Investigating the Effects of Madden-Julian Oscillation on Climate Elements of Iran (1980-2020)

The Madden-Julian Oscillation is one of the large-scale climate change patterns in the maritime tropics,with sub-seasonal time periods of 30 to 60 days affecting tropical and subtropical regions.This phenomenon can cause changes in various quantities of the atmosphere and ocean,such as pressure,sea surface temperature,and the rate of evaporation from the ocean surface in tropical regions.In this research,the effects of Madden-Julian fluctuation on the weather elements of Iran have been investigated with the aim of knowing the effects of different phases in order to improve the quality of forecasts and benefits in territorial planning.At first,the daily rainfall data of 1980-2020 were received from the National Meteorological Organization and quality controlled.Using the Wheeler and Hendon method,the two main components RMM1 and RMM2 were analyzed,based on which the amplitude of the above two components is considered as the main indicator of the intensity and weakness of this fluctuation.This index is based on the experimental orthogonal functions of the meteorological fields,including the average wind levels of 850 and 200 hectopascals and outgoing long wave radiation(OLR)between the latitudes of 20 degrees south and 20 degrees north.The clustering of the 7-day sequence with a component above 1 was used as the basis for clustering all eight phases,and by calculating the abnormality of each phase compared to its long term in the DJF time frame,the zoning of each phase was produced separately.In the end,phases 1,2,7,8 were concluded as effective phases in Iran’s rainfall and phases 3,4,5,6 as suppressive phases of Iran’s rainfall.

Impacts of Sea Surface Temperature in the Tropical Pacific on Interannual Variability of Madden-Julian Oscillation in Precipitation

The Madden-Julian Oscillation (MJO) is investigated in two sets of 11-year records of observed precipitation, the daily mean Microwave Sounding Units (MSU) oceanic rainfall (Spencer, 1993) data and the pentad Climate Prediction Center Merged Analysis of Precipitation (CMAP) data (Xie and Arkin, 1997). Obvious interannual variability is found in the MJO in the tropical Pacific. MJO is limited to the west of dateline in normal years while extends more east during the year of warm sea surface temperature (SST) appeared in the eastern Pacific (i.e., El Ni?o years of 1982–1983, 1986–1988, 1991–1992) and manifested in the central-eastern Pacific for several months. The most significant correlation between interannual variability of MJO in the central-eastern Pacific and SST was found in the vicinity of the Ni?o3 region. Forced by observed SST, CCM3 presents a realistic trend of interannual variability to MJO in the 11 years, with a smaller magnitude than that from the observation. Comparison between the two realizations of the CCM3 simulation, which are forced by weekly and monthly mean SST respectively, showed that the MJO activities resemble each other in central-eastern Pacific while there is discrepancy in the western Pacific. It is suggested that the interannual variability of MJO is controlled, to certain extent bythe powerful interannual variability of SST in the central-eastern Pacific. In the western Pacific, however, there were remarkable impacts of the intraseasonal oscillation of SST on the MJO, where there was active MJO around the year. The notable disagreement between simulated and observed MJO in the western Pacific may come from the lack of high frequency variation of SST force, or from the shortage of air sea interaction for the intraseasonal time scale. It might be of importance to the MJO which is unable to be represented in the atmospheric model. Key words Madden-Julian Oscillation - Precipitation - Sea surface temperature - Interannual variability This study was sponsored by Chinese Academy of Sciences under grant “Hundred Talents” for “Validation of Coupled Climate Models”, the National Natural Science Foundation of China (Grant No. 49823002), and Project G1999043808.

An Overview of the Madden-Julian Oscillation and Its Relation to Monsoon and Mid-Latitude Circulation

In the past decade there has been extensive research into tropical intraseasonal variability, one of the major components of the low frequency variability of the general atmospheric circulation. This paper briefly reviews the state-of-the-art in this research area: the nature of the Madden-Julian Oscillation, its relation to monsoonal and extratropical circulations, and the current theoretical understandings.

