Contrasted surface mass balances of debris-free glaciers observed between the southern and the inner parts of the Everest region (2007-15)

Three debris-free glaciers with strongly differing annual glaciological glacier-wide mass balances (MBs) are monitored in the Everest region (central Himalaya, Nepal). The mass budget of Mera Glacier (5.1 km 2 in 2012), located in the southern part of this region, was balanced during 2007–15, whereas Pokalde (0.1 km 2 in 2011) and West Changri Nup glaciers (0.9 km 2 in 2013), ~30 km further north, have been losing mass rapidly with annual glacier-wide MBs of −0.69 ± 0.28 m w.e. a −1 (2009–15) and −1.24 ± 0.27 m w.e. a −1 (2010–15), respectively. An analysis of high-elevation meteorological variables reveals that these glaciers are sensitive to precipitation, and to occasional severe cyclonic storms originating from the Bay of Bengal. We observe a negative horizontal gradient of annual precipitation in south-to-north direction across the range (≤−21 mm km −1 , i.e. −2% km −1 ). This contrasted mass-balance pattern over rather short distances is related (i) to the low maximum elevation of Pokalde and West Changri Nup glaciers, resulting in years where their accumulation area ratio is reduced to zero and (ii) to a steeper vertical gradient of MB for glaciers located in the inner arid part of the range.