Medium-range oscillations in the summer tropical easterlies at 200 hPa

By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range Oscillations in the summer tropical easterlies (10oS-20o) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from May to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward moving waves was relatively small in the 200 hPa easterlies; the total energy of the eastward moving waves was at minimum at 10oN. Three kind of the medium-range oscillations with about 50 day, 25 day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variation and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1-3), which all may make important contributions to the 50 day and 25 day oscillations in different years or different latitudes. The quasi-biweekly oscillation is mainly related to the synoptic waves (4-6). In equatorial region, the 50 day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25day period with a westward phase propagations in 1980. Both of them are of the mode of zonal wavenumber 1. Strong westward 50 day oscillation was found in 10oN-20oN in these two years. Regular propagations of the meridional wind 50 day oscillation were also found in the easterlies.The 50 day and 25 day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near interational date line, where are the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT), respectively.     

北半球夏季热带季节内振荡影响我国夏季降水的规律和预测方法

利用1979—2018年夏季逐日观测和再分析数据,对北半球夏季热带季节内振荡影响我国夏季降水的规律和预测方法开展了研究。首先,利用非传统滤波即异常相对倾向（Anomalous Relative Tendency,ART）方法获取了气象要素的次季节变化分量,并采用EOF分析方法提取了北半球夏季热带主要季节内振荡信号,结果表明向外长波辐射（Outgoing Longwave Radiation,OLR）异常相对倾向EOF前两个模态共同反映了北半球夏季起源于印度洋并向东和向北传播的、具有30~60 d周期的季节内振荡（Boreal Summer Intraseasonal Oscillation,BSISO）信号。回归分析表明,该季节内振荡信号能够导致当地及其北面地区低层风场和位势高度场异常,影响该地区及其北面地区的水汽辐合辐散,从而能引起我国尤其是我国南方地区季节内旱涝变化,并一定程度上反映了我国异常雨带的向北推进过程。而后,将提取的热带主要季节内振荡信号作为预测因子,将降水异常相对倾向作为先行预板对象,利用多元线性回归方法构建了我国夏季旬降水异常相对倾向的预报模型,将预报的旬降水异常相对倾向加上观测已知的降水近期背景距平,从而得到旬降水距平的预报结果。通过历史回报和交叉检验,评估了该模型对梅雨期我国江淮流域降水（包括2020年梅汛期异常降水）的次季节预测能力。     

Propagation and mechanisms of the quasi-biweekly oscillation over the Asian summer monsoon region

The propagation and underlying mechanisms of the boreal summer quasi-biweekly oscillation (QBWO) over the entire Asian monsoon region are investigated, based on ECMWF Interim reanalysis (ERA-Interim) data, GPCP precipitation data, and an atmospheric general circulation model (AGCM). Statistical analyses indicate that the QBWO over the Asian monsoon region derives its main origin from the equatorial western Pacific and moves northwestward to the Bay of Bengal and northern India, and then northward to the Tibetan Plateau (TP) area, with a baroclinic vertical structure. Northward propagation of the QBWO is promoted by three main mechanisms: barotropic vorticity, boundary moisture advection, and surface sensible heating (SSH). It is dominated by the barotropic vorticity effect when the QBWO signals are situated to the south of 20°N. During the propagation taking place farther north toward the TP, the boundary moisture advection and SSH are the leading mechanisms. We use an AGCM to verify the importance of SSH on the northward propagation of the QBWO. Numerical simulations confirm the diagnostic conclusion that the equatorial western Pacific is the source of the QBWO. Importantly, the model can accurately simulate the propagation pathway of the QBWO signals over the Asian monsoon region. Simultaneously, sensitivity experiments demonstrate that the SSH over northern India and the southern slope of the TP greatly contributes to the northward propagation of the QBWO as far as the TP area.     

Interannual meridional displacement of the East Asian upper-tropospheric jet stream in summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS‘s meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-July-August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.     

