This study investigates the physical mechanisms for the persistent heavy rainfall (PHR) events over South China (SC) associated with the 10–30-day intraseasonal oscillation (ISO) before the South China Sea Summer Monsoon (SCSSM) onset (SCSSM-I) and after the SCSSM onset (SCSSM-II) in the first rainy season (April–June). About 25.0

The north edge of the East Asian summer monsoon (EASM) is defined by using the isoline with average total column water vapor (TCWV) equal to 25 mm, and the relationship between the north edge of the EASM and summer extreme precipitation in North China (NC) from 1961 to 2020 is discussed. Self-Organizing Maps (SOM) method is used to cluster extreme precipitation in the abnormal years of the north edge in NC, and the circulation characteristics of different types of extreme precipitation are discussed. The results show that the north edge of the EASM is closely related to extreme precipitation in NC. When the north edge is northerly, both the frequency and total amount of extreme precipitation increase significantly in the monsoon marginal zone. Based on different monsoon circulation background, summer extreme precipitation events in NC are divided into four categories, namely, the central type and southern type in the northerly years of north edge, and the central type and eastern type in the southerly years of north edge. The occurrence of different types of extreme precipitation is closely related to the different characteristics of local trough, the western Pacific subtropical high (WPSH), and wave trains in the middle and high latitudes. Specifically, the central type extreme precipitation is affected by the mid-latitude wave train in the northerly years of north edge. As for the central type extreme precipitation in the southerly years of north edge, both high-latitude and mid-latitude wave trains transfer from west to east and converge over NC. However, the wave disturbance energy is relatively weak, leading to the extreme precipitation with weak intensity.

The first rainy season (April–May–June) extreme precipitation events over South China tend to frequently occur in specific hotspots, resulting in a great threat to local society and economy. Abundant moisture supply is essential to precipitation formation. In this study, we employ the FLEXPART model to investigate the geographical moisture sources for extreme precipitation events in three hotspots of Guangdong from west to east (termed as Area‐A, B, and C). During both the periods before and after the onset of the South China Sea Summer Monsoon (hereafter SCSSM), the moisture contributions of ocean regions are larger than that of land regions for all the hotspots, but the main moisture source regions are different. Before the onset of SCSSM, the main source regions are South China Sea and Southeastern Asia, which contribute 36.50 and 31.79% to Area‐A, 35.85 and 27.83% to Area‐B, while 42.27 and 23.81% to Area‐C, respectively. After the onset of SCSSM, the contribution of moisture from Indian Ocean increases a lot. Thus, the main source regions are Indian Ocean, Southeastern Asia and South China Sea, which contribute 31.52, 24.58 and 18.13% to Area‐A, 29.21, 21.82 and 14.99% to Area‐B, 26.62, 20.70%, and 28.19% to Area‐C, respectively. Among the three hotspots, the contribution of ocean regions for Area‐C is the largest. Southeastern Asia is the main land contributor before and after the onset of SCSSM for all the hotspots. This study implies the important role of both ocean and land moisture sources on extreme precipitation variations over South China.

The impact of sea surface temperature(SST)on winter haze in Guangdong province(WHDGD)was analyzed on the interannual scale.It was pointed out that the northern Indian Ocean and the northwest Pacific SST play a leading role in the variation of WHDGD.Cold(warm)SST anomalies over the northern Indian Ocean and the Northwest Pacific stimulate the eastward propagation of cold(warm)Kelvin waves through the Gill forced response,causing Ekman convergence(divergence)in the western Pacific,inducing abnormal cyclonic(anticyclonic)circulation.It excites the positive(negative)Western Pacific teleconnection pattern(WP),which results in the temperature and the precipitation decrease(increase)in Guangdong and forms the meteorological variables conditions that are conducive(not conducive)to the formation of haze.ENSO has an asymmetric influence on WHDGD.In El Ni?o(La Ni?a)winters,there are strong(weak)coordinated variations between the northern Indian Ocean,the northwest Pacific,and the eastern Pacific,which stimulate the negative(positive)phase of WP teleconnection.In El Ni?o winters,the enhanced moisture is attributed to the joint effects of the horizontal advection from the surrounding ocean,vertical advection from the moisture convergence,and the increased atmospheric apparent moisture sink(Q2)from soil evaporation.The weakening of the atmospheric apparent heat source(Q1)in the upper layer is not conducive to the formation of inversion stratification.In contrast,in La Ni?a winters,the reduced moisture is attributed to the reduced upward water vapor transport and Q2 loss.Due to the Q1 increase in the upper layer,the temperature inversion forms and suppresses the diffusion of haze.

