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

北方最大的淡水储备--相当于海平面上升约7米--被保存在格陵兰冰盖（GrIS）内。在过去的十年中，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因素。在过去十年中，质量损失的速度加快，冰盖现在每年对海平面上升的贡献约为1毫米。这种现象是由于：1）许多大型出口（“冰川”）冰川的速度急剧增加，从而增加了流向海洋的冰通量; 2）融化速率和冰盖面积的大幅增加经历夏季融化，从而增加了从冰盖表面到海洋的径流。这些冰运动和表面融化速度的增加与格陵兰岛及其周围的温暖空气和海洋温度有关。政策制定者、社会和科学家的一个主要关注点是，GrIS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入海洋的过程。该项目将提高我们对为什么海洋冰川以加速的速度失去质量的理解。最近的观测表明，温暖的海洋温度导致了冰川加速通过一个过程被称为“动态变薄”，这里温暖的海洋沃茨访问冰川的前端，使他们薄的增强融化和退缩通过增加冰山崩解率。然而，虽然格陵兰海岸外的海洋沃茨在过去十年中已经变暖，但尚不清楚这些温暖的沃茨实际上如何进入冰川的前端，这些冰川通常位于距离温暖的海洋架沃茨数十公里的狭长峡湾的顶端。一种越来越流行的理论认为，夏季冰川融水径流建立了峡湾环流，由此，沿峡湾向下的融水强烈外流建立了从沿海大陆架吸引温水的反向流动。随着径流量的增加，峡湾环流的强度，从而吸引到峡湾的温暖的近海水的体积也增加。这一理论得到了峡湾水流和温度的一些观察结果的支持，但该过程的敏感性尚未经过测试。该提案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率和动态来解决这一限制。潮汐冰川的动态行为。更具体地说，该项目将使用以前测试过的模型（卑尔根海洋模型（BOM）），以确定峡湾几何形状和融水径流的变化如何影响“温暖”的货架沃茨的海洋终端的tibal冰川的季节性交付。该项目将通过以下方式实现这一目标：1）使用一系列合成模型参数和边界条件运行一套峡湾建模实验（例如峡湾的几何形状（长度、宽度、深度）;径流量;沿海海洋温度），这将系统地变化，以建立沿峡湾热传输的敏感性（因此，潮汐终端融化率）的不同参数。2）调查的程度，（2000-2009）冰川径流和近海海洋温度的变化可以解释在格陵兰岛东海岸沿着10个冰川边缘位置观测到的变化。3）调查2100年海洋和大气变暖将如何影响沿峡湾的热量输送，从而影响我们10个试验的终点融化率。通过实现这些目标，该项目将为我们理解GrIS如何动态应对未来气候变化，特别是大气和海洋温度的变化做出根本性贡献。

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.