全球变化背景下厄尔尼诺的模态变异及其与东亚和北极夏季气候异常的遥相关关系

发布时间：2018-08-05 16:25

【摘要】：在过去的三十几年,中太平洋型的厄尔尼诺在全球变暖背景下越来越频繁地出现,并且其强度也越来越强。但是中太平洋型厄尔尼诺的模态特征还存在一些争议。同时,伴随着厄尔尼诺的模态转变,东亚夏季风降水和北极的夏季气候异常变得十分不同,而它们之间的联系目前尚不完全清楚。本研究主要关注夏季热带太平洋海温的年际变化特征及其与东亚和北极区域夏季年际气候异常的联系。论文的主要研究结论如下:一、重新审视了两类厄尔尼诺的年际变化特征,并提出了一对新的Nino指数以改进ENSO的实时监测首先利用一种叫作RC-REOF的联合方法,分别从逐月和季节的角度重新审视了不同ENSO的主模态。得到以下新的认识:(1)东太平洋型ENSO和中太平洋型ENSO的解释方差是大体相当的,分别在33-43%和23-28%范围内。(2)利用传统EOF得到的El Nino Modoki的解释方差只有11-12%,远小于东部型El Nino的解释方差,这不符合近几十年中部型ENSO频繁发生的这一事实。(3)东部型ENSO和中部型ENSO的空间模态都是关于赤道非对称的,这一点没有被传统的Nino指数考虑进去。据此,这里推出一对新的Nino指数,叫作Nino3b和Nino4b,它们具有以下优点:(1)计算简单,(2)和两类ENSO模态的关系强且稳定,(3)对太平洋年代际信号有较好的代表性,(4)易于实时区分ENSO的类型,(5)没有数学上的正交限制,等。因此,Nino3b和Nino4b在科学研究和两类ENSO的实时监测方面具有潜在的应用价值。此外,还定义了 Nino3.4b指数用于描述混合型的ENSO事件,相比于Nino3.4指数,它与两类ENSO具有更加均等的协方差,并且也包含更多的年代际信号。二、揭示了中部型厄尔尼诺使得盛夏东亚季风和降水减弱的物理机制研究表明东亚夏季风和长江流域盛夏降水异常的年际变化和中部型厄尔尼诺具有密切的联系。通过对比分析中部型厄尔尼诺指数和东亚夏季风指数的回归或相关结果,我们揭示了中部型厄尔尼诺使得盛夏东亚季风和长江流域降水减弱的物理机制。对比分析结果表明,中部型厄尔尼诺可引起西太平洋整个对流层的显著增暖,从而使得东亚-西太平洋之间的海陆热差异减弱,进而通过减弱东亚夏季风环流(比如南亚高压、西北太平洋副热带高压、及南海-菲律宾海的低层反气旋,等),最终改变对流层水汽输送方向,导致长江流域盛夏降水减少。中部型厄尔尼诺指数和东亚夏季风指数还与太平洋-日本遥相关(或东亚-太平洋遥相关)紧密联系,这可能是东亚夏季风降水年际变率的内部动力因素。并且,显著的中太平洋拉尼娜型海温异常模态可以直接由东亚夏季风指数回归得到,进一步证明了中部型厄尔尼诺的确扮演着使东亚夏季风降水减弱的重要角色。三、发现了变异的中部型厄尔尼诺能抑制夏季北极变暖和北极海冰融化夏季北极的变化不仅包括气候态和气候趋势的变化,还包括强烈的年际变化。虽然热带-热带外遥相关对厄尔尼诺的变化很敏感是众所周知的,但北极气候的年际变率是否与厄尔尼诺的模态转变有关,目前尚不清楚。这里我们证明了中部型厄尔尼诺与夏季北极气候之间存在大尺度遥相关关系:观测结果表明中部型厄尔尼诺引起的中太平洋海温增暖可导致夏季北极涡加深,和绕极西风急流减弱,从而使得北极夏季变冷、海冰增加。大气模式模拟的结果基本能捕捉到类似的北极环流响应和强烈的北极变冷异常。据此我们认为:(1)如果在全球变暖背景下不考虑中部型厄尔尼诺对北极增暖的抑制作用,夏季北极海冰的融化将更加剧烈;(2)这个赤道-北极遥相关的发现——通过"海洋记忆"和"大气桥"作用——对提高北极气候异常的预报具有潜在的贡献。
[Abstract]:In the past three decades, the mid Pacific El Nino has appeared more and more frequently in the context of global warming, and its intensity is becoming stronger and stronger. However, there are still some controversies in the modal characteristics of the Middle Pacific type EL Nino, and the East Asian summer monsoon precipitation and the summer climate of the Arctic are different with the El Nino modal transformation. The relationship between the interannual variations of the tropical Pacific sea temperature in summer and the interannual climate anomalies in summer in East Asia and the Arctic region are mainly concerned. The main conclusions of this paper are as follows: first, the interannual variation of the two types of El Nino is reexamined. A new Nino index is proposed to improve the real-time monitoring of ENSO. First, a joint method called RC-REOF is first used to reexamine the main modes of different ENSO from a monthly and seasonal point of view. The following new understandings are obtained: (1) the explanatory variances of the eastern Pacific ENSO and the mid Pacific ENSO are roughly equivalent, respectively in 33-43% And within the range of 23-28%. (2) the explanatory variance of the El Nino Modoki obtained by the traditional EOF is only 11-12%, far less than the interpretation variance of the eastern El Nino, which does not conform to the fact that the central ENSO frequently occurs in recent decades. (3) the spatial modes of the Eastern ENSO and central ENSO are all about the equatorial asymmetries, and this is not traditional. The Nino index is taken into account. Here, a new pair of Nino exponents, called Nino3b and Nino4b, has the following advantages: (1) simple calculation, (2) strong and stable relations with the two ENSO modes, (3) a better representation of the Pacific interdecadal signals, (4) easy to distinguish the type of ENSO in real time, (5) no mathematical orthogonal constraints, etc. Therefore, Nino3b and Nino4b have potential application value in