THE EXCITING MECHANISM OF TROPICAL INTRASEASONAL OSCILLATION TO EL NINO EVENT

The data analyses indicated that the occurrence of D Nino event is closely related to intraseasonal oscillation (ISO) in the tropical atmosphere : The intraseasonal oscillation is very strong in tile tropics (particularly over the equatorial western Pacific) prior to the occurrence of El Nino; But the ISO is evidently reduced and the quasistationary system is enhanced after the outbreak of El Nino. A simple air-sea coupled model study shows that the periodical self-excited oscillation can be produced in the air-sea-coupled system, but the pattern is different from the observed ENSO mode. When there is external (atmospheric) forcing with interannual time scale, a coupled mode, which looks like the ENSO mode, will be excited in the air-sea system. Synthesizing the results in data analyses and the theoretical investigation. the mechanism of ISO in the tropical atmosphere exciting the EI Nino event can be suggested : The interannual anomalies (variations) of the tropical ISO play an important role in the exciting EI Nino event through the air-sea interaction.

The Tropical Intraseasonal Oscillation in SAMIL Coupled and Uncoupled General Circulation Models

Simulations of tropical intraseasonal oscillation （TISO） in SAMIL, the Spectral Atmospheric Model from the Institute of Atmospheric Physics （IAP） State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics （LASG） coupled and uncoupled general circulation models were comprehensively evaluated in this study. Compared to the uncoupled model, the atmosphere-ocean coupled model improved the TISO simulation in the following aspects： （1） the spectral intensity for the 30-80-day peak eastward periods was more realistic; （2） the eastward propagation signals over western Pacific were stronger; and （3） the variance distribution and stronger signals of Kelvin waves and mixed Rossby gravity waves were more realistic. Better performance in the coupled run was assumed to be associated with a better mean state and a more realistic relationship between precipitation and SST. In both the coupled and uncoupled runs, the unrealistic simulation of the eastward propagation over the equatorial Indian Ocean might have been associated with the biases of the precipitation mean state over the Indian Ocean, and the unrealistic split of maximum TISO precipitation variance over the Pacific might have corresponded to the exaggeration of the double Intertropical Convergence Zone （ITCZ） structure in precipitation mean state. However, whether a better mean state leads to better TISO activity remains questionable. Notably, the northward propagation over the Indian Ocean during summer was not improved in the mean lead-lag correlation analysis, but case studies have shown some strong cases to yield remarkably realistic northward propagation in coupled runs.

Sea surface temperature anomaly in the tropical North Atlantic during El Nino decaying years

Based on reanalysis data from 1979 to 2016,this study focuses on the sea surface temperature(SST)anomaly of the tropical North Atlantic(TNA)in El Nino decaying years.The TNA SST exhibits a clear warm trend during this period.The composite result for 10 El Nino events shows that the TNA SST anomaly reaches its maximum in spring after the peak of an El Nino event and persists until summer.In general,the anomaly is associated with three factors-namely,El Nino,the North Atlantic Oscillation(NAO),and a long-term trend,leading to an increase in local SST up to 0.4℃,0.3℃,and 0.35℃,respectively.A comparison between 1983 and 2005 indicates that the TNA SST in spring is affected by El Niño,as well as the local SST in the preceding winter,which may involve a long-term trend signal.In addition,the lead-lag correlation shows that the NAO leads the TNA SST by 2-3 months.By comparing two years with an opposite phase of the NAO in winter(i.e.,1992 and 2010),the authors further demonstrate that the NAO is another important factor in regulating the TNA SST anomaly.A negative phase of the NAO in winter will reinforce the El Nino forcing substantially,and vise versa.In other words,the TNA SST anomaly in the decaying years is more evident if the NAO is negative with El Nino.Therefore,the combined effects of El Nino and the NAO must be considered in order to fully understand the TNA SST variability along with a long-term trend.

Contribution of El Nino amplitude change to tropical Pacific precipitation decline in the late 1990s

Equatorial central Pacific precipitation experienced a prominent decline in the late 1990 s.This change was previously attributed to a La Nina-like mean sea surface temperature(SST)change in the Pacific Ocean associated with a phase switch of the Interdecadal Pacific Oscillation.Here,using a series of model experiments,the authors reveal that the El Nino-related interannual SST anomalies contributed largely to the precipitation decrease over the equatorial central Pacific.This El Nino SST effect was due to the change in the amplitude of El Nino events in the late 1990 s.The 1980-98 decade had more large-amplitude El Nino events than the 1999-2014 decade.The nonlinear precipitation response to SST anomalies resulted in a larger decadal mean precipitation in the 1980-98 decade than in the 1999-2014 decade.The results highlight the importance of El Nino amplitude change in future climate change related to global warming.

