For a White Christmas, head to Greenland
After an unusually warm Arctic this year, large amounts of snow have fallen on the Greenland ice sheet.
Keen observers of the Polar Portal data products will notice an unusually high accumulated surface mass balance in Greenland right now as shown here. This is in large part due to a series of big storms with high amounts of snowfall in October, particularly on the eastern coast of Greenland. These storms included the remnant of tropical hurricane Nicole and led to piteraq events (a cold katabatic wind sweeping down from the ice sheet associated with hurricane force winds) in both eastern and, more unusually, north-west Greenland.
The weather plots from the Polar Portal show this anomalously high precipitation in October 2016, though November and December so far have been closer to the average.
So what does this mean for the health of the ice sheet? Climate scientist at DMI, Peter Langen said “This year’s accumulation season appears so far to be quite spectacular with large amounts of winter accumulation. It will be interesting to see how this continues and what the total winter accumulation will be” He continues, “but by far the largest differences from year to year in the total annual SMB are determined by the summer melt. You can easily have a high winter accumulation that can in just the three months of the melt season be more than overcome if the summer is warm. The opposite can also happen, but in the end it is typically the summer that ends up determining if it is a big or a small SMB year.”
So why is this year’s early accumulation season so intense? We know that increased accumulation on Greenland (particularly in the Southeast) is a pretty consistent response in a wide range of models to a warming climate. But whether this year’s high accumulation may be attributable to the warm Arctic as it has been observed this year is more uncertain. Weather really plays a central role and attribution of individual events is always tricky. There is some evidence that reducing sea ice in the Arctic makes it much more likely that warm periods associated with storms can penetrate further north more frequently but this remains a very active area of research at all of the Polar Portal partner institutions.
Here on the Polar portal we show both the surface mass balance and the total mass balance, which is measured by the GRACE satellite. What is the difference between these two?
The graph here shows the GRACE-derived curve for total mass balance (taking into account all the contributions). It clearly shows a downward trend. Every winter the ice sheet gains more mass and every summer it loses some. The loss in the summer by both melt and calving dominates over all the other processes and the net effect is an average annual loss of about 250 Gt from Greenland over the last decade.
The surface mass balance considers only the processes at the surface: snowfall, rainfall, sublimation, deposition, melting along with percolation, refreezing and runoff of meltwater. It does not take into account the loss due to iceberg calving and direct submarine melting of marine terminating glaciers. It is literally only half of the story. The total mass balance, observed with the GRACE satellites, includes all of these processes.
A healthy glacier which does not have a net mass loss or gain should have a positive accumulated SMB at the end of the year to balance the losses due to melt at the margins. If that positive SMB becomes smaller – not necessarily negative, just a smaller positive – it can no longer also balance the calving and submarine melt losses, and the total mass balance becomes negative. That is what has happened in Greenland over the last roughly 15 years.
Polar Portal is collaboration between DMI, GEUS, DTU-Space and DTU-Byg with funding from the Danish Energy Agency (Energistyrelsen).
Glædelig Jul og Godt Nytår!