Interannual and interdecadal variability of ocean temperature along the equatorial Pacific in conjunction with ENSO

In this paper, the leading modes of ocean temperature anomalies (OTA) along the equatorial Pacific Ocean are analyzed and their connection with El Niño-Southern Oscillation (ENSO) and interdecadal variation is investigated. The first two leading modes of OTA are connected with the different phases of the canonical ENSO and display asymmetric features of ENSO evolution. The third leading mode depicts a tripole pattern with opposite variation of OTA above the thermocline in the central Pacific to that along the thermocline in the eastern and western Pacific. This mode is found to be associated with so-called ENSO-Modoki. Insignificant correlations of this mode with the first two leading modes suggest that ENSO-Modoki may be a mode that is independent to the canonical ENSO and also has longer time scales compared with the canonical ENSO. The fourth mode reflects a warming (cooling) tendency above (below) the thermocline since 2000. Both the first and second modes have a large contribution to the interdecadal change in thermocline during 1979–2012. Also, the analysis also documents that both ENSO and OTA shifted into higher frequency since 2000 compared with that during 1979–1999. Interestingly, the ENSO-Modoki related OTA mode does not have any trend or significant interdecadal shift during 1979–2012. In addition, it is shown that first four EOF modes seem robust before and after 1999/2000, suggesting that the interdecadal shift of the climate system in the tropical Pacific is mainly a frequency shift and the changes in spatial pattern are relatively small, although the mean states over two periods experienced some significant changes.     

CMIP5模式对青藏高原中东部夏季降水双极型模拟能力的评估

青藏高原中东部夏季降水主要表现为东北和东南反位相变化的双极型特征。采用经验正交函数(empirical orthogonal function,EOF)分解方法,系统性地评估参与第五次耦合模式比较计划 (Coupled Model Intercomparison Project Phase 5,CMIP5)历史模拟试验的 47 个模式对青藏高原中东部夏季降水双极型变化特征的模拟能力。结果表明,大多数模式基本可以反映青藏高原中东部夏季降水东北部和东南部反位相的变化特征。模式间 EOF 分析结果表明在35°N 以南的东西向模拟偏差是 CMIP5 模式模拟降水空间型态的主要偏差,且大多数模式对时间系数的模拟效果差于空间型态。文中定义了一个综合评估指标 Snew 来定量描述模式对空间型态、时间系数以及方差贡献的综合模拟效果。由定量评估结果来看,MIROC-ESM、HadGEM2-CC 和 ACCESS1-0 (FIO-ESM、 HadGEM2-AO 和 MIROC-ESM-CHEM)模式对观测降水的 EOF1(EOF2)模态的综合模拟能力相对较好,而 GISS 系列模式、CESM1-CAM5 和 MPI-ESM-LR (CMCC-CESM、MPI-ESM-MR 和 GFDL- CM3)模式对观测降水的 EOF1(EOF2)模态的综合模拟效果较差。由 EOF1 和 EOF2 的综合评估结果来看,MIROC-ESM-CHEM模式对观测降水的 EOF1 和 EOF2 模态的综合模拟效果最好。     

How predictable is the northern hemisphere summer upper-tropospheric circulation?

The retrospective forecast skill of three coupled climate models (NCEP CFS, GFDL CM2.1, and CAWCR POAMA 1.5) and their multi-model ensemble (MME) is evaluated, focusing on the Northern Hemisphere (NH) summer upper-tropospheric circulation along with surface temperature and precipitation for the 25-year period of 1981–2005. The seasonal prediction skill for the NH 200-hPa geopotential height basically comes from the coupled models’ ability in predicting the first two empirical orthogonal function (EOF) modes of interannual variability, because the models cannot replicate the residual higher modes. The first two leading EOF modes of the summer 200-hPa circulation account for about 84% (35.4%) of the total variability over the NH tropics (extratropics) and offer a hint of realizable potential predictability. The MME is able to predict both spatial and temporal characteristics of the first EOF mode (EOF1) even at a 5-month lead (January initial condition) with a pattern correlation coefficient (PCC) skill of 0.96 and a temporal correlation coefficient (TCC) skill of 0.62. This long-lead predictability of the EOF1 comes mainly from the prolonged impacts of El Niño-Southern Oscillation (ENSO) as the EOF1 tends to occur during the summer after the mature phase of ENSO. The second EOF mode (EOF2), on the other hand, is related to the developing ENSO and also the interdecadal variability of the sea surface temperature over the North Pacific and North Atlantic Ocean. The MME also captures the EOF2 at a 5-month lead with a PCC skill of 0.87 and a TCC skill of 0.67, but these skills are mainly obtained from the zonally symmetric component of the EOF2, not the prominent wavelike structure, the so-called circumglobal teleconnection (CGT) pattern. In both observation and the 1-month lead MME prediction, the first two leading modes are accompanied by significant rainfall and surface air temperature anomalies in the continental regions of the NH extratropics. The MME’s success in predicting the EOF1 (EOF2) is likely to lead to a better prediction of JJA precipitation anomalies over East Asia and the North Pacific (central and southern Europe and western North America).     

