Reanalysis of the surface mass balance of Mittivakkat Gletsjer (Southeast Greenland): Synthesizing data sources

The contribution of Arctic glaciers and ice caps (GICs) to sea level rise in the last decades was similar to that of the Greenland Ice Sheet, however, their mass loss per unit area was larger. Between 2006 and 2015, mass changes were largest for GICs in Greenland when compared to other regions in the Arctic. Mittivakkat Gletsjer (Southeast Greenland) has the longest surface mass balance (SMB) record from field-based observations (since 1995/1996) for peripheral Greenland and is significantly out of balance with the current climate. Synthesizing ablation stake records (glaciological SMB), 1 km-downscaled RACMO 2.3p2 SMB output (modelled SMB) and volume changes from photogrammetrically-derived digital elevation models (geodetic MB) indicate a change from an almost balanced state for 1959-1995 to a negative mass balance regime for 1996-2022. RACMO is a regional atmospheric climate model forced by meteorological (reanalysis) data and estimates SMB from multi-layer snow cover simulations and albedo scheme. The model output shows SMB to be between 0.10 ± 0.14 m w.e. yr-1 and -0.56 ± 0.14 m w.e. yr-1 for the periods 1959-1995 and 1996-2022, respectively. The modelled SMB for the latter period is contrasted by the glaciological SMB of -1.06 ± 0.16 m w.e. yr-1 for 1996-2022. The model output allows for assessing monthly and elevation-dependent changes in SMB between 1959-1995 and 1996-2022. Most months experienced a reduction in specific SMB with highest decreases in Jul (-0.19 m w.e.), Jun (-0.15 m w.e.) and Aug (-0.14 m w.e.), but Apr and Dec experienced no change (0.00 m w.e.) or an increase (0.10 m w.e.), respectively. The equilibrium line altitude increased from 600-650 to 800-850 m a.s.l., while there was a SMB decrease at each of the 11 altitude sections between 300-950 m a.s.l ranging from -0.59 to -0.70 m w.e yr-1. The modeled SMB correlates well with the glaciological SMB (R2 = 0.74; p < 0.01) but underestimates the glacier-wide mass loss by 47 % in the overlapping period.   The geodetic MB yields estimates of -0.73 ± 0.20 m w.e. yr-1 for 1981-2013 (modelled SMB: -0.33 ± 0.14 m w.e. yr-1) and -1.41 ± 0.76 m w.e. yr-1 for 2014-2021 (modelled SMB: -0.59 ± 0.14 m w.e. yr-1; glaciological SMB:  -1.15 ± 0.17 m w.e. yr-1).  These differences highlight the challenges of synthesizing results of different mass balance methods such as spatial coverage, density assumptions, data quality, scaling and spatial extrapolation. We ran different configurations for the geodetic and modelled SMB outputs with varying agreement. The change to a more negative regime in the mid-1990s is discussed in the context of climate indices and are in line with modelled and ablation stake SMBs being negative in 24 out of 27 years between 1996 and 2022. The three years with a slightly positive balance can be associated with unusually high winter precipitation.