Title: Ocean-induced Melting of Greenland Ice Shelves
The Greenland Ice Sheet plays a crucial role in regulating the climate system. It has been experiencing rapid changes during the last two decades caused by enhanced surface melt and ice discharge into the ocean from the termini of Greenland glaciers. Both in-situ and remote sensing observations revealed ongoing acceleration of glaciers and significant calving events at the front of ice shelves. Ocean-induced melt is a potential trigger for destabilizing the glaciers and ice shelves and consequently contributing to global sea level rise. However, the ocean processes causing ice melt and how surface melt from the ice sheet may contribute to those processes is uncertain.
In this dissertation, we employ observational and numerical methods to improve our understandings of ocean-induced melt in sub-shelf cavities under the ice shelves of two major Greenland glaciers: Petermann Glacier and Zachariæ Isstrøm. Using improved ice velocity estimates, ice shelf thickness and surface mass balance data, we calculate ocean-induced melt rates beneath ice shelves with an improved accuracy. We then employ the Ocean Model of Massachusetts Institute of Technology general circulation model (MITgcm) to study ice-ocean interactions beneath an ice shelf. We include thermal forcing from the ocean, cavity shape, and for the first time subglacial water discharge at the grounding line. We investigate ocean-induced melt in a 2-D configuration at a high resolution (40 m × 20 m spacing). For evaluation, we compare the model results with the remote sensing estimates, and in particular optimize the heat and salt transfer coefficients. The model replicates the general pattern of melting: high near the grounding zone, decreasing rapidly downstream. The injection of cold, fresh subglacial water increases the buoyancy and entrainment speed of the melt water plume, and increases the melt rates of the ice shelf. As a result, we calculate a 2-3 times increase in ice shelf melt rate in the summer months. Melt increases below linear with subglacial discharge and above linear with thermal forcing from the ocean. Next, we investigate the role of the slope of the ice shelf draft in controlling ice shelf melt. The simulations indicate that the melt rate is sensitive to the slope, hence is larger for steeper ice shelves; and the location of the region of high melt migrates toward the grounding line as the slope becomes steeper. In the limit case of a vertical wall, no ice shelf, we know that the locus of ice melt undercuts the glacier.
This study provides major new insights on the sensitivity of ice shelf melt to (1) subglacial water discharge: a direct product of ice sheet surface melt (2) thermal forcing from the ocean: a direct product of changes in ocean circulation as a result of wind forcing, and (3) a time-evolving cavity which affects the melt regimes: shallow, nearly flat cavities do not favor high melt; deep, steep cavities favor high melt. These results are important to interpret recent changes on the ice shelves and to inform ice sheet numerical models how to parameterize ice shelf melt in a changing climate.