A possible new mechanism for northward propagation of boreal summer intraseasonal oscillations based on TRMM and MERRA reanalysis

Boreal summer intraseasonal oscillations (BSISOs) manifest in the active and break spells and act as the primary building block of the Indian summer monsoon. Although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the BSISO, the role of different hydrometeors in modulating these processes remains poorly explored. In this study, TRMM-2A12 retrievals and Modern Era Retrospective-analysis for Research and Applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the BSISOs. The study reveals that the cloud liquid water leads the deep convection during the northward propagation of BSISOs in the lower troposphere, while the cloud ice slightly lags the convection. This distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer (PBL) convergence, followed by triggering of the deep convection. The analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and PBL convergence of moisture anomaly north of the convection center. A new internal dynamical-thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation. It is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub-grid scale eddies within the PBL induces lower level instability required to precondition the lower atmosphere for triggering the deep convection. Vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere. To check the robustness of the proposed hypothesis, similar analysis is performed for the weak northward propagating BSISO cases.     

Effects of air�Csea coupling on the boreal summer intraseasonal oscillations over the tropical Indian Ocean

The effects of air?Csea coupling over the tropical Indian Ocean (TIO) on the eastward- and northward-propagating boreal summer intraseasonal oscillation (BSISO) are investigated by comparing a fully coupled (CTL) and a partially decoupled Indian Ocean (pdIO) experiment using SINTEX-F coupled GCM. Air?Csea coupling over the TIO significantly enhances the intensity of both the eastward and northward propagations of the BSISO. The maximum spectrum differences of the northward- (eastward-) propagating BSISO between the CTL and pdIO reach 30% (25%) of their respective climatological values. The enhanced eastward (northward) propagation is related to the zonal (meridional) asymmetry of sea surface temperature anomaly (SSTA). A positive SSTA appears to the east (north) of the BSISO convection, which may positively feed back to the BSISO convection. In addition, air?Csea coupling may enhance the northward propagation through the changes of the mean vertical wind shear and low-level specific humidity. The interannual variations of the TIO regulate the air?Csea interaction effect. Air?Csea coupling enhances (reduces) the eastward-propagating spectrum during the negative Indian Ocean dipole (IOD) mode, positive Indian Ocean basin (IOB) mode and normal years (during positive IOD and negative IOB years). Such phase dependence is attributed to the role of the background mean westerly in affecting the wind-evaporation-SST feedback. A climatological weak westerly in the equatorial Indian Ocean can be readily reversed by anomalous zonal SST gradients during the positive IOD and negative IOB events. Although the SSTA is always positive to the northeast of the BSISO convection for all interannual modes, air?Csea coupling reduces the zonal asymmetry of the low-level specific humidity and thus the eastward propagation spectrum during the positive IOD and negative IOB modes, while strengthening them during the other modes. Air?Csea coupling enhances the northward propagation under all interannual modes due to the persistent westerly monsoon flow over the northern Indian Ocean.     

全球变化背景下北半球冬季MJO传播的年代际变化

利用1979~2013年实时多要素MJO（Madden-Julian Oscillation）监测（RMM）指数,美国NOAA逐日长波辐射资料和NCEP/NCAR再分析资料等,分析了全球变化背景下北半球冬季MJO传播的年代际变化特征。从全球平均气温快速增暖期（1985~1997）到变暖趋缓期（2000~2012）,MJO 2~4位相频次减少,5~7位相频次增多,即MJO对流活跃区在热带印度洋地区停留时间缩短、传播速度加快,而在热带西太平洋停留时间加长、传播明显减缓。进一步分析发现,以上MJO的年代际变化特征与全球变化年代际波动有关。当太平洋年代际涛动（PDO）处于负位相时,全球变暖趋缓,热带东印度洋—西太平洋海温异常偏暖,使其上空对流加强,垂直上升运动加强,对流层低层辐合,大气中的水汽含量增多,该区域的湿静力能（MSE）为正异常。当MJO对流活跃区位于热带印度洋地区时,MJO异常环流对季节平均MSE的输送在强对流中心东侧为正、西侧为负,有利于东侧MSE扰动增加,使得MJO对流扰动东移加快;而当MJO对流活跃区在热带西太平洋地区,MJO异常环流对平均MSE的输送形成东负西正的形势,东侧MSE扰动减小,不利于MJO快速东传。因此,全球变化背景下PDO引起的大气中水汽含量及MSE的变化可能是MJO传播年代际变化的重要原因。     

