OCEAN TIDAL INFLUENCE ON SUBGLACIAL HYDROLOGY AND ITS EFFECTS ON ICE SLIP

Ice streams and outlet glaciers act as major pathways through which ice sheets discharge to the oceans, and increases in discharge have the potential to thin inland ice dramatically. Because of this potential, the dynamics of these outlets are especially scrutinized. Many ice streams and outlet glaciers have active subglacial hydraulic networks that can modulate sliding at the ice--bed interface. These networks are often sensitive to ocean forcing and, in particular, tides. Ocean tides create a periodic pressure condition at the ice-ocean boundary. The periodicity allows in-depth understanding of the effects of the subglacial system where ice flow is sensitive to water drainage. This sensitivity allows inference of the importance of the subglacial system relative to overall ice discharge in other areas of the ice sheet.

Here, we show how tides affect subglacial till using mathematical and numerical models of subglacial water flow. Tides modulate effective pressure that, under certain circumstances, aids in till failure and slip. The critical parameter for tidal influence on slip is the compressibility of the water system. The compressibility is linked to the amount of water stored along the ice--bed interface. For ice streams, observations show two starkly different tidally-modulated behaviors. In one case, Rutford Ice Stream has a smoothly varying displacement with the tides. Whillans Ice Stream, however, shows discrete stick-slip events. We show how both of these types of behaviors can be related to hydraulic weakening of the slip interface and discuss implications of this phenomenon. Furthermore, we use a multiple stable state drainage model to understand how the drainage system is affected by morphological switches. Finally, we discuss how the ice stream cases relate to Greenland outlet glaciers and highlight the major differences.