Impact of MJO on the intraseasonal variation of summer monsoon rainfall over India

The summer monsoon rainfall over India exhibits strong intraseasonal variability. Earlier studies have identified Madden Julian Oscillation (MJO) as one of the most influencing factors of the intraseasonal variability of the monsoon rainfall. In this study, using India Meteorological Department (IMD) high resolution daily gridded rainfall data and Wheeler?CHendon MJO indices, the intra-seasonal variation of daily rainfall distribution over India associated with various Phases of eastward propagating MJO life cycle was examined to understand the mechanism linking the MJO to the intraseasonal variability. During MJO Phases of 1 and 2, formation of MJO associated positive convective anomaly over the equatorial Indian Ocean activated the oceanic tropical convergence zone (OTCZ) and the resultant changes in the monsoon circulation caused break monsoon type rainfall distribution. Associated with this, negative convective anomalies over monsoon trough zone region extended eastwards to date line indicating weaker than normal northern hemisphere inter tropical convergence zone (ITCZ). The positive convective anomalies over OTCZ and negative convective anomalies over ITCZ formed a dipole like pattern. Subsequently, as the MJO propagated eastwards to west equatorial Pacific through the maritime continent, a gradual northward shift of the OTCZ was observed and negative convective anomalies started appearing over equatorial Indian Ocean. During Phase 4, while the eastwards propagating MJO linked positive convective anomalies activated the eastern part of the ITCZ, the northward propagating OTCZ merged with monsoon trough (western part of the ITCZ) and induced positive convective anomalies over the region. During Phases 5 and 6, the dipole pattern in convective anomalies was reversed compared to that during Phases 1 and 2. This resulted active monsoon type rainfall distribution over India. During the subsequent Phases (7 and 8), the convective and lower tropospheric anomaly patterns were very similar to that during Phase 1 and 2 except for above normal convective anomalies over equatorial Indian Ocean. A general decrease in the rainfall was also observed over most parts of the country. The associated dry conditions extended up to northwest Pacific. Thus the impact of the MJO on the monsoon was not limited to the Indian region. The impact was rather felt over larger spatial scale extending up to Pacific. This study also revealed that the onset of break and active events over India and the duration of these events are strongly related to the Phase and strength of the MJO. The break events were relatively better associated with the strong MJO Phases than the active events. About 83% of the break events were found to be set in during the Phases 7, 8, 1 and 2 of MJO with maximum during Phase 1 (40%). On the other hand, about 70% of the active events were set in during the MJO Phases of 3 to 6 with maximum during Phase 4 (21%). The results of this study indicate an opportunity for using the real time information and skillful prediction of MJO Phases for the prediction of break and active conditions which are very crucial for agriculture decisions.     

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。     

MJO活动轨迹对贵州区域强降水过程延伸期预报的影响分析北大核心

通过对贵州省主汛期季节内振荡(Intra-Seasonal Oscillation,ISO)活跃年进行低频对流场和降水的合成分析,确定了影响贵州主汛期ISO和降水的热带印度洋(Indian Ocean,IO)低频对流关键区和南海(South China Sea,SCS)低频对流关键区,并利用MJO活动轨迹对贵州区域强降水过程开展了延伸期预报试验。将贵州省主汛期ISO位相划分为发展、峰值、减弱、抑制、谷值和恢复6个位相,发现贵州主汛期ISO活跃年的降水与本地区低频对流具有较好的对应关系,即在峰值位相时低频对流最强、降水正异常强度最强;在谷值位相时低频对流最弱、降水负异常强度最强。同时,热带和副热带低频对流场在贵州主汛期ISO波动的第1、4位相、第2、5位相及第3、6位相均呈反位相特征。在热带印度洋低频对流发展、并东传的过程中,有两条传播路径分别激发了孟加拉湾西南季风ISO活跃和南海热带季风ISO活跃共同影响贵州主汛期降水;在贵州主汛期有3个低频对流活跃期,IO关键区和SCS关键区ISO都有3次提前的低频对流加强。基于上述研究,分析MJO活动轨迹对贵州主汛期区域强降水过程的影响,发现热带印度洋MJO活动中心强度在贵州区域强降水过程发生前15 d~前3 d具有较好的持续性预报信号,提前9 d时正相关性最好。与延伸期预报业务规定的预报时段(未来11~30 d)相结合,通过确定贵州典型区域强降水过程发生前(提前量为10 d)至过程结束时段的MJO活动轨迹在历年中的最相似时段,发现MJO活动中心轨迹和强度对贵州区域强降水过程的趋势预报具有较好的指示意义。     

