Seasonal Forecasts of Precipitation during the First Rainy Season in South China Based on NUIST-CFS1.0

Current dynamical models experience great difficulties providing reliable seasonal forecasts of regional/local rainfall in South China.This study evaluates seasonal forecast skill for precipitation in the first rainy season(FRS,i.e.,April–June)over South China from 1982 to 2020 based on the global real-time Climate Forecast System of Nanjing University of Information Science and Technology(NUIST-CFS1.0,previously known as SINTEX-F).The potential predictability and the practical forecast skill of NUIST-CFS1.0 for FRS precipitation remain low in general.But NUIST-CFS1.0 still performs better than the average of nine international models in terms of correlation coefficient skill in predicting the interannual precipitation anomaly and its related circulation index.NUIST-CFS1.0 captures the anomalous Philippines anticyclone,which transports moisture and heat northward to South China,favoring more precipitation in South China during the FRS.By examining the correlations between sea surface temperature(SST)and FRS precipitation and the Philippines anticyclone,we find that the model reasonably captures SST-associated precipitation and circulation anomalies,which partly explains the predictability of FRS precipitation.A dynamical downscaling model with 30-km resolution forced by the large-scale circulations of the NUIST-CFS1.0 predictions could improve forecasts of the climatological states and extreme precipitation events.Our results also reveal interesting interdecadal changes in the predictive skill for FRS precipitation in South China based on the NUIST-CFS1.0 hindcasts.These results help improve the understanding and forecasts for FRS precipitation in South China.

Temporal Variations of the Frontal and Monsoon Storm Rainfall during the First Rainy Season in South China

The temporal variations in storm rainfall during the first rainy season (FRS) in South China (SC) are investigated in this study. The results show that the inter-annual variations in storm rainfall during the FRS in SC seem to be mainly influenced by the frequency of storm rainfall, while both frequency and intensity affect the inter-decadal variations in the total storm rainfall. Using the definitions for the beginning and ending dates of the FRS, and the onset dates of the summer monsoon in SC, the FRS is further divided into two sub-periods, i.e., the frontal and monsoon rainfall periods. The inter-annual and inter-decadal variations in storm rainfall during these two periods are investigated here. The results reveal a significant out-of-phase correlation between the frontal and monsoon storm rainfall, especially on the inter-decadal timescale, the physical mechanism for which requires further investigation.

INFLUENCES OF LOW-FREQUENCY MOISTURE TRANSPORTATION ON LOW FREQUENCY PRECIPITATION ANOMALIES IN THE ANNUALLY FIRST RAINY SEASON OF SOUTH CHINA IN 2010

85-station daily precipitation data from 1961-2010 provided by the National Meteorological Information Center and the NCEP/NCAR 2010 daily reanalysis data are used to investigate the low-frequency variability on the precipitation of the first rain season and its relationships with moisture transport in South China,and channels of low-frequency water vapor transport and sources of low-frequency precipitation are revealed.The annually first raining season precipitation in 2010 is mainly controlled by 10-20 d and 30-60 d oscillation.The rainfall is more(interrupted) when the two low-frequency components are in the same peak(valley) phase,and the rainfall is less when they are superposed in the inverse phase.The 10-20 d low-frequency component of the moisture transport is more active than the 30-60 d.The10-20 d water vapor sources lie in the South India Ocean near 30° S,the area between Sumatra and Kalimantan Island(the southwest source),and the equatorial middle Pacific region(the southeast source),and there are corresponding southwest and southeast moisture transport channels.By using the characteristics of 10-20 d water vapor transport anomalous circulation,the corresponding low-frequency precipitation can be predicted 6 d ahead.

Simulation of Potential Productivity of Early Season Rice Varieties in Different Reqions of South China

Rice is a staple food crop in China.Since the 1950’s,many new varieties havebeen used and resulted in great increase ofyield.However there were still some barriersin the nationwide extension of new varietiesdue to the insufficient information about thecharacteristics of varieties.So,it is impor-tant to find ways of determining the potential

Analysis of Paths and Sources of Moisture for the South China Rainfall during the Presummer Rainy Season of 1979–2014