Simulation of the Madden-Julian Oscillation in Wintertime Stratospheric Ozone over the Tibetan Plateau and East Asia: Results from the Specified Dynamics Version of the Whole Atmosphere Community Climate Model

The authors examined the Madden-Julian Oscillation(MJO) in stratospheric ozone during boreal winter using a simulation from the Specified Dynamics version of the Whole Atmosphere Community Climate Model(SD-WACCM) in 2004 and 2010. Comparison with European Centre for Medium-Range Weather Forecasts Interim Reanalysis(ERA-Interim) data suggested that the model simulation represented well the three-dimensional structure of the MJO-related ozone anomalies in the upper troposphere and stratosphere(i.e., between 200 and 20 h Pa). The negative ozone anomalies were over the Tibetan Plateau and East Asia in MJO phases 3–7, when the MJO convective anomalies travelled from the equatorial Indian Ocean towards the equatorial western Pacific Ocean. Due to the different vertical structures of the MJO-related circulation anomalies, the MJO-related stratospheric ozone anomalies showed different vertical structure over the Tibetan Plateau(25–40°N, 75–105°E) and East Asia(25–40°N, 105–135°E). As a result of the positive bias in the model-calculated ozone in the upper troposphere and lower stratosphere, the amplitude of MJO-related stratospheric ozone column anomalies(10–16 Dobson Units(DU)) in the SD-WACCM simulation was slightly larger than that(8–14 DU) in the ERA-Interim reanalysis.

Distinct influential mechanisms of the warm pool Madden-Julian Oscillation on persistent extreme cold events in Northeast China

This study investigates whether and how the Madden-Julian Oscillation(MJO)influences persistent extreme cold events(PECEs),a major type of natural disaster in boreal winter,over Northeast China.Significantly increased occurrence probabilities of PECEs over Northeast China are observed in phases 3 and 5 of the MJO,when MJOrelated convection is located over the eastern Indian Ocean and the western Pacific,respectively.Using the temperature tendency equation,it is found that the physical processes resulting in the cooling effects required for the occurrence of PECEs are distinct in the two phases of the MJO when MJO-related convection is consistently located over the warm pool area.The PECEs in phase 3 of the MJO mainly occur as a result of adiabatic cooling associated with ascending motion of the low-pressure anomaly over Northeast Asia.The cooling effect associated with phase 5 is stronger and longer than that in phase 3.The PECEs associated with phase 5 of the MJO are linked with the northwesterly cold advection of a cyclonic anomaly,which is part of the subtropical Rossby wave train induced by MJO-related convection in the tropical western Pacific.

Research on the Response of the Upper Layer Heat Structure in the Western Pacific Warm Pool to the Mean Madden-Julian Oscillation

By using the long-term observed hydro-meteorological data (1985-2002) from the Tropical Atmosphere Ocean System (TAO) during the international Tropical Ocean and Global Atmosphere (TOGA) experiment, the key parameters of the Sea Surface Temperature (SST), thermocline depth, surface sensible heat flux and latent heat flux, and the pseudo wind stress in the Westen Equatorial Ocean are calculated in this paper. On the basis of the calculation, the response of upper layer heat structure in the Westen Pacific Warm Pool to the mean Madden-Julian Oscillation (MJO) and its relation to the El Nio events are analyzed. The results show that within the MJO frequency band (42-108 d), the distributions of sea surface wind stress and upper ocean temperature have several spatial-temporal variation structures. Among these structures, the type-I surface pseudo wind stress field plays the role of inhibiting the eastward transport of ocean heat capacity, while the type-II strengthens the heat capacity spreading eastward. Therefore the type-II surface pseudo wind stress field is the characteristic wind field that provokes El Nio events. During calm periods (July-September) of the wind stress variations, the sensible and latent heat capacity fluxes change considerably, mostly in the region between 137°-140°E, while to the east of 150°E, the heat capacity flux changes less.\ In the mean MJO state, the type-I surface pseudo wind stress field structure dominates in the Western Pacific. This is why El Nio events can not occur every year. However, when the type-II and type-III surface pseudo wind stress field structures are dominant, an El Nio event is likely to occur. In this case, if the heat capacity of the Western Pacific Warm Pool is transported eastward and combined with the Equatorial Pacific heat capacity spreading eastward, El Nio events will soon occur.

MJO对2020年夏季中国东部持续性降水的维持作用

本文利用美国NOAA的CMORPH降水和NCEP的CFSv2再分析资料、欧洲中心ERA-5再分析数据、Wheeler和Hendon的MJO指数等资料和敏感性数值试验分析了MJO对2020年夏季中国东部持续性降水的影响机制。通过相关、回归、波源分析等方法发现:(1)与往年相比,2020年夏季风雨带在我国华南和长江中下游地区长时间维持,使得该地区季风降水具有明显的极端性特征;(2)同时,我国东部大气中高层位势高度出现负异常,伴随着水汽辐合和低层大气不稳定,均有利于2020年夏季强降水的维持;(3)2020年夏季MJO活跃于1、2位相,其振幅在10~30天具有显著的周期。MJO在非洲和印度洋产生显著的Rossby波源,该波源激发的波列沿急流传播,造成了我国东部位势高度负异常的维持,同时也有利于我国东部夏季降水的维持;(4)通过敏感性试验进一步验证,MJO的在10~30天的信号有利于在我国东部500 hPa上空形成负的位势高度异常,有利于该区域降水增加。