印度洋偶极子对中国南海夏季西南季风水汽输送的影响

利用NCEP/NCAR再分析资料和中科院大气物理研究所PIAP3大气环流模式,分析了印度洋偶极子对夏季中国南海西南季风水汽输送的影响。结果表明,印度洋偶极子正位相期间夏季中国南海西南水汽输送较强,负位相期间则较弱。原因可归结为以下:正位相期间,MJO（Madden-Julian Oscillation）多活动于热带西印度洋,其向东传播受到阻碍,但经向传播明显,通常可传播至孟加拉湾地区,同时PIAP3显示印度洋季风槽位置偏北,且印尼以西过赤道气流较强,从而使得这一地区气旋性环流得到建立与加强。孟加拉湾地区对应着较强的对流活动以及深厚积云对流加热,从而通过对流加热的二级热力响应使西太平洋副热带高压位置向北推进,进而使得南海地区西南季风水汽输送得到建立与加强。在此期间孟加拉湾、中南半岛至南海地区对流活动较强,而苏门答腊沿岸对流活动受到抑制,由此增强了Reverse-Hadley环流,使低层经向风较强,进而增强了南海西南季风的水汽输送,PIAP3大气环流模式证实了Reverse-Hadley环流的增强。负位相期间,MJO多活动于热带东印度洋,在东传过程中受到Walker环流配置影响,在140°E赤道附近形成东西向非对称积云对流加热热源,其东侧Kelvin波响应加强了东风异常并配合副热带高压南缘东风压制了中国南海的西南季风水汽输送。在此期间,MJO在南海地区的经向传播较强,但经向传播常止步于南海地区15°N附近,虽携带大量水汽,但深厚积云对流强烈地消耗水汽使大气中水汽含量降低,PIAP3大气环流模式证实负位相期间深厚积云对流对水汽消耗加大,从而使得负位相期间南海地区水汽含量与正位相期间大体相近,但由于经向风不足使水汽向北输送较弱。     

Vertical cloud structures of the boreal summer intraseasonal variability based on CloudSat observations and ERA-interim reanalysis

The boreal summer intraseasonal variability (BSISV), which is characterized by pronounced meridional propagation from the equatorial zone to the Indian Continent, exerts significant modulation of the active/break phases of the south Asian monsoon. This form of variability provides a primary source of subseasonal predictive skill of the Asian summer monsoon. Unfortunately, current general circulation models display large deficiencies in representing this variability. The new cloud observations made available by the CloudSat mission provide an unprecedented opportunity to advance our characterization of the BSISV. In this study, the vertical structures of cloud water content and cloud types associated with the BSISV over the Indian Ocean and subcontinent are analyzed based on CloudSat observations from 2006 to 2008. These cloud structures are also compared to their counterparts as derived from ERA-interim reanalysis. A marked vertical tilting structure in cloud water is illustrated during the northward propagation of the BSISV based on both datasets. Increased cloud liquid water content (LWC) tends to appear to the north of the rainfall maximum, while ice water content (IWC) in the upper troposphere slightly lags the convection. This northward shift of increased LWC, which is in accord with local enhanced moisture as previously documented, may play an important role in the northward propagation of the BSISV. The transition in cloud structures associated with BSISV convection is further demonstrated based on CloudSat, with shallow cumuli at the leading edge, followed by the deep convective clouds, and then upper anvil clouds. Some differences in cloud water structures between CloudSat and ERA-interim are also noted, particularly in the amplitudes of IWC and LWC fields.     