The paths and sources of moisture supplied to South China during two periods of the presummer rainy season （April-June） of 1979-2014, i.e., before and after the onset of the summer monsoon over the South China Sea （SCS）, are investigated by using the Hybrid Single-Particle Lagrangian Integrated Trajectory （HYSPLIT） model. During the premonsoon-onset period, the moisture transport trajectories are clustered into 6 groups, with four ocean-originating paths providing 83.9% and two continent-originating paths （originating over Lake Baikal and the Persian Gulf） con- tributing the remaining 16.1% of the total moisture. The two Pacific-originating paths, from the western Pacific Ocean and the East China Sea, combined account for about 46%, the SCS-originating path contributes about 24.3%, while the Bay of Bengal-originating path accounts for 13.6% of the total moisture over South China. The trajectories during the postmonsoon-onset period are clustered into 4 groups, with three southwesterly paths （from the Arabian Sea, the central Indian Ocean, and the western Indian Ocean, respectively） accounting for more than 76% and the sole Pacific-originating path accounting for 23.8% of the total moisture. The formation of the moisture transport tra-jectories is substantially affected by the topography, especially the Tibetan Plateau and the Indian and Indo-China Peninsulas. The SCS region contributes the most moisture during both periods （35.3% and 31.1%）. The Pacific Ocean is ranked second during the former period （about 21.0%） but its contribution is reduced to 5.0% during the lat-ter period, while the contribution from the Bay of Bengal and the Indian Ocean combined increases from 17.1% to 43.2%.

NUMERICAL SIMULATION OF A MESOSCALE CONVECTIVE SYSTEM(MCS)DURING THE FIRST RAINY SEASON OVER SOUTH CHINA

Based on the NCEP/NCAR reanalysis data and observations collected during the SCSMEX,a mesoscale convective system (MCS) occurring over South China during 23-24 May 1998 has been studied with a numerical simulation using the Fifth Generation Penn-State/NCAR Mesoseale Modeling System (MMS).The successful simulations present us some interesting findings.The simulated MCS was a kind of meso-β scale system with a life cycle of about 11 hours.It generated within a small vortex along a cold front shear line.The MCS was characterized by severe convection.The simulated maximum vertical velocity was greater than 90 cm s^(-1),and the maximum divergence at about 400 hPa.The rainfall rate of MCS exceeded 20 mm h^(-1).To the right of the simulated MCS,a mesoscale low-level jet (mLLJ) was found.A strong southwesterly current could also be seen to the right of MCS above the mLLJ.This strong southwesterly current might extend up to 400 hPa.A column of cyclonic vorticity extended through most part of the MCS in the vertical direction.Additionally,the simulated MCS was compared favorably with the observational data in terms of location,precipitation intensity and evolution.

Assimilating Doppler radar observations with an ensemble Kalman filter for convection-permitting prediction of convective development in a heavy rainfall event during the pre-summer rainy season of South China

This study examines the effectiveness of an ensemble Kalman filter based on the weather research and forecasting model to assimilate Doppler-radar radial-velocity observations for convection-permitting prediction of convection evolution in a high-impact heavy-rainfall event over coastal areas of South China during the pre-summer rainy season. An ensemble of 40 deterministic forecast experiments(40 DADF) with data assimilation(DA) is conducted, in which the DA starts at the same time but lasts for different time spans(up to 2 h) and with different time intervals of 6, 12, 24, and 30 min. The reference experiment is conducted without DA(NODA).To show more clearly the impact of radar DA on mesoscale convective system(MCS)forecasts, two sets of 60-member ensemble experiments(NODA EF and exp37 EF) are performed using the same 60-member perturbed-ensemble initial fields but with the radar DA being conducted every 6 min in the exp37 EF experiments from 0200 to0400 BST. It is found that the DA experiments generally improve the convection prediction. The 40 DADF experiments can forecast a heavy-rain-producing MCS over land and an MCS over the ocean with high probability, despite slight displacement errors. The exp37 EF improves the probability forecast of inland and offshore MCSs more than does NODA EF. Compared with the experiments using the longer DA time intervals, assimilating the radial-velocity observations at 6-min intervals tends to produce better forecasts. The experiment with the longest DA time span and shortest time interval shows the best performance.However, a shorter DA time interval(e.g., 12 min) or a longer DA time span does not always help. The experiment with the shortest DA time interval and maximum DA window shows the best performance, as it corrects errors in the simulated convection evolution over both the inland and offshore areas. An improved representation of the initial state leads to dynamic and thermodynamic conditions that are more conducive to earlier initiation of the inland MCS and longer maintenance of the offshore MCS.