东亚夏季经向水汽输送的变异及其对极端降水的影响

东亚夏季降水的异常与水汽输送的变异密切相关。基于1958—2016年资料,研究了夏季东亚季风区经向水汽输送的主要变异特征及其对东亚夏季极端降水的影响。经向水汽输送的第一主变异模态表现出中国东部和西北太平洋上的水汽经向输送呈现反向异常,以年际变化为主。当中国东部向北输送的水汽增强(减弱)而西北太平洋向北输送减弱(增强),则中国东部大范围的极端降水量及频次增加(减少)。该模态与西太(西太平洋)副高西伸(东撤)有关,并主要受到热带中东印度洋海温的影响。第二变异模态以年代际变化为主兼有年际变化,表现在1980年后中国东部及邻近海域上空的经向水汽输送减弱,使得环渤海地区和华南沿海的极端降水量及频次减少而长江上、下游和贵州的极端降水量及频次增加。该模态与西太副高的减弱有关,并受到热带西太海温年代际增温的影响。第三变异模态以年际变化为主兼有年代际变化,反映中国长江以北地区和日本南部及附近区域的经向水汽输送的反相变化结构。长江以北水汽输送减弱(增强),可导致华北、东北的极端降水量及频次减少(增加)和长江下游及江南地区的极端降水量及频次的减少(增加)。该模态主要受欧亚大陆上空中高纬度纬向遥相关波列和热带印太（印度洋太平洋）海温异常的影响。      

Interdecadal changes in the Asian winter monsoon variability and its relationship with ENSO and AO

Interdecadal changes in the Asian winter monsoon (AWM) variability are investigated using three surface air temperature datasets for the 55-year period of 1958–2012 from (1) the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis 1 (NCEP), (2) combined datasets from the European Centre for Medium-range Weather Forecasts (ECMWF) 40-yr reanalysis and interim data (ERA), and (3) Japanese 55-year reanalysis (JRA). Particular attention has been paid to the first four empirical orthogonal function (EOF) modes of the AWM temperature variability that together account for 64% of the total variance and have been previously identified as predictable modes. The four modes are characterized as follows: the first mode by a southern warming over the Indo-western Pacific Ocean associated with a gradually increasing basin-wide warming trend; the second mode by northern warming with the interdecadal change after the late 1980s; the third and fourth modes by north-south triple pattern, which reveal a phase shift after the late 1970s. The three reanalyses agree well with each other when producing the first three modes, but show large discrepancy in capturing both spatial and temporal characteristics of the fourth mode. It is therefore considered that the first three leading modes are more reliable than the rest higher modes. Considerable interdecadal changes are found mainly in the first two modes. While the first mode shows gradually decreasing variance, the second mode exhibits larger interannual variance during the recent decade. In addition, after the late 1970s, the first mode has a weakening relationship with the El Niño-Southern Oscillation (ENSO) whereas the second mode has strengthening association with the Artic Oscillation (AO). This indicates an increasing role of AO but decreasing role of ENSO on the AWM variability. A better understanding of the interdecadal change in the dominant modes would contribute toward advancing in seasonal prediction and the predictability of the AWM variability.     