1998年区域性水汽输送及对流活动与副高活动变异的相关特征(英)

根据1998年NCEP逐日资料和TBB逐日资料,探讨了低纬度对流活动和副高周边水汽输送及其对流活动对夏季西太平洋副热带高压季节性北跳、南撤的影响效应。研究表明:低纬热带对流加强,且110°-150°E地区的南北向垂直经圈环流下沉区北移,夏季西太平洋副热带高压有北跳现象。另外,诊断结果亦表明西太平洋副高周边纬向水汽输送的显著减弱亦预示将出现副高的北跳,而西太平洋地区低纬经向水汽输送减少一候之后,副高南撤。研究结果表明西太平洋副高北跳、南撤与低纬度的对流潜热释放、中纬西太平洋副高周边的水汽输送及其对流活动存在密切的关系。数值模拟结果进一步证实上述副高活动变异与前期水汽输送及其对流特征的相关关系。     

印度洋海温年际异常与热带夏季季节内振荡的关系及其数值模拟研究

利用观测分析资料和SINTEX-F海气耦合长时间(70年)数值模拟结果,分析了印度洋海温年际异常与热带夏季季节内振荡(BSISO)各种传播模态之间关系及其物理过程。结果表明,印度洋海温年际异常与热带BSISO关系密切,当印度洋为正(负)偶极子情况,中东印度洋北传BSISO减弱(加强);当印度洋为正(负)海盆异常(BWA)情况,印度洋西太平洋赤道地区(40°E -180°)东传BSISO加强(减弱)。印度洋海温年际变化通过大气环流背景场和BSISO结构影响热带BSISO不同传播模态强度的年际变化。在负(正)偶极子年夏季,由于对流层大气垂直东风切变加强(减弱),对流扰动北侧的正压涡度、边界层水汽辐合加强更明显(不明显),导致形成BSISO较强(弱)的经向不对称结构,因此北传BSISO偏强(减弱)。印度洋BWA模态通过影响赤道西风背景以及海气界面热力交换,导致赤道东传BSISO强度产生变化。在正BWA年夏季,赤道地区西风较明显,当季节内振荡叠加在这种西风背景下,扰动中心的东侧(西侧)风速减弱(加强)更明显,海面蒸发及蒸发潜热减弱(加强)更明显,导致扰动中心的东侧(西侧)海温升高(降低)幅度更大,从而使边界层产生辐合(辐散)更强、水汽更多(少),因此赤道东传BSISO偏强;而在负BWA年,赤道地区西风背景减弱,以上物理过程受削弱使赤道东传BSISO偏弱。     

Role of the 10–20-Day Oscillation in Sustained Rainstorms over Hainan,China in October 2010

Hainan, an island province of China in the northern South China Sea, experienced two sustained rainstorms in October2010, which were the most severe autumn rainstorms of the past 60 years. From August to October 2010, the most dominant signal of Hainan rainfall was the 10–20-day oscillation. This paper examines the roles of the 10–20-day oscillation in the convective activity and atmospheric circulation during the rainstorms of October 2010 over Hainan. During both rainstorms,Hainan was near the center of convective activity and under the influence of a lower-troposphere cyclonic circulation. The convective center was initiated in the west-central tropical Indian Ocean several days prior to the rainstorm in Hainan. The convective center first propagated eastward to the maritime continent, accompanied by the cyclonic circulation, and then moved northward to the northern South China Sea and South China, causing the rainstorms over Hainan. In addition, the westward propagation of convection from the tropical western Pacific to the southern South China Sea, as well as the propagation farther northward, intensified the convective activity over the northern South China Sea and South China during the first rainstorm.     

Seasonal variation and physical properties of the cloud system over southeastern China derived from CloudSat products

Based on the National Centers for Environmental Prediction(NCEP) and Climate Prediction Center(CPC) Merged Analysis of Precipitation(CMAP) data and Cloud Sat products, the seasonal variations of the cloud properties, vertical occurrence frequency, and ice water content of clouds over southeastern China were investigated in this study. In the Cloud Sat data, a significant alternation in high or low cloud patterns was observed from winter to summer over southeastern China. It was found that the East Asian Summer Monsoon(EASM) circulation and its transport of moisture leads to a conditional instability, which benefits the local upward motion in summer, and thereby results in an increased amount of high cloud. The deep convective cloud centers were found to coincide well with the northward march of the EASM, while cirrus lagged slightly behind the convection center and coincided well with the outflow and meridional wind divergence of the EASM. Analysis of the radiative heating rates revealed that both the plentiful summer moisture and higher clouds are effective in destabilizing the atmosphere. Moreover, clouds heat the mid-troposphere and the cloud radiative heating is balanced by adiabatic cooling through upward motion, which causes meridional wind by the Sverdrup balance. The cloud heating–forced circulation was observed to coincide well with the EASM circulation, serving as a positive effect on EASM circulation.     

