Fragmentation and melting of the seasonal sea ice cover

近年来，北极海冰的夏季范围迅速缩小，重新引起了人们对夏季海冰退缩的物理过程的兴趣。大气-海洋耦合全球气候模式的海冰分量包含海冰覆盖边缘物理过程的简化表示。特别是海冰气候模式将浮冰大小视为一个常数，而观测表明浮冰大小是有分布的，具有显著的时空变异，特别是在季节性和边缘冰带。较小的浮冰有更强的横向融化，所以当海洋变暖时，提供了一个反馈，潜在地加速了夏季海冰的退缩。这个博士项目将研究观测到的浮冰大小的非均匀性对海冰夏季融化和退缩的影响。海冰覆盖的季节性减少是由浮冰的机械破碎、浮冰边缘和上下表面的融化共同驱动的。机械碎裂的发生是由于在施加的风应力、围应力的非均质性作用下的脆性破坏，以及（特别是在冰缘附近）在入射海浪和潮汐运动作用下的弯曲破坏，例如Squire[2007]。随着浮冰的破裂，海冰的总周长或边缘长度会增加。冰体尺寸的减小影响了冰体对入射波场的响应，促进了冰体的侧向融化。目前，气候模型中缺少浮冰破碎的过程：新的基于物理的模型开发将使其对冰-气候反馈的贡献，相对于表面融化，首次得到评估。本博士项目将：(1)研究控制季节性冰带和边缘冰带浮冰大小分布的因素。观测结果表明，边缘冰区的浮冰粒径分布（FSD）遵循幂律分布[Toyota et al ., 2011]。来自ICESAT和即将到来的边缘冰区运动（海军研究办公室资助的北极运动，2014年）的新证据将被审查。将开发一个包括碎裂和絮凝（浮冰冻结在一起）在内的FSD演化模型[扩展Dumont等人，2011]。在夏季的海冰边缘，缺乏浮冰重新冻结，对风暴驱动的海浪的快速反应，将使我们能够从现有气候模型中包含的总变量中诊断出FSD。这个FSD模型将被移植到海冰模型CICE上。(2)在CICE模型中检验侧向熔体。CICE模型将用于检验FSD对浮冰周缘侧融的影响。该模式将受到夏季融化季节大气强迫的驱动，并用于研究太阳热量在冰（和雪）表面融化、混合层变暖和侧向融化中的分布。由于太阳热量在混合层中被吸收，冰融化，预计混合层将使热量在混合层中集中。将探讨冰的质量平衡（浓度和厚度）对FSD的敏感性、侧向融化的公式和强迫条件。HadGEM3气候模式将被修改为包含FSD模式的新版本CICE，并用于在历史模拟中检查季节性和边缘冰带（如巴伦支海，Fram海峡等）的北极海冰退缩，以确定任何增强的海冰-气候反馈。[8]杜蒙，李建平，李建平（2011），基于波的边缘冰带模型及其破碎参数化，地球物理学报。, 116年,jc006682 doi: 10.1029/2010。《海浪与海冰重访》，《寒潮科技》。, 49110 - 133年,2007年。Toyota T,C Haas, T Tamura(2011)，南极边缘冰带相对小浮冰的尺寸分布和形状特性，深海科学进展，58,1182-1193。

Recent years have seen a rapid reduction in the summer extent of Arctic sea ice, renewing interest in the physical processes responsible for summer sea ice retreat. Sea ice components of atmosphere-ocean coupled Global Climate Models contain a simplified representation of physical processes at the edge of the sea ice cover. In particular sea ice climate models treat the floe size as a constant whereas observations show there to be a distribution of floe sizes, with significant spatial and temporal variability, especially in the seasonal and marginal ice zones. Smaller floes have an enhanced lateral melt, so providing a feedback when ocean the ocean is warm, potentially accelerating summer sea ice retreat. This PhD project will examine the impact of the observed heterogeneity in floe size on the summer melt and retreat of sea ice.The seasonal reduction of the sea ice cover is driven by a combination of mechanical floe break up, and melting at the floe edges and upper and lower surfaces. Mechanical floe breakup occurs due to brittle failure in response to heterogeneity in imposed wind stresses, confining stresses, and, especially near the ice edge, by failure in flexure in response to incoming ocean waves and tidal motions, e.g. Squire[2007]. As floes break up, the total perimeter, or edge length, of a given area of sea ice increases. The reduction in floe size affects the response of the ice cover to the incoming wave field and promotes lateral melting. The process of floe breakup is currently absent from climate models: new, physics-based model development will allow its contribution to ice-climate feedbacks, relative to surface melt, to be evaluated for the first time. This PhD project will:(1)Examine factors controlling the floe size distribution in the seasonal and marginal ice zone.Observations show that the floe size distribution(FSD)in the marginal ice zone follows a power law distribution [Toyota et al, 2011]. New evidence from ICESAT and forthcoming Marginal Ice Zone campaigns (Office of Naval Research funded Arctic campaign, 2014) will be examined. A model of FSD evolution will be developed including fragmentation and flocculation (freezing together of floes[extending Dumont et al, 2011]. At the sea ice edge in summer, the lack of floe refreezing, and rapid response to storm-driven incoming ocean waves,will allow us to diagnose the FSD from the gross variables included in existing climate models. This FSD model will be ported to the sea ice model CICE.(2)Examine lateral melt in CICE model.The CICE model will be used to examine the impact of FSD on lateral melt at floe perimeters. The model will be driven with summer melt season atmospheric forcing and used to examine solar heat distribution into surface melt of the ice (and snow),warming of the mixed layer, and lateral melt. As solar heat is absorbed in the mixed layer and as ice melt occurs, the mixed layer is expected to shallow enabling concentration of heat in the mixed layer. The sensitivity of ice mass balance (concentration and thickness) to the FSD,formulation of lateral melt, and forcing conditions will be explored.(3)Examine the impact of lateral melting in a coupled climate model The HadGEM3 climate model will be modified to include the new version of CICE, containing the FSD model, and used to examine Arctic sea ice retreat in the seasonal and marginal ice zones (e.g. Barent's Sea, Fram Strait etc) in historical simulations to identify any enhanced sea ice - climate feedback. ReferencesDumont D,A Kohout, and L Bertino (2011), A wave-based model for the marginal ice zone including a floe breaking parameterization, J.Geophys.Res.,116, doi:10.1029/2010JC006682.Squire, VA, Of ocean waves and sea-ice revisited,Cold Reg.Sci.&Tech.,49,110-133, 2007.Toyota T,C Haas, and T Tamura (2011),Size distribution and shape properties of relatively small sea-ice floes in the Antarctic marginal ice zone in later winter,Deep Sea Res. II, 58, 1182-1193.