海温和MJO对2023年西南春旱的协同影响

2023年春季,我国西南地区发生了严重的气象干旱,对当地社会经济造成严重影响。为深入认识这次干旱事件的成因、并为未来西南地区春旱的预测提供科学依据,本文利用站点观测数据、美国国家环境预测中心和国家大气研究中心(National Centers for Environmental Prediction/National Center for Atmospheric Research,NCEP/NCAR)再分析数据、美国国家海洋和大气管理局(National Oceanic and Atmospheric Administration,NOAA)的海表温度等,采用T-N波作用通量和合成分析等方法,从海温和热带大气季节内振荡(Madden-Julian Oscillation,MJO)的角度深入探讨此次春旱成因。结果表明:(1)2023年我国西南春旱是高温干旱复合事件,3月干旱发生在中部,4月干旱加剧并向西扩展,5月干旱持续。(2)3月北太平洋的马蹄形海温异常导致西风急流偏南偏西,抑制了西南地区的降水。(3)4月印度洋暖海温通过Kelvin波导致孟加拉湾附近的反气旋式环流异常,西北太平洋暖海温通过Rossby波导致南海至菲律宾的气旋式环流异常,造成西南地区南部出现偏北风,导致水汽辐散,加剧干旱。(4)5月MJO长时间维持在西太平洋,通过Gill响应引发南海至菲律宾对流层低层的气旋异常,减少偏南水汽的输送,从而使得西南干旱持续。

Recent Advance in Understanding the Dynamics of the Madden-Julian Oscillation

The Madden-Julian oscillation （MJO） is a dominant atmospheric low-frequency mode in the tropics. In this review article, recent progress in understanding the MJO dynamics is described. Firstly, the fundamental physical processes responsible for MJO eastward phase propagation are discussed. Next, a recent modeling result to address why MJO prefers a planetary zonal scale is presented. The effect of the seasonal mean state on distinctive propagation characteristics between northern winter and summer is discussed in a theoretical framework. Then, the observed precursor signals and the physical mechanism of MJO initiation in the western equatorial Indian Ocean are further discussed. Finally, scale interactions between MJO and higher- frequency eddies are delineated.

Influence of Springtime Atlantic SST on ENSO:Role of the Madden-Julian Oscillation

Increased evidence has shown the important role of Atlantic sea surface temperature(SST) in modulating the El Nio-Southern Oscillation(ENSO). Persistent anomalies of summer Madden-Julian Oscillation(MJO) act to link the Atlantic SST anomalies(SSTAs) to ENSO. The Atlantic SSTAs are strongly correlated with the persistent anomalies of summer MJO, and possibly affect MJO in two major ways. One is that an anomalous cyclonic(anticyclonic) circulation appears over the tropical Atlantic Ocean associated with positive(negative) SSTA in spring, and it intensifies(weakens) the Walker circulation. Equatorial updraft anomaly then appears over the Indian Ocean and the eastern Pacific Ocean, intensifying MJO activity over these regions. The other involves a high pressure(low pressure) anomaly associated with the North Atlantic SSTA tripole pattern that is transmitted to the mid-and low-latitudes by a circumglobal teleconnection pattern, leading to strong(weak) convective activity of MJO over the Indian Ocean. The above results offer new viewpoints about the process from springtime Atlantic SSTA signals to summertime atmospheric oscillation, and then to the MJO of tropical atmosphere affecting wintertime Pacific ENSO events, which connects different oceans.