In this study, persistent rainfall (PR) over South China (SC) is divided into two types. One type occurs multiple times in succession [defined as multiple PR (MPR)]; another type represents isolated PR (IPR), for which no new PR occurs for 10 days after the previous PR. The spatiotemporal structures of the 10-30-day intraseasonal oscillations (ISOs) associated with the two types of PR are compared and analyzed. The results reveal that the low-level moisture and air temperature perturbations always have a leading phase relative to the anomalous precipitation. In addition, the positive low-level moisture tendency appears in the MPR ending phase, whereas that in the IPR is close to zero. This difference results in convective development after the MPR ending phase, though not after the IPR. The moisture budget shows that the difference in moisture tendency between MPR and IPR is mainly due to meridional advection, including advections by the mean meridional flow across the perturbation moisture gradient and by the perturbation meridional flow across the mean moisture gradient. For the former, the difference is attributed to the perturbation moisture gradients, while the mean moisture gradients are responsible for the difference of the latter. Furthermore, an essential cause of the difference is the influence of higher-latitude disturbances that affect the IPR more significantly than the MPR. Two associated mechanisms are proposed. One is the perturbation stacking effect, and the other is the effect of angular momentum conservation. By contrast, the low-level temperature anomalies are not the key factor causing the difference between MPR and IPR.

本文利用近 60年我国气象站的日最高气温、降水资料以及NCEP/NCAR大气多要素等全球格点资料,基于华南区域性持续高温历史过程定义夏季区域性持续高温指数(RPH),分析影响华南夏季区域性持续高温年际异常的大气环流及下垫面海温因子.主要结果表明,在华南夏季持续高温偏重年,华南区域对流层高、中、低层均受反气旋性环流异常控制,在华南上空垂直方向上表现出深厚的大气动力和热力异常,华南夏季持续性高温的年际异常也与欧亚东传波列和热带大范围环流异常密切联系.热带印度洋、热带太平洋以及北大西洋三大洋海温异常共同影响华南区域性持续高温的年际异常,其中热带印度洋、太平洋海温异常主要有利于南海—西太平洋反气旋性环流异常的形成,使副热带高压偏西偏强,北大西洋海温异常则有利于高层欧亚波列的加强,使南亚高压在东亚地区加强东伸.在各区域海温异常共同作用下,华南在对流层高、低层反气旋性环流的控制下,大气产生强烈的下沉运动,一方面大气绝热增温,另一方面使大气晴空少云、地面接收更多的太阳辐射,从而有利于高温偏重.

In this study, the flood season in South China (SC) was divided into three stages: two first rainy seasons (FRSs) around the South China Sea summer monsoon onset and one second rainy season when Typhoon prevails, denoted as FRS1, FRS2, and SRS, respectively, and then we diagnosed the moist static energy (MSE) budget associated with the 10–30‐day persistent heavy rainfall (PHR) over SC during these periods. The results indicate that there are great differences in the recharge of PHR‐related MSE in different stages of the flood season in SC: The FRS1 MSE associated with PHR moves southeastward from midlatitude; the large MSE is maintained in SC during the FRS2; during the SRS, the MSE perturbation propagates from the tropical western North Pacific to SC. From the perspective of the local MSE budget in SC, meridional and zonal advection play a key role in the maximum MSE change in the FRS1; the FRS2 and SRS MSE tendency is mainly determined by zonal advection and meridional advection, respectively. In contrast, the 10–30‐day propagating perturbation of MSE changes during both the FRS1 and FRS2 are mainly affected by the zonal advection, while the meridional circulation is dominant in the SRS. The cumulative contribution of external forcing (including radiation and surface heat fluxes) during the SRS to the propagation of PHR‐related MSE perturbation can reach more than 30%, and the closer to the land, the stronger the external forcing. During the FRS (including FRS1 and FRS2), however, the external forcing contributes little, even negatively.

