Investigating the mechanisms leading to the deglaciation of past continental northern hemisphere ice sheets with the CLIMBER–GREMLINS coupled model

A coupling procedure between a climate model of intermediate complexity (CLIMBER-2.3) and a 3-dimensional thermo-mechanical ice-sheet model (GREMLINS) has been elaborated. The resulting coupled model describes the evolution of atmosphere, ocean, biosphere, cryosphere and their mutual interactions. It is used to perform several simulations of the Last Deglaciation period to identify the physical mechanisms at the origin of the deglaciation process. Our baseline experiment, forced by insolation and atmospheric CO₂, produces almost complete deglaciation of past northern hemisphere continental ice sheets, although ice remains over the Cordilleran region at the end of the simulation and also in Alaska and Eastern Siberia. Results clearly demonstrate that, in this study, the melting of the North American ice sheet is critically dependent on the deglaciation of Fennoscandia through processes involving switches of the thermohaline circulation from a glacial mode to a modern one and associated warming of the northern hemisphere. A set of sensitivity experiments has been carried out to test the relative importance of both forcing factors and internal processes in the deglaciation mechanism. It appears that the deglaciation is primarily driven by insolation. However, the atmospheric CO₂ modulates the timing of the melting of the Fennoscandian ice sheet, and results relative to Laurentide illustrate the existence of threshold CO₂ values, that can be translated in terms of critical temperature, below which the deglaciation is impeded. Finally, we show that the beginning of the deglaciation process of the Laurentide ice sheet may be influenced by the time at which the shift of the thermohaline circulation from one mode to the other occurs.     

基于中等复杂地球系统模式的全新世 气候突变事件模拟试验*

全新世暖湿气候背景下的代表性气候突变是发生在约8.2kaB.P. 和4.2kaB.P. 的气候突然变冷变干的事件。对于气候突变事件的出现,海洋环流被普遍认为是其中的关键环节。科学家们利用不同类型的气候模式进行过个例和机理模拟试验,但还未能对气候突变机理做出合理的解释。本研究使用一个中等复杂程度的地球系统模式EMIC ( Earth-system Models Intermediate Complexity) MPM-2,模拟研究了全新世气候背景下北大西洋区域,淡水强迫机制作用对温盐环流以及全球气候的可能影响、气候系统响应和恢复时间特征以及南北半球的差异。模拟结果表明海洋对北大西洋淡水通量异常的强度响应敏感,海洋环流对热量和盐份的输送与淡水通量异常强度之间的平衡决定了温盐环流能否对淡水强迫产生不可逆转变,当淡水通量异常达到某一阈值后可能引起海洋和气候平衡态的改变。同时还发现,在气候变化过程中,两次淡水通量异常对海洋和大气系统的强迫作用是不同的,首次淡水通量异常对气候平衡态的影响要远大于其后的淡水通量异常。模拟结果还表明,北大西洋海洋的异常状况可以通过海洋、大气等各圈层相互作用传递到不同圈层和空间区域,在南北半球出现不同的响应特征。上述模拟结果对于认识全新世气候变化特征、不同突变事件之间的关系及其对现代气候的可能影响具有启发意义。     

波粒相互作用导致环电流质子沉降的卫星共轭观测

波粒相互作用是环电流损失的重要机制之一,但波粒相互作用导致的环电流离子沉降而损失迄今为止缺乏直接的观测证据.基于磁层及电离层卫星的协同观测,本文报道了发生在2015年9月7日,由电磁离子回旋波(EMIC波)导致环电流质子沉降的共轭观测事件.在等离子体层的内边界,Van Allen Probe B卫星观测到,存在EMIC波的区域和不存在EMIC波的区域相比,离子通量的投掷角分布的各向异性变弱.我们将Van Allen Probe B卫星沿着磁力线投影到电离层高度,同时在该投影区域内DMSP 16卫星在亚极光区域观测到环电流质子沉降.而且,通过从理论上计算质子弹跳平均扩散系数,我们进一步证实观测的EMIC波确实能将环电流质子散射到损失锥中.本文的研究工作为EMIC波导致环电流质子沉降提供了直接的观测证据,揭示了环电流衰减的重要物理机制:EMIC波将环电流质子散射到损失锥中,从而沉降到低高度大气层中而损失.     

基于CARISMA地磁台链观测的IPDP事件的统计分析

亚暴期间磁尾等离子体片离子注入内磁层能够激发电磁离子回旋(EMIC)波.对应于这种EMIC波,地面磁力仪可观测到周期逐渐减小的地磁脉动(IPDP).利用GOES卫星数据,地磁指数和加拿大CARISMA地磁台站的数据,我们研究了IPDP事件的产生与亚暴磁尾注入的关系.同时利用CARISMA地磁台链中的MCMU和MSTK两个台站,从2005年4月到2014年5月期间的观测数据,统计分析了亚暴期间的IPDP事件,研究了IPDP事件的出现率关于季节和磁地方时的分布特征.我们总共获得128个两个台站同时观测的IPDP事件.该类事件关于季节分布的发生率,冬季最小,为13.28%,春季最大,为32.81%,结果表明IPDP事件关于季节分布的发生率受到电离层电导率及亚暴发生率的影响.两个台站同时观测到的IPDP事件最大出现率出现在15—18 MLT(磁地方时),结果表明IPDP事件主要由亚暴期间产生的能量离子注入内磁层,西向漂移遇到等离子体层羽状结构(Plume)区的高密度等离子体所激发.