Structures and mechanisms of the first-branch northward-propagating intraseasonal oscillation over the tropical Indian Ocean

The first-branch northward-propagating intraseasonal oscillation (FNISO) over the tropical Indian Ocean (IO) often triggers the onset of the Asian summer monsoon. In this study we investigate the structures and mechanisms associated with FNISO through the diagnosis of ERA-Interim reanalysis data for the period of 1990–2009. A composite analysis is conducted to reveal the structure and evolution characteristics of the FNISO and associated background circulation changes. It is found that the FNISO convection originates from the southwestern IO and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern Bay of Bengal (BoB). Two possible mechanisms may contribute to the northward propagation of the FNISO. One is the meridional asymmetry of the background convective instability. A greater background convective instability over the northern BoB may destabilize Rossby waves and cause convection to shift northward. The other is the meridional phase leading of perturbation humidity in the planetary boundary layer (PBL). Maximum PBL moisture appears to the north of the convection center, which promotes a convectively unstable stratification ahead of the convection and leads to the northward propagation of the FNISO. A PBL moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry.     

北半球夏季大气季节内振荡对中国东南沿海热带气旋暴雨的影响

本文采用美国台风联合警报中心整编的1981—2012年的best-track热带气旋资料、中国大陆743站逐日降水数据、亚太经合组织气候中心的北半球夏季季节内振荡(BSISO)指数和美国国家环境预报中心及大气研究中心的再分析资料,分析了中国东南部地区热带气旋暴雨特征及其与BSISO 8个位相的联系。结果表明:7、8月,中国东南沿海地区的热带气旋暴雨发生的频次最多,多个站点热带气旋暴雨占总暴雨频次的比例达40%以上。7、8月热带气旋暴雨主要发生于BSISO1的第1、2、7、8位相,发生的暴雨频次占总频次的78.4%,主要分布于福建省沿海地区和西南部;BSISO2的第5、6、7位相热带气旋暴雨发生的频次也较多,占总频次的73.6%,主要分布于福建省沿海地区及西南部和广东省西南沿海地区。对发生较多热带气旋暴雨的BSISO1和BSISO2位相背景下的环流合成异常的诊断显示,西北太平洋伸向中国东南沿海地区,大尺度引导气流为显著的异常东风带,有利于热带气旋登陆中国大陆;显著异常的纬向东风切变、气旋性相对涡度和整层水汽的异常辐合,有利于热带气旋登陆过程强度的维持,促使热带气旋暴雨发生在中国大陆。     

Predictability of the summer East Asian upper-tropospheric westerly jet in ENSEMBLES multi-model forecasts

The interannual variation of the East Asian upper-tropospheric westerly jet(EAJ) significantly affects East Asian climate in summer. Identifying its performance in model prediction may provide us another viewpoint,from the perspective of uppertropospheric circulation,to understand the predictability of summer climate anomalies in East Asia. This study presents a comprehensive assessment of year-to-year variability of the EAJ based on retrospective seasonal forecasts,initiated from1 May,in the five state-of-the-art coupled models from ENSEMBLES during 1960–2005. It is found that the coupled models show certain capability in describing the interannual meridional displacement of the EAJ,which reflects the models' performance in the first leading empirical orthogonal function(EOF) mode. This capability is mainly shown over the region south of the EAJ axis. Additionally,the models generally capture well the main features of atmospheric circulation and SST anomalies related to the interannual meridional displacement of the EAJ. Further analysis suggests that the predicted warm SST anomalies in the concurrent summer over the tropical eastern Pacific and northern Indian Ocean are the two main sources of the potential prediction skill of the southward shift of the EAJ. In contrast,the models are powerless in describing the variation over the region north of the EAJ axis,associated with the meridional displacement,and interannual intensity change of the EAJ,the second leading EOF mode,meaning it still remains a challenge to better predict the EAJ and,subsequently,summer climate in East Asia,using current coupled models.     