Variations in the starting date of the pre-summer rainy season in South China, 1736-2010

The starting dates of the pre-summer rainy season during historical times （1736- 1911） in Fuzhou and Guangzhou of South China, were determined and reconstructed on the basis of historical documents in the Yu-Xue-Fen-Cun archive, together with observed features of precipitation during the pre-summer rainy season. In addition, starting dates of the pre-summer rainy season from 1953 in Fuzhou and from 1952 in Guangzhou were reconstructed for the instrumental period. These data allowed for analyses of inter-annual and inter-decadal changes in the starting dates of the pre-summer rainy season in South China over the past 300 years. Results show that the mean starting date of the pre-summer rainy season in South China was the first pentad of May; in addition, periodicities in the starting dates of 2-3 years, 10 years, and 40 years were detected during the period 1736-1911, and of 2-3 years, 10 years, and 22 years during the instrumental period. From 1736 to 1911, the earliest starting dates at Fuzhou and Guangzhou both occurred at the fourth pentad of April, while the latest starting dates were at the sixth pentad of May in Fuzhou and the first pentad of June in Guangzhou. During the instrumental period, the earliest and latest starting dates were at the fourth pentad of April and the first pentad of June, respectively, in both Fuzhou during 1953-2010 and Guangzhou during 1952-2010. The maximum difference between neighboring decades during 1736-1911 was 2.2 and 1.6 pentads in Fuzhou and Guangzhou, respectively, and during the instrumental period it was 2.5 and 2.4 pentads in Fuzhou and Guangzhou, respectively.

IOD Modoki事件对华南后汛期降水异常的影响及可能机理

利用1979—2019年CN05.1中国区域高分辨率降水格点数据、英国Hadley中心观测海温数据、ERA5逐月大气再分析资料及大气环流模式,研究了华南后汛期期间(7—9月)IOD Modoki事件与华南后汛期降水异常的关系及可能机理。观测资料结果表明,华南后汛期降水异常与热带印度洋中部(东和西部)海温异常呈显著正(负)相关关系,表现为印度洋IOD Modoki或印度洋三极子事件的空间分布型。滤除ENSO信号影响后,华南后汛期降水异常仍和IOD Modoki存在较为密切的联系。IOD Modoki正异常对华南后汛期降水异常的影响有以下途径,一方面,异常水汽从热带印度洋东部向西输送至热带中印度洋后,在北半球受科氏力作用向东输送至华南地区,为华南地区提供了充足的水汽条件,并且对华南地区降水正异常的主要水汽辐合贡献为平均水汽的水平扰动散度项和扰动引起的平均水汽垂直平流项。另一方面,热带东南印度洋海温负异常,通过Mastuno-Gill响应引起对流层低层自热带东南印度洋至热带中印度洋有东南风异常,增强了70°E附近的越赤道气流,在北半球向东输送至西北太平洋,这引起了华南地区对流层低层气旋式环流异常。另外,热带东印度洋对流层低(高)层异常辐散(辐合),华南地区低(高)层异常辐合(辐散)增强了东亚地区的局地Hadley环流,有利于华南地区降水的产生。再者,IOD Modoki引起南亚季风区受异常下沉运动控制,并通过季风-荒漠机制引起副热带北大西洋东部、北非荒漠区及地中海西部周围正涡度异常,激发了沿急流向下游传播的准静止Rossby波,增强了日本海高压异常和华南及邻近地区对流层低层气旋式环流异常。上述原因均有利于华南地区降水的产生,反之亦然。上述结果在数值模式中亦得到了验证。

非典型大西洋尼诺对中国东部前、后汛期降水的影响

采用1979—2020年共42 a的降水观测和再分析资料,使用多变量EOF、傅立叶滤波、线性回归、T-N波通量等方法,研究了非典型大西洋尼诺(Non-Canonical Atlantic Ni1o,NCA)对中国东部夏季前、后汛期降水的影响及物理机制。结果显示,NCA在前汛期引起中国南方降水增加,但在后汛期导致华南沿海地区降水减少而北方降水增加。这是由于在前汛期,NCA的暖海温异常位于热带北大西洋,可通过“风-蒸发-海表温度”正反馈引起拉尼娜,进而激发西北太平洋异常反气旋,其西北侧的异常西南气流有助于将暖湿水汽输送至我国南方,导致前汛期南方降水增加。在后汛期,NCA的暖海温异常南移至赤道大西洋,通过调节沃克环流增强拉尼娜,使得西北太平洋异常反气旋进一步增强并覆盖华南地区,导致华南降水减少而北方降水增加。此外,与NCA相关的东传罗斯贝波列在前/后汛期位于副热带/中高纬地区,其在我国东部引起的环流异常也有助于NCA在前/后汛期影响我国南/北方降水。

