Warm winds cause ice-shelf melt

Any skier will know that strong winds over a mountain range usually bring warm, balmy weather to its leeward side, causing rapid snowmelt and generally deteriorating skiing conditions. We find that this föhn effect plays an important role in the melting of ice shelves in Antarctica. Moist air is first pushed up onto the mountain, where moisture is lost through snowfall. The dried-out air heats up as it descends along the leeward slope of the mountain. On the Larsen C Ice Shelf (Antarctic Peninsula), föhn winds create a remarkable pattern of meltwater lakes on the glacier inlets just downslope of the Antarctic Peninsula mountain range. These meltwater lakes could make the ice shelf unstable, by leaking into and opening up cracks and fissures within the ice shelf.

Length of the melt season. Red = long, green = short. Note the long melt season in the inlets of Larsen C.

Detecting melt and lakes

Making ground observations in Antarctica is a risky and costly affair. Enter the power of satellites. Between 2006 and 2012, the European satellite Envisat carried ASAR, an advanced radar device capable of generating extremely detailed maps (150x150 m resolution) of melt onset, duration, and termination on ice shelves in Antarctica. For the first time, my co-worker Adrian Luckman uses these data to show that on the Larsen C Ice Shelf, there is a fascinating pattern of melt intensity. Of course, more melt occurs in the north, where temperatures are higher. But very prominently, the longest melt seasons are found in the inlets just east of the Antarctic Peninsula mountains. Satellite pictures from the MODIS and LandSat satellites confirm that these inlets are riddled with large, elongated lakes of standing water on the ice.

MODIS satellite images of melt lakes (in black) over a period of 3 weeks in January 2007.

Modelling the föhn

Föhn winds are almost always accompanied by clear and sunny skies, increasing the amount of available melt energy. Apart from that, my colleague Andy Elvidge used the detailed MetOffice MetUM model to demonstrate that temperatures and wind speeds are highest in the inlets during föhn events. The constant flow of warm air over the cold snow surface causes a significant extra amount of melt. This confirms what I found before by analyzing weather station data from the Larsen C.

Wind speed (left) and temperature (right) during a föhn event.

Implications

In earlier research, I suggested that catastrophic collapse of ice shelves could be preconditioned by the loss of snow. Standing water at the surface could provide sufficient pressure to open up crevasses in the ice shelf. It would be interesting to investigate further if the meltwater lakes that we found in the inlets of the Larsen C could play a role in its future destabilization.

Read more

This research was published in November 2014 in Antarctic Research: Luckman, A., A. Elvidge, D. Jansen, B. Kulessa, P. Kuipers Munneke, J. King and N. E. Barrand. 2014. Surface melt and ponding of Larsen C Ice Shelf and the impact of foehn winds. Antarct. Sci., 26(6), 625-635. . (Open Access)