利用1961-2017年中国地面观测站日降水资料、全球大气多要素和海表温度月资料,分析华南区域持续性强降水过程的气候特征,诊断并比较与华南前汛期、后汛期区域持续性强降水年际变化相关的大气环流和海表温度异常特征.结果表明,3-12月华南都可能出现持续性强降水过程,其中汛期4-9月的占了 94.4％.伴随着区域持续性强降水的年际变化,华南本地垂直上升运动显著异常是前汛期和后汛期的共同点,但前汛期、后汛期在华南及周边环流异常、水汽输送来源以及海温异常分布等方面都存在一定差异.在前汛期华南区域持续性强降水偏重年,赤道西太平洋区域海温偏低,由于大气罗斯贝波响应使西太平洋副热带高压偏强,热带西太平洋向华南区域水汽输送加强,从而有利于区域持续性强降水偏重.后汛期华南区域持续性强降水偏重年的海温异常分布是赤道中东太平洋区域正异常、东印度洋至西太平洋暖池区负异常,海温异常通过西北太平洋副热带高压、南海热带季风强度、水汽输送和垂直环流等多方面,导致后汛期区域持续性强降水偏重.

A persistent heavy rainfall (PHR) process usually undergoes a suppressed phase, a developing phase, an active phase, a decaying phase, and an ending phase. This study tries to determine the PHR phase related to 10–30-day (high-frequency) intraseasonal oscillation (HF-ISO) over Southeastern China (including South China (SC) and the Yangtze River Basin (YRB)), by using the daily precipitation from the China Meteorological Administration (CMA) surface stations in China. We found that the HF-ISO associated with the PHR has a larger amplitude in the positive phase relative to that in the negative phase. Considering the asymmetry of the PHR process, we divide the PHR process into 5 phases with 1std in the positive phase and − 0.75std in the negative one as the threshold, that is, the suppressed phase, developing phase, active phase, decaying phase, and ending phase. During the different phases, the variables are in a wave-train-like pattern and strike from southeast to northwest. In addition to water vapor, convective instability (CI), and moisture static energy (MSE), the anomalies of other variables are not evident in the developing phase and decaying phase. Furthermore, the extremes of moisture, CI, and MSE are found ahead of the peak precipitation. In this study, we found that the moisture tendency is much larger in the suppressed phase than in the ending phase over both SC and the YRB. Thus, in the following phase, more (less) moisture leads to a stronger (weaker) convective instability and a more (less) recharge of MSE, and favoring (unfavoring) for the development of PHR. Over SC, the main reason for the difference of the moisture tendency during the suppressed phase and the ending phase is the meridional advection based on the mean state and eddy-eddy flow interactions. Over the YRB, the difference of the moisture tendency is due to the advection by the anomalous zonal wind across the mean moisture gradient.

In the present study, the structures and mechanisms of the heatwaves (HWs) associated with the quasi-biweekly (QBW; 10-20-day period) variability (QBW-HW) over southern China (SC; 106 degrees-120 degrees E, 21 degrees-30 degrees N) are investigated by using observation data from surface stations in China and the related gridded dataset (CN05.1), and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis. We found that the strongest anticyclonic anomaly and subsidence appear over SC during the developing phase of QBW-HW, and then induced excess solar radiation at surface and significant diabatic heating lead to a positive surface air temperature change, thus favoring occurrence of QBW-HW over SC. In addition, we found a wet near-surface atmosphere in the QBW-HW events over SC, and further confirmed that near-surface moisture should play an important role in the occurrence of QBW-HW, via absorptions of longwave and shortwave radiation. This result is quite different from previous studies since they did not pay attention to the near-surface moisture. On the other hand, warmer SAT favors more water vapor evaporated from the moist soil when considering the Clausius-Clapeyron relationship. Then, the positive feedback processes promote the occurrence of QBW-HW over SC. In contrast, during the developing and warm phases of QBW-HW over SC, except for the near-surface level, the troposphere is in a dry condition, even at 850 and 700 hPa. In the QBW-HW events over SC, the factor responsible for the wet near-surface atmosphere is the enhanced surface evaporation, which is attributed to strengthened surface wind speed and background moist soil. Significance StatementUnder the background of global warming, heatwaves over Southern China are experiencing an increasing trend. In this study, we want to understand the structures and mechanisms of the heatwaves related to 10-20-day (quasi-biweekly) variability. We that found some structures of heatwaves (e.g., anticyclonic anomalies along with subsidence) are consistent with previous studies. In addition, we also show that the moist soil and increased induced near-surface moisture play a key role in the occurrence of heatwaves over Southern China, via enhanced absorptions of longwave and shortwave radiation. This study is helpful for understanding the processes and prediction of heatwaves over Southern China. Future work should examine the findings by some numerical experiments with a climate model.

