Arctic Sea Ice Cover and Winter Weather
Thursday, February 2, 2012 4:30:18 PM
The albedo effect: Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth's surface. Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space. Water is much more absorbent and less reflective. So, if there is a lot of water, more solar radiation is absorbed by the ocean than when snow or ice dominates.
The lid effect: Ice cover prevents release into the atmosphere of heat stored in the ocean.
Such effects make Arctic sea ice an important component in the global climate system.
A study recently published in the scientific journal Tellus A shows that the probability of cold winters with much snow in Central Europe rises when the Arctic is covered by less sea ice in summer. A shrinking summertime sea ice cover changes the air pressure zones in the Arctic atmosphere and impacts the European winter weather.
Retreat of the light ice surface reveals the darker ocean, causing it to warm up more in summer from the solar radiation (ice-albedo feedback). The diminished ice cover can no longer prevent the heat stored in the ocean being released into the atmosphere (lid effect). As a result of the decreased sea ice cover the air is warmed more greatly than it used to be particularly in autumn and winter because during this period the ocean is warmer than the atmosphere. The warming of the air near to the ground leads to rising movements and the atmosphere becomes less stable. Winds are driven by air pressure difference between the Arctic and mid-latitudes: the so-called Arctic oscillation with the Azores highs and Iceland lows. If this difference is high, a strong westerly wind will result which in winter carries warm and humid Atlantic air masses right down to Europe (cf. the severe January storms we had in North Western Europe last month (January 2012) - January on average in fact being the most stormy in this region). If the wind does not come, cold Arctic air can penetrate down through to Europe, as was the case in the winters of 2009/2010 and 2010/2011. Model calculations show that the air pressure difference with decreased sea ice cover in the Arctic summer is weakened in the following winter, enabling Arctic cold to push down to mid-latitudes.
It must be pointed out that other factors of course also play a role. The sea ice-atmosphere relationship suggests however a potential for use in operational Northern Hemisphere seasonal forecasts.
Jaiser et al.
Impact of sea ice cover changes on the Northern Hemisphere atmospheric winter circulation
Tellus A 2012, 64, 11595
(NB: published under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported License -- and thus not behind a pay-wall).