热带大气季节内振荡激发El Nino的机制

资料分析表明，热带大气季节内振荡同ＥｌＮｉｎｏ的发生有密切的关系。在ＥｌＮｉｎｏ事件发生之前，热带大气（尤其是赤道西太平洋地区）季节内振荡异常加强；伴随着ＥｌＮｉｎｏ的发生，热带大气季节内振荡动能明显减弱，而准定常系统动能明显增强。简单海－气耦合模式的分析表明，只有在年际时间尺度大气外强迫作用下，海－气系统才可以产生类似ＥＮＳＯ模的耦合波。资料和理论模式分析的结果相结合，说明热带大气季节内振荡激发ＥｌＮｉｎｏ的机制是：热带大气季节内振荡的年际异常通过海－气相互作用而对ＥｌＮｉｎｏ事件起重要的激发作用。

El Nino影响热带大气季节内振荡的动力学研究

根据Ｅ１Ｎｉｎｏ期间热带大气状态的异常特征，确定了在简单大气动力模式中描写海温异常的参数化方法。然后利用斜压半地转两层模式从动力学上分析研究了Ｅ１Ｎｉｎｏ对热带大气低频波的影响。理论分析同有关热带大气季节内振荡的资料分析结果完全一致，Ｅ１Ｎｉｎｏ事件使热带大气季节内振荡减弱。

大气季节内振荡的活动与El Nino

对三次ＥｌＮｉｎｏ发生前后的ＥＣＭＷＦ资料用滤波方法（带通，低通）进行分析，得到的结果清楚地表明，在ＥｌＮｉｎｏ发生前热带季节内振荡较强，而伴随ＥｌＮｉｎｏ发生，季节内振荡明显减弱，这种能量变化最明显的地区是赤道东太平洋地区；对于周期在９０天以上的热带准定常波，伴随ＥｌＮｉｎｏ的发生其能量明显增加，这种增加反映最显著的区域是从大西洋往西一直到西太平洋。对中纬度（２５－３５°Ｎ）及中高纬度（４０－５０°Ｎ）地区的准定常波能量分析表明，中纬度地区的准定常波能量比低纬大，准定常波能量变化在中纬度（２５－３５°Ｎ）表现出与低纬能量变化一致的情形，即伴随着ＥｌＮｉｎｏ的发生，准定常波能量增加；中高纬度（４０－５０°Ｎ）只在亚洲大陆（１００－１７０°Ｅ）表现出与热带一致的能量变化。结果还显示，热带季节内振荡在低层的东传对赤道西风异常及对ＥｌＮｉｎｏ的发生、发展起着很重要的作用。

不同PDO位相下El Nino发展年和La Nina年东亚夏季风的季节内变化

基于1957~2017年观测和再分析资料,合成分析了北太平洋年代际振荡(Pacific decadal oscillation,PDO)不同位相下El Ni?o发展年和La Nina年东亚夏季风的环流、降水特征及季节内变化。结果表明,PDO正、负位相作为背景场,分别对El Ni?o发展年、La Nina年东亚夏季风及夏季降水具有加强作用。PDO正位相一方面可增强El Ni?o发展年夏季热带中东太平洋暖海温异常信号,另一方面通过冷海温状态加强中高纬东亚大陆与西北太平洋的环流异常,从而在一定程度上增强了东亚夏季风环流的异常程度;反之,PDO负位相则增强了La Nina年热带海气相互作用以及中高纬环流(如东北亚反气旋)的异常。在季节内变化方面,El Ni?o发展年6月贝湖以东反气旋性环流为东亚地区带来稳定的北风异常,东北亚位势高度减弱;7月开始,环流形势发生调整,日本以东洋面出现气旋性异常,东亚大陆偏北风及位势高度负异常均得到加强;8月,随着东亚夏季风季节进程和El Ni?o发展,西太平洋出现气旋性环流异常,东亚副热带位势高度进一步降低,西北太平洋副热带高压(简称副高)明显东退。La Nina年6月异常较弱,主要环流差异自7月西北太平洋为大范围气旋性异常控制开始,东亚-太平洋遥相关型显著,副高于季节内始终偏弱偏东。上述两种情况下,均造成东亚地区夏季降水总体上偏少,尤其是中国北方降水显著偏少。