Spatial and interannual variations of spring rainfall over eastern China in association with PDO–ENSO events

The spatio-temporal variations of eastern China spring rainfall are identified via empirical orthogonal function (EOF) analysis of rain-gauge (gridded) precipitation datasets for the period 1958–2013 (1920–2013). The interannual variations of the first two leading EOF modes are linked with the El Niño–Southern Oscillation (ENSO), with this linkage being modulated by the Pacific Decadal Oscillation (PDO). The EOF1 mode, characterized by predominant rainfall anomalies from the Yangtze River to North China (YNC), is more likely associated with out-of-phase PDO–ENSO events [i.e., El Niño during cold PDO (EN_CPDO) and La Niña during warm PDO (LN_WPDO)]. The sea surface temperature anomaly (SSTA) distributions of EN_CPDO (LN_WPDO) events induce a significant anomalous anticyclone (cyclone) over the western North Pacific stretching northward to the Korean Peninsula and southern Japan, resulting in anomalous southwesterlies (northeasterlies) prevailing over eastern China and above-normal (below-normal) rainfall over YNC. In contrast, EOF2 exhibits a dipole pattern with predominantly positive rainfall anomalies over southern China along with negative anomalies over YNC, which is more likely connected to in-phase PDO–ENSO events [i.e., El Niño during warm PDO (EN_WPDO) and La Niña during cold PDO (LN_CPDO)]. EN_WPDO (LN_CPDO) events force a southwest–northeast oriented dipole-like circulation pattern leading to significant anomalous southwesterlies (northeasterlies) and above-normal (below-normal) rainfall over southern China. Numerical experiments with the CAM5 model forced by the SSTA patterns of EN_WPDO and EN_CPDO events reproduce reasonably well the corresponding anomalous atmospheric circulation patterns and spring rainfall modes over eastern China, validating the related mechanisms.     

我国东部夏季降水异常主模态的年代际变化及其与东亚水汽输送的关系

本文利用1958～2000年ERA-40再分析每日资料和我国516台站降水资料以及EOF方法,分析了我国东部季风区夏季降水异常主模态的年代际变化特征及其与东亚上空水汽输送通量时空变化的关系.分析结果表明了我国东部季风区夏季降水的时空变化存在两种主模态:第1主模态不仅显示出明显的准两年周期振荡的年际变化特征且也有明显的年...     

ROLE OF EQUATORIAL PACIFIC OCEAN SUBSURFACE OCEANIC TEMPERATURE MODE IN ENSO CYCLE

Based on the simple ocean data assimilation (SODA) reanalysis dataset from the University of Maryland and the method of Empirical Orthogonal Functions (EOF), the characteristics of interannual and interdecadal variabilities of the equatorial Pacific subsurface oceanic temperature anomaly (SOTA) are captured. The first and second modes of the equatorial Pacific SOTA in the interannual and interdecadal variations are found respectively and the effect of the second mode on the ENSO cycle is discussed. Results show that the first mode of SOTA’s interannual and interdecadal variabilities exhibit a dipole pattern, indicating that the warm and cold temperature anomalies appear simultaneously in the equatorial subsurface Pacific. The second mode shows coherent large-scale temperature anomalies in the equatorial subsurface Pacific, which is a dominant mode in the evolution of ENSO cycle. The temporal series of the second mode has a significant lead correlation with the Ni?o-3.4 index, which can make a precursory prediction signal for ENSO. The function of this prediction factor in SOTA is verified by composite and case analyses.     

西南季风期间斯里兰卡降水的年代际、年际变化及其与热带印度洋的联系

本文主要研究了1979—2016期间斯里兰卡在西南季风期间降水的年代际、年际变化以及与印度洋海温的联系.首先用经验正交的方法分析了斯里兰卡以及周边地区降水的时空分布,发现前两个模态能够解释超过70%的方差.其中第一模态为均一模态,且其PC1以及斯里兰卡7 a滑动平均降水序列都有年代际变化,降水异常在2000年前后异常偏多和偏少.通过合成分析发现2000年之后降水的异常减少与热带西部、中部印度洋的暖海温异常有关.暖海温异常通过调整经向环流引起了斯里兰卡上空的下沉运动,抑制了降水.在第二模态中,负的信号出现在斯里兰卡大部分地区,只有在斯里兰卡北部海角很小地区出现了正的信号.PC2表现出了年际变化,且与热带东南印度洋海温异常有显著的关系.通过Gill-Matsuno响应,热带东南印度洋海温异常造成热带北印度洋上空的气旋性环流异常,引起了水汽的辐合,从而利于降水.     