热带夏季风场与对流场季节内振荡传播模比较

利用1979-2007年卫星观测日平均OLR资料以及NCEP/DOE第2套再分析资料中的风场资料,采用有限区域波一频分析、合成分析等方法,分析对比对流层高、低层风场与对流场所表征的热带北半球夏季季节内振荡(BSISO)各种传播模态谱分布气候特征及其年际异常。结果表明:各要素反映的BSISO各种模态的气候特征及其年际变化存在一定差异,总体而言对流层低层风(850hPa纬向风或经向风)与对流比较一致。850hPa经向风(纬向风)所反映的纬向(经向)传播BSISO谱分布气候特征与对流情况最相似。在ENSO发展年,850hPa经向风反映的赤道东传波加强趋势与对流较为一致;850hPa纬向风、经向风反映的北传波变化趋势都与对流相似。在ENSO衰减年,850hPa纬向风(经向风)反映的赤道东传波(赤道外西传波)减弱趋势与对流较为一致;对流以及850hPa经向风、200hPa纬向风和200hPa经向风4种要素都能体现南海及周边地区北传波明显减弱这一特征。对流和850hPa纬向风所反映的北传波与印度洋偶极子模态之间关系一致。     

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.     

气候系统模式FGOALS模拟的南亚夏季风：偏差和原因分析

本文通过与观测和再分析资料的对比,评估了LASG/IAP发展的气候系统模式FGOALS的两个版本FGOALS-g2和FGOALS-s2对南亚夏季风的气候态和年际变率的模拟能力,并使用水汽收支方程诊断,研究了造成降水模拟偏差的原因。结果表明,两个模式夏季气候态降水均在陆地季风槽内偏少,印度半岛附近海域偏多,在降水年循环中表现为夏季北侧辐合带北推范围不足。FGOALS-g2中赤道印度洋"东西型"海温偏差导致模拟的东赤道印度洋海上辐合带偏弱,而FGOALS-s2中印度洋"南北型"海温偏差导致模拟的海上辐合带偏向西南。水汽收支分析表明,两个模式中气候态夏季风降水的模拟偏差主要来自于整层积分的水汽通量,尤其是垂直动力平流项的模拟偏差。一方面,夏季阿拉伯海和孟加拉湾的海温偏冷而赤道西印度洋海温偏暖,造成向印度半岛的水汽输送偏少;另一方面,对流层温度偏冷,冷中心位于印度半岛北部对流层上层,同时季风槽内总云量偏少,云长波辐射效应偏弱,对流层经向温度梯度偏弱以及大气湿静力稳定度偏强引起的下沉异常造成陆地季风槽内降水偏少。在年际变率上,观测中南亚夏季风环流和降水指数与Ni?o3.4指数存在负相关关系,但FGOALS两个版本模式均存在较大偏差。两个模式中与ENSO暖事件相关的沃克环流异常下沉支和对应的负降水异常西移至赤道以南的热带中西印度洋,沿赤道非对称的加热异常令两个模式中越赤道环流季风增强,导致印度半岛南部产生正降水异常。ENSO相关的沃克环流异常下沉支及其对应的负降水异常偏西与两个模式对热带南印度洋气候态降水的模拟偏差有关。研究结果表明,若要提高FGOALS两个版本模式对南亚夏季风气候态模拟技巧,需减小耦合模式对印度洋海温、对流层温度及云的模拟偏差;若要提高南亚夏季风和ENSO相关性模拟技巧需要提高模式对热带印度洋气候态降水以及与ENSO相关的环流异常的模拟能力。     