El Nino衰减年夏季西太平洋副热带高压的季节内变化

本文分析了El Nino衰减年夏季西太平洋副热带高压(副高)的季节内变化,发现其季节内变化存在两种模态,一种是6~8月的一致偏西,另一种是6~7月偏西,而8月逆转为偏东,其中偏西模态的异常要远大于偏东模态。对偏西模态而言,由于热带北大西洋海温正异常的强迫影响,激发出一个从北大西洋经过欧亚大陆高纬度到东亚的遥相关,抑制了暖池地区的对流,东亚地区位势高度增加,从而导致副高加强西伸。在偏东模态下,热带印度洋海温异常演变与偏西模态类似,但强度偏低,同时热带北大西洋海温正异常在4月达到峰值后衰减,导致两大洋对8月暖池地区对流的抑制作用减弱。此外,由于6~7月暖池海温持续升高,在局地海气相互作用下,8月暖池对流发展,位势高度场降低,从而造成副高减弱东退。因此,副高8月异常主要取决于热带北大西洋海温异常。在预测El Nino衰减年副高异常变化时,要综合考虑两大洋海温异常的影响。

Activities of Low-Frequency Waves in the Tropical Atmosphere and ENSO

ENSO, particularly the occurrence of ENSO is still an important research object in climatic variation. Using the ECMWF data, the relationship between ENSO and the activities of low-frequency waves in the tropical atmosphere is analyzed in this paper. It is shown that the occurrence of ENSO is closely related to the intraseasonal oscillation and the quasi-stationary waves (the period >90 days) in the tropical atmosphere. Associated with the occurrence of El Nino event, the kinetic energy of low-frequency waves has obvious variation: the kinetic energy of atmospheric intraseasonal (30-60 days) oscillation (ISO) decreases abruptly and the kinetic energy of quasi-stationary waves increases abruptly. Moreover, the ISO and quasi-stationary waves propagate eastward clearly corresponding to El Nino; but they clearly propagate westward in La Nina cases.

热带大气季节内振荡的异常与1997年El Nio 事件的发生

利用美国NCEP资料分析热带大气季节内振荡（ISO）与ENSO的关系，揭示了1997年El Ni o事件的发生，与赤道中西太平洋地区大气 ISO在 1996年冬到 1997年春期间的异常增强有重要关系。而且该地区 ISO的异常增强并不主要由赤道印度洋地区移来，主要表现为ISO在印度尼西亚上空被激发增强和东传。印度尼西亚地区ISO的被激发，则是由于东亚冬季风强异常在该地区引起的强对流活动的结果。

两类（反）El Nio年与热带风暴活动的初步分析

本文分析了两类（反）ＥＩＮｉｎｏ年１９４９～１９８８年西北太平洋热带风暴活动的气候特点，包括热带风暴生成数和登陆数、风暴生成位置及路径，并讨论了其与太平洋副热带高压的关系。分析结果表明，在ＥＩＮｉｎｏⅠ类年，１０５°～１１９．９°Ｅ经度范围内的热带风暴生成数偏多，而１２０°～１７９．９°Ｅ范围内的风暴生成数偏少；风暴生成位置偏西、偏北；移动趋势多转向路径而少西行路径；另外，登陆我国的热带风暴数也偏多，此时，副高脊线偏南，西伸脊点偏东，强度及面积指数均偏弱，Ⅱ类年则相反。同样，分析反ＥＩＮｉｎｏⅠ、Ⅱ类年亦可得到类似的结果。

Multichannel singular spectrum analysis of the axial atmospheric angular momentum

Earth＇s variable rotation is mainly produced by the variability of the AAM（atmospheric angular momentum）. In particular, the axial AAM component X_3, which undergoes especially strong variations,induces changes in the Earth＇s rotation rate. In this study we analysed maps of regional input into the effective axial AAM from 1948 through 2011 from NCEP/NCAR reanalysis. Global zonal circulation patterns related to the LOD（length of day） were described. We applied MSSA（Multichannel Singular Spectrum Analysis） jointly to the mass and motion components of AAM, which allowed us to extract annual, semiannual, 4-mo nth, quasi-biennial, 5-year, and low-frequency oscillations. PCs（Principal components） strongly related to ENSO（El Nino southern oscillation） were released. They can be used to study ENSO-induced changes in pressure and wind fields and their coupling to LOD. The PCs describing the trends have captured slow atmospheric circulation changes possibly related to climate variability.

