The role of atmospheric forcing on the dynamic stability of Greenland's outlet glaciers

该项目将量化地表产生的融水波动对格陵兰冰盖边缘冰运动的影响。更具体地说，它将提供数据，使冰盖建模者能够改进他们对未来GrIS对海平面上升的贡献的预测，以应对全球变暖。为了实现这一目标，需要对GrIS进行专门的实地活动，以调查从冰盖边缘延伸到潮汐和陆地终止冰川的平衡线高度（ELA）的平行断面上的季节性冰流动态和径流过程。北半球最大的淡水储存量——相当于海平面上升7米——储存在格陵兰冰盖（GrIS）内，但它目前和未来对海平面的贡献却很少受到限制（IPCC, 2007）。最近的观测表明，由于地表融化增加（如Steffen等人，2004年）和动态变薄（如Rignot和Kanagaratnam， 2006年）的共同作用，GrIS边缘附近的质量损失正在加速。然而，控制动态变薄的关键过程尚未确定（Alley等人，2005年），因此没有纳入构成IPCC海平面预估基础的冰盖模式。这在一定程度上反映了这样一个事实，即揭示冰川普遍加速的卫星数据无法以解决因果机制所需的时间分辨率获得。我们目前对GrIS质量平衡的理解尤其复杂，因为冰川边缘动力学对冰上融水渗透到冰川床所引起的融水润滑变化的敏感性存在不确定性（Zwally等，2002）。最近对GrIS的观测表明，在7月的5天期间，地表水文和动力学之间存在强烈而直接的耦合，其中流速的日波动为100%，并且最大日流速随温度变化。这些观测证实，需要获得全年高时间（次小时）和空间分辨率的水文和动力数据，以参数化冰融化和冰流之间的耦合。该项目将以必要的分辨率收集数据，以量化融水产生与冰盖动态之间的关系，从而使冰盖建模者能够改进GrIS对气候变化反应的预测。我们将沿着GrIS西部边缘相邻陆地和海洋终止流域的两条平行流样带进行地面实验，以解决以下目标：与冰上排水系统（包括冰上湖泊）的季节演变有关的任何水文动力联系是否存在时空格局？2. 在什么区域表面产生的融水渗透到冰盖的底部？3. 冰川表面融水输入量与动力响应的大小之间是否存在关系？4. 在融化季节，潮水冰川和陆地冰川的表现是否不同？在2008年和2009年期间将开展实地活动，以确定：1)融化的速度、程度和持续时间。(2)冰上湖泊储水量和释放水量以及向自由排水的冰碛湖输送水量的时空变化。3)冰动力的季节、日、时变化。4)前冰期流量和水化学变化（罗素冰川）。由于我们的工作，将有可能确定GrIS边缘的冰动力学是否受到地表融水排水后冰川床润滑的显著影响。我们的研究结果将提交给冰盖建模者，以帮助他们限制对GrIS未来的预测

This project will quantify the effect of surface generated melt-water fluctuations on ice motion at the margin of the Greenland Ice Sheet (GrIS). More specifically, it will provide data that will enable ice-sheet modellers to improve their predictions of the future contribution of the GrIS to sea level rise in response to a warming world. To achieve this aim requires a dedicated field campaign to the GrIS to investigate seasonal ice flow dynamics and runoff processes along flow parallel transects extending from the ice sheet margin to the equilibrium line altitude (ELA) at both tidewater and land-terminating glaciers. The greatest store of fresh water in the northern hemisphere - equivalent to 7m of eustatic sea level rise - is held within the Greenland Ice Sheet (GrIS), and yet its present and future contribution to sea level is poorly constrained (IPCC, 2007). Recent observations suggest that mass loss near the margin of the GrIS is accelerating through a combination of increased surface melting (e.g. Steffen et al, 2004) and dynamic thinning (e.g. Rignot and Kanagaratnam, 2006). However, the key processes controlling dynamic thinning have yet to be identified (Alley et al, 2005), and in consequence, are not incorporated in the ice-sheet models which form the basis of the IPCC sea level projections. This in part reflects the fact that the satellite data that has revealed the widespread speed-up of glaciers cannot be acquired at the temporal resolution needed to resolve the causal mechanisms. Our present understanding of GrIS mass balance is especially complicated by uncertainties in the sensitivity of ice-marginal dynamics to changes in melt-water induced lubrication resulting from penetration of supraglacial melt-waters to the glacier bed (Zwally et al, 2002). Recent observations on the GrIS Shepherd et al, in review) reveal, over a five day period in July, a strong and direct coupling between surface hydrology and dynamics where diurnal fluctuations in velocity of >100% occur and where maximum daily velocities scale with temperature. Such observations confirm the need to acquire hydrological and dynamic data at high temporal (sub-hourly) and spatial resolution throughout the year to parameterise the coupling between ice melting and flow. This project will collect data at the necessary resolution to quantify the relationship between melt-water production and ice sheet dynamics thereby enabling ice-sheet modellers to improve predictions of the GrIS's response to climate change. We will conduct ground based experiments along two flow-parallel transects at the western margin of the GrIS in adjacent land and marine terminating drainage basins to address the following objectives: 1. Is there a temporal and spatial pattern to any hydrology-dynamic link associated with the seasonal evolution of the supraglacial drainage system (including supraglacial lakes)? 2. Over what area does surface generated meltwater penetrate to the base of the ice sheet? 3. Is there a relationship between the volume of meltwater input at the glacier surface and the magnitude of the dynamic response? 4. Do tidewater and land-terminating glaciers behave differently during the course of a melt-season? Field campaigns will be undertaken during 2008 and 2009 to determine: 1) The rate, extent and duration of melt. 2) The temporal and spatial variations in water volumes stored in and released from supraglacial lakes and delivered to freely draining moulins. 3) The seasonal, diurnal and hourly variations in ice dynamics. 4) The variations in proglacial discharge and water chemistry (at Russell Glacier). As a result of our work, it will be possible to determine whether ice dynamics at the margin of the GrIS is significantly affected by lubrication of the glacier bed following the drainage of surface derived meltwaters. Our results will be delivered to ice sheet modellers to help them constrain predictions for the future of the GrIS