In the present study, the moist static energy (MSE) budget associated with a persistent rainfall event that occurred over South China in late August 2018 (PR1808) was analyzed using ERA-Interim reanalysis data. The results revealed that the PR1808 event was closely related to a 12–30-day intraseasonal oscillation (ISO), with a significant westward-propagating mode. The recharge of MSE occurred before ISO deep convection, and the MSE was discharged during and after the peak precipitation. This result indicates that the recharge-discharge cycle of MSE played an important role in regulating the high-frequency ISO (HF-ISO) process during the PR1808 event. However, the mid-level MSE tendency, rather than the low-level tendency, controlled the column-integrated MSE change in this case, and there was no low-level MSE tendency ahead of HF-ISO convection, which is different from previous studies, implying that shallow convection was not a key factor in this case. The recharge of MSE related to the mid-level MSE change mode was mainly attributed to zonal advection, while the forcing related to radiative heating (longwave and shortwave radiation) and heat fluxes (latent and sensible heat fluxes) contributed little to the change in MSE. Furthermore, for the zonal advection of MSE, the main contribution originated from the advection by the low-frequency zonal flow across the low-frequency MSE gradient, and interactions between high- (low-) frequency zonal flow and low- (high-) frequency MSE. In addition, the disturbances from higher latitudes enhanced the persistent rainfall in this case over South China through the southward shift of baroclinic vorticity.

The first rainy season (April-June) of South China includes the phases before and after the onset of the South China Sea summer monsoon (SCSSM). Abundant moisture supply is the key dynamic process for precipitation formation. Thus, we employ the FLEXPART model to explore the corresponding moisture sources for the two phases. Before the onset of SCSSM, land regions contribute more moisture to the precipitation over South China than the ocean sources. The main source regions are Southeast Asia (27.01%), the South China Sea (25.96%), South China (11.12%), and the southern part of the northwestern Pacific (10.23%). Land sources (66.87%) play a more important role than ocean sources (33.13%) in the interannual variations, with the contributions mainly from Southeast Asia (47.56%) and the South China Sea (28.79%). After the onset of SCSSM, the climatological contribution of ocean sources is larger than that of land regions, and the main source regions are the South China Sea (20.78%), Southeast Asia (17.51%), the Bay of Bengal (13.76%), and South China (11.21%). For the interannual variations, the contributions of land sources and ocean regions are comparable, and mainly from Southeast Asia (33.53%) and the Bay of Bengal (32.26%). The moisture transports for the interannual variations in first rainy season precipitation over South China before and after the onset of SCSSM are significantly correlated with the east-west contrast of sea surface temperature anomalies over northern part of North Pacific and the uniform warming over Indian Ocean, respectively. This study provides important guidance in improving the regional precipitation predictions and understanding the water resources changes.

利用重建的华南区域黑碳气溶胶(Black Carbon,BC)浓度资料,分析其与南海夏季风在年际尺度上的关系.结果 表明,华南区域BC浓度与南海夏季风的关系在2000年前后有明显的突变,由显著负相关变为显著正相关,即由高BC浓度弱季风变为高BC浓度强季风.通过合成对比分析,发现1988-1999年(第一时间段)的华南BC主要气候效应是间接辐射强迫作用:华南BC使云粒子半径减小,抑制华南区域春季降水,增加了云的生命期,从而使到达地面的短波辐射减少,表面和低层大气降温.负温度异常激发了异常反气旋,在南海区域即有东风异常.到夏季,东风异常减弱了季风强度,同时抑制了南海地区的降水.2000-2010年(第二时间段)的华南BC主要气候效应是直接辐射强迫作用:春季高BC浓度通过直接气候效应,增暖大气,加强降水,但是雨日减少,从而使到达地面的短波辐射增多,表面和低层大气增温.正温度异常激发了异常气旋,在南海区域即有西风异常一直维持到夏季,增大了季风强度,同时增强了南海地区的降水.