Simulations of the Tropical Intraseasonal Oscillation by the Atmospheric General Circulation Model of the Beijing Climate Center

The performance of BCC （Beijing Climate Center） AGCM 2.0.1 （Atmospheric General Circulation Model version 2.0.1） in simulating the tropical intraseasonal oscillation （TIO） is examined in this paper.The simulations are validated against observation and compared with the NCAR CAM3 （Community Atmosphere Model version 3） results.The BCC AGCM2.0.1 is developed based on the original BCC AGCM （version 1） and NCAR CAM3.New reference atmosphere and reference pressure are introduced into the model.Therefore,the original prognostic variables of temperature and surface pressure become their departures from the reference atmosphere.A new Zhang-McFarlane convective parameterization scheme is incorporated into the model with a few modifications.Other modifications include those in the boundary layer process and snow cover calculation.All simulations are run for 52 yr from 1949 to 2001 under the lower boundary conditions of observed monthly SST.The TIOs from the model are analyzed.The comparison shows that the NCAR CAM3 has a poor ability in simulating the TIO.The simulated strength of the TIO is very weak.The energy of the eastward moving waves is similar to that of the westward moving waves in CAM3.While in observation the former is much larger than the latter.The seasonal variation and spatial distribution of the TIO produced by CAM3 are also much different from the observation.The ability of the BCC AGCM2.0.1 in simulating the TIO is significantly better.The simulated TIO is evident.The strength of the TIO produced by the BCC AGCM2.0.1 is close to the observation.The energy of eastward moving.waves is much stronger than that of the westward moving waves,which is consistent with the observation.There is no significant difference in the seasonal variation and spatial distribution of the TIO between the BCC model simulation and the observation.In general,the BCC model performs better than CAM3 in simulating the TIO.

基于海表温度遥感资料重构的热带不稳定波特征分析

[目的]研究赤道东太平洋热带不稳定波(tropical instability waves,TIWs)的时空分布特征及其驱动机制.[方法]利用数据插值经验正交函数方法对MODIS卫星2003-2018年赤道东太平洋海表面温度(sea surface temperature,SST)数据进行重构,进而提取TIWs的波长和速度,并分析其时空分布特征以及与赤道风场和流场的关系.[结果]研究发现:TIWs在纬向上呈正负相位交替分布,在经向上则表现为双峰结构.季节性方面,TIWs在3-5月强度最弱,而在8-10月强度最强.纬向风场通过影响赤道纬向流场,特别是南赤道流(South Equatorial Current,SEC)的北分支(SEC-N)和北赤道逆流(North Equatorial Countercurrent,NECC)的剪切调控TIWs的生成;经向风场则通过调制“冷舌”边缘的SST梯度影响TIWs的生成.在年际尺度上,TIWs在厄尔尼诺(El Nino)年受到抑制,在拉尼娜(La Nina)年得到加强.纬向风场、纬向流场和TIWs三者的异常均超前于厄尔尼诺-南方涛动(El Nino-Southern Oscillation,ENSO)指数所刻画的赤道东太平洋SST异常,这表明纬向风场影响下的纬向流场剪切同样驱动TIWs的年际变化.NECC上空纬向风异常较弱,而SEC-N上空纬向风异常较强,这使得SEC-N年际变化主导流场剪切的强度,对TIWs的年际特征有重要作用.[结论]这一发现强调了大气风场和海洋动力过程共同驱动TIWs的季节性特征和年际变化.

