Fragmentation and melting of the seasonal sea ice cover

近年来，北极海冰的夏季范围迅速减少，重新引起人们对夏季海冰退缩的物理过程的兴趣。海气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。特别是海冰气候模型将浮冰大小视为一个常数，而观测结果表明浮冰大小有一个分布，具有显著的空间和时间变异性，特别是在季节性和边缘冰区。较小的浮冰具有增强的横向融化，因此在海洋温暖时提供反馈，可能加速夏季海冰退缩。这个博士项目将研究所观察到的浮冰大小的异质性对夏季海冰融化和退缩的影响。海冰覆盖的季节性减少是由机械浮冰破碎，浮冰边缘和上下表面融化的组合驱动的。机械性絮凝物破碎是由于对施加的风应力、围压应力的不均匀性的响应而发生的脆性破坏，特别是在冰缘附近，是由于对进入的海浪和潮汐运动的响应而发生的弯曲破坏，例如Squire[2007]。随着浮冰的破碎，给定面积的海冰的总周长或边缘长度会增加。浮冰尺寸的减小影响冰盖对入射波场的响应，并促进侧向融化。目前气候模型中没有浮冰破碎的过程：新的基于物理学的模型开发将首次评估其对冰气候反馈的贡献，相对于表面融化。本博士项目将：（1）研究控制季节性和边缘冰区浮冰大小分布的因素。观测表明，边缘冰区浮冰大小分布（FSD）遵循幂律分布[Toyota et al，2011]。将审查来自ICESAT和即将开展的边缘冰区活动（海军研究办公室资助的北极活动，2014年）的新证据。将开发FSD演变模型，包括破碎和絮凝（絮凝物冻结在一起[扩展Dumont等人，2011]。在夏季的海冰边缘，缺乏浮冰重新冻结，以及对风暴驱动的传入海浪的快速反应，将使我们能够从现有气候模型中包含的总变量诊断FSD。该FSD模型将移植到海冰模型CICE。（2）考察CICE模型中的侧向融流，CICE模型将用于考察FSD对絮体周边侧向融流的影响。该模型将与夏季融化季节的大气强迫驱动，并用于检查太阳能热分布到冰（和雪）的表面融化，混合层的变暖，和横向融化。由于混合层吸收了太阳能热量，并且由于冰融化，预计混合层会变浅，从而使热量能够集中在混合层中。将探讨冰质量平衡（浓度和厚度）对FSD、侧向融化公式和强迫条件的敏感性。HadGEM 3气候模式将进行修改，以包括新版本的CICE，其中包含FSD模式，并用于在历史模拟中研究季节性和边缘冰区（例如巴伦海、弗拉姆海峡等）的北极海冰退缩，以确定任何增强的海冰-气候反馈。Dumont D、A Kohout和L Bertino（2011年），包括浮冰破碎参数化的边缘冰区波浪模型，J.Geophys.Res.，116，doi：10.1029/2010JC 006682. Squire，VA，Of ocean waves and sea-ice revisited，Cold Reg.Sci.&技术人员：Toyota T，C哈斯和T Tamura（2011），在晚冬南极边缘冰区相对小的海冰的尺寸分布和形状特性，深海研究II，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.