PRE-MELT: Preconditioning the trigger for rapid Arctic ice melt

在过去的一年里，北极“最后冰区”最古老、最厚的海冰--该地区被认为对气候变暖的适应能力最强--出人意料地两次破裂。我们目前的理论假设，随着未来几十年全球变暖的加速，夏末冰盖地区将稳步退回到北极中部盆地。然而，2018年发生的动态分手事件挑战了这一主流观点。在这里，我们假设，北极中部较弱、流动性越来越强的冰层现在容易受到动态碎裂事件的影响，这可能是夏季冰层迅速融化的先决条件。这种动态的季节性预适应机制在全球气候模型中没有被考虑在内，所以我们目前最好的预测忽略了北极最后一块冰区迅速解体的可能性。我们的团队已经证明，季节性预适应已经导致邻近的波弗特海在过去五年中两次实现无冰。甚至在十年前，该地区还拥有厚厚的常年海冰，与北冰洋中部相映成趣，但现在它已经过渡到边缘的北冰洋。导致波弗特海冰层减少的过程会开始在北极中部显现吗？目前，卫星对北极夏季积冰的观测不足，使我们无法验证我们的假设。我们迫切需要泛北极地区对冰川融化速度的观测，但到目前为止，卫星对海冰厚度的观测只能在冬季进行。因此，我们的项目将通过两颗卫星：欧空局的Cryosat-2和NASA的ICESat-2，在夏季几个月内首次测量北极海冰的厚度。我们设计了一种新的分类算法，用于分离冰和海洋雷达高度计回波，而不考虑表面融化状态，提供了填补现有夏季观测“空白”所需的突破。利用最近启动的多个搜救任务进行极地侦察，我们的项目将整合有关冰盖消融、运动和变形的信息，以生成独特的北极高地全年海冰量预算。这一记录将用于高分辨率冰动力学模拟，使用一套最先进的海冰模型进行模拟，从独立海冰(CICE)、海-海冰(NEMO/CICE)到完全耦合的区域高分辨率(RASM)和全球粗分辨率(HadGEM)模型，所有这些模型现在都配备了我们团队开发的各向异性(EAP)海冰流变学。使用区域和独立模型，我们将分析格陵兰岛以北这一关键地区的力学作用，以仔细审查冬季分手事件--如2018年目睹的--与夏季融化速度的耦合和预调节。使用耦合模型，我们将量化北极最后一块冰区由于海洋和大气反馈而比预期更早破裂的可能性，以及这将如何影响冰和淡水流入北大西洋。

The oldest, thickest sea ice in the 'last ice area' of the Arctic - a region thought to be most resilient to climate warming - unexpectedly broke up twice in the past year. Our current theories assume that the end-of-summer ice-covered area will steadily retreat into the Central Arctic Basin as global warming accelerates over coming decades. However, the dynamic break-up events witnessed in 2018 challenge this prevailing view. Here we hypothesise that a weaker, increasingly mobile Central Arctic ice pack is now susceptible to dynamic episodes of fragmentation which can precondition the ice for rapid summer melt. This mechanism of dynamic seasonal preconditioning is unaccounted for in global climate models, so our best current projections are overlooking the possibility for rapid disintegration of the Arctic's last ice area.Our team has demonstrated that seasonal preconditioning is already responsible for the neighbouring Beaufort Sea becoming ice-free twice in the past five years. Even ten years ago this region contained thick perennial sea ice, mirroring the Central Arctic Ocean, but it has now transitioned to a marginal Arctic sea. Could the processes responsible for the decline of the Beaufort Sea ice pack start to manifest themselves in the Central Arctic? Currently, a shortfall in satellite observations of the Arctic pack ice in summer prevents us from testing our hypothesis. We desperately require pan-Arctic observations of ice melting rates, but so far satellite observations of sea ice thickness are only available during winter months. Our project will therefore deliver the first measurements of Arctic sea ice thickness during summer months, from twin satellites: ESA's Cryosat-2 & NASA's ICESat-2. We have designed a new classification algorithm for separating ice and ocean radar altimeter echoes, regardless of surface melting state, providing the breakthrough required to fill the existing summer observation 'gap'. Exploiting the recent launch of multiple SAR missions for polar reconnaissance, our project will integrate information on ice-pack ablation, motion and deformation to generate a unique year-round sea ice volume budget in the High Arctic. This record will inform high-resolution ice dynamics simulations, performed with a suite of state-of-the-art sea ice models from stand alone (CICE), ocean-sea ice (NEMO/CICE), to fully coupled regional high resolution (RASM), and global coarser resolution (HadGEM) models, all now equipped with the anisotropic (EAP) sea ice rheology developed by our team. Using the regional and stand-alone models we will analyse the role of mechanics in this keystone region north of Greenland to scrutinise the coupling and preconditioning of winter breakup events - such as those witnessed in 2018 - to summer melting rates. Using the coupled models, we will quantify the likelihood of the Arctic's last ice area breaking up much sooner than expected due to oceanic and atmospheric feedbacks and how this will affect the flushing of ice and freshwater into the North Atlantic.