Dynamics of Greenland outlet glaciers with strong ocean interaction: studies of Petermann and Jakobshavn Glaciers from an observational and numerical modeling approach

The Greenland Ice Sheet is of great scientific interest for the role it plays in the global climate system, including its contribution to global mean sea-level changes due to mass exchange with the world ocean. Two outlet glaciers, regions where the interior ice sheet drains into the surrounding ocean, of particular interest are Petermann Glacier and Jakobshavn Glacier, which both terminate in the ocean and are profoundly influence by the ocean. We present results of numerical modeling of the coupled ice and ocean system that reproduce several of the salient features of Petermann Glacier, including the pronounced basal channels that are observed along the underside of the floating ice shelf at the glacier's terminus. The model shows that the large melt rates caused by oceanic heat transfer are organized spatially in such a way that, in combination with the motion of the glacier, deep channels are carved into the glacier's base. We thus present new results regarding the coupled interplay between ice shelf dynamics and morphology and ocean circulation underneath an ice shelf. Sensitivity experiments show that the amplitude of these basal channels vary with the width of the initial perturbations that lead to channels and also with ocean temperature. The coupled evolution is also strongly influence by the details of how vertical mixing occurs between the upper mixed-layer of the ocean and the deeper waters, which are the main reservoir of heat for melting. As a second case study, we present ocean observations collected at Jakobshavn Glacier in 2009, 2010 and 2011. These observations confirm the presence of warm water of North Atlantic origin in the deep pro-glacial fjord at Jakobshavn. The data show that the deep fjord water experiences complete renewal on a sub-annual timescale. To explain this rapid exchange we present results of numerical simulations using MITgcm in a two-dimensional non-rotating and non-hydrostatic configuration. These experiments indicate that the primary driver of the rapid deep-water renewal is a buoyancy-driven overturning circulation stimulated by melting at the glacier's marine terminus and especially by the discharge of freshwater from underneath the glacier into the fjord.