Impacts of the MJO on Winter Rainfall and Circulation in China

Impacts of the MJO on winter rainfall and circulation in China are investigated using a real-time multivariate MJO index.Composite results using the daily rainfall anomalies and "rainy day" anomalies according to eight different MJO phases show that the MJO has considerable influence on winter rainfall in China. Rainfall anomalies show systematic and substantial changes(enhanced/suppressed) in the Yangtze River Basin and South China with the eastward propagation of the MJO convective center from the Indian Ocean to the western Pacific.When the MJO is in phase 2 and 3(MJO convective center is located over the Indian Ocean),rainfall probability is significantly enhanced.While in phase 6 and 7(MJO convective center is over the western Pacific),rainfall probability is significantly reduced. MJO in winter influences the rainfall in China mainly through modulating the circulation in the subtropics and mid-high latitudes.For the subtropics,MJO influences the northward moisture transport coming from the Bay of Bengal and the South China Sea by modulating the southern trough of the Bay of Bengal and the western Pacific subtropical high.For the mid-high latitudes,the propagation of the low frequency perturbations associated with the eastward-propagating MJO convection modulate the circulation in the mid-high latitudes,e.g.the East Asian winter monsoon and the low trough over central Asia.     

Influence of the Madden Julian Oscillation on precipitation and surface air temperature in South America

The regional influence of the Madden–Julian oscillation (MJO) on South America is described. Maps of probability of weekly-averaged rainfall exceeding the upper tercile were computed for all seasons and related statistically with the phase of the MJO as characterized by the Wheeler–Hendon real-time multivariate MJO (RMM) index and with the OLR MJO Index. The accompanying surface air temperature and circulation anomalies were also calculated. The influence of the MJO on regional scales along with their marked seasonal variations was documented. During December–February when the South American monsoon system is active, chances of enhanced rainfall are observed in southeastern South America (SESA) region mainly during RMM phases 3 and 4, accompanied by cold anomalies in the extratropics, while enhanced rainfall in the South Atlantic Convergence Zone (SACZ) region is observed in phases 8 and 1. The SESA (SACZ) signal is characterized by upper-level convergence (divergence) over tropical South America and a cyclonic (anticyclonic) anomaly near the southern tip of the continent. Impacts during March–May are similar, but attenuated in the extratropics. Conversely, in June–November, reduced rainfall and cold anomalies are observed near the coast of the SACZ region during phases 4 and 5, favored by upper-level convergence over tropical South America and an anticyclonic anomaly over southern South America. In September–November, enhanced rainfall and upper-level divergence are observed in the SACZ region during phases 7 and 8. These signals are generated primarily through the propagation of Rossby wave energy generated in the region of anomalous heating associated with the MJO.     

MJO对春季中国东部大气环流和降水的影响

利用再分析资料分析了MJO（Madden-Julian Oscillation）不同位相对春季中国东部降水的影响。结果表明：MJO处于位相3时对应长江及其以南地区降水增多,处于位相7时该区域降水减小。当热带MJO对流从位相1东传至位相4,与其相伴的北向辐散辐合流会在印度东北部—长江及副热带西北太平洋地区的对流层中低层产生明显的辐合异常,且在MJO位相3时中国东部的辐合异常达到最大。从Rossby波源角度分析,这种辐合异常会增强对流层中低层气旋性环流,导致MJO处于位相3时长江流域及其以南地区降水增多。同时,利用现代次季节和季节预报业务系统探讨了MJO与降水的关系对降水预报技巧的影响,发现预报降水和再分析产品的相关系数在MJO处于位相3和7时有所增加。     

