Collaborative Research: Understanding the controls on spatial and temporal variability in ice discharge using a Greenland-wide ice sheet model

据观察，海平面正在以越来越快的速度上升。在过去的几十年里，山地冰川是一个重要的贡献，但预计格陵兰冰盖的贡献在不久的将来将占主导地位。这种贡献以融水和冰山的形式在格陵兰岛周围的峡湾和沿海海洋中融化。这些贡献在空间上有所不同。拟议的工作将开发一个格陵兰岛对海平面上升贡献的模型，使用观测数据约束该模型，并根据未来气候情景估计贡献。该项目将为两名研究生的培训提供支持，从而促进STEM劳动力的发展。它也将提供支持，为一个开始调查员在他的职业生涯的形成岁月。它将通过维护和增强开源并行冰盖模型（PISM）代码以供社区使用，从而为社区资源做出贡献。确定了出口冰川系统动力学的四种可能控制因素：1)变暖的地下海水和/或增加的冰下径流可能会增加冰川-峡湾界面的海底冰融化；2)坚硬的海冰和冰凌（海冰和冰山的混合物）可能会抑制产犊，允许终端前进；3)相对于冰下地形的末端位置（例如，过深或底岩）影响退缩速度；4)与峡湾壁和/或冰川床接触引起的阻力应力变化可导致终端前进、后退和/或变薄。以往对冰盖对海平面上升贡献的模拟受到模式和观测数据空间分辨率不足的限制，这使得全冰盖模拟无法忠实地捕捉冰川出口流量。迄今为止的结果要么来自区域模型，要么来自高度理想化的流线模型，这些模型被放大到冰盖尺度。冰盖模拟的最新进展，以及高分辨率冰下地形的可用性，现在允许人们在整个冰盖的模拟中解决单个出口冰川流动。该项目将使用开源的平行冰盖模型（PISM）的框架，与新的大气和海洋的高分辨率预测单向耦合。这将为在冰川的基础上评估提供一个试验台：1)四个控制因素对冰川出口流量和冰流量的相对影响是什么；2) 21世纪冰流量大幅增加的可能性是什么；3)什么条件会导致大的变化（如海洋变暖的时空分布、地表径流增强）；4)为了支持格陵兰冰盖海洋观测系统捕捉大变化的强迫或开始，需要哪些观测资料？与现有的遥感和原位观测进行比较，包括但不限于地表速度、地表高程和质量变化的时间序列，将作为成功的衡量标准。然后，根据现有的大气-海洋预估，将对格陵兰冰盖21世纪的演变进行模拟，以提供对未来冰流量的现实估计。

Sea level is observed to be rising at an increasing rate. A significant contribution during the past decades has been from mountain glaciers, but the contribution from the Greenland Ice Sheet is anticipated to become dominant in the near future. This contribution is delivered to the ocean as both meltwater and icebergs that melt in the fjords and coastal ocean around Greenland. These contributions vary spatially. The proposed work will develop a model of Greenland's contribution to sea level rise, constrain the model using observed data, and estimate contributions based on scenarios of future climate.The project will contribute to STEM workforce development by providing support for the training of two graduate students. It will also provide support for a beginning investigator during the formative years of his career. It will contribute to the community resources by maintaining and enhancing the open source Parallel Ice Sheet Model (PISM) code for community use.Four possible controls on outlet glacier systems dynamics have been identified: 1) Warming subsurface ocean water and/or increased subglacial runoff may increase submarine ice melting at the glacier-fjord interface;2) Rigid sea ice and ice mélange (a mixture of sea ice and icebergs) may suppress calving, allowing for terminus advance; 3) The terminus position relative to subglacial topography (e.g., over-deepenings or sills) influences rates of retreat; and 4) Changes in the resistive stress caused by contact with the fjord walls and/or glacier bed can lead to terminus advance, retreat, and/or thinning.Previous simulations of ice sheet contributions to sea level rise have been limited by the insufficient spatial resolution of models and observational data, which prevented whole-ice sheet simulations to faithfully capture outlet glacier flow. Results to date are either obtained from regional models or from highly idealized flow line models that were upscaled to ice-sheet scale. Recent advances in ice sheet modeling, and the availability of high-resolution subglacial topography, now allow one to resolve individual outlet glacier flow in ice sheet-wide simulations. This project will use the framework of the open-source Parallel Ice Sheet Model (PISM), uni-directionally coupled to new high-resolution hindcasts of the atmosphere and ocean. This will provide a test bed for assessing, on a glacier-by-glacier basis: 1) what is the relative present-day influence of the four controls on outlet glacier flow and ice discharge; 2) what is the potential for a substantial increase in 21st century ice discharge; 3) what conditions would precipitate large changes (e.g., spatio-temporal distribution of ocean warming, enhanced surface runoff); and 4) what observations are required in support of a Greenland Ice Sheet Ocean Observing System to capture the forcing or onset of large changes? Comparison to available remotely-sensed and in-situ observations, including, but not limited to, time-series of surface velocities, surface elevation, and mass changes will serve as metrics of success. Simulations of the 21st century evolution of the Greenland Ice Sheet will then be performed, forced by available atmosphere-ocean projections, to provide realistic estimates of future ice discharge.