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Wednesday, May 8, 2013

Jason Box: Greenland “snow drought” makes big 2013 melt more likely

by Jason Box, Melt Factor blog, May 3, 2013

A friend in Greenland’s capital Nuuk reported (with a frown) that the backcountry skiing this year was poor due to a “snow drought.”

Figure 1. Western ice sheet snowfall totals are 30%-70% of normal. Brown areas have less than ‘normal’ precipitation. Blue/purple areas are anomalously ‘wet’. The precipitation anomalies are calculated from ‘re-analyses’ data after Kalnay et al. (1996).

Multiple melt factors combine to increase the odds of more melt water runoff from the ice sheet during the 2013 melt season:
  1. less ‘cold content’ of snow to melt away (ablate) for a given energy input before bare ice is exposed;
  2. a longer period of exposed darker bare ice, in this case weeks earlier bare ice exposure is likely unless a big snow dump before or during the coming warm season;
  3. Less snow leads to a smaller refreezing capacity in the lower accumulation area. Thanks Robert Fausto of GEUS for reminding me of this one.
  4. a possible higher concentration of light absorbing impurities per unit volume of snow, assuming that the impurities are deposited whether or not it snows.
This pattern results from a persistent atmospheric anomaly, blocking cold air transport southward along west Greenland, producing relatively warm temperatures there while northwestern Europe has had a cold winter (Figure 2).

Figure 2. The data after Kalnay et al. (1996) indicate tendencies toward offshore flow over western Greenland, opposite for what is needed to produce normal snowfall. 

The precipitation anomaly is manifesting in abnormally low land and ice sheet reflectivity (albedo) (Figure 3).

Figure 3. April 2013 surface albedo (a.k.a. reflectivity) anomaly. Substantially lower albedo anomalies on land are due to the dearth of snow revealing a much darker underlying tundra. The red areas across the northern 1/3 of Greenland are uncertain due to low solar illumination angles.

Low snowfall anomalies precondition Greenland ice for enhanced melt (Mote, 2003; Box et al. 2005, 2012), especially for the western ice sheet where the snowfall amounts are less than over the east.

From 20 March to 20 April, the snow drought drove ice sheet reflectivity well below values in 13 years of (NASA MODIS sensor) satellite observations since 2000 (Figure 4). Negative North Atlantic Oscillation (NAO) has promoted Greenland heating, melting and snow drought for now 6 summers in a row (Tedesco et al. 2013, Fettweis et al. 2013). Negative late winter NAO packs a similar punch. Negative NAO has prevailed much of the past decade and is largely to blame for Greenland’s astonishing melt increase. Whether negative NAO is promoted by an earlier loss of snow on land and declining Arctic sea ice area is something I’ve been wondering about.

Figure 4. Greenland ice sheet (land excluded) reflectivity or albedo updated after Box et al. (2012).

Then the weather flipped and ice sheet reflectivity rebounded toward normal values in the latest 10 days (Figure 4). Ice sheet reflectivity and accumulated precipitation remains lower than average for the year to date through 1 May (not shown), it therefore remains more likely than not that we’ll see a big melt in 2013.

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