The seasonal predictability of the Asian summer monsoon in a two-tiered forecast system

An extensive set of boreal summer seasonal hindcasts from a two tier system is compared with corresponding seasonal hindcasts from two other coupled ocean–atmosphere models for their seasonal prediction skill (for precipitation and surface temperature) of the Asian summer monsoon. The unique aspect of the two-tier system is that it is at relatively high resolution and the SST forcing is uniquely bias corrected from the multi-model averaged forecasted SST from the two coupled ocean–atmosphere models. Our analysis reveals: (a) The two-tier forecast system has seasonal prediction skill for precipitation that is comparable (over the Southeast Asian monsoon) or even higher (over the South Asian monsoon) than the coupled ocean–atmosphere. For seasonal anomalies of the surface temperature the results are more comparable across models, with all of them showing higher skill than that for precipitation. (b) Despite the improvement from the uncoupled AGCM all models in this study display a deterministic skill for seasonal precipitation anomalies over the Asian summer monsoon region to be weak. But there is useful probabilistic skill for tercile anomalies of precipitation and surface temperature that could be harvested from both the coupled and the uncoupled climate models. (c) Seasonal predictability of the South Asian summer monsoon (rainfall and temperature) does seem to stem from the remote ENSO forcing especially over the Indian monsoon region and the relatively weaker seasonal predictability in the Southeast Asian summer monsoon could be related to the comparatively weaker teleconnection with ENSO. The uncoupled AGCM with the bias corrected SST is able to leverage this teleconnection for improved seasonal prediction skill of the South Asian monsoon relative to the coupled models which display large systematic errors of the tropical SST’s.     

热带大气季节内振荡对华南前汛期降水的影响

本文基于实时的热带大气季节内振荡(MJO)指数和中国台站降水资料,研究了MJO对中国华南前汛期(4～6月)降水的影响.结果表明,随着MJO的活跃中心从印度洋进入西太平洋,华南地区的降水由偏多转为偏少.最显著的降水正负异常分别位于第4位相和第7位相,其区域平均的最大正负异常值相对于气候平均值的变化约为17%和11%.与降...     

THE IMPACTS OF MADDEN-JULIAN OSCILLATION ON SPRING RAINFALL IN EAST CHINA

Phase composite analyses are conducted to investigate the possible effect of the Madden–Julian oscillation(MJO)on the spring rainfall anomalies in East China by using the Real-time Multivariate MJO(RMM)index from Australian Meteorological Bureau.The results show that the rainfall anomalies over the mid-and lower-valley of Yangtze River are positive when the MJO shifts eastward to the mid-and eastern-Indian Ocean,and anomalous precipitation over South China are positive when the MJO moves further eastward to the maritime continent,whereas spring rainfall anomalies over East China are negative in the other MJO episodes.The MJO impacts on the precipitation over East China result from the changes in large-scale atmospheric circulation as well as vorticity and water vapor transportation in the mid-and lower-troposphere.     

THE RELATIONSHIP OF THE PRECEDING WINTER MJO ACTIVITIES AND THE SUMMER PRECIPITATION IN YANGTZE-HUAIHE RIVER BASIN OF CHINA

