Investigating controls on flow variability in Greenland's tidewater glaciers: the impact of runoff on fjord circulation and termini melt rates

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The greatest store of fresh water in the northern hemisphere - equivalent to ~7m of sea level rise - is held within the Greenland Ice Sheet (GrIS). In the last decade, it has become clear that the ice sheet is losing mass and has become a significant contributor to global sea-level rise. The rate of mass loss has accelerated in the last decade and the ice sheet is now contributing about 1 mm/yr to sea level rise. This behaviour is a result of: 1) dramatic increases in the speed of many large outlet ('tidewater') glaciers, thereby increasing ice flux to the sea; and 2) substantial increases in both melt rates and the area of the ice sheet experiencing summer melting thereby increasing runoff from the ice-sheet surface to the ocean. These increases in ice motion and surface melt rate have been linked with warmer air and ocean temperatures over and around Greenland. A major concern for policymakers, society and scientists is how the GrIS will continue to react to the temperature rises that are predicted during the coming centuries. However, to better understand ice sheet sensitivity to projected global warming, the processes which affect how ocean terminating glaciers flow and discharge into the sea must be better understood.This project will improve our understanding of why tidewater glaciers are losing mass at an accelerating rate. Recent observations suggest warmer ocean temperatures have resulted in tidewater glacier acceleration through a process known as 'dynamic thinning'; here warm ocean waters access the front of tidewater glaciers causing them to thin by enhanced melting and to retreat through increased rates of iceberg calving. However, while ocean waters off the coast of Greenland have warmed in the last decade, it is unclear how these warm waters actually access the front of glaciers which are often located at the head of long narrow fjords tens of kilometres from the warm ocean shelf waters. An increasingly popular theory contends that summer meltwater runoff from the glaciers establishes fjord circulation whereby strong ouflow of meltwater down the fjord sets up an opposing flow drawing in the warm water from the coastal shelf. As the runoff increases, the fjord circulation strength and thus volume of warm offshore water drawn into the fjord also increases. This theory is supported by some observations of water flow and temperature in fjords but the sensitivity of the process has not been tested.This proposal aims to address this limitation by investigating how variations in glacial runoff perturb fjord circulation and thus the submarine melt rate and dynamic behaviour of tidewater glaciers. More specifically, the project will use a previously tested model (the Bergen Ocean Model (BOM)) to determine how variations in fjord geometry and meltwater runoff affect the seasonal delivery of 'warm' shelf waters to the marine termini of tidewater glaciers. The project will achieve this by:1) Running a suite of fjord modeling experiments using a range of synthetic model parameters and boundary conditions (e.g. fjord geometry (length, width, depth); runoff volume; coastal ocean temperature) which will be varied systematically to establish the sensitivity of along-fjord heat transportation (and thus tidewater terminus melt-rates) to the different parameters.2) Investigating the extent to which decadal (2000-2009) changes in glacier runoff and offshore ocean temperatures can explain observed changes in glacier margin position at ten tidewater glaciers along Greenland's east coast3) Investigating how projected ocean and atmospheric warming by 2100 will affect the along-fjord heat transportation and thus terminus melt-rates at our ten 'test-case' glaciers.Through the delivery of these objectives, the project will make a fundamental contribution to our understanding of how the GrIS will likely respond dynamically to future climate change and specifically, changes in atmospheric and ocean temperatures.