Bimodal representation of the tropical intraseasonal oscillation

The tropical intraseasonal oscillation (ISO) shows distinct variability centers and propagation patterns between boreal winter and summer. To accurately represent the state of the ISO at any particular time of a year, a bimodal ISO index was developed. It consists of Madden-Julian Oscillation (MJO) mode with predominant eastward propagation along the equator and Boreal Summer ISO (BSISO) mode with prominent northward propagation and large variability in off-equatorial monsoon trough regions. The spatial–temporal patterns of the MJO and BSISO modes are identified with the extended empirical orthogonal function analysis of 31?years (1979–2009) OLR data for the December–February and June–August period, respectively. The dominant mode of the ISO at any given time can be judged by the proportions of the OLR anomalies projected onto the two modes. The bimodal ISO index provides objective and quantitative measures on the annual and interannual variations of the predominant ISO modes. It is shown that from December to April the MJO mode dominates while from June to October the BSISO mode dominates. May and November are transitional months when the predominant mode changes from one to the other. It is also shown that the fractional variance reconstructed based on the bimodal index is significantly higher than the counterpart reconstructed based on the Wheeler and Hendon’s index. The bimodal ISO index provides a reliable real time monitoring skill, too. The method and results provide critical information in assessing models’ performance to reproduce the ISO and developing further research on predictability of the ISO and are also useful for a variety of scientific and practical purposes.     

2015年夏季东亚和南亚降水异常与热带太平洋海温异常的联系及机理

2015年我国东部夏季降水呈现南北反位相的空间分布,河套地区降水异常偏少、长江中下游地区降水异常偏多,同期印度中部地区降水负异常,上述三个区域2015年夏季降水距平百分率绝对值极大值均超过55%。东亚和南亚地区2015年夏季降水异常的形成机理主要是由于该年夏季处于El Niňo事件的发展位相,菲律宾群岛及邻近区域反气旋环流异常,江淮地区至日本列岛气旋式环流异常,对流层低层位势高度异常场和整层水汽异常输送场亦存在相一致的空间分布,表现为负位相的EAP(East Asian-Pacific)/PJ(Pacific-Japan)型遥相关,有利于河套地区降水偏少和长江流域降水偏多。热带太平洋海温异常引起热带地区Walker环流负异常,热带西太平洋地区上空受异常下沉气流控制,热带印度洋区域对流层盛行东风异常,减弱了印度夏季风,并造成了印度中部地区夏季降水偏少。另一方面,印度上空对流层低层受异常反气旋控制,该异常反气旋北侧的西风异常沿着青藏高原南麓向东运动,增强了与EAP/PJ型遥相关相联系的异常水汽输送,有利于维持和增强河套地区降水负异常和长江中下游地区降水正异常。     

Quasi-two weeks oscillation in the tropical atmosphere

Summary Based on analysis of ECMWF data (1981–1987) and numerical simulations using a general circulation model (GCM), a quasi-two-week (10–20 day) oscillation in the tropical atmosphere is studied in this paper. It is shown that the kinetic energy of the quasi-two-week oscillation is larger than that of the intraseasonal oscillation, and is another important low-frequency system in the tropical atmosphere. By comparing it with the intraseasonal oscillation, some obvious differences can be found. For example, the zonal scale of the quasi-two-week oscillation is dominated by perturbations with wavenumber 2–4; its vertical structure mainly shows barotropic features; the zonal propagation is basically westward; and its meridional and zonal components the same size.With 10 Figures     

Intraseasonal Oscillation in the Tropical Indian Ocean

1. Introduction The intraseasonal oscillation (ISO or Madden- Julian Oscillation, MJO) in the tropical atmosphere has been studied extensively, including its existence, structure, evolution and propagation (Madden and Ju- lian, 1971; Murakami, et al., 198…     