The first two series(RMM1 and RMM2) of RMM Index(all-Season Real-time Multivariate MJO Index) are computed to obtain the interannual variation of the preceding winter(preceding December to current February) MJO strength,according to which active(or inactive) years of preceding winter MJO are divided.By utilizing the data provided by NCEP/NCAR,CMAP and China’s 160 stations from 1979 to 2008,we studied the preceding winter MJO strength and discovered that the summer precipitation in the basin are of significantly negative correlation,i.e.when the preceding winter MJO is relatively active,the summer precipitation in the basin decreases,and vise verse.We also analyzed the causes.When the preceding winter MJO is relatively active,its release of potential heat facilities Inter-Tropical Convergence Zone(ITCZ) to strengthen and locate northward in winter and propagate northeastward.This abnormal situation lasts from winter to summer.In mid-May,ITCZ jumps northward to the South China Sea,the western Pacific subtropical high withdraws eastward,and the South China Sea summer monsoon sets off and strengthens.In summer,ITCZ propagates to South China Sea-subtropical western Pacific,the zonal circulation of subtropical Pacific strengthens,and a local meridional circulation of the South China Sea to the basin area forms,giving rise to the East Asia Pacific teleconnection wave-train.An East Asian monsoon trough and the Meiyu front show opposite features from south to north,the East Asian summer monsoon strengthens and advances northward.As a result,the summer monsoon is weakened as the basin is controlled by the subtropical high continually,with less rain in summer.On the contrary,when the preceding winter MJO is inactive,ITCZ weakens and is located southward,the subtropical high is located southward in summer,and the basin is in a region of ascending airflow with prevailing southwest wind.The East Asian monsoon trough and EASM weaken so that summer monsoon is reduced in the basin where precipitation increases.     

Indian summer monsoon rainfall predictability and variability associated with Northwest Pacific circulation in a suit of coupled model hindcasts

The Northwest Pacific (NWP) circulation (subtropical high) is an important component of the East Asian summer monsoon system. During summer (June–August), anomalous lower tropospheric anticyclonic (cyclonic) circulation appears over NWP in some years, which is an indicative of stronger (weaker) than normal subtropical high. The anomalous NWP cyclonic (anticyclonic) circulation years are associated with negative (positive) precipitation anomalies over most of Indian summer monsoon rainfall (ISMR) region. This indicates concurrent relationship between NWP circulation and convection over the ISMR region. Dry wind advection from subtropical land regions and moisture divergence over the southern peninsular India during the NWP cyclonic circulation years are mainly responsible for the negative rainfall anomalies over the ISMR region. In contrast, during anticyclonic years, warm north Indian Ocean and moisture divergence over the head Bay of Bengal-Gangetic Plain region support moisture instability and convergence in the southern flank of ridge region, which favors positive rainfall over most of the ISMR region. The interaction between NWP circulation (anticyclonic or cyclonic) and ISMR and their predictability during these anomalous years are examined in the present study. Seven coupled ocean–atmosphere general circulation models from the Asia-Pacific Economic Cooperation Climate Center and their multimodel ensemble mean skills in predicting the seasonal rainfall and circulation anomalies over the ISMR region and NWP for the period 1982–2004 are assessed. Analysis reveals that three (two) out of seven models are unable to predict negative (positive) precipitation anomalies over the Indian subcontinent during the NWP cyclonic (anticyclonic) circulation years at 1-month lead (model is initialized on 1 May). The limited westward extension of the NWP circulation and misrepresentation of SST anomalies over the north Indian Ocean are found to be the main reasons for the poor skill (of some models) in rainfall prediction over the Indian subcontinent. This study demonstrates the importance of the NWP circulation variability in predicting summer monsoon precipitation over South Asia. Considering the predictability of the NWP circulation, the current study provides an insight into the predictability of ISMR. Long lead prediction of the ISMR associated with anomalous NWP circulation is also discussed.     

How are seasonal prediction skills related to models’ performance on mean state and annual cycle?

