Start of the melt season

By Peter Langen, DMI/Polar Portal
May 23, 2014

The melt season has started on the Greenland Ice Sheet at the usual time. However, the so-called ablation season, where net mass is lost from the surface of the ice sheet as a whole, has not yet begun.

There is no clear definition of when the melting season starts. Is it when melting first occurs somewhere on the ice sheet? When a certain fraction of the ice sheet area sees melting? Or when net mass starts to be lost from the surface of the ice sheet as a whole?

Playing with different definitions brings out different results. Here, we will use two definitions:

  • Melting season onset: The first day of a period of at least three consecutive days where more than 5% of the ice sheet experiences melting. In our model, we define melting to occur in a place when the melt rate is greater than 1 mm/day.

  • Ablation season onset: The first day of a period of at least three consecutive days where surface mass balance (SMB) is negative and below –1 Gt/day (1 Gt is one billion tons and corresponds to 1 cubic kilometer of water). See this article for a discussion of the difference between SMB and melt.

We have looked into these events in the model run behind the SMB figures on Polar Portal. May 19-21 (illustrated below) was the first stretch with more than 5% melt area for three days in a row. This year, our definition therefore marks May 19 as the melting season onset.

Melt extent in the model used for the SMB figures on Polar Polar on May19, 20 and 21.

Comparing with the summers in the rest of the 1990-2014 period that we have run the model for, this is not particularly early or late. The top-three on the list is

Earliest melting season onset (1990-2014)

1. 1996 Apr 29
2. 2010 May 2
3. 1990 May 6

The year 2012, which eventually became a big melt year with record large mass loss from the ice sheet, is number 10 on the list with May 18. The latest melt onset was last year (2013) with June 5. This year’s onset on May 19 places 2014 at the middle of the list.

Looking instead at the ablation season onset, that is, when we start losing mass from the surface of the ice sheet as a whole, this year’s season has not yet begun. The ablation onset is typically some weeks later than the melt onset, and the top-three (with a tie for second place) looks like this

Earliest ablation season onset (1990-2013)

1. 2010 May 24
2. 2012 May 29
2. 1998 May 29

This year’s accumulated SMB since Sep 1 2013 (in blue) compared to the same in the 2011-2012 season, which had very low accumulated SMB at the end of the season (red). The ablation season onset corresponds to the point where the curve goes from upward to downward. Click on the figure for details.

Both of the years 2010 and 2012 were years with large surface mass loss in summer and low net SMB at the end of the season. The latest ablation onset was in 1999 with June 28. Last year gave a medium net SMB season and the ablation season onset was also medium with June 12. There appears to be some predictive power in the ablation onset: the later the onset, the longer time we will have mass accumulating and the higher the end-of-season net SMB. Ablation seasons can, however, be long and weak or short and intense, so there is no one-to-one correspondence.

At the time of writing, this year’s ablation season has not yet started. In fact, we have not even seen one day with SMB below -1 Gt/day. It will be interesting to follow the development over the next few weeks. The coming week is the time of the top-three on the list.