Principal oscillation pattern analysis of the tropical 30- to 60-day oscillation

In Part I (Storch and Xu 1990) the principal oscillation pattern (POP) analysis of 200 mb equatorial velocity potential leads to the definition of a bivariate (POP-) index of the tropical 30- to 60-day oscillation. Using the POP prediction scheme this index is predictable for a few days in advance. In Part 11, the prediction of the equatorial velocity potential field, made by the POP method and made by two GCMs, is investigated. The POP index forecast can incorporate skillful forecasts of the equatorial velocity potential () field. Its ensemble correlation skill score passes the 0.50 level at 7 days, whereas persistence passes after 3 days. If there is a strong 30- to 60-day oscillation signal in the initial state, useful forecasts of more than 20 days are sometimes possible; if the initial signal is weak, the POP forecast fails. Also, the forecast skill of two GCMs is considered. The NCAR T31 GCM appears to be quite skillful in predicting the equatorial -field, and in particular the 30- to 60-day oscillation. Its skill, however, is less than that of the POP scheme. The CNRM T42 GCM seems not to be able to predict the regular development associated with the tropical 30- to 60-day oscillation. The power of the POP index in explaining the equatorial x-field is a measure of the strength and dominance of the 30- to 60-day oscillation. This measure at day 0 is an a priori indicator of the NCAR T31 GCM's skill in predicting the equatorial velocity potential field.The National Center for Atmospheric Research is sponsored by the National Science Foundation     

Boreal summer quasi-monthly oscillation in the global tropics

The boreal summer intraseasonal oscillation (ISO) in the global tropics is documented here using a 7-year suite (1998–2004) of satellite measurements. A composite scenario was made of 28 selected events with reference to the oscillation in the eastern equatorial Indian Ocean (EIO), where the oscillation is most regular and its intensity is indicative of the strength of the subsequent northward propagation. The average oscillation period is about 32 days, and this quasi-monthly oscillation (QMO) is primarily confined to the tropical Indian and Pacific Oceans. Topics that were investigated are the partition of convective versus stratiform clouds, the vertical structure of precipitation rates, and the evolution of cloud types during the initial organization and the development of intraseasonal convective anomalies in the central Indian Ocean. During the initiation of the convective anomalies, the stratiform and convective rains have comparable rates; the prevailing cloud type experiences a trimodal evolution from shallow to deep convection, and finally to anvil and extended stratiform clouds. A major northwest/southeast-slanted rainband forms as the equatorial rainfall anomalies reach Sumatra, and the rainband subsequently propagates northeastward into the west Pacific Ocean. The enhanced precipitation in the west Pacific then rapidly traverses the Pacific along the Intertropical Convergence Zone, meanwhile migrating northward to the Philippine Sea. A seesaw teleconnection in rainfall anomalies is found between the southern Bay of Bengal (5–15°N, 80–100°E) and the eastern Pacific (5–15°N, 85–105°W). Local sea-surface temperature (SST)-rainfall anomalies display a negative simultaneous correlation in the off-equatorial regions but a zero correlation (quadrature phase relationship) near the equator. We propose that atmosphere–ocean interaction and the vertical monsoon easterly shear are important contributors to the northeastward propagation component of the intraseasonal rainband. The observed evidence presented here provides critical information for validating the numerical models, and it supports the self-induction mechanism theory for maintenance of the boreal summer ISO.     

Decadal change in the boreal summer intraseasonal oscillation

A decadal change in activity of the boreal summer intraseasonal oscillation (BSISO) was identified at a broad scale. The change was more prominent during August–October in the boreal summer. The BSISO activity during 1999–2008 (P2) was significantly greater than that during 1984–1998 (P1). Compared to P1, convection in the BSISO was enhanced and the phase speed of northward-propagating convection was reduced in P2. Under background conditions, warm sea surface temperature (SST) anomalies in P2 were apparent over the tropical Indian Ocean and the western tropical Pacific. The former supplied favorable conditions for the active convection of the BSISO, whereas the latter led to a strengthened Walker circulation through enhanced convection. This induced descending anomalies over the tropical Indian Ocean. Thermal convection tends to be suppressed by descending anomalies, whereas once an active BSISO signal enters the Indian Ocean, convection is enhanced through convective instability by positive SST anomalies. After P2, the BSISO activity was weakened during 2009–2014 (P3). Compared to P2, convective activity in the BSISO tended to be inactive over the southern tropical Indian Ocean in P3. The phase speed of the northward-propagating convection was accelerated. Under background conditions during P3, warmer SST anomalies over the maritime continent enhance convection, which strengthened the local Hadley circulation between the western tropical Pacific and the southern tropical Indian Ocean. Hence, the convection in the BSISO over the southern tropical Indian Ocean was suppressed. The decadal change in BSISO activity correlates with the variability in seasonal mean SST over the tropical Asian monsoon region, which suggests that it is possible to predict the decadal change.