Contributions to the Interannual Summer Rainfall Variability in the Mountainous Area of Central China and Their Decadal Changes

Using a high-resolution precipitation dataset,the present study detected that the mountainous area of central China(MACA)is a hotspot of ENSO’s impact on the summer rainfall variability.Further analysis suggests that both ENSO and atmospheric forcing make contributions to the summer rainfall variability in MACA.The dominant rainfall-related SST mode features as a seasonal transition from an El Niño-like warming in the preceding winter to a La Nina-like cooling in the following autumn,and it explains about 29%of the total variance of the rainfall during 1951–2018.It indicates that ENSO with a rapid phase transition is responsible for inducing summer rainfall anomalies in MACA.Besides,an upperlevel circumglobal wave mode in the Northern Hemisphere during summer also explains about 29%of the summer rainfall variance.Contributions of both the SST and the atmospheric modes have experienced interdecadal changes.The influence of the SST mode gradually increases and plays a dominant role in the recent decades,suggesting that ENSO with a rapid phase transition becomes more important for rainfall prediction in MACA.     

10月份热带印度洋海气耦合的统计动力诊断

本文将热带印度洋10月份的大气风场和海洋上层流场看作一个整体, 对其作了动力统计诊断, 即作了复EOF分析, 考察了其年际和年代际变化, 并揭示其与印度洋偶极子 (IOD) 和ENSO的关系。结果表明: 在同一模态中, 海洋模态表现出很强的赤道俘获现象, 而大气则无此现象; 第一模态为印度洋偶极子模态; 第二模态为ENSO前期在印度洋的延伸模态。前2个模态的风场都揭示了Walker环流异常的结构; 印度洋海温的年际变化主要取决于印度洋地区的海气耦合状态, 但太平洋的ENSO循环对其也有一定影响。     

The Major Interannual Modes of Late-Summer Precipitation Over the Northern China Monsoon Region

ABSTRACT

Summer precipitation in the northern China monsoon region (NCMR; 35°–55°N, 108°–135°E) shows significant intraseasonal variability. The early-summer (June) and late-summer (July–August) precipitation patterns show clear differences in their formation mechanisms and the systems that affect them. We used empirical orthogonal function (EOF) analysis to investigate the two leading modes of July–August precipitation over the NCMR and their associated atmospheric circulation anomalies using linear regression. The results show that the first (EOF1) and second (EOF2) modes correspond to a pan-NCMR precipitation variation pattern and a precipitation oscillation pattern between North China (NC) and Northeast China (NEC), respectively. These two modes account for 22.1% and 10.1% of the total variance, respectively. The associated principal components (PCs) both have significant interannual variability with a period of 2–4 years. In addition, PC1 has significant interdecadal variability with a period of 20–30 years. Further analysis suggests that EOF1 and EOF2 clearly have a different relationship with the summer monsoon circulation system. In the positive phase of PC1, the East Asian subtropical westerly jet stream (EAWJS) shows a northward trend with higher intensity than normal the blocking high at mid- to high latitudes is inactive; and the western Pacific subtropical high (WPSH) is located to the north of its normal position. The NCMR is controlled by stronger southerly winds, which cause the convergence of water vapour, favouring more precipitation in this region and vice versa. In the positive phase of PC2, the EAWJS swings to the south of Lake Baikal. Significant positive height anomalies exist from western NC to NEC. Significant negative height anomalies occur to the subtropical northwestern Pacific. This indicates that the cold vortex in Northeast China is inactive, the WPSH tends to be weaker and located to the south of its normal position, and NEC (NC) is dominated by anomalous northeasterly (southeasterly) winds. The convergence (divergence) of water vapour in NC (NEC) favours more (less) precipitation in NC (NEC) and vice versa. Therefore, EOF1 is related to the large-scale circulation anomalies over East Asia and the northwest Pacific in July and August, whereas EOF2 is more closely related to the anomalies in the regional circulation over the NCMR and the subtropical northwestern Pacific.     