THE ROLES OF BOUNDARY-LAYER DYNAMICS ON THE DEVELOPMENT OF MADDEN-JULIAN OSCILLATION

In this paper,a tropical atmospheric model of relevance to shorts-term climate variations(Wang and Li 1993)is util- ized for study of the development of Madden-Julian oscillation.The model contains an interactive process of boundary-layer Ekman convergence and precipitation heating.The model is solved by expanding dependent variables in terms of parabolic cylindrical functions in the meridional direction and truncating three meridional modes n=0,2,4 for equatorial symmetric solutions.The free wave solutions obtained under long-wave approximation are induced as a Kelvin wave and two Rossby waves.After considering the effect of boundary-layer dynamic process,the modified Kelvin wave becomes unstable in long-wave bands with a typical growth rate on an order of 10^(-6) s^(-1)and an eastward phase speed of 10 m s^(-1);the most unstable mode is wavenumber one.These theoretical results are consistent with the ob- served Madden-Julian oscillation in equatorial area.For the two modified Rossby waves,one with a smaller meridional scale(n=4)decays except for extra long-waves;the other with a larger meridional scale(n=2)grows in short-wave bands.This may be relevant to explaining the westward propagation of super cloud clusters in the Madden-Julian oscillation.The theory suggests that the boundary-layer dynamic process is an important mechanism in the develop- ment of the Madden-Julian oscillation.

MJO与中间层-低热层风场潮汐DE3季节内变化性的关联

本文结合TIMED-TIDI卫星与流星雷达子午链风场观测数据,分析了中间层-低热层(MLT)区域非迁移潮汐DE3的季节内变化性及其与对流层MJO的可能关联.结果揭示了DE3风场潮汐广泛存在显著的宽频带季节内变化信号,且强度存在季节依赖.纬向风DE3潮汐(DE3-U)季节内变化在北半球冬季具有较强幅值,可达季节平均的1~2倍,而在其他季节通常在20%以内.结合MJO指数进一步讨论了不同季节下DE3-U对MJO的响应.统计结果表明,在影响较大的北半球冬季,DE3-U通常在MJO第4—6位相时有更大的幅值(+10%~+40%),而在其余位相时更小(-10%~-40%).这表明对流层与MJO相关的对流活动通过潜热释放的纬向变化可影响高空大气非迁移潮汐的强度.

南海夏季风爆发对广西6月暴雨的影响

利用1961—2020年广西地面气象观测站逐日降水资料、NCEP/NCAR再分析资料,研究了南海夏季风爆发对广西6月暴雨的影响。结果表明,当南海夏季风爆发偏早时,东亚大槽显著偏强,中高纬度地区经向环流增强;华南沿海西南风显著偏强,配合中高纬度偏强的经向型环流引导北风南下,南北风在广西上空交汇;印度洋到海洋性大陆热带季节内振荡(MJO)处于对流活跃位相,且向东移动明显,低频对流带在西南季风引导下向广西输送;广西上升气流显著偏强,暴雨日数偏多。反之,暴雨日数偏少。

Progress of MJO Prediction at CMA from Phase I to Phase II of the Sub-Seasonal to Seasonal Prediction Project

As one of the participants in the Subseasonal to Seasonal(S2S)Prediction Project,the China Meteorological Administration(CMA)has adopted several model versions to participate in the S2S Project.This study evaluates the models’capability to simulate and predict the Madden-Julian Oscillation(MJO).Three versions of the Beijing Climate Center Climate System Model(BCC-CSM)are used to conduct historical simulations and re-forecast experiments(referred to as EXP1,EXP1-M,and EXP2,respectively).In simulating MJO characteristics,the newly-developed high-resolution BCC-CSM outperforms its predecessors.In terms of MJO prediction,the useful prediction skill of the MJO index is enhanced from 15 days in EXP1 to 22 days in EXP1-M,and further to 24 days in EXP2.Within the first forecast week,the better initial condition in EXP2 largely contributes to the enhancement of MJO prediction skill.However,during forecast weeks 2–3,EXP2 shows little advantage compared with EXP1-M because the increased skill at MJO initial phases 6–7 is largely offset by the degraded skill at MJO initial phases 2–3.Particularly at initial phases 2–3,EXP1-M skillfully captures the wind field and Kelvin-wave response to MJO convection,leading to the highest prediction skill of the MJO.Our results reveal that,during the participation of the CMA models in the S2S Project,both the improved model initialization and updated model physics played positive roles in improving MJO prediction.Future efforts should focus on improving the model physics to better simulate MJO convection over the Maritime Continent and further improve MJO prediction at long lead times.