     

The tropical very low-frequency oscillation on interannual scale

This is a review on the studies of tropical very low-frequency oscillation (VLFO) on interannual scale, mainly on the recent researches undertaken by Chinese scientists which are not well known outside of the country.This paper summarizes the basic features of VLFO in the tropics, the characteristic time and spacial structure of oscillation, especially the new concept of Low Latitude Oscillation consisted of two components: the well-known Southern Oscillation (SO) and the so-called Northern Oscillation (NO). A large number of evidences have been provided to illustrate the relationship between VLFO in tropics and the climate variation in China, such as the long-term variation of north Pacific high, the frequency of typhoon and the cyclone over the East China Sea, the summer monsoon rainfall in Yangtze valley basin and the cold summer disaster in Northeast China, and so on. Finally throw some lights on the nature of VLFO on imerannual scale.     

Pacific decadal oscillation and variability of the Indian summer monsoon rainfall

Recent studies have furnished evidence for interdecadal variability in the tropical Pacific Ocean. The importance of this phenomenon in causing persistent anomalies over different regions of the globe has drawn considerable attention in view of its relevance in climate assessment. Here, we examine multi-source climate records in order to identify possible signatures of this longer time scale variability on the Indian summer monsoon. The findings indicate a coherent inverse relationship between the inter-decadal fluctuations of Pacific Ocean sea surface temperature (SST) and the Indian monsoon rainfall during the last century. A warm (cold) phase of the Pacific interdecadal variability is characterized by a decrease (increase) in the monsoon rainfall and a corresponding increase (decrease) in the surface air temperature over the Indian subcontinent. This interdecadal relationship can also be confirmed from the teleconnection patterns evident from long-period sea level pressure (SLP) dataset. The SLP anomalies over South and Southeast Asia and the equatorial west Pacific are dynamically consistent in showing an out-of-phase pattern with the SLP anomalies over the tropical central-eastern Pacific. The remote influence of the Pacific interdecadal variability on the monsoon is shown to be associated with prominent signals in the tropical and southern Indian Ocean indicative of coherent inter-basin variability on decadal time scales. If indeed, the atmosphere–ocean coupling associated with the Pacific interdecadal variability is independent from that of the interannual El Niño-Southern Oscillation (ENSO), then the climate response should depend on the evolutionary characteristics of both the time scales. It is seen from our analysis that the Indian monsoon is more vulnerable to drought situations, when El Niño events occur during warm phases of the Pacific interdecadal variability. Conversely, wet monsoons are more likely to prevail, when La Niña events coincide during cold phases of the Pacific interdecadal variability.     

Teleconnections associated with Northern Hemisphere summer monsoon intraseasonal oscillation