Mesoscale Observational Analysis of Lifting Mechanism of a Warm-Sector Convective System Producing the Maximal Daily Precipitation in China Mainland during Pre-Summer Rainy Season of 2015

A long-lived, quasi-stationary mesoscale convective system （MCS） producing extreme ramtall （maximum of 542 mm） over the eastern coastal area of Guangdong Province on 20 May 2015 is analyzed by using high-resolution surface observations, sounding data, and radar measurements. New convective ceils are continuously initiated along a mesoscale boundary at the surface, leading to formation and maintenance of the quasi-linear-shaped MCS from about 2000 BT 19 to 1200 BT 20 May. The boundary is originally formed between a cold dome generated by previous convection and southwesterly flow from the ocean carrying higher equivalent potential temperature （θe） air. The boundary is subsequently maintained and reinforced by the contrast between the MCS-generated cold outflow and the oceanic higher-θe air. The cold outflow is weak （wind speed ≤ 5 m s-1）, which is attributable to the characteristic environmental conditions, i.e., high humidity in the lower troposphere and weak horizontal winds in the middle and lower troposphere. The low speed of the cold outflow is comparable to that of the near surface southerly flow from the ocean, resulting in very slow southward movement of the boundary. The boundary features temperature contrasts of 2-3℃ and is roughly 500-m deep. Despite its shallowness, the boundary appears to exert a profound influence on continuous convection initiation because of the very low level of free convection and small convection inhibition of the near surface oceanic air, building several parallel rainbands （of about 50-kin length） that move slowly eastward along the MCS and produce about 80% of the total rainfall. Another MCS moves into the area from the northwest and merges with the local MCS at about 1200 BT. The cold outflow subsequently strengthens and the boundary moves more rapidly toward the southeast, leading to end of the event in 3 h.

The Record-Breaking Extreme Drought in Yunnan Province, Southwest China during Spring-Early Summer of 2019 and Possible Causes

In spring and early summer of 2019, Yunnan Province experienced the most severe seasonal drought on record,with days of extreme drought area exceeding 10^5 km^2 far more than normal. Consistently, the precipitation in each month from February to June is over 30% less than normal, and about 50% less in the most severe drought period(April–June). The rainy season in Southwest China(SWC) started on the third pentad in June 2019, which is the second latest in history. The rainy season in Yunnan started on 24 June, which is the latest(29 days later than normal). On the contrary, the onset of the South China Sea summer monsoon(SCSSM) is abnormally early. The lag time between the start of the rainy season in SWC and the onset of the SCSSM in 2019 is 7 pentads, which is the largest since 1961, much longer than the climate mean(less than 1 pentad). The present study analyzes the possible reasons why the rainy season came extremely late and the drought disaster persisted and intensified after a much early SCSSM, at both seasonal and subseasonal timescales. The abnormally late onset of the rainy season and the second greatest potential evapotranspiration(PET) since 1981 are the direct reasons for the persistent drought. Statistical results show that the water vapor from southwest of Yunnan in April–June contributes more than that from the east at the seasonal scale. In April–June 2019, however, the southern branch trough(SBT) was abnormally weak, the large and strong anticyclonic wind anomaly prevailed over the Bay of the Bengal(BOB), and the meridional water vapor transport to Yunnan was weak. At the subseasonal scale, the weaker SBT lasted the longest, and the strong convection over the BOB came up late despite of an early onset of the SCSSM, which resulted in reduced low-level moisture convergence in Yunnan and development of drought prior to the SCSSM onset. From the onset of SCSSM to the start of rainy season in SWC, the SBT and meridional water vapor transport from the BOB were still weak, and the water vapor was mainly transported into the coastal area of South and Southeast China rather than Yunnan. After the start of the rainy season in SWC, the SBT was still weak. This led to less moisture transport in the westerlies to the west of Yunnan and the persistent extreme drought. Both the statistical results and case analysis indicate that the stronger Australian high in spring and early summer of 2019 was associated with the abnormally strong anticyclone over the BOB and the always weak SBT. In sum, the anomalous weakness of SBT played a critical role in the extreme drought occurrence and persistence in Yunnan of Southwest China in 2019.