scientific research and real-time monitoring of two types of ENSO. In addition, the Nino3.4b index is also defined to describe mixed ENSO events. Compared with the Nino3.4 index, it has a more homogeneous covariance with the two class of ENSO and also contains more decadal signals. Two, revealing the central part. The study of the physical mechanism of the weakening of the East Asian monsoon and precipitation in the summer shows that the interannual variation of the East Asian summer monsoon and the anomaly of summer precipitation in the Yangtze River Basin is closely related to the middle type EL Nino. By comparing the regression and correlation results of the central El Nino index and the East Asian summer monsoon index, we reveal that The central type EL Nino makes the physical mechanism of the weakening of the precipitation in the East Asian monsoon and the Yangtze River Basin in the summer. The comparative analysis shows that the central El Nino can cause a significant warming of the whole troposphere in the Western Pacific, which makes the difference between the sea and land heat between East Asia and the West Pacific weaken and then weaken the East Asian summer monsoon circulation (for example, South). The subtropical high, the subtropical high in the Northwest Pacific and the low level anticyclone in the South China Sea and the Philippines sea eventually change the direction of the tropospheric water vapor transport, resulting in a decrease in the summer precipitation in the Yangtze River Basin. The central El Nino index and the East Asian summer monsoon index are also closely related to the Pacific Japan teleconnection (or the East Asia Pacific teleconnection). It is the internal dynamic factor of the interannual variability of the East Asian summer monsoon precipitation. And the remarkable anomalous mode of the La Nina type sea temperature in the central Pacific can be directly regressed by the East Asian summer monsoon index. It is further proved that the central type EL Nino does play an important role in weakening the East Asian summer monsoon precipitation. Three, the central type of variation is found. Nino can suppress the summer Arctic warmer and Arctic sea ice melting in summer, which includes not only changes in climate and climate trends, but also strong interannual variations. Although tropical extratropical teleconnection is well known to the changes in El Nino, the interannual variability of the Arctic climate is associated with the El Nino mode. The transition is not clear. Here we prove that there is a large scale teleconnection between the central El Nino and the summer Arctic climate: observations show that the warming of the central Pacific Ocean temperature caused by the central El Nino can lead to the deepening of the Arctic vortex in summer and the weakening of the winds around the polar west wind, which makes the Arctic summer cold. Ice increases. The results of atmospheric model simulation can basically capture similar Arctic circulation responses and intense Arctic cooling anomalies. Accordingly, we think: (1) the melting of Arctic sea ice will be more intense in summer if it does not consider the inhibitory effect of central El Nino on Arctic warming in the context of global warming; (2) this equatorial Arctic tele phase Guan's discovery, through "ocean memory" and "atmospheric bridge" functions, has the potential to contribute to improving the prediction of Arctic climate anomalies.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P732;P461.2

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