The boreal summer intraseasonal oscillation (BSISO) has strong convective activity centers in Indian (I), Western North Pacific (WNP), and North American (NA) summer monsoon (SM) regions. The present study attempts to reveal BSISO teleconnection patterns associated with these dominant intraseasonal variability centers. During the active phase of ISM, a zonally elongated band of enhanced convection extends from India via the Bay of Bengal and Philippine Sea to tropical central Pacific with suppressed convection over the eastern Pacific near Mexico. The corresponding extratropical circulation anomalies occur along the waveguides generated by the North African-Asian jet and North Atlantic-North European jet. When the tropical convection strengthens over the WNPSM sector, a distinct great circle-like Rossby wave train emanates from the WNP to the western coast of United States (US) with an eastward shift of enhanced meridional circulation. In the active phase of NASM, large anticyclonic anomalies anchor over the western coast of US and eastern Canada and the global teleconnection pattern is similar to that during a break phase of the ISM. Examination of the evolution of the BSISO teleconnection reveals quasi-stationary patterns with preferred centers of teleconnection located at Europe, Russia, central Asia, East Asia, western US, and eastern US and Canada, respectively. Most centers are embedded in the waveguide along the westerly jet stream, but the centers at Europe and Russia occur to the north of the jet-induced waveguide. Eastward propagation of the ISO teleconnection is evident over the Pacific-North America sector. The rainfall anomalies over the elongated band near the monsoon domain over the Indo-western Pacific sector have an opposite tendency with that over the central and southern China, Mexico and southern US, providing a source of intraseasonal predictability to extratropical regions. The BSISO teleconnection along and to the north of the subtropical jet provides a good indication of the surface sir temperature anomalies in the NH extratropics.     

Influences of ENSO on boreal summer intraseasonal oscillation over the western Pacific in decaying summer

Climate Dynamics - Characteristics of boreal summer intraseasonal oscillation (BSISO) over the western Pacific in ENSO decaying summer are revealed in this study. BSISO activity over the western...     

北半球夏季季节内振荡年代际变化对西北太平洋群发台风突变减少的影响

利用台风最佳路径、bimodal IntraSeasonal Oscillation(bimodal ISO)指数和全球逐日向外长波辐射数据,研究了北半球夏季季节内振荡（Boreal Summer IntraSeasonal Oscillation,BSISO）年代际变化造成1996/1997年后西北太平洋群发台风突变减少的可能机制。分析显示,仅有包含3个及以上个数台风成员的“MTC3”群发出现了突变减少,此类台风更倾向在传播速度较慢、低频对流维持时间较长的BSISO活跃位相内出现,对次季节信号强度的要求相对较低。1996/1997年后,BSISO东传范围减小、周期延长、对流活跃位相日数缩短,导致西北太平洋长时间连续维持对流抑制位相,低频对流在145°E以东海域的强度减弱。当偏西海域有先导台风活动时,它向东南侧激发的罗斯贝波频散波列在（5°—20°N,145°—165°E）海域因没有低频对流耦合而快速消散,导致MTC3群发台风年代际突变减少。     

Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region

The boreal summer intraseasonal oscillation (BSISO) of the Asian summer monsoon (ASM) is one of the most prominent sources of short-term climate variability in the global monsoon system. Compared with the related Madden-Julian Oscillation (MJO) it is more complex in nature, with prominent northward propagation and variability extending much further from the equator. In order to facilitate detection, monitoring and prediction of the BSISO we suggest two real-time indices: BSISO1 and BSISO2, based on multivariate empirical orthogonal function (MV-EOF) analysis of daily anomalies of outgoing longwave radiation (OLR) and zonal wind at 850 hPa (U850) in the region 10°S–40°N, 40°–160°E, for the extended boreal summer (May–October) season over the 30-year period 1981–2010. BSISO1 is defined by the first two principal components (PCs) of the MV-EOF analysis, which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward MJO with quasi-oscillating periods of 30–60 days. BSISO2 is defined by the third and fourth PCs, which together mainly capture the northward/northwestward propagating variability with periods of 10–30 days during primarily the pre-monsoon and monsoon-onset season. The BSISO1 circulation cells are more Rossby wave like with a northwest to southeast slope, whereas the circulation associated with BSISO2 is more elongated and front-like with a southwest to northeast slope. BSISO2 is shown to modulate the timing of the onset of Indian and South China Sea monsoons. Together, the two BSISO indices are capable of describing a large fraction of the total intraseasonal variability in the ASM region, and better represent the northward and northwestward propagation than the real-time multivariate MJO (RMM) index of Wheeler and Hendon.