Seasonal and annual mass balances of Mera and Pokalde glaciers (Nepal Himalaya) since 2007

In the Everest region, Nepal, ground-based monitoring programmes were started on the debris-free Mera Glacier (27.7 degrees N, 86.9 degrees E; 5.1 km(2), 6420 to 4940 m a.s.l.) in 2007 and on the small Pokalde Glacier (27.9 degrees N, 86.8 degrees E; 0.1 km(2), 5690 to 5430 m a.s.l., similar to 25 km north of Mera Glacier) in 2009. These glaciers lie on the southern flank of the central Himalaya under the direct influence of the Indian monsoon and receive more than 80% of their annual precipitation in summer (June to September). Despite a large inter-annual variability with glacier-wide mass balances ranging from -0.67 +/- 0.28 m w.e. in 2011-2012 (Equilibrium-line altitude (ELA) at similar to 5800 m a.s.l.) to +0.46 +/- 0.28 m w.e. in 2010-2011 (ELA at similar to 5340 m a.s.l.), Mera Glacier has been shrinking at a moderate mass balance rate of -0.08 +/- 0.28 m w.e. yr(-1) since 2007. Ice fluxes measured at two distinct transverse cross sections at similar to 5350 m a.s.l. and similar to 5520 m a.s.l. confirm that the mean state of this glacier over the last one or two decades corresponds to a limited mass loss, in agreement with remotely-sensed region-wide mass balances of the Everest area. Seasonal mass balance measurements show that ablation and accumulation are concomitant in summer which in turn is the key season controlling the annual glacier-wide mass balance. Unexpectedly, ablation occurs at all elevations in winter due to wind erosion and sublimation, with remobilised snow potentially being sublimated in the atmosphere. Between 2009 and 2012, the small Pokalde Glacier lost mass more rapidly than Mera Glacier with respective mean glacier-wide mass balances of -0.72 and -0.23 +/- 0.28 m w.e. yr(-1). Low-elevation glaciers, such as Pokalde Glacier, have been usually preferred for in-situ observations in Nepal and more generally in the Himalayas, which may explain why compilations of ground-based mass balances are biased toward negative values compared with the regional mean under the present-day climate.