Melt record of the Antarctic Ice sheet during summer 2019-2020
While the summer of 2019 had already led to a melting record of the Greenland Ice sheet, the 2019-2020 summer in the southern hemisphere continued the same trend with an unprecedented surface melting record over the Antarctic continent. The surface melt over this 2019-2020 summer was about twice as high as the 1981-2010 average.
The surface melting of the Antarctic ice sheet has been slightly decreasing since 1980 while it has accelerated over Greenland since the end of the 1990's. The Antarctic trend, partly explained by stronger westerlies protecting the Antarctic ice sheet from warm air intrusions, is then in opposite to the Global Warming induced changes. This does not mean that the Antarctic continent was not suffering the consequences of climate change as the ice sheet had already melt strongly in contact with a warmer ocean (basal melting), which notably contributes to the collapse of large ice shelves surrounding the continent (such as Larsen A and B in the Antarctic Peninsula).
This is in any case the first time that the surface is melting so strongly over the satellite area (1979-2020) and on such a large scale. Indeed, the surface melt was about 300 billion tons, corresponding twice as high as the average of the baseline 1980-2010. “This summer marks a real break in the time series. This is not just a noise in the time series but something likely much bigger" says Christoph KITTEL, PhD student at the ULiège (UR SPHERES) studying the Antarctic ice sheet. He extends his point : “Such warm air intrusions already occurred in the past but without causing so much melting because the air was not so warm than today around Antarctica“.
Summer (DJF) meltwater production (in red) as well as the amount of that surface meltwater that runs off to the ocean (in blue), as simulated by the climate model MAR forced by the reanalysis ERA5, for each year (value for 1979 are for December 1979 and January and February 1980, similar 2019 is for December last year and January + February this year). Units are GT/3 months.
Anomaly of the number of melt days in summer 2019-2020 as simulated by MAR forced by ERA5.
Researchers of the Climatology laboratory used the regional climate model MAR they are developing at ULiège to reconstruct the climate over both ice sheet in order to analyze the variability of temperature, snowfall and surface melting of the ice sheet. They found that the summer of 2019-2020 had been abnormally warm over all of Antarctica. Nearly every day was warmer than normal even inducing record-breaking temperatures on the Peninsula These same temperatures led to a continuous increased melting with some very strong events.
The researchers of the Climatology laboratory are now trying to understand why the temperatures were so high. The first leads by Xavier FETTWEIS (heading the Climatology Laboratory and the PhD supervisor of Christoph KITTEL) reveal that, as it was the case over Greenland last summer, this summer was dominated by more anticyclonic conditions than normal, as a results of a weaker polar vortex than usual, allowing advection of warm air masses towards the ice sheet from tropical mid-latitude. This anomaly is particularly strong over Peninsula, where the largest melt anomaly has been suggested by MAR and where anticyclonic blocking events (as the ones observed over Greenland this 2019 summer) have dominated through the whole summer. Moreover, the sea surface temperature around Antarctica has been higher than normal, associated to an extension of the ice pack which was less this summer than in previous summers. Weaker westerlies, usually isolating Antarctica from warm air intrusions, combined to a warmer ocean around Antarctica mainly explain these anomalies of surface melt simulated by MAR over this 2019-2020 summer.
Anomaly of sea level pressure in summer 2019-2020 from ERA5 reanalysis. Units are hPa.
Although it is still too early to claim that global warming could be the main cause of this record the repetition of years with more and more melting is what is projected for the Antarctic ice sheet as well as over Greenland. “This is a matter of concern as Antarctica is by far the largest reservoir of water in the form of ice and therefore a potential contributor to sea-level rise“ concludes Christoph KITTEL, adding that although snowfall will also increase in the near future and compensate for the loss of surface ice, this melting could accelerate the flow of ice to the ocean and destabilize the ice sheet. It is nevertheless important to note that the current surface mass loss from meltwater runoff, even if a record has been reached this summer, remains negligible compared to the mass gain by snowfall.
Contacts :
Christoph KITTEL
Xavier FETTWEIS
