic processes involve changes in ice circulation in response to winds and ocean currents. Such as the Beaufort Gyre and transition of the Polar drift Stream. The seasonal variations of surface air temperature should be noted. Rigor et al (2000) showed positive surface air temperature changes from 1979 to 2000 (year of study) combining data from coastal station observations, drifting buoys and high resolution radiometer satellite imagery. So an overall warming trend has been identified over the discussed period having implications such as extended melt seasons and earlier spring melt. This may be due to positive feedbacks such as the loss of sea ice allows for stronger heat fluxes from the ocean to the atmosphere which in turn promotes a stronger long wave flux to the surface.
The North Atlantic Oscillation (NAO), Northern Annular Mode (NAM) and other atmospheric patterns are associated with ice circulation patterns which have played a large role in sea ice loss over the discussed period (Rigor and Wallace, 2004. Rothrock and Zhang, 2005). Briefly the NAO refers to the covariabiltiy between the strength of Azores high and Icelandic low while NAM refers to the oscillation of atmospheric mass between the Arctic midlatitudes and middle latitudes. Serreze et al (2007) states how these both can largely be viewed as expressions of the same phenomenon. From about 1970 through to the mid 1990s the NAO-NAM indices shifted from negative to strongly positive. This altered surface winds resulting in enhance sea ice transport away from the Alaskan and Siberian coasts. This led to openings in the sea ice cover which would quickly freeze during the winter. However during summer these patches of young, thin ice would quickly melt leading to large reductions in sea ice. The enhanced absorption of solar energy in the open water promotes stronger melt. Furthermore younger thinner ice promoted stronger heat fluxes to the atmosphere resulting in higher air temperatures leading to the onset of earlier melt.
Since the 1990s NAO-NAM indices have regressed back to a neutral state (Overland and Wang, 2005). As ice loss has continued with a declining September minima yet the aforementioned indices were neutral questions of the NAO-NAM influence on sea ice occurred. Rigor and Wallace (2004) reasoned it was the delayed impact of the very strong NAO-NAM state as wind flushed much of the Arctic’s thick ice store into the North Atlantic. While Rothrock and Zhang built upon this using a model to reason wind forcing was a dominant driver of declining ice volume and thickness. However the ice response to rising air temperatures declined with time over the phase thus concluding there would have been a decline in ice extent even without such a phase.
The summer of 2007 if of note as the observed decrease of sea ice resulted in a record minimum of 4.2 million km (squared) breaking the previous record of 4.3million km (squared) in 2005 (Perovich and Menge, 2009). The causes of the extraordinary event involve the simultaneous workings of the aforementioned mechanisms. The preconditioning of the ice played an important role, paving the way for atmospheric and ice-albedo feedback mechanisms. Warm temperatures from the south, increased ice export, reduced cloudiness and increased downwelling radiation (500% positive anomaly (Perovich, 2008) when taken together explain the record minimum.
To summarize, the observed sea ice loss can be connected to arctic warming. The warming is part of a global signal suggesting a link with Green House Gas (GHG) loading. The picture is however blurred by factors such atmospheric and oceanic forcings.
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