两类厄尔尼诺背景下MJO对太平洋阻塞频率的调节作用

本文基于1979~2019年ERA-interim逐日再分析数据和二维阻塞指数,探究了冬季两类(中部型和东部型)厄尔尼诺背景下热带季节内振荡(MJO)对太平洋地区阻塞频率的调节作用。本研究选取出现频次较高且平均振幅较强的位相3和7进行研究。结果表明,在两类厄尔尼诺背景下MJO第3位相期间,MJO激发的遥相关位置相似,均对应极地地区(白令海地区)正(负)的位势高度异常,从而使高纬度太平洋地区均出现正的阻塞频率异常。在东部型厄尔尼诺背景下MJO第7位相(EP7)期间中高纬太平洋地区存在正的阻塞频率异常。但是在中部型厄尔尼诺背景下MJO第7位相(CP7)期间没有大范围显著的异常阻塞频率。这是因为EP7期间MJO激发的异常Rossby波源位于急流核区的西北部,使得MJO的遥相关可以传至50°N以北,引起中高纬度地区有利于阻塞频率增加的位势高度异常。然而CP7期间MJO激发的异常Rossby波源位于急流核区内部,使得对应的遥相关仅在副热带急流中传播,对高纬度地区的位势高度影响较小,导致该时期内没有大范围显著的阻塞频率异常。最后本文使用ECHAM4.6气候模式验证了上述结论。

一次云南干季极端暴雨成因分析

利用NCEP再分析资料、常规及非常规气象观测资料,综合分析2015年1月9日云南干季极端暴雨天气过程.结果表明:深厚的南支槽、低空切变线及地面冷锋共同作用导致此次过程;强位相热带季节内振荡(Madden-Julian Oscillation,MJO)处于“湿窗口”期,对水汽输送及对流发展有指示作用;南支槽前西南低空急流输送充沛水汽,暴雨落区与水汽强辐合区基本一致;高低空急流耦合,云南处于高空急流次级环流上升支,受锋面抬升,锋区次级环流上升支与高空急流次级环流上升支叠加,形成极强的上升运动;西南低空急流输送不稳定能量,滇西南暴雨区以强对流天气为主,滇中暴雨区以稳定性降水为主;结合稳定度参数的综合分析,有助于判断云南干季强对流天气落区.

MJO研究新进展

MJO与ENSO的关系、MJO的数值模拟(预报)以及MJO对天气气候的影响是近些年来国内外大气科学研究的重要前沿问题。本文将综合介绍国内有关MJO对天气气候的影响以及MJO的数值模拟(预报)方面的近期研究进展,因为过去已介绍过MJO与ENSO相互作用的研究结果。利用澳大利亚气象局的RMM-MJO指数研究MJO与西北太平洋台风活动的关系,结果表明大气MJO对西北太平洋台风的生成有比较明显的调制作用,在MJO的活跃期与非活跃期西北太平洋生成台风数的比例为2:1;而在MJO活跃期,对流中心位于赤道东印度洋(即MJO第2、3位相)与对流中心在西太平洋地区(即MJO第5、6位相)时的比例也为2:1。对大气环流的合成分析显示,在MJO的不同位相西太平洋地区的动力因子和热源分布形势有极其明显不同。在第2、3位相,各种因子均呈现出抑制西太平洋地区对流及台风发展的态势;而在第5、6位相则明显有促进对流发生发展,并为台风生成和发展创造了有利的大尺度环流动力场。对多台风年与少台风年850hPa的30～60d低频动能距平的合成分析表明,在多台风年最显著的是低频动能正异常位于菲律宾以东15?N以南的西北太平洋地区,表明那里有强MJO的活动;而少台风年的情况与多台风年相反,菲律宾以东的西北太平洋上与季风槽位置对应区域是低频动能的负距平区,那里MJO偏弱。即赤道西北太平洋上MJO活动的强(弱)年对应西北太平洋的台风偏多(偏少)。对应MJO的不同活动位相,无论冬季、春季或夏季,中国东部的降水都将出现特殊的异常形势。在春季,MJO的第2、3位相有利我国东部长江中下游地区多雨、华南地区少雨;MJO的第4、5位相有利我国华南地区多雨而长江中下游地区少雨;在MJO的其它位相,我国东部地区都为降水负异常。在冬季,对应MJO的第1～3位相(特别是第2、3位相)中国华南降水偏多;而对应MJO的第6～8位相(特别是第6、7位相)中国华南降水偏少。在夏季,MJO位于印度洋时,MJO可以通过低层西风急流的波导效应影响到中国东南部地区,造成该地区降水偏多;当MJO位于西太平洋地区时,可以造成经向环流的上升支向北偏移,导致西北太平洋副高的东撤、以及中国东南部地区水汽输送减弱,降水减少。资料分析还表明,在年际变化尺度上,热带中、东印度洋MJO指数的持续异常对云南夏季降水有明显的影响。大气环流和数值模拟都表明,MJO活动不同位相的强对流会在东亚/西北太平洋地区激发产生不同形势的遥响应(Rossby波列),导致在中国不同地区出现有利(或不利)降水的环流形势和条件,是MJO活动影响中国降水的主要机制。用数值模式对MJO进行数值模拟(预报)是尚未很好解决的困难问题,原因也没有完全搞清楚。我们的一系列数值模拟清楚表明,MJO的模拟(预报)效果对模式所用对流参数化方案有很强的依赖关系;模式能否很好描写(再现)热带大气非绝热加热廓线,是极其关键的问题,只有当加热廓线在对流层中低层有最大加热时,模式才能得到同实际观测大体一致的MJO及其活动特征。这些数值模拟结果与我们过去从理论研究得到的结论相吻合,彼此得到应证。