登陆台风Matmo(2014)北侧对流雨带的触发和发展过程

登陆台风Matmo（2014）北侧弱回波区突发多条强对流雨带,造成严重影响。本文利用NECP/NCAR的FNL（Final）再分析资料,地面自动站加密观测资料、雷达拼图资料等对台风北侧连续出现的3条对流雨带的触发和发展过程进行研究。结果表明:初始对流是在弱的对流不稳定和条件对称不稳定条件下,由锋面强迫和地形抬升作用共同触发。接二连三发生的3条对流雨带是在台风北部高湿环境,长时间稳定的水汽辐合,明显的对流不稳定,局部弱条件对称不稳定条件下,由近地面辐合线在辐合稳定并加强后触发的,有利的环境和多层不稳定的叠加有利于雨带的发展和维持。先后触发对流雨带的3条辐合线在发生位置上相当接近。辐合线的形成一方面由海陆下垫面差异、江苏和山东南部海岸线曲率、台风风速随半径分布特点所决定;另一方面,辐合线2和3的形成还与它们北侧对流雨带的冷出流密切相关。对流带上的强降水中心往往对应着稳定的辐合中心,辐合中心主要位于风速大小梯度明显的位置上。而辐合线上的风速梯度主要受海岸线、地形附近的降水分布影响。     

东亚—西北太平洋地区夏季季节内振荡的年代际变化

本文重点分析对比热带夏季季节内振荡(Boreal Summer Intraseasonal Oscillation, BSISO)1987—1995年(P1),1996—2007年(P2)和2008—2017年(P3)三阶段东亚—西北太平洋地区(East Asian-Western North Pacific, EAWNP)5—9月BSISO年代际变化的季节内差异特征。结果表明,在P1和P3两阶段,5—7月EAWNP BSISO强度几乎相同,但P2中每个月均显著增强,表明5—7月EAWNP BSISO经历了P1—P2增强和P2—P3减弱的年代际变化。8月,EAWNP BSISO强度从P1到P3逐渐增强,P3阶段比P1有显著增强,孟加拉湾和东亚副热带区域的BSISO活动增强。和P1相比,南海地区BSISO活动在P2阶段异常活跃,在5—7月强度增强,并且北传显著。在P2阶段,负位相的太平洋年代际(Interdecadal Pacific Oscillation, IPO)对应的赤道西太平洋和印度洋海温增暖,及Walker环流的增强为5—7月BSISO活动提供了水汽和对流发展的有利条件,而南...     

A Study on the Mesoscale Convective Systems during the Summer Monsoon Onset over the South China Sea in 1998 Part Ⅰ: Analysis of Large-Scale Fields for Occurrence and Development of the Mesoscale Convective Systems

The summer monsoon onset over the northern South China Sea (SCS) in May 16-20, 1998 was characterized by the abrupt onset of mesoscale convective activities and rapid increase of precipitation. The possible mechanism for formation of the mesoscale convective systems (MCSs) and related rain belts were revealed through discussing their forming physical conditions under the large-scale background: (1) The high pseudo-equivalent potential temperature and the convective instability in the lower troposphere, the low-level southwesterly confluence and the high-level divergence over South China and the northern SCS provided the favorable large-scale thermodynamic and dynamic conditions for development of MCSs. The southwesterly flow from the Bay of Bengal (BOB) interacted with that to the western flank of the subtropical high, which constituted the major moisture channels, thus bringing about deep wet layers and strong moisture convergence;(2) triggered by several cold troughs from high and mid latitudes, the convectively unstable energy was released and the convective activities over the northern SCS broke out abruptly;(3)analysis of retrieved precipitation based on the dual-Doppler radar during South China Sea Monsoon Experiment (SCSMEX) indicated that active convection influenced by the monsoon trough and corresponding wind shear line organized and formed continually some mesoscale convective rainbelts. During May 15-19,about 12 precipitation processes with 6-12-hour life span or more were observed;and (4) under the favorable synoptic conditions, establishment of the monsoon trough and shear line in the low levels, as well as production and development of mesoscale low vortex were all necessary conditions for the formation and maintenance of MCSs.     