Given observed initial conditions, how well do coupled atmosphere–ocean models predict precipitation climatology with 1-month lead forecast? And how do the models’ biases in climatology in turn affect prediction of seasonal anomalies? We address these questions based on analysis of 1-month lead retrospective predictions for 21 years of 1981–2001 made by 13 state-of-the-art coupled climate models and their multi-model ensemble (MME). The evaluation of the precipitation climatology is based on a newly designed metrics that consists of the annual mean, the solstitial mode and equinoctial asymmetric mode of the annual cycle, and the rainy season characteristics. We find that the 1-month lead seasonal prediction made by the 13-model ensemble has skills that are much higher than those in individual model ensemble predictions and approached to those in the ERA-40 and NCEP-2 reanalysis in terms of both the precipitation climatology and seasonal anomalies. We also demonstrate that the skill for individual coupled models in predicting seasonal precipitation anomalies is positively correlated with its performances on prediction of the annual mean and annual cycle of precipitation. In addition, the seasonal prediction skill for the tropical SST anomalies, which are the major predictability source of monsoon precipitation in the current coupled models, is closely link to the models’ ability in simulating the SST mean state. Correction of the inherent bias in the mean state is critical for improving the long-lead seasonal prediction. Most individual coupled models reproduce realistically the long-term annual mean precipitation and the first annual cycle (solstitial mode), but they have difficulty in capturing the second annual (equinoctial asymmetric) mode faithfully, especially over the Indian Ocean (IO) and Western North Pacific (WNP) where the seasonal cycle in SST has significant biases. The coupled models replicate the monsoon rain domains very well except in the East Asian subtropical monsoon and the tropical WNP summer monsoon regions. The models also capture the gross features of the seasonal march of the rainy season including onset and withdraw of the Asian–Australian monsoon system over four major sub-domains, but striking deficiencies in the coupled model predictions are observed over the South China Sea and WNP region, where considerable biases exist in both the amplitude and phase of the annual cycle and the summer precipitation amount and its interannual variability are underestimated.     

Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean

El Ni?o Southern Oscillation (ENSO) and given phases of the Madden?CJulian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (1) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (2) the intraseasonal (30?C60?days) and interannual (4?C5?years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (3) extreme values of the Indian Ocean wind shear result from the combination of El Ni?o and the MJO phase enhancing atmospheric convection over Africa, or La Ni?a and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.     

MJO活动对云南5月降水的影响

本文分析了1979～2008年5月MJO(Madden and Julian Oscillation)不同位相上大尺度环流对流和水汽输送的异常情况及其对云南5月降水的影响。按MJO活动中心位置从西向东分为8个位相, 在不同位相上, 云南5月降水呈现出明显的差异:第4～6位相(MJO对流中心位于赤道印度洋中部至西太平洋)降水偏多, 而第7～8位相(赤道太平洋中部以东)和第1～3位相(赤道印度洋中西部)降水偏少, 其中以第6位相的降水正异常和第2位相的负异常最为显著。在MJO 1～8位相中, 对流主体从热带印度洋东移。在第1～3位相, 孟加拉湾还未形成西南向水汽输送, 而云南又处于水汽辐散区, 降水较少;第4位相时对流主体到达90°N附近, 部分对流云系向孟加拉湾北传, 并在孟加拉湾生成气旋性环流, 向云南输送水汽, 云南降水增多;第5位相时对流主体传到南海, 部分对流云系在南海北传, 同时在南海形成北传的气旋性环流;第6位相时赤道MJO对流主体虽然东移出孟加拉湾, 但孟加拉湾和南海的两个气旋性环流依然继续北传, 孟加拉湾气旋东部的西南风和南海气旋西部的东北风在云南交汇, 云南被强烈的水汽辐合区控制, 降水最充沛。第7～8位相时, 对流主体减弱, 东移到南海和西太平洋一带, 孟加拉湾转向为偏北风, 停止向云南输送水汽, 且云南处于水汽辐散区控制, 降水偏少。因此, MJO主体在东传过程中, 激发了热带对流在孟加拉湾和南海两条通道上的北传, 强盛的水汽输送和两个海区气旋环流的有利配置是造成云南5月降水的重要原因。     

