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.
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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