GPS data reveal the evolution of liquid water retention within the Greenland Ice Sheet at the seasonal and inter-annual time scale

The Greenland ice sheet (GrIS) is currently losing mass, as a result of complex mechanisms of ice-climate interaction that need to be understood for reliable projections of future sea level rise. Previous authors have intensively investigated the total GrIS mass balance, Surface Mass Balance (SMB), and ice discharge at different temporal scales. However, knowledge of the remaining mass variations due to supra-, en- and sub-glacier meltwater retention, is limited. In this study, we make a first attempt to quantify the temporal pattern of meltwater retention at different locations within the GrIS by analyzing the bedrock’s elastic response caused by meltwater mass variations. To that end, we use vertical displacements observed by Global Positioning System (GPS) stations. We estimate, for the first time, the evolution of meltwater storage variations at both the seasonal and inter-annual time scale. We find, among others, that the annual cycle of vertical displacements, after subtracting the signals related to SMB and other known mass transport processes, demonstrates at many GPS stations a consistent subsidence from May to July. This is an indication of a mass gain, which starts in May and peaks in July. We infer that this mass gain signal is due to the meltwater accumulation within the ice sheet. An in-depth investigation of this process by Geodetic data is critical for better understanding the hydrological cycle and the further evolution of the GrIS.