MRI-CGCM模式气候预测回报试验在东亚夏季的检验和降尺度订正

应用1979—2010年MRI-CGCM模式回报、NCEP/NCAR再分析数据和中国东部降水观测资料检验了模式对东亚夏季风的模拟能力,并利用模式500 hPa高度场回报资料建立了中国东部夏季降水的奇异值分解（SVD）降尺度模型。模式较好地模拟了亚洲季风区夏季降水的气候态,但模拟的季风环流偏弱、偏南,导致降水偏弱。模拟降水的方差明显偏小,且模拟降水的外部、内部方差比值低,模拟降水受模式初值影响较大。模式对长江雨型的模拟能力最高,华南雨型次之,华北雨型最低。模式对东亚夏季风第1模态的模拟能力明显高于第2模态。对于东亚夏季风第1模态,模式模拟出了西太平洋异常反气旋,但强度偏弱,且未模拟出中高纬度的日本海气旋、鄂霍次克海反气旋,导致长江中下游至日本南部降水偏弱。各时次模拟环流均能反映但低估了ENSO衰减、印度洋偏暖对西太平洋反气旋的增强作用。对于东亚夏季风第2模态,模式对西太平洋的“气旋-反气旋”结构有一定的模拟能力,但未模拟出贝加尔湖异常反气旋和东亚沿海异常气旋,导致中国东部“北少南多”雨型在模拟中完全遗漏。仅超前时间小于4个月的模拟降水能够反映ENSO发展对降水分布的作用。通过交叉检验选取左场时间系数可以提高降尺度模型的预测技巧,SVD降尺度模型在华南、江南、淮河、华北4个区域平均距平相关系数分别为0.20、0.23、0.18、0.02,明显高于模式直接输出。      

Interdecadal aridity variations in Central Asia during 1950–2016 regulated by oceanic conditions under the background of global warming

As a typical inland arid and semiarid region, Central Asia (CA) is vulnerable to the forced global warming (FGW) due to anthropogenic activity. Aiming at the interdecadal variation of the FGW-forced aridity pattern (FAP) in CA, we try to extract the associated oceanic and atmospheric modes by analyzing observations, reanalysis data and multi-model simulations during 1950–2016. The FAP in CA features a tripolar pattern with wetting–drying-wetting responses arranging from southeast to northwest and shows strong interdecadal-to-interannual amplitude variations. It is found that the sea surface temperature (SST) in the tropical South Atlantic (TSA) well correlates with the amplitude variation of FAP on interdecadal time scale, possibly through modulating the interannual SST modes characterized by the North Atlantic horseshoe-like dipole (NAHD) and the El Ninõ and South Oscillation (ENSO). Corresponding to the enhancing FAP from the middle 1970s to early 2000s, the TSA-modulated NAHD and ENSO, together with the Pacific Decadal Oscillation-modulated Indian Ocean Dipole-like mode, show connections with an Eurasian middle-latitude wave train coupled with the North Arctic Oscillation and equatorial low, which favors the moisture transport to strengthen the tripolar FAP by forming a local circulation dipole with positive/negative anomaly over the northwest/southeast CA. But after the early 2000s, the increasing FAP amplitude is decelerated due to the interdecadal decline of TSA accompanied by the weakened/reversed relationship between FAP and the NAHD/ENSO. Because of the corresponding breakdown of the wave train, the favorable local circulation is unavailable to support the sustained enhancement of FAP. Therefore, the multiscale coupling between the above oceanic and atmospheric modes is significantly related to the characteristic of stage of the forced aridity change in CA under the background of global warming.