春夏季热带东印度洋海温的季节内振荡及大气响应

基于1982—2020年OISSTv2.1数据集、NOAA Interpolated OLR数据集和NCEP/DOE II再分析数据资料,通过功率谱分析、位相合成法等方法研究了春夏季(3—8月)热带东印度洋海温季节内振荡的特征及大气的响应。结果发现春夏季热带东印度洋海温季节内振荡的显著周期主要集中在20~50天,强海温振荡事件主要分布在4—7月,关键区内对流抑制超前暖海温异常1/8周期,滞后冷海温异常3/8周期,反之亦然。春夏季海温异常激发的对流异常呈经向偶极子分布,且有明显的北移演变特征,但北侧的对流中心较南侧弱,即对流在北移过程中迅速衰减。使用K-means聚类法提取了两种具有显著差异的环流响应类型:北移型和局地型。北移型主要分布在夏季,表现为更有组织性的对流偶极子对,南北中心的强度和影响范围相当且北移特征更为清晰。在对流偶极子北移的过程中显著影响阿拉伯海上西南季风的强弱,对印度半岛夏季风季节内变化的影响较大。局地型主要分布在春夏转换季节,对流异常呈单一中心结构,移动性较弱,主要在热带东印度洋上空发展和消亡。热带东印度洋海温季节内振荡耦合的大气环流异常主要影响赤道印度洋地区,对热带外的影响较弱。进一步探究发现,背景东风切变的差异是导致两种对流响应差异的原因。夏季,北印度洋东风切变强,垂直风切变机制导致对流明显北移,春夏转换季节,东风切变弱,垂直风切变机制不成立,较弱的正压涡度平流机制引起对流微弱的北移,但主要在局地生消。

A FURTHER INQUIRY ON 30—60 DAY OSCILLATION IN THE TROPICAL ATMOSPHERE

Based on the ECMWF data (1980--1983) and others, a further inquiry on the activities and the structure feature of 30--60 day oscillation in the tropical atmosphere has been completed. The following results are obtained: There is stronger perturbation kinetic energy of 30--60 day atmospheric oscillation (AO) in the equatorial eastern Pacific. This means the equatorial eastern Pacific is a stronger activity region of 30--60 day AO in the tropics. Analyses also show that the AO system with the time scale of 30-60 days might consist of various spatial scale disturbances. The zonal propagation of 30-60 day oscillation in the tropical atmosphere is not all eastward, Some differences are found for different spatial scales, and for propagations in upper and lower tropospheres. The meridional propagation of the oscillation is even more different in the various regions and might be related to the low-frequency wave train in the atmosphere. The stronger activities of 30-60day AO in the equatorial middle-western Pacific are related to the El Nino events and the weaker ones are correspondent to the inverse El Nino phenomena.

大气季节内振荡的数值模拟比较研究

用国内外两个较好的大气环流模式、在观测海表温度的强迫下进行了长时间(1978—1989年)的数值积分,然后对数值模拟结果与NCAR/NCEP再分析资料进行比较分析,其结果清楚表明,模式模拟结果的均方根误差中有30%—40%是来自于模拟的大气季节内振荡的均方根误差。尤其是,大气季节内振荡模拟的均方根误差的分布形势与总的均方根误差的分布形势几乎完全一致。对热带地区大气季节内振荡动能的模拟结果与NCAR/NCEP再分析资料的比较分析表明,其差异也十分明显,说明模式对热带大气季节内振荡的模拟能力也还比较差。因此可以认为,大气季节内振荡在天气气候模拟中极为重要,而如何在数值模式中模拟好大气季节内振荡还需要进行很好地研究。

大气季节内振荡对热带气旋活动影响的研究进展

近30年来,大气季节内振荡对热带气旋活动影响的研究已逐渐成为人们关注的焦点,并取得了显著的进展。就大气季节内振荡对热带气旋生成、路径及登陆的影响进行概述,并揭示了这种影响的主要机制。同时回顾了与季节内振荡密切相关的季风槽活动对热带气旋的影响。在此基础上简单讨论了该领域中存在的问题及未来研究前景,并提出当前该研究领域中一些亟需研究的科学问题。

热带季节内振荡研究新进展

季节内振荡既是热带大气活动的强信号,也是热带海洋中较为普遍的现象。本文叙述了热带大气和热带海洋季节内振荡研究的进展,包括热带大气季节内振荡的观测研究、机制研究以及热带海洋季节内振荡方面的研究;讨论了海气相互作用、不同尺度间相互作用对季节内振荡的影响;在总结上述研究成果的基础上提出了热带季节内振荡研究进一步的发展方向。