【作者】 李静；

【导师】 牟林；

【作者基本信息】 中国地质大学 ， 海洋科学， 2022， 博士

【摘要】 在全球变暖背景下,北极地区的升温速率远高于全球平均水平,被称为“北极放大效应”。北极的快速增暖不仅伴随极冰的融化,还对北半球中纬度地区特别是东亚的气候产生了显著影响。不论从科学层面还是国家的战略需求,都对掌握北极的气温变化提出了更高的要求。北极气温在显著增温的同时,还表现出强烈的年际变化。El Ni(?)o-Southern Oscillation(ENSO)作为全球气候系统最显著的年际振荡信号,是全球年际变率可预测性的主要来源之一,其能够通过遥相关作用对中高纬度地区的气候产生重要影响。研究ENSO对北极的影响,对提高北极气温的预测能力具有重要的科学意义。在北极快速增暖的同时,以热带太平洋海温异常为主要特征的ENSO事件,在振幅、空间模态和生命周期等时空特征上都表现出明显的多样性,并具有不同的气候影响。然而,目前在热带太平洋海温异常以及ENSO事件多样性对北极气温的影响上,现有的认识还比较有限。基于此,本文开展了热带太平洋海温异常多样性对北极气温影响的研究,期待提高对ENSO多样性的气候影响差异和北极气温变化规律的认识。论文得到的主要结论如下:(1)北极地区的增温趋势存在显著的季节锁相,秋冬两季增温速率最大,但秋季的增温趋势呈现显著的阶段性特征,冬季则呈现明显的线性趋势。同时,秋冬季节的气温变化均伴随强烈的年际振荡。在垂直分布上,北极地区从地表到对流层上层均表现出显著的升温趋势。除夏季以外,其他三个季节的最大升温速率均出现在近地面。对冬季北极气温进行经验正交分解,得到第一模态为趋势模态,变温中心位于格陵兰岛、巴伦支海-喀拉海一带;第二、三模态以年际振荡为主,分别呈现欧亚-北美偶极子分布和极地内外反位相的跷跷板分布特征。北极地区冬季气温强烈的年际振荡和区域性差异指示,除去温室效应造成的长期增温,包含ENSO在内的气候系统内部变率能够造成极地气温强烈的年际变化。(2)为了分析热带太平洋海温异常对北极气温的可能影响,初步分析了极端El Ni(?)o事件下冬季北极气温的响应。1950年以来,三次极端El Ni(?)o事件分别发生在1982-1983、1997-1998和2015-2016,三次事件均表现为东太平洋海温显著正异常,也都伴随正位相的PNA(Pacific-North American)波列,但北极地区的温度响应却存在显著差异。进一步分析发现,三次极端El Ni(?)o事件的海温分布也存在一定差异,其中,1982/83年与1997/98年同属于典型的东部型ENSO事件,但其东太平洋海温的差异引发了不同的非绝热加热分布,造成向极波列位相的差异,导致北极地区出现相反的气温变化。以上工作确定了热带太平洋海温异常对北极气温的影响,指出海温的多样性分布能够造成对北极气温的多样性影响,因此,需要进一步划分热带太平洋海温分布的多样性特征。(3)为了探究热带太平洋海温异常的多样性,本文提出了经验正交分解(EOF)和k-means聚类相结合的分类方法,该方法既弥补了EOF分解法的强制正交局限性,又利用了EOF主模态的物理含义。热带太平洋海温异常的前三个主海温模态包含两个以年际变率为主的内部模态(EOF1和EOF3)和一个背景模态(EOF2)。其中,背景模态的时间变化趋势与全球平均气温的变化趋势一致,被认为是热带太平洋海温平均态对全球变暖的响应。通过引入总方差距离和总轮廓系数,客观地将热带太平洋海温异常变化分为10类,其中包括了近年来出现频率较高的全海盆增温型、冷舌型模态和极端冷ENSO模态。研究表明,背景模态的位相变化伴随热带太平洋东-西部之间的海温梯度增加,其内部模态之间的关系也相应地发生了改变。进一步通过比较1950-1969年和1997-2016年两个时间段的海温异常变化,发现热带太平洋海温的系统性变化表现为:C-index指代的极端冷异常/中等强度的暖异常海温形态与E-index指代的极端暖异常海温形态,在全球变暖下,两种形态的区分度变得更加明显。(4)北极地区的升温趋势在冬季最显著,并伴随强烈的年际振荡。通常,ENSO事件也在冬季达到峰值。因此,基于(3)的研究,分析冬季不同海温类型对北极气温的影响差异。对暖事件海温类型的研究发现,全海盆暖异常(第2类)和中部型暖事件类型(第5类)对北极气温的影响,除了在位置上存在东西偏差,整体上比较相似。这两种海温类型在传统的类型划分中,通常都归为中部型事件,其对北极气温的影响与前人的研究基本一致。而冷事件海温类型的变化比暖事件更加明显:与过去频繁发生的全海盆冷异常类型相比,自80年代以来,经典La Ni(?)a类型(第6类)和中部型La Ni(?)a类型(第7类)的发生频率明显增加,冷中心明显西移和加强。其中,中部型La Ni(?)a类型能够引发西太平洋地区对流中心向北偏移,从而诱发波弗特海到格陵兰岛地区明显升温,这与经典La Ni(?)a类型所产生的影响完全不同。进一步,利用数值模式敏感性实验证明了对流中心北移的作用。该部分工作通过观测分析和数值实验揭示了ENSO冷暖事件对北极气温的非对称影响,以及ENSO多样性变化对北极加速变暖的可能贡献。更多还原

