The North Atlantic Oscillation and the ice albedo feedback deliver a 1-2-3-4-5 punch, leading to extreme Greenland melting
The North Atlantic Oscillation (NAO) is measured as the atmospheric sea level pressure difference between the sub-tropical high and the low pressure that prevails over the North Atlantic. This difference ‘oscillates’ in time from positive to negative. From late May until ~20 July, 2012, the NAO was negative (the low pressure in the North Atlantic was relatively high and the sub-tropical high may have been relatively low). This summer pattern has persisted since 2007. year 2012 is the 6th year in a row with this anomalous pattern.
Negative summer NAO allowing more north-south heat exchange, northward (warm air) along west Greenland heated the ice at the same time the surface reflectivity (or albedo) was trending low.
NAO index 1 April to ~20 July, 2012 from NOAA's Climate Prediction Center
The negative NAO combined with the ice albedo feedback delivered a 1-2-3-4-5 punch… 1.) the high pressure suppressed cloud formation that could reduce (slightly, its a minor effect) the solar energy reaching the surface; 2.) less cloud development reduces snowfall which can brighten the surface reducing absorbed solar heating; 3.) the “cold content” of the snowpack and ice surface had been reduced from the previous years of warming and in summer the NAO had been negative just like in 2012; 4.) warm south air (and enhanced solar absorption) heated the snowpack and ice surface to the melting point; and 5.) the heating rounded the jagged snow crystal edges, reducing the snow’s reflectivity, allowing more solar absorption, a process that amplifies melting.
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