基于季节内振荡的延伸预报试验

2～4周的延伸预报是近年来国际上天气和气候业务预报发展的一个重要方向。本文以江淮梅雨区降水为例,在利用集合经验模态分解(EEMD)及多变量EOF方法获取梅雨区降水及其影响系统低频信号的基础上,采用最优子集回归方法、经验波传播(EWP)方法及全球海气耦合模式产品,对梅雨季节内演变的延伸期预报方法进行了预报和试验,以期为建立延伸期预报业务提供科学依据。试验结果表明:(1)大气季节内振荡对梅雨区降水的延伸预报具有重要的应用价值,可能是联系天气过程和异常的重要系统。(2)通过EEMD方法提取前期降水演变及影响因子的季节内振荡信号,采用最优子集回归统计学方法对梅雨区逐候降水量演变进行超前30天预报是有可能的。(3)EWP经验动力方法对热带ITCZ活跃异常的未来40天东传可能具有较好的预报效果,还可能较好地预报出延伸期的梅雨区风场距平演变,具有一定应用价值。(4)全球海气耦合动力模式输出产品在延伸期环流形势趋势预报及20天左右的MJO指数预报方面有一定的参考价值。

边界层动力过程在Madden－Julian振荡发展中的作用

利用包括边界层爱克曼辐合－降水加热过程在内的短期气候变化模式［１］，研究了Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡［１１，１２］形成的机制。当经圈方向用抛物圆柱函数展开，并对赤道偶对称解取ｎ＝０，２，４三个截断模时，在长波近似下的自由波解，一为向东的Ｋｅｌｖｉｎ波，另外两支为向西的Ｒｏｓｓｂｙ波。当考虑边界层动力影响后，修正后的Ｋｅｌｖｉｎ波，其向东传播的速度约为１０ｍｓ－１，且在长波波段是不稳定的，最不稳定的波出现在纬向一波附近，不稳定增长率的量级约为Ｏ（１０ｍ－６ｓ－１），理论结果和Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡在赤道附近主体环流东传的观测事实接近。另外两支修正后的Ｒｏｓｓｂｙ波，其中经圈尺度较小的那支波，除波长极长的波外，波是阻尼的；但另一支经圈尺度较大的波，在短的波段是不稳定增长的，这似乎可以用来解释Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡中大尺度超级云团的西传过程。这些结果表明边界层动力过程在Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡的形成中是一种重要的机制。

夏季印度洋MJO活跃时间对中国长江流域降水日数的影响

利用1980—2020年中国753站逐日降水资料、NCEP/NCAR大气再分析资料以及哈得来中心的海表温度资料和实时多变量Madden-Julian振荡(MJO)指数,研究了MJO在印度洋地区(1—3位相)活跃日数对长江流域夏季降水日数的影响。结果表明两者存在显著的统计联系,在MJO活跃日数偏多的年份,MJO相关的西北太平洋反气旋环流异常有利于向长江中下游地区输送水汽,进而导致长江流域中下游范围内降水日数的增加,且这种影响主要体现在降水等级为大雨(25 mm/d)及以上强度的日数上。进一步研究发现,MJO在印度洋活跃日数与长江中下游夏季降水日数的关系存在年代际变化,两者显著的联系仅出现在2000年之后,之前的时段两者联系则较弱。这种关系的转变可能与印度洋海表温度变率减弱的背景有关,印度洋海洋年际变率变弱导致其对于长江中下游地区的影响减弱,进而使得MJO的调控作用凸显出来。夏季季节平均的印度洋MJO活跃日数可以对长江中下游的大雨以上的降水日数产生影响,且两者的关系在大约2000年之后变得尤为显著。