热带北太平洋海温异常对2018年华南前汛期降水负异常事件的影响

本文利用1979—2018年中国地面气象台站1767个站点降水资料、HadISST海温数据及NCEP/NCAR再分析资料,滤除同期ENSO影响后,分析了2018年华南前汛期降水负异常与热带北太平洋海温异常之间的联系。研究结果显示,2018年华南前汛期降水负异常事件和热带北太平洋(Tropical Northern Pacific,TNP)海温偏暖有关。热带北太平洋海温正异常,通过Mastuno-Gill响应引起西北太平洋气旋式环流异常,中国南海东风异常,减弱了南海夏季风,华南至中国南海地区对流层低层存在异常反气旋环流。水汽存在由中纬度北太平洋经黑潮海区-华南地区-中国南海-菲律宾群岛向热带太平洋的异常输送,且华南地区为水汽的异常辐散区域。另一方面,TNP区域暖海温异常引起了该区域低层异常辐合、高层异常辐散、异常上升运动,使得华南地区高层异常辐合、低层异常辐散、异常下沉运动。这样的环流配置对华南地区降水的产生有不利影响,引起了2018年华南前汛期降水异常偏少。2018年TNP区域暖海温异常引起华南前汛期降水负异常的物理机制还在ECHAM5模式30个成员集合平均的试验结果中得以验证。

青藏高原冬季积雪与华南前汛期降水年际变率的联系

利用1979—2018年青藏高原(简称高原,下同)卫星积雪数据集、华南地区261站逐日降水及ERA5再分析资料,探讨了高原冬季积雪与华南前汛期降水的联系。结果表明:1)高原西部积雪与华南前汛期降水的正相关关系最为稳定,其主要影响前汛期的锋面降水,对夏季风降水的影响较小;2)华南前汛期在高原西部积雪偏多年比偏少年偏早20 d,使得前汛期降雨日数偏多,持续时间偏长,总降水量偏多,而降水强度受积雪的影响较小;3)高原积雪偏多年,积雪的冷却作用形成了低层异常反气旋环流,而东亚沿岸为“+-+”的位势高度异常,中纬度“西高东低”的环流配置有利于中高纬冷空气南侵,使得华南上空温度偏低,同时偏强偏南的西太平洋副热带高压加强了低纬地区偏南气流和水汽输送。3—4月锋面在华南北部南北摆动,4月初偏北干冷空气南侵和偏南暖湿气流的持续北推使得锋面加强,触发了前汛期的较早建立;积雪偏少年冷空气和偏南暖湿气流均较弱,华南北部锋面在4月初中断,4月中下旬华南北部锋面在偏北弱冷空气和偏南暖湿气流的共同作用下重新建立,从而华南前汛期开始偏晚。

汛期我国主要雨季进程成因及预测应用进展

汛期内我国中东部地区的雨季是东亚夏季风推进过程中的重要产物,主要包括华南前汛期、梅雨、华北雨季和华西秋雨等,各地雨季决定了我国中东部地区汛期的旱涝布局和旱涝演变,是我国汛期预测和服务的重点.该文回顾了4个雨季特征及影响因子方面的研究进展,在此基础上梳理物理概念预测模型.研究显示:海温异常是影响各区域雨季的重要先兆信号,但不同雨季的年际和年代际变化特征不同,海温作为外强迫信号的影响程度和时空形式也有差异.利用热带太平洋东西海温差指标能更好地解释华南前汛期降水的年际变化.而与梅雨的年际变化分量相关联的海温关键区主要分布于热带,与年代际或多年代际变化分量相联系的海温关键区则来自中高纬度.华北雨季降水的强弱不仅与ENSO循环的位相有关,更多受到ENSO演变速率的影响.而影响华西秋雨的海温关键区随着年代际背景的变化发生了改变,需要重新诊断和建模.

