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

在北极的“最后冰区”——一个被认为对气候变暖最具弹性的地区——最古老、最厚的海冰在去年意外地破裂了两次。我们目前的理论假设，随着未来几十年全球变暖的加速，夏末的冰雪覆盖地区将稳步退缩到北极中部盆地。然而，2018年发生的动态分手事件挑战了这一普遍观点。在这里，我们假设，一个较弱的、日益移动的北极中部浮冰现在容易受到动态破碎事件的影响，这可能是冰在夏季迅速融化的先决条件。这种动态季节性预调节机制在全球气候模型中没有考虑到，因此我们目前最好的预测忽略了北极最后一片冰区迅速解体的可能性。我们的团队已经证明，在过去的五年里，季节性的预调节已经导致邻近的波弗特海两次无冰。甚至在十年前，这一地区还覆盖着厚厚的常年海冰，与北冰洋中部相呼应，但现在它已经转变为北冰洋的边缘。导致波弗特海冰减少的过程是否会开始在北极中部显现出来？目前，由于夏季北极浮冰的卫星观测不足，我们无法验证我们的假设。我们迫切需要对整个北极地区的冰融化速度进行观测，但到目前为止，海冰厚度的卫星观测只能在冬季进行。因此，我们的项目将通过两颗卫星（ESA的Cryosat-2和NASA的ICESat-2）在夏季提供北极海冰厚度的首次测量数据。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，无论表面融化状态如何，为填补现有夏季观测的“空白”提供了必要的突破。利用最近发射的多个SAR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，以生成北极高地区独特的全年海冰体积预算。这一记录将为高分辨率冰动力学模拟提供信息，通过一套最先进的海冰模型进行模拟，从独立的（CICE），海洋-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北部这一关键地区的力学作用，以仔细研究冬季破裂事件（如2018年目睹的事件）与夏季融化速度的耦合和预处理。使用耦合模型，我们将量化由于海洋和大气反馈，北极最后一个冰区比预期更快破裂的可能性，以及这将如何影响冰和淡水涌入北大西洋。

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.