The evolution of isolated cavities and hydraulic connection at the glacier bed. Part 1: steady states and friction laws

Abstract

Abstract. Models of subglacial drainage and of cavity formation generally assume that the glacier bed is pervasively hydraulically connected. A growing body of field observations indicates that this assumption is frequently violated in practice. In this paper, I use an extension of existing models of steady state cavitation to study the formation of hydraulically isolated, uncavitated low-pressure regions of the bed, which would become flooded if they had access to the subglacial drainage system. I also study their natural counterpart, hydraulically isolated cavities that would drain if they had access to the subglacial drainage system. I show that connections to the drainage system are made at two different sets of critical effective pressure, a lower one at which uncavitated low-pressure regions connect to the drainage system, and a higher one at which isolated cavities do the same. I also show that the extent of cavitation, determined by the history of connections made at the bed, has a dominant effect on basal drag while remaining outside the realm of previously employed basal friction laws: Changes in basal effective pressure alone may have a minor effect on basal drag until a connection between a cavity and an uncavitated low-pressure region of the bed is made, at which point a drastic and irreversible drop in drag occurs. These results point to the need to expand basal friction and drainage models to include a description of basal connectivity.

Numerical and physical instability of subglacial water flow

“…The speed of ice flow is in large part due to sliding at the bed (Rignot & Mouginot, 2012;MacGregor et al, 2016;Maier et al, 2019), the rate of which depends strongly on the effective pressure, defined as the difference between the pressure exerted by the overlying ice and the water pressure, N = p i − p w (e.g. Schoof, 2005;Helanow et al, 2021;Schoof, 2023;Warburton et al, 2023). Thus, understanding the future of the Greenland Ice Sheet requires an understanding of the way subglacial water pressure evolves in time, over a melt-season and over several decades (Nienow et al, 2017;Aschwanden et al, 2019).…”