国家气候中心MJO监测预测业务产品研发及应用

热带大气低频振荡 (MJO) 和北半球夏季季节内振荡 (BSISO) 对全球范围天气气候事件有重要影响,是次季节-季节 (S2S) 预报最主要的可预报性来源之一。国家气候中心 (BCC) 基于我国完全自主的T639全球分析场数据、风云三号气象卫星射出长波辐射 (OLR) 资料以及BCC第2代大气环流模式系统的实时预报,发展了MJO实时监测预测一体化业务技术,建立了ISV/MJO监测预测业务系统 (IMPRESS1.0),已投入实时业务运行,在全国气象业务系统得到应用。该文着重介绍该系统提供的MJO和BSISO指数监测预测数据和图形产品,并描述了这些业务产品在2015年对MJO典型个例的实时监测预测应用情况。监测分析和预报检验表明,基于我国自主资料的监测结果能够较为准确地表征MJO和BSISO指数的振荡和演变过程,该系统对MJO和BSISO事件分别至少具备16 d和10 d左右的预报技巧。因此,基于IMPRESS1.0的MJO/BSISO监测预测一体化业务产品可为制作延伸期预报提供重要的参考依据。     

上海梅汛期候降水异常的低频信号及延伸期预报

提高汛期降水过程的延伸期预报能力是目前天气预报和气候预测发展的重要方向。本文以上海梅汛期降水为例,利用非传统滤波方法提取多变量季节内分量,分析了梅汛期季节内候降水异常及其相联系的延伸期关键低频信号,进一步综合多变量低频信号建立了梅汛期候降水异常延伸期预报方法,并开展了多年的回报和试报检验。结果表明:①梅汛期候降水异常季节内分量具有显著的40～60d低频振荡周期,与降水异常实况具有显著的正相关和较高的符号一致率;②梅汛期季节内候降水异常与超前10～35d的热带及中高纬低频信号有关,主要包括:热带MJO(Madden Julian Oscillation)自阿拉伯海的向东传播、西太平洋副热带高压季节内活动的西北向传播、PNA(Pacific-North American)遥相关型的季节内位相转换以及东北亚冷空气的持续性异常影响;③综合上述多变量低频信号建立了延伸期候降水异常预报模型,对提前10～35d的延伸期候降水异常的季节内分量具有预报技巧,也能较好地预报实际的候降水异常趋势。     

IMPACT OF TROPICAL INTRASEASONAL OSCILLATIONS ON THE PRECIPITATION OF GUANGDONG IN JUNES

The impact of tropical intraseasonal oscillations on the precipitation of Guangdong in Junes and its physical mechanism are analyzed using 30-yr (1979 to 2008), 86-station observational daily precipitation of Guangdong and daily atmospheric data from NCEP-DOE Reanalysis. It is found that during the annually first rainy season (April to June), the modulating effect of the activity of intraseasonal oscillations propagating eastward along the equator (MJO) on the June precipitation in Guangdong is different from that in other months. The most indicative effect of MJO on positive (negative) anomalous precipitation over the whole or most of the province is phase 3 (phase 6) of strong MJO events in Junes. A Northwest Pacific subtropical high intensifies and extends westward during phase 3. Water vapor transporting along the edge of the subtropical high from Western Pacific enhances significantly the water vapor flux over Guangdong, resulting in the enhancement of the precipitation. The condition is reverse during phase 6. The mechanism for which the subtropical high intensifies and extends westward during phase 3 is related to the atmospheric response to the asymmetric heating over the eastern Indian Ocean. Analyses of two cases of sustained strong rainfall of Guangdong in June 2010 showed that both of them are closely linked with a MJO state which is both strong and in phase 3, besides the effect from a westerly trough. It is argued further that the MJO activity is indicative of strong rainfall of Guangdong in June. The results in the present work are helpful in developing strategies for forecasting severe rainfall in Guangdong and extending, combined with the outputs of dynamic forecast models, the period of forecasting validity.     

