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

北半球最大的淡水储备相当于海平面上升约7米，储存在格陵兰冰盖(GRIS)内。在过去的十年里，很明显，冰盖正在失去质量，并已成为全球海平面上升的重要贡献者。在过去的十年里，质量损失的速度加快了，现在冰盖每年对海平面上升的贡献约为1毫米。造成这种现象的原因是：1)许多大型出口(“潮水”)冰川的速度急剧增加，从而增加了流入海洋的冰流量；2)融化速度和经历夏季融化的冰盖面积大幅增加，从而增加了从冰盖表面向海洋的径流。冰层运动和表面融化速度的增加与格陵兰岛及其周围空气和海洋温度的升高有关。政策制定者、社会和科学家的一个主要担忧是，地球资源研究所将如何继续对未来几个世纪预测的气温上升做出反应。然而，为了更好地了解冰盖对预计的全球变暖的敏感性，必须更好地了解影响海洋终止冰川流动和排放到海洋中的过程。这个项目将提高我们对潮水冰川为什么正在加速失去质量的理解。最近的观察表明，海洋温度变暖导致潮水冰川加速，这一过程被称为动态变薄；在这里，温暖的海水接触到潮水冰川的前部，通过增强融化导致它们变薄，并通过冰山崩解速度的增加而后退。然而，尽管格陵兰沿海的海水在过去十年里变暖了，但尚不清楚这些温暖的海水实际上是如何进入冰川前沿的，这些冰川往往位于距离温暖的大陆架水域数十公里的狭长峡湾的源头。一种越来越流行的理论认为，夏季来自冰川的融水径流建立了峡湾环流，由此融水沿着峡湾强烈流出，形成了从海岸陆架吸收温暖水的相反流动。随着径流量的增加，峡湾的环流强度也会增加，因此进入峡湾的温暖近海海水的体积也会增加。这一理论得到了对峡湾水流和温度的一些观测的支持，但这一过程的敏感性尚未得到测试。这项建议旨在通过研究冰川径流的变化如何扰乱峡湾环流，从而扰乱海底融化速度和潮水冰川的动态行为，来解决这一局限性。更具体地说，该项目将使用一个先前测试过的模型(卑尔根海洋模型(BOM))来确定峡湾几何形状和融水径流的变化如何影响陆架水的季节性输送到潮水冰川的海洋终点站。该项目将通过以下方式实现这一点：1)使用一系列合成模型参数和边界条件(例如峡湾几何形状(长度、宽度、深度)；径流量；2)调查冰川径流和近海温度的年代际变化在多大程度上可以解释格陵兰东海岸10个潮水冰川冰川边缘位置的观测变化3)调查预计到2100年海洋和大气变暖将如何影响沿峡湾的热量传输，从而终止我们的10个‘测试案例’冰川的融化速率。通过这些目标的实现，该项目将对我们理解全球资源信息系统如何动态应对未来的气候变化，特别是大气和海洋温度的变化做出根本性的贡献。

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.