北极海冰快速减退对太平洋扇区海洋环流和温盐结构的影响

The rapid decline of Arctic sea ice and its climate and ecosystem effects have inspired extensively interests. The coupled climate system models projected that there is probably an ice-free Arctic Ocean during summer within 30-40 years. Understanding the ecosystem and climate effects requires knowledge of impacts of sea ice decline on ocean circulation and thermal structure. Pacific sector of the Arctic Ocean is a gateway of water exchange between the Pacific and the Arctic Oceans, a marginal ice zone, a region with active air-sea-ice interaction. Especially , Pacific water transiting across the region is a major driving force for the physical environmental state, ice extent and thickness in the western Arctic Ocean. Significant changes have been observed associated with the rapid decline of the sea ice, such as, the increase of Bering Strait inflow, decrease of transport of Beaufort Sea slope current during summer, the thickening, warming and freshening of the Pacific summer water in the Canada basin. Such changes are related to the knock-on effects of sea ice decline, such as ice-albedo feedback, strengthened air-sea exchanges, the modification and pathways of Pacific Water etc. The impacts of sea ice decline on ocean circulation and thermal structure are still unclear. This project is to utilize historical in-situ observations, satellite observations and coupled ice-ocean model to investigate the impacts of the sea ice decline on the key physical processes: Bering Strait transport, the modification and pathways of Pacific water, heat content, mixing and stratification and upwelling in the Pacific Sector Arctic Ocean. The project is also to examine the responses of the physical environmental state in Pacific Sector of the Arctic Ocean to the projected future climate.

北极海冰快速减退及其气候和生态效应引起了广泛关注。气候模式预测未来三四十年将有可能出现夏季无冰的北冰洋。太平洋扇区作为与太平洋与北冰洋进行水交换的通道、海冰变化的边缘，是海-冰-气相互作用的关键海域。流经该海域的太平洋水是西北冰洋物理环境和生态系统变化的主要驱动力。伴随着海冰的减少,太平洋扇区已经出现显著变化:如白令海峡流量增加、陆坡急流的流量减少、加拿大海盆太平洋夏季水增厚、增温以及变淡等。这些变化与海冰快速减退引起的连锁效应，如冰-水反照率反馈、海气交换加强等有关。目前对海冰减退导致的海洋环流和温盐结构的变化还缺乏系统 的了解。本项目拟利用观测资料和海洋-海冰耦合模式，探讨海冰减退对白令海峡流量、太平洋水的变性及其向深水海盆的输送路径、陆坡上升流、上层海洋混合和层化和热含量等关键海洋过程和变量的影响。还拟通过模式研究太平洋扇区物理环境对预测的未来气候的响应。

由于全球变暖，北冰洋海冰快速减退，太平洋扇区的温盐结构和环流正在发生显著的变化，对北极生态和气候系统有重要的影响。本项目收集北冰洋海冰、海洋和大气的现场调查、卫星遥感和再分析资料，主要包括海水温度和盐度、流速、海冰厚度、密集度资料以及大气参量等。建立了北冰洋高分辨率区域耦合海洋-海冰模式,能较准确模拟北冰洋上层海洋结构；在模式中设置示踪物追踪太平洋水在北冰洋中的路径。研究了海冰快速减退背景下波弗特高压的年际和年代际变化、太平洋扇区楚科奇海的水团特征和温盐垂直结构的变化机制、太平洋入流的路径和去向、海底地形对环流斜压不稳定的影响、海冰减退对北冰洋海平面变化的影响机制以及地球系统模式CMIP6对未来北冰洋上层热含量的预测。重要结果有：指出近十年来波弗特高压冬季持续西移、春季持续东移；由于白令海峡水流量增加和北侵范围扩展，楚科奇海的水团特征和温盐结构发生了明显的变化并有区域特征；揭示了楚科奇陆坡流的来源和去向，陆坡流68%来自白令海峡水驱动的巴罗海谷出流，32%来自波弗特流涡的向西急流。示踪剂试验表明楚科奇陆坡流一部分向西延伸至东西伯利亚海，另一部分则越过北风海脊后向北进入楚科奇边陲地带，最终卷入波弗特流涡。海底坡度对最不稳定波的尺度和传播有重要影响。当海底坡度与等密度线倾斜方向相同，最不稳定波能穿越基本流；当底坡与等密度线倾斜相反，最不稳定波沿着流线传播。海冰减少不改变整个北冰洋的平均海面高度，但它会通过增加淡水含量、改变海表面应力分布，使淡欧亚海盆淡水减少、加拿大海盆淡水增加，引起加拿大海盆海面高度上升而欧亚海盆海面高度下降。本项目关键数据的积累和耦合海洋-海冰模式的建立和优化为以后的北极研究奠定了坚实的基础。取得的研究成果加深对北极太平洋扇区海洋环流的理解以及海冰减退对北冰洋海洋环境的影响。