【Abstract】 In the context of global warming,Arctic has exhibited a significant warming trend,with a warming rate much above the Earth average,namely the “Arctic amplification”.Along with the melting of the sea ice,the rapid Arctic warming has a significant impact on the climate in the central latitudes of the Northern Hemisphere,especially in East Asia.Both from the scientific level and the strategic needs of the country,higher requirements for forecasting the changes of Arctic surface air temperature(SAT)are needed.Besides significant warming trend,Arctic SAT also shows a strong interannual variability,which is suggested to be related to the interannual oscillations of climate system.El Ni(?)oSouthern Oscillation(ENSO)acts as the most significant interannual oscillation signal in the global climate system,which is also one of the main sources of global interannual variation predictability.ENSO events can trigger a remote effect on the climate in high latitudes.Therefore,it is important to investigate the relationship between ENSO and Arctic in order to improve the prediction ability of Arctic SAT.However,with the rapid warming of Arctic,ENSO exhibits distinct diversity in spatiotemporal features such as amplitude,spatial modes and life cycle,with different climatic effects.The scientific community currently has a very limited understanding of the impacts of ENSO diversity on Arctic SAT.Based on this,this paper conducts a study on the impact of ENSO diversity on Arctic SAT,looking forward to improving the understanding of the climate impact difference of ENSO diversity and the changes in Arctic SAT.The main results are summarized as follows:(1)Arctic warming trend has significant seasonal lock.Autumn and winter show greatest warming rate.At the same time,both the surface and the upper troposphere over the Arctic show significant warming trend,and during other seasons except summer,the maximum warming rate concentrates near the ground.Among them,the warming trend in autumn shows significant periodic characteristics,while which in winter shows obvious linear warming trend.Both autumn and winter are accompanied by strong interannual oscillations.Through empirical orthogonal function(EOF)decomposition,the warming trend mode(EOF1)in winter is obtained and the SAT warming center is located in the area of Greenland and the Barents-Kara Seas;The second and third modes are dominated by interannual variability,showing the seesaw distribution of EurasiaNorth America and the reverse phase inside and outside the polar regions.The strong interannual oscillations and regional differences of Arctic SAT in winter indicate that excluding the long-term warming from greenhouse effects,the internal variation of the climate system including ENSO is able to cause the interannual SAT changes in the polar region.(2)In order to analyze the possible impact of the tropical Pacific sea surface temperature anomalies(SSTAs)on Arctic SAT,the response of Arctic SAT in winter during extreme El Ni(?)o events was analyzed.Since 1950,three extreme El Ni(?)o events have occurred in 1982-1983,1997-1998 and 2015-2016.The three events are manifested as significant positive SSTAs in the east tropical Pacific,and all three events are also accompanied by typical Pacific-North American(PNA)waves.However,Arctic SATs in the three events showed significant difference.In further analysis,the SSTA distributions of the three extreme El Ni(?)o events are different.Among them,the ENSO events in1982/83 and 1997/98 are usually classified to the so-called eastern Pacific(EP)ENSO events.However,the difference in their eastern sea surface temperature for these two events triggered different non-adiabatic thermal distributions,resulting in differences in the polarward wave fluxes and the opposite SAT distribution in the Arctic.The above work has confirmed the impacts of the tropical Pacific SSTAs on the Arctic region,pointed out that the diversity distribution of SSTAs can induce the diversity of the influence on Arctic SAT.Therefore,further dividing the diversity characteristics of the tropical Pacific SSTAs distribution is needed.(3)ENSO events are usually monitored by the tropical Pacific sea surface temperature anomaly(SSTA)patterns with dramatic impacts on the global climate.To explore the diversity of the tropical Pacific SSTA,a novel method combined EOF decomposition and the k-means clustering algorithm is carried out to classify SSTA patterns during 1950-2016.Meanwhile,the total distance variance and total silhouette value are introduced to determine an optimal number of distinguishable representative SSTA patterns.Ten SSTA patterns are obtained,which shows frequent basin-wide warming and extreme cold ENSO events in recent decades.It may be attributed to the changes in composition of the intrinsic modes along with the background mode of slowly increasing east–west SST gradient.The comparative analysis between periods 1950-1969 and 1997-2016 suggests that the two regimes of tropical Pacific SSTA,featured as extreme warm and moderate warm/extreme cold patterns respectively,become more distinct under recent global warming.(4)Including significant warming trend,the Arctic SAT changes also exhibit strong interannual variations in various fields,which is suggested to be related to ENSO events.Previous studies have demonstrated the different impacts on the Arctic of central Pacific(CP)and eastern Pacific(EP)ENSO events,and suggested these impacts are largely of opposite sign for ENSO warm and cold phases.Our results illustrate asymmetrical changes for the cold and warm ENSO events,especially for the La Ni(?)a events.Compared to the past frequent basin-wide cooling La Ni(?)a events,since the 1980 s the cooling center for the La Ni(?)a event has strengthened and moved westward along with the increasing frequency for the canonical and CP La Ni(?)a events.Contrary to the basinwide cooling and canonical La Ni(?)a events,the frequent CP La Ni(?)a events induce significant warming from the Beaufort Sea to Greenland via the convection center moving northward over the western Pacific.Observation analysis and numerical experiments both suggest that the changes in La Ni(?)a type may also accelerate Arctic warming.更多还原