M. Tedesco et al., Year 2011 Greenland melting remains well above the (1979–2010) average; close-to-record mass loss

Year 2011 Greenland melting remains well above the (1979–2010) average; close-to-record mass loss

M. Tedesco¹, X. Fettweis², T. Mote³ , N. Steiner¹ and  J. E. Box⁴

1) City College of New York, NYC, NY, USA

2) University of Liege, Liege, Belgium

3) University of Athens, Athens, Georgia, USA

4) Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA

Summary: Melting in Greenland in 2011 was still above the average (1979–2010 baseline period), exceptionally high over the west coast and reaching close-to-record simulated surface mass balance, bare ice exposure, albedo and runoff anomalies.

The melting index (e.g., the number of melting days times the area subject to melting) in 2011 estimated from spaceborne microwave observations using the approach in (Tedesco, 2007) did not break the previous record set in 2010 (e.g., Tedesco et al., 2011). However, 2011 is positioning itself 6th in terms of melting index, after 2010, 2007, 1998, 2002, 2005. An alternative approach using microwave data as well (Mote & Anderson, 2005) indicates that melt extent for the period June through August 2011 ranked third since 1979, following 2010 and 2007. Satellites data cannot produce estimates of runoff and liquid water content. However, these can be analyzed by means of models. The model used in this analysis (MAR, e.g., Tedesco et al., 2011) indicates that 2011 was comparable to the record season of 2010 with respect to runoff, surface mass balance, albedo and bare ice exposure. Strong negative surface mass balance anomalies occurred in 2011, according to MAR (e.g., the loss in 2011 and 2010 were much higher than the gained mass because of accumulation). Surface albedo simulated by MAR was consistently below or around 2 standard deviations below the mean for the period June–August (e.g., more solar radiation absorbed supporting more melting, see Figure 4 for a diagram). The bare ice area exposed during the summer of 2011 was also large with respect to the mean (close to up 3 standard deviations for the month of July), similarly to what happened in 2010.

Figure 1 Map of the 2011 anomaly for the number of melting days

Figure 2 illustrates the snowfall, runoff and surface mass balance for the period 1958 – 2011 obtained from the MAR model. MAR indicates that 2011 runoff and surface mass balance was comparable to the record setting year 2010, with strong negative surface mass balance anomalies. The loss in 2011 and 2010 was much higher than the mass gain from snow accumulation.

Figure 2. Snowfall, runoff and surface mass balance for the period 1958 - 2011 simulated by the MAR model.

Figure 3 illustrates the 2011 standardized anomalies for near surface air temperature, surface albedo, snowfall, melt water production, bare ice area, and melt area all obtained from MAR for the months of April through August.  Anomalies for the North Atlantic Oscillation (NAO) index are also reported.  The temperatures in 2011 were higher than normal from mid June to mid August, according to a constant negative NAO index during these months. The constant negative NAO index induced anticyclonic and then dry conditions over the ice sheet, allowing to maintain a large bare ice extent through the whole melt season compared to 2010, when two snowfall events reduced temporarily the bare ice extent.

Figure 3. 2011 standardized anomalies for the the 3m temperature, surface albedo, snowfall, meltwater, bare ice area, melt area obtained from MAR for the months of April through August.

According to MAR, 2011 was characterized by an anomalously cold spring. Year 2011 melt onset was relatively late, beginning in June. The melt area for 2011 simulated by MAR is above 1 and 2 standard deviations in June and August, respectively, but within the mean during July. Nevertheless, surface albedo was consistently below or around 2 standard deviations below the mean for the period June–August (e.g., more solar radiation absorbed enhancing melting). The bare ice area exposed during the summer of 2011 was also anomalous with respect to the mean (close to up 3 standard deviations for the month of July), similarly to 2010. Figure 4 illustrates the simulated melt water, runoff, melt extent for the period May through August, and the bare ice extent for the same period simulated by MAR for the years 1958 through 2011. Both runoff and bare ice extent in 2011 are close to the 2010 record.

Figure 4. Feedback mechanisms diagram.

As mentioned above, the exposure of bare ice plays a major role on the increase of runoff and mass loss, either because it promotes enhanced absorption of solar radiation, much more than snow and because ice can melt faster than snow (see Figure 4 for a diagram explaining some of the feedback mechanisms).

How can we explain the differences between the record simulated by MAR and the results from spaceborne microwave data ?

Strong positive melting anomalies occurred in 2010 mainly because melting started earlier and lasted longer than usual. This aspect was captured by spaceborne microwave data because melting anomalies were largely driven by the length of the melting season. Because of this, the surface mass balance and runoff simulated by MAR for 2010 were in agreement with the melting index record derived from spaceborne observations. In 2011, however, melting did not start until late in the season and it did not last as long as in 2010. The exposure of bare ice promoted strong melting with the 2011 season being characterized by relatively short but intense melting. This was not captured by spaceborne microwave data because of their limitation in estimating the amount of liquid water within the snowpack. In summary: the 2010 season was largely driven by a longer season and therefore captured by microwave data, where the 2011 season was characterized by a relatively short but intense melting season, with the albedo feedback mechanism playing a major role (as in 2010) and large bare ice areas subject to melting.

http://greenland2011.cryocity.org/