Regional persistent heat event is extensively studied and its intensity is normally investigated from the view of regional average. Climatic characteristics of persistent heat event in 4 regions in the eastern China are analyzed on the basis of distinguishing historical persistent heat event in the South China, the Yangtze River, the Huanghuai and the North China, using the daily maximum temperature data of 2407 stations in China during 1961-2019. The definition index of regional persistent heat event is established, in which the proximity of high temperature stations, the spatial range, the temporal consistency, the regional climate characteristics and the universality of methods are synthesized. Average times of regional persistent heat event in the South China, the Yangtze River, the Huanghuai and the North China are 3.3, 2.8, 2.2 and 0.8 per year respectively, with an average duration of 5.1 d, 6.4 d, 5.0 d and 3.9 d for per event, and the average annual cumulative days in climatology are 16.8 d, 17.8 d, 11.0 d, 3.1 d. Both the duration and the average annual cumulative days of persistent heat event are the longest in the Yangtze River, indicating that the persistent heat event in the Yangtze River is the most serious in climatology among these regions. The average annual cumulative days of persistent heat event in the North China are the least, and it usually ends at the end of July or before. There is a significant linear growing trend in the persistent heat event index in the South China, the North China, and the Yangtze River. There are only 4 non-summer persistent heat events in the Huanghuai in recent 20 years. The difference in strength of the persistent heat event index between the Yangtze River and the South China has significant inter-decadal trends. The index in the Yangtze River is obviously stronger than that in the South China during the first period of 1961-1978, while the index in the South China is slightly stronger than that in the Yangtze River during the second period of 1979-2019. The difference of persistent heat event between these two regions changes by nearly 10 d over these years.

采用观测分析和数值试验等方法,分析夏季南亚高压与热带季节内振荡(ISO)之间的关系,并对两者之间的相互作用进行量化诊断,探讨其物理过程.主要结果表明:南亚高压ISO与热带ISO活动关系密切,当热带ISO处于印度洋位相(第1、2、3位相),则南亚高压东脊点位置偏西,当ISO处于太平洋位相(第5、6、7位相),则南亚高压东脊点位置偏东.与热带ISO关系最密切的是南亚高压东部附近区域,即东亚—西太平洋地区(15°～25°N,110°～140°E),该关键区也是南亚高压ISO最显著区域.在热带ISO的调制下,关键区对流层大气垂直结构产生斜压性异常变化,导致高层南亚高压东脊点的东伸(西退)对应中低层西太平洋副热带高压西脊点的东退(西伸).在南亚高压与热带ISO之间关系中,主要是热带ISO对南亚高压的影响,南亚高压东部关键区ISO强度40％来源于热带ISO的贡献,而南亚高压对热带ISO平均强度的影响很弱.热带ISO影响南亚高压的物理过程如下,热带ISO从印度洋向东传播至西太平洋时,强对流产生分支,部分由于东亚—西太平洋的有利夏季风背景转为向北传播,ISO向北传播过程中对流强度进一度加强,这就相当于存在一个赤道非对称热源.在热源的作用下,大气产生异常响应,在热源的西北侧,即东亚—西太平洋地区,对流层低层为气旋性环流异常、位势高度负异常,对流层高层为反气旋性环流异常、位势高度正异常,从而导致南亚高压东脊点偏东.而当热带ISO处于印度洋位相时,大气异常响应与上述相反,南亚高压东部位势高度降低,南亚高压东脊点西撤.

We investigate a record-breaking heat wave in southern China (SC) and explore its association with the South China Sea (SCS) summer monsoon (SCSSM), using station observations and various reanalysis products. This heat wave event started in mid-May 2018 and persisted more than 15 days, when the maximum air temperature anomaly exceeded 6 °C. Meanwhile, the onset of the SCSSM was extremely late in 2018, primarily due to the intrusion of easterly winds over the SCS. We show that the anomalous easterlies over the SCS associated with the delayed SCSSM could block the moisture transport from the tropical oceans to SC, which led to decreased rainfall, increased surface radiation, and elevated probability of heat wave in SC during May 2018. Further analysis reveals that both SC heat waves and SCSSM were significantly affected by the Pacific subtropical high (PSH). The westward extension of the PSH can hinder the establishment of SCSSM via inducing easterly anomalies over the SCS, which reduces SC rainfall and results in a drier surface condition. Moreover, the westward displacement of the PSH may cause anomalously high pressures, descending air motions, and divergent winds over SC, which triggers above-normal air temperatures that are conducive to the occurrences of SC heat waves. This study also underlines the importance of Eurasian planetary wave trains in bridging the upstream climate variability and the changes in PSH during boreal spring, which could be used to