The role of equatorial waves in the onset of the South China Sea summer monsoon and the demise of El Niño during 1998

The observed sequence of events leading to the onset of the summer monsoon in the South China Sea (SCS) is described, with a particular focus on conditions during the South China Sea monsoon experiment (SCSMEX) in May–June 1998. During SCSMEX, SCS monsoon onset occurred within the context of a multitude of scale interactions within the ocean-atmosphere system on intraseasonal time scales. Results from the 1998 SCSMEX case study illustrate that SCS monsoon onset is preceded by the development of an eastward-propagating Madden-Julian Oscillation (MJO) in the Indian Ocean, as suggested by previous authors, and the subsequent emanation of a convectively coupled Kelvin wave into the Pacific. Remarkably similar results are obtained in an independent composite of 25 years of data. Since both the MJO and Kelvin waves generate westerly surface winds in their wake, it is suggested that these waves may accelerate or trigger the monsoon onset process in the southern SCS. A detailed analysis of the Kelvin wave that propagated through the SCS during SCSMEX shows that it was responsible for a large portion of the surface wind shift leading to monsoon onset in 1998. Finally, easterly wind anomalies in the eastern Pacific associated with the Indian Ocean MJO event during the SCSMEX period are shown to result in the sudden demise of the 1997–1998 El Niño event.     

Deficiencies and possibilities for long-lead coupled climate prediction of the Western North Pacific-East Asian summer monsoon

Long-lead prediction of waxing and waning of the Western North Pacific (WNP)-East Asian (EA) summer monsoon (WNP-EASM) precipitation is a major challenge in seasonal time-scale climate prediction. In this study, deficiencies and potential for predicting the WNP-EASM precipitation and circulation one or two seasons ahead were examined using retrospective forecast data for the 26-year period of 1981–2006 from two operational couple models which are the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) and the Bureau of Meteorology Research Center (BMRC) Predictive Ocean–Atmosphere Model for Australia (POAMA). While both coupled models have difficulty in predicting summer mean precipitation anomalies over the region of interest, even for a 0-month lead forecast, they are capable of predicting zonal wind anomalies at 850 hPa several months ahead and, consequently, satisfactorily predict summer monsoon circulation indices for the EA region (EASMI) and for the WNP region (WNPSMI). It should be noted that the two models’ multi-model ensemble (MME) reaches 0.40 of the correlation skill for the EASMI with a January initial condition and 0.75 for the WNPSMI with a February initial condition. Further analysis indicates that prediction reliability of the EASMI is related not only to the preceding El Niño and Southern Oscillation (ENSO) but also to simultaneous local SST variability. On other hand, better prediction of the WNPSMI is accompanied by a more realistic simulation of lead–lag relationship between the index and ENSO. It should also be noted that current coupled models have difficulty in capturing the interannual variability component of the WNP-EASM system which is not correlated with typical ENSO variability. To improve the long-lead seasonal prediction of the WNP-EASM precipitation, a statistical postprocessing was developed based on the multiple linear regression method. The method utilizes the MME prediction of the EASMI and WNPSMI as predictors. It is shown that the statistical postprocessing is able to improve forecast skill for the summer mean precipitation over most of the WNP-EASM region at all forecast leads. It is noteworthy that the MME prediction, after applying statistical postprocessing, shows the best anomaly pattern correlation skill for the EASM precipitation at a 4-month lead (February initial condition) and for the WNPSM precipitation at a 5-month lead (January initial condition), indicating its potential for improving long-lead prediction of the monsoon precipitation.     

南海西南季风异常与广东省汛期重要天气的关系

南海西南季风的活动直接影响广东省前、后汛期重要天气的异常,因此深入探索南海西南季风的活动规律及其与广东省各种重大天气异常的关系十分必要.利用合成分析和相关统计方法,探讨和分析了南海西南季风建立早晚、强弱与广东省前、后汛期降水量趋势,初、终台的早晚及登陆广东省的热带气旋个数等重要天气的关系.指出南海西南季风爆发早的年份,前汛期雨量以正常偏少为主、后汛期雨量以偏多为主、登陆广东热带气旋偏多;南海西南季风偏强的年份,后汛期雨量以偏多为主,登陆广东热带气旋以正常偏多为主.还分析了4～6月、7～9月以及前冬(12～2月)的海温场、500 hPa高度场与西南季风建立早晚、强弱的关系,初步探索了西南季风建立的早晚、强弱与广东省汛期重要天气气候异常的关系的一些机理,其结果可供短期气候业务预测参考.