     

夏季纬向平均气流变动的主要模态及其与AO和ENSO的联系

使用NCEP-NCAR月平均再分析资料,研究了夏季纬向平均气流变动的主要模态及其与北极涛动(AO)和ENSO的联系,探讨了夏季[u]主要模态维持的可能机制。结果表明,北半球夏季纬向平均[u]的异常分布表现为两个主要模态。EOF1反映了与AO相对应的纬向平均[u]的分布,EOF2反映了与ENSO相对应的纬向平均[u]的分布。周期分析显示了EOF1、EOF2分别具有与AO及ENSO相似的周期。滞后相关分析表明,除了夏季之外的其他季节EOF1与ENSO存在显著相关,而EOF2与AO亦存在显著相关。夏季,EOF1与ENSO、EOF2与AO的同期相关量值小且不显著。这些说明,北半球夏季纬向平均气流变化的主要模态可以有效分离,其分别代表了AO和ENSO有关的信号。这对于更好地理解AO和ENSO的全球影响具有重要意义。利用准地转无加速定理,发现EOF1和EOF2对应的时间系数高低值年的E-P通量散度的合成差值场分别与纬向平均[u]的EOF1、EOF2分布表现出很好的对应关系,其中行星波对纬向平均气流主要模态的维持起着主要作用。余差环流与E-P通量散度分布型相似,量值相当,但符号相反。在一些地区,大气涡动的摩擦耗散也起着一定的作用。这些平衡了波动的能量输送作用,使纬向平均[u]的结构得以维持。     

中国南方夏季降水的年代际变率主模态特征及机理研究

在气候变暖背景下,中国南方夏季降水存在明显的年代际变化特征。本文利用1920~2014年的逐月降水,以分析南方夏季降水年代际变率主模态为切入点,以研究南方夏季降水年代际变率空间分布型的年代际变化特征为重点,进一步研究了印度洋、北太平洋及北大西洋海温的年代际变率对南方夏季降水主模态年代际变率的可能影响机制。得到的主要结论包括:（1）指出中国南方夏季降水年代际变率的两个主模态为全区一致型和东西反相型降水模态。两个主导模态在1971/1972年发生了显著的年代际转变,在1925~1971年的第一主模态为东西反相型降水;在1972~2009年的第一主模态为一致型降水。不同主模态对应的海温异常关键区也在1971/1972年发生了相应的年代际变化。（2）揭示了全区一致型和东西反相型降水模态对应的环流场异常特征。一致多（少）型降水对应着中国南海及西北太平洋低空的反气旋（气旋）性异常,有（不）利于水汽自南海向南方地区输送。而贝加尔湖东侧低空的反气旋（气旋）性异常,有（不）利于冷空气向南方输送,并与来自南海地区的水汽在南方地区辐合,有利于南方地区降水一致偏多（少）。东多西少（西多东少）型降水对应着中国东南地区高空的正（负）异常中心,有利于高空辐散（辐合）及异常的上升（下沉）运动,其与南方地区东部低空的气旋（反气旋）性异常共同作用,有利于东部降水偏多（偏少）。与此同时,低空中南半岛反气旋（气旋）性异常及菲律宾地区反气旋（气旋）性异常,不（有）利于水汽自孟加拉湾及南海地区输送向南方地区西部,有利于形成东多西少（西多东少）的降水型。（3）揭示了印度洋海温、北太平洋海温和北大西洋海温协同影响南方地区东西反相型降水和一致型降水的机制。     

Interannual variability of winter precipitation over northwest India and adjoining region: impact of global forcings

The role of El Niño/Southern Oscillation (ENSO) and the mechanism through which ENSO influences the precipitation variability over northwest India and the adjoining (NWIA) region is well documented. In this study, the relative role of North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) and ENSO in modulating the Asian jet stream in the Northern Hemisphere winter and their relative impact on the precipitation variability over the region have been estimated through analysis of observed data. It is seen that interannual variations of NWIA precipitation are largely influenced by ENSO. An empirical orthogonal function (EOF) analysis has been carried out to understand dominant modes of interannual variability of zonal wind at 200 hPa of the Northern Hemisphere. The EOF-1 pattern in the tropical region is similar to that of an ENSO pattern, and the principal component (PC) time series corresponds to the ENSO time series. The EOF-2 spatial pattern resembles that of NAO/AO with correlation of PC time series with AO and NAO being 0.74 and 0.62, respectively. The precipitation anomaly time series over the region of interest has marginally higher correlation with the PC-2 time series as compared to that of PC-1. Regression analysis of precipitation and circulation parameters indicates a larger contribution of the second mode to variability of winds and precipitation over the NWIA. Moisture transport from the Arabian Sea during the active phase of NAO/AO and the presence of a cyclonic anomaly lead to higher precipitation over the NWIA region.     

