Superimposed ice formation reduces meltwater runoff from ice slab areas of the Greenland Ice Sheet 

Abstract

Refreezing is an important component of the Greenland Ice Sheet's surface mass balance. At higher elevations of the ice sheet which are underlain by porous firn, meltwater percolates into the firn pore space where it refreezes in-situ, and therefore does not run off. However, in the last three decades, surface melting has increased at a faster rate than accumulation. In the percolation zone, this has caused anomalous densification of the firn pore space by meltwater percolation and refreezing, leading to metres-thick near-impermeable ice 'slabs' forming and causing the runoff limit to rise. In-situ hydrological observations on ice slabs show that surface meltwater percolates through the seasonal snowpack to flow laterally at metres an hour through a slush matrix atop the ice slab. A saturated slush matrix on top of a cold ice slab therefore has the potential to accrete superimposed ice onto the slab surface. We present the first measurements of superimposed ice formation (SIF) on top of ice slabs in the vicinity of the runoff limit on the K-Transect, south-west Greenland and quantify the impact of SIF on local surface mass balance and runoff. Next, we use vertical heat flow modelling to calculate the ability of an ice slab to refreeze surface melt during a melt season. With synthetic aperture radar observations, we estimate the contribution of residual stored meltwater to autumn-time refreezing. Finally, we assess the importance of SIF across all regions of the ice sheet underlain by ice slabs. Our findings reveal widespread and substantial refreezing in areas that, at a first glance of recent satellite imagery, can appear to be dominated by runoff. Ice slabs undoubtedly enable surface runoff from higher elevations of the ice sheet. However, their cold content and their shallow surface slopes, promoting water retention, can also enable substantial refreezing. In our field area at the 2022-2023 runoff limit, net refreezing corresponded to roughly 50 % of melt. Ice-sheet-wide, ice slabs enable runoff but are also hotspots of refreezing, retaining around 27 Gt of melt as superimposed ice between 2017 and 2022.