华南前汛期区域持续性暴雨的分布特征及分型

利用广东和广西两省共175个台站的日降水观测资料,采用计算机检索的方法,对1961-2005年间华南前汛期区域持续性暴雨进行了定义。对南海夏季风爆发前后区域持续性暴雨的气候分布特征的分析发现,季风爆发前持续性暴雨频数从60年代至今呈现出正态分布的年代际变化特征,而季风爆发后的区域持续性暴雨频数变化则几乎相反;广东省前汛期区域持续性暴雨降水明显比广西强。此外,通过EOF方法和相关分析得到了夏季风爆发前后出现频率较高的几种分布雨型,它们能较好地代表季风爆发前后华南降水分布的特点。

华南前汛期的锋面降水和夏季风降水II.空间分布特征

利用中国730站降水资料和第I部分(郑彬等,2006)得到的华南前汛期锋面降水和季风降水的划分日期,计算出1958-2000年华南前汛期锋面降水量(强度)和季风降水量(强度)的序列,采用EOF和扩展EOF分析方法,得到华南前汛期降水的几个主要分布型,并探讨锋面降水与季风降水的可能联系。分析结果表明:华南前汛期的锋面降水和季风降水分布主要有三种类型——全区旱涝型、西南涝(旱)东北旱(涝)型、东南涝(旱)西北旱(涝)型。各分布型的时间系数与850 hPa风场的相关结果表明不同的分布对应着不同的低层环流形势。统计结果显示华南前汛期锋面降水的分布形式与季风降水的分布形式有一定的对应关系。

华南前汛期持续暴雨环流分型初步研究

采用1961—2010年NCEP/NCAR逐日再分析资料和台站观测降水量资料,按一定标准选取了华南前汛期24个持续暴雨过程;并且按基本判据确定逐年华南夏季风降水开始日期。然后依据南亚高压环流型和相对于该年夏季风降水开始的早晚,将这些暴雨过程划分为夏季风降水前、后南亚高压东部型,夏季风降水后南亚高压带状、西部型共4个类型;其中,夏季风后南亚高压西部型次数最多、平均持续时间最长。所有类型持续暴雨的相同点是:广东东北部附近均为暴雨频率和雨量高值区;暴雨期间华南150 h Pa位势高度增加、500 h Pa位势高度减少;华南处在150 h Pa偏西风急流南侧辐散区中;850 h Pa华南沿海有明显的西南气流,低层辐合在华南东北部最明显;两广沿海为可降水量大值区;华南的整层水汽输送主要呈现西南向。不同点是:夏季风后南亚高压西部型平均雨量较小,夏季风后南亚高压带状型与西部型在印度洋上存在明显的偏东风高空急流;夏季风后南亚高压类型在两广沿海的可降水量数值较大。

云南雨季开始期与热带OLR的联系

利用NCEP1979-2003年1～12月再分析的OLR逐日、月平均资料及同期云南雨季开始期资料，通过相关统计、对比合成等方法，重点分析了南海和孟湾海区的OLR场变化对云南雨季开始期的影响。结果表明，南海和孟湾海区OLR在云南雨季开始偏早年与偏晚年之间差异明显，南海海区前期的对流活动对云南雨季开始期有更显著的指示性，但孟湾海区对流的强弱仍是影响云南雨季开始期的重要因素之一。

东亚副热带西风急流位置变化及其对中国东部夏季降水异常分布的影响

利用1951-2004年美国国家环境预报中心（NCAR/NCEP）再分析候平均资料和同期中国740站日降水资料,分析了东亚副热带西风急流轴位置的变化趋势及其对中国东部夏季降水分布的影响,结果表明：（1）夏季7-8月东亚副热带西风急流位置和形态在1975-1980年间出现转折,1980年后西风急流中心逐渐向西移动的同时伴随有西风急流向南偏移。（2）在1980年以后华北地区降水量减少和降水强度减弱,雨季开始时间推迟、雨季变短,而长江中下游地区入梅提前、梅雨期变长,降水量增加,从而形成南涝北旱的降水分布形势。（3）1957-1964年华北典型多雨时期,西风急流呈纬向分布,在华北地区有高低空急流耦合,强辐合上升区正好位于华北,并有充足的水汽条件供应,使得华北降水偏多。而1980-1987年和1997-2002年华北地区为典型少雨时期,1980-1987年西风急流中心位置偏南和1997-2002年急流位置显著偏西,在华北地区均无高低空急流耦合的环流形势,水汽辐合区位于长江流域,强辐合上升区位置在30°N以南区域,有利于江南地区降水增加而华北地区少雨,这表明西风急流位置变化导致环流调整对中国东部降水分布有显著影响。因而,在讨论东亚副热带西风急流位置与中国东部地区降水异常的关系时,不仅要考虑西风急流南北位置变化,还需要综合分析西风急流的东西位置和形态的变化。