Spatio-Temporal Variations of Blue Slush and Water Flow in the Percolation Zone of Greenland: the Role of Local Topography

The presence of thick ice layers in firn (so-called ice slabs) has the potential to increase the contribution to sea-level rise of the Greenland ice sheet. These impermeable ice layers prevent water percolation in the firn, leading to more efficient runoff by favoring lateral movement of water on top of the ice slabs. Here we use optical images from the Sentinel-2 satellites to track the seasonal and interannual evolution of snow fully saturated with water to the surface (blue slush) in southwest Greenland. Furthermore, we use a high resolution digital elevation model to assess the role of local topography on the formation of ice slabs and on lateral movement of water. 

We find that blue slush can reach elevations up to 1900 m a.s.l. in years with above average melt with maxima in August. Blue slush appears preferentially in areas where the surface slope approaches 0°, which is also where the ice slabs are thicker. The propagation of blue slush to lower elevation following local slope indicates water movement on top of the impermeable layer. Thus, we suggest that the process of formation of thick ice slabs is a self-sustaining positive feedback system.