A Study on the Mesoscale Convective Systems during the Summer Monsoon Onset over the South China Sea in 1998Part I: Analysis of Large-Scale Fields for Occurrence and Development of the Mesoscale Convective Systems

1. Introduction China is located in the East Asian monsoon re- gion. Every year's weather and climate in this region is deeply affected by the monsoon activities. Es- pecially, during flooding season (May to September), the summer monsoon controls large-scale precipitation patterns, the movement of seasonal rain belt and oc- currence of drought/flood disasters. The Asian mon- soon can be divided into two systems (Tao and Chen, 1987). As a major component and its unique location, the South …     

Three-type MJO initiation processes over the Western Equatorial Indian Ocean

Thirty strong Madden-Julian Oscillation (MJO) events in boreal winter 1982-2001 are selected to investigate the triggering processes of MJO convection over the western equatorial Indian Ocean (IO). These MJO events are classified into three types, according to their dynamic and thermodynamic precursor signals in situ. In Type I, a remarkable increase in low-level moisture occurs, on average, 7 days prior to the convection initiation. This low-level moistening is mainly due to the advection of the background mean moisture by easterly wind anomalies over the equatorial IO. In Type II, lower-tropospheric ascending motion anomalies develop, on average, 4 days prior to the initiation. The cause of this ascending motion anomaly is attributed to the anomalous warm advection, set up by a suppressed MJO phase in the equatorial IO. In Type III, there are no clear dynamic and thermodynamic precursor signals in situ. The convection might be triggered by energy accumulation in the upper layer associated with Rossby wave activity fluxes originated from the midlatitudes.     

On the maintenance and initiation of the intraseasonal oscillation in the NCEP/NCAR reanalysis and in the GLA and UKMO AMIP simulations

 In this study, satellite-derived outgoing longwave radiation (OLR) and the reanalysis from the National Centers for Environmental Prediction/National Center for Atmospheric Research are used as verification data in a study of intraseasonal variability in the Goddard Laboratory for Atmospheres (GLA) and the United Kingdom Meteorological Office (UKMO) atmospheric general circulation models. These models simulated the most realistic intraseasonal oscillations (IO) of the 15 Atmospheric Model Intercomparison Project models previously analyzed. During the active phase of the intraseasonal oscillation, convection is observed to migrate from the Indian Ocean to the western/central Pacific Ocean, and into the South Pacific Convergence Zone (SPCZ). The simulated convection, particularly in the GLA model, is most realistic over the western/central Pacific Ocean and the SPCZ. In the reanalysis, the baroclinic structure of the IO is evident in the eddy-stream function, and eastward migration of the anticyclone/cyclone pairs occurs in conjunction with the eastward development of convection. Both the GLA and UKMO models exhibit a baroclinic structure on intraseasonal time scales. The GLA model is more realistic than the UKMO model at simulating the eastward migration of the anticyclone/cyclone pairs when the convection is active over the western/central Pacific. In the UKMO model, the main heating is located off the equator, which contributes to the irregular structures seen in this model on intraseasonal time scales. The maintenance and initiation of the intraseasonal oscillation has also been investigated. Analysis of the latent heat flux indicates that evaporative wind feedback is not the dominant mechanism for promoting the eastward propagation of the intraseasonal oscillation since evaporation to the west of the convection dominants. The data suggest a wave-CISK (conditional instability of the secondkind) type mechanism, although the contribution by frictional convergence is not apparent. In the GLA model, enhanced evaporation tends to develop in-place over the west Pacific warm pool, while in the UKMO simulation westward propagation of enhanced evaporation is evident. It is suggested that lack of an interactive ocean may be associated with the models systematic failure to simulate the eastward transition of convection from the Indian Ocean into the western Pacific Ocean. This hypothesis is based upon the examination of observed sea surface temperature (SST) and its relationship to the active phase of the intraseasonal oscillation, which indicates that the IO may evolve as a coupled ocean-atmosphere mode. The eastward propagation of convection appears to be related to the gradient of SST, with above normal SST to the east of the convection maintaining the eastward evolution, and decreasing SST near the western portion of the convective envelope being associated with the cessation of convection. Received: 13 September 1996/Accepted: 14 April 1997     

The Combined Effects of the Tropical and Extratropical Quasi-biweekly Oscillations on the Record-setting Mei-yu Rainfall in the Summer of 2020

During June–July 2020, the strongest recorded mei-yu rainfall occurred in the middle and lower reaches of the Yangtze River. The rainfall processes exhibited an obvious quasi-biweekly(biweekly in brief) variability, and there are altogether five cycles. It is found that the biweekly rainfall cycle mainly arises from the collaborative effects of biweekly variabilities from both the tropics and extratropics. As for the tropics, the biweekly meridional march and retreat of the western Pacific subtr...     

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.