我国东部梅雨期降水的年际和年代际变化特征及其与大气环流和海温的关系

基于我国160站59年(1951～2009年)的月降水观测资料、美国气象环境预报中心和国家大气研究中心(NCEP/NCAR)提供的再分析资料和Hadley中心的海表温度(Sea Surface Temperature,简称SST)资料,对我国东部(100°E以东,15°N～40°N)梅雨期(6月和7月)降水的时空变化特...     

东北亚地区初夏、盛夏和传统夏季降水特征及环流型的异同性研究

本文利用1979~2015年GPCP（Global Precipitation Climatology Project）逐月降水资料,采用经验正交函数（EOF）分解和Morlet小波分析方法,对东北亚地区初夏、盛夏和传统夏季降水的时空分布特征以及环流型开展了系统性的研究,揭示了东北亚地区传统夏季降水表现为盛夏降水贡献占主导,其年际和年代际特征以及环流特征同盛夏降水特征相一致,而初夏降水和盛夏降水特征及形成机制则具有显著差异。空间分布上,初夏的降水EOF第一模态表现为“+－+”的三极型分布,而盛夏和传统夏季则表现为南北相反的偶极型特征;时间演变方面,初夏降水表现为5~6 a振荡周期,盛夏为2~3 a为主的振荡周期,传统夏季则兼具上述两类振荡周期;在年代际调整方面,在1990年代末,盛夏降水和传统夏季降水在华北和东北地区发生了显著的年代际转折。此外,分析降水与环流的联系发现:初夏,由于西太平洋上空异常反气旋将西太平洋等地的水汽向北方地区输送,且受欧亚Ⅱ型（EUII）遥相关的作用,东北亚地区初夏降水异常具有明显纬向特征。盛夏,东北亚地区降水主要受到西太平洋副热带高压西伸北进、孟加拉湾和南海等地水汽加强的影响。欧亚Ⅰ型（EUI）遥相关和亚洲太平洋型（EAP）遥相关与我国东北以西和沿海地区的降水具有显著相关性。EU型遥相关的作用使东北亚夏季降水的异常中心存在西北—东南向的波列特征,EAP型遥相关的作用则使夏季降水存在经向三极型或偶极型特征。     

Future change of the Indian Ocean basin-wide and dipole modes in the CMIP5

The Indian Ocean sea surface temperature (SST) variability has been represented with the two dominant variability modes: the Indian Ocean basin-wide (IOBW) and dipole (IOD) modes. Here we investigate future changes of the two modes together with mean state and El Niño and Southern Oscillation (ENSO) relationship under the anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project by comparing the historical run from 1950 to 2005 and the RCP 4.5 run from 2050 to 2099. The five best models are selected based on the evaluation of the 20 models’ performances in simulating the two modes and Indian Ocean basic state for the latest 56 years. They are capable of capturing the IOBW and IOD modes in their spatial distribution, seasonal cycle, major periodicity, and relationship with ENSO to some extent. The five best models project the significant changes in the Indian Ocean mean state and variability including the two dominant modes in the latter part of twenty-first century under the anthropogenic warming scenario. First, the annual mean climatological SST displays an IOD-like pattern change over the Indian Ocean with enhanced warming in the northwestern Indian Ocean and relatively weaker warming off the Sumatra–Java coast. It is also noted that the monthly SST variance is increased over the eastern and southwestern Indian Ocean. Second, the IOBW variability on a quasi-biennial time scale will be enhanced due to the strengthening of the ENSO–IOBW mode relationship although the total variance of the IOBW mode will be significantly reduced particularly during late summer and fall. The enhanced air-sea coupling over the Indian-western Pacific climate in response to El Nino activity in the future projection makes favorable condition for a positive IOD while it tends to derive relatively cold temperature over the eastern Indian Ocean. This positive IOD-like ENSO response weakens the relationship between the eastern Indian Ocean and El Nino while strengthens the relationship with western Indian Ocean. Third, the IOD mode, intrinsic coupled mode of the Indian Ocean may not be changed appreciably under the anthropogenic global warming.