The impact of volcanic aerosols on the energy- and mass balance of Langjökull ice cap, SW-Iceland

The mass balance of the Langjökull ice cap in SW-Iceland has been monitored since 1997. Since the summer 2001 two automatic weather stations (AWSs) have been used to estimate the full energy balance of the glacier surface; one near to the terminus the other close to ELA. Following the subglacial eruptions in the Eyjafjallajökull volcano 14 April to 22 May 2010, and Grímsvötn volcano 21 - 28 May 2011, airborne tephra was deposited on the surface of all the major ice caps in Iceland. The mass balance and AWS records for Langjökull provide valuable data to study the impact of aerosol deposition on albedo and radiative forcing. Here we compare the mass- and energy balance of the Langjökull ice cap during the exceptional circumstances of 2010 and 2011 to the more climatically controlled mass- and energy balance of 1997-2009. Dark tephra from the 2010 Eyjafjallajökull eruption, deposited on Langjökull 18-19 May (3-4 days before the eruption stopped), produced a thin layer of fine grained dark trachyandesite tephra particles on the surface (fraction of a mm). This highly reduced the surface albedo and greatly enhanced melting, especially within the accumulation area, where up to three years of accumulation melted. The resulting negative net balance in 2010 was threefold the average during the preceding warm decade. In 2011, basalt tephra dust from the Grímsvötn eruption was deposited on Langjökull (on 11 June, 2 weeks after the end of the eruption, deposited by wind). The quantity of dust was less than in 2010, and the color of the basaltic tephra particles not as dark, hence smaller reduction in albedo. The weather during the summer 2011 was not favorable to ablation on Langjökull; exceptionally cold period from early May to mid June with occasional snowfall, delayed the start of the ablation season by 3 weeks compared to typical years. Thus, despite the lower albedo, causing fast melt rates after 11 June, the total summer ablation was close to the average for the years 1997 to 2009.