【作者】 徐康；

【导师】 何金海； 祝从文；

【作者基本信息】 南京信息工程大学 ， 气象学， 2013， 博士

【摘要】 厄尔尼诺-南方涛动(ENSO)是热带太平洋大尺度海气耦合作用的主要模态,其对全球气候异常有着极其重要的影响。研究证据表明,热带太平洋存在两种不同类型且相对独立的ENSO事件,即以热带东太平洋海表面温度(SST)异常为主的东部(Eastern-Pacific, EP)型ENSO和以热带中太平洋SST异常增暖(变冷),热带西太平洋和热带东太平洋SST变冷(增暖)的中部(Central-Pacific, CP)型ENSO。本论文以EP和CP型ENSO为研究核心,从海洋和大气多个角度深入分析两类ENSO的海气耦合模态和周期变化等主要特征,重点探讨EP和CP型ENSO事件分别对热带大气响应和东亚气候异常的关联,并利用国际耦合模式比较计划5(CMIP5)发布的多情景下全球气候系统模式模拟结果,通过对EP型和CP型ENSO模拟能力的评估,试图揭示两种类型ENSO自然振荡属性及其未来对全球气候变化的可能响应。主要结论概括如下：(1)东部型和中部型ENSO事件分别表现出显著的年际尺度(2-7年)和年代际尺度(10-15年)周期变化特征,并以纬向偶极型和三极型模态于热带太平洋SST和次表层海温(SOT)场之间相互耦合。当CP型ENSO处于成熟位相时,暖SOT异常中心位于日界线(180°)以西地区,其可能导致Nino4区的SOT增温,同时引起热带中太平洋SST增暖。根据SVD分析结果中SOT场的第一和第三模态所对应的时间系数定义为东部型次表层指数和中部型次表层指数,该指标不仅与其它学者定义的两类ENSO指数具有较高的相关性,而且它们具备良好地描述和区分EP型和CP型的ENSO事件的能力。此外,20世纪80年代后SOT在年代际时间尺度上振幅明显增强这一事实说明,CP型ENSO事件的频率和强度均呈现出上升趋势,且在过去30年里CP型ENSO事件明显加强了对全球气候的影响。(2)EP型和CP型El Nino对热带纬向和垂直环流的响应存在显著差异,根据热带地区海气耦合关系重新定义EP型和CP型SO指数。EP-SO和CP-SO分别与EP型和CP型El Nino模态相互耦合,并呈现出年际(2-7年)和年代际(10-15年)的周期振荡特征。两种不同类型SO指数还将有效地描述热带地区不同的海气耦合作用：当EP-ElNino事件发生时,热带太平洋Walker环流异常表现为大尺度偶极型单圈环流结构；当CP-E1Nino事件发生时,Walker环流异常呈现出纬向三极型双圈环流结构。此外,EP型和CP型SO事件与中国冬春季节降水的关系在全国范围均呈现相反特征,而两类SO事件与中国夏秋季节的降水呈相反特征的地区却分别只集中于长江中下游地区和华南地区。以中国南方冬季降水为例,EP型ENSO往往造成我国南方冬季降水偏多,而CP型ENSO时,南方降水异常偏少,这种相反关系主要体现于年际变化尺度上,同时造成冬季降水异常显著差异的主要原因是西北太平洋异常反气旋环流差异。因此,进行预测东亚季风区季节性降水时必须考虑两类ENSO与我国降水之间关联的差异性。(3)通过评估CMIP5发布的20个气候模式在工业革命前(Pre-industrial)控制实验中模拟EP型和CP型ENSO耦合模态的能力发现,当前气候系统模式对热带太平洋SST年际变率的模拟能力强于年代际变率,且气候系统模式模拟EP-ENSO的能力相对强于CP-ENSO。所有CMIP5气候模式能够模拟出EP型ENSO的耦合模态,但仅12个模式能够真实反映CP型ENSO的耦合模态。基于这12个气候模式集合结果发现,EP型ENSO模态主要表现出显著的年际变率,而CP型ENSO模态主要表现为多年代际振荡。由于Pre-industrial控制实验仅仅考虑自然因子,因此,CP-ENSO的多年代际周期振荡和EP-ENSO的年际变化特征均属于气候系统中自然变率的结果。(4)CMIP5气候系统模式对两类ENSO的空间耦合模态和周期变化等主要特性具有较好的历史再现能力,即EP-ENSO呈现为纬向偶极型海气耦合模态,并以年际尺度振荡为主要特征,而CP-ENSO呈现为纬向三极型海气耦合模态分布,并主要以年代际尺度变化为特征,同时可以反映CP型El Nino事件发生频率增多和强度增强的趋势。21世纪在未来全球变暖背景下,热带太平洋EP型和CP型ENSO的空间耦合模态和周期变化等主要特性仍将维持,它们的模态强度较历史模拟结果有所增强,而且未来EP型和CP型El Nino事件亦呈现频率增多和强度增强的趋势。此外,造成热带太平洋SST长期趋势变化的主要原因是外源强迫因子的作用,CP型ENSO的年代际周期变化不仅是气候系统中的自然振荡,而且人为温室气体强迫对CP-ENSO年代际周期变化的影响并不显著。更多还原

【Abstract】 The El Nino-Southern Oscillation (ENSO) is the dominant mode of air-sea coupled interaction in the tropical Pacific, manifesting important impacts on the global climate. It has been increasingly recognized that there are two distinct types of ENSO events prevailing in the tropical Pacific:one is the Eastern-Pacific (EP) type that has sea surface temperature (SST) anomalies centered over the eastern Pacific, while the other is the Central-Pacific (CP) type with the SST anomalies situated over the central Pacific. Based on the phenomena of the two types of ENSO events, the air-sea coupled modes and the periodic variation of atmosphere and ocean signals in different ENSO category are analyzed in this thesis. The key point is the different tropic atmosphere responses and East Asian climate anomaly associated with the two types of ENSO. Also the simulations of climate system models in multi-scenarios (i.e., the pre-industrial, historical, and future climate projection, namely Representative Concentration Pathways4.5, RCP4.5) released by the Coupled Model Intercomparison Project phase5(CMIP5) are used to evaluate the simulating capability of both EP-ENSO and CP-ENSO, with an attempt to reveal the natural oscillation of EP-and CP-ENSO and their possible responses to the global climate change in future. The major conclusions are summarized as follows:(1) The EP-and CP-ENSO events show the2-7and the10-15years oscillation in the tropical SST anomalous field, respectively. The coupling between subsurface ocean temperature (SOT) and SST anomalies is featured in a zonal dipole mode in the EP-ENSO group, while in a zonal tripole mode in the CP-ENSO category. During the mature phase of CP-ENSO, the warm center of SOT anomalies is located to the west of dateline to increase the SOT in the Nino4region, causing the CP-ENSO event simultaneously. The eastern and central Pacific subsurface indices are defined by the expansion coefficients of the first and third SVD mode for SOT. Such indices are not only highly correlated with those deduced by other researchers, but they can well describe and distinguish the EP and CP-ENSO events. In addition, the SOT amplitude on the interdecadal time scale has enhanced since the late1980s, indicating the increment of frequency and intensity of CP-ENSO events. This result suggests that the impact of CP-ENSO events on the global climate has been more significant in the past decades.(2) There are differences in the responses of EP-and CP-El Nino to the zonal and vertical circulation in the tropics. Accordingly, a new eastern and central Pacific southern oscillation index (i.e., EP-and CP-SOI) is defined based on the air-sea coupled relationship in the tropics, respectively. Results suggest that EP-SOI and CP-SOI is coupled with the EP-and CP-El Nino, respectively. The EP-SOI exhibits interannual variability (2-7yr), while decadal (10-15yr) variations in the CP-SOI are more dominant. During the EP-ENSO, the Walker circulation shows a dipole structure with a signal cell over the Pacific. However, the Walker circulation shows a tripole structure with double cells over the Pacific when CP-ENSO prevails. Furthermore, the correlation of seasonal precipitation across China’s mainland with EP-ENSO is opposite to that with CP-ENSO during winter and spring. But the differences in relationship between rainfall and the two types of ENSO is only restricted in the lower reaches of Yangtze River and Southern China (SC) during summer and fall, respectively. Such contrary relationship associated with the EP-and CP-ENSO events is primarily reflected on the interannual time scale. For example, EP-ENSO events generally lead to more rainfall over SC in winter, while the negative rainfall anomalies are observed in SC during the CP-ENSO events. And the distinct winter rainfall anomalies over SC responses to EP-and CP-ENSO can be attributed to the difference in both strength and location of the Northwestern Pacific anticyclone circulation. Therefore, the different response of Chinese rainfall to EP-and CP-ENSO should be taken into consideration when predicting the seasonal rainfall in East Asian monsoon regions.(3) The control runs of20climate models in the pre-industrial are examined to evaluate the capability of simulating air-sea coupling modes in EP-and CP-ENSO. The results point out that the current models have higher skills in simulating the interannual variances than the interdecadal variability of SST anomalies in the tropical Pacific. And the EP-ENSO can be better simulated than the CP-ENSO by the current climate models. All the20models can simulate the observed EP-ENSO modes, but only12in them exhibits good performance in simulating the CP-ENSO modes. The composite result of the12model outputs suggests that the model-simulated EP-and CP-ENSO exhibits an interannual and multi-decadal oscillation, respectively. Because there is no external forcing in the control runs during the pre-industrial, such results support that both the decadal oscillation of CP-ENSO mode and the interannual oscillation of EP-ENSO mode are due to the air-sea interaction in natural climate system.(4) The climate system models in CMIP5can well simulate the spatial distribution of coupling modes and their periodic changes associated with EP-and CP-ENSO in the historical runs. The historical simulated EP-ENSO shows an air-sea coupling mode in zonal dipole distribution and exhibits interannual variability, while the simulated air-sea coupling mode of CP-ENSO is a zonal tripole pattern with evident decadal oscillation. The frequency and intensity of CP-E1Nino events simulated by the CMIP5models have been enhanced as that in observation. Results of four global climate system models for the21st century assessment indicate that primary characteristics of air-sea coupling modes and periodic variation associated with the two types ENSO events will be still maintained under the background of global warming, and the their strengths would increase in the future. Moreover, the major reason for changes in the long-term trend of SST over the tropical Pacific is the external forcing factors. Although the decadal variation of CP-ENSO is a natural oscillation of air-sea interaction in climate system, it is little influenced by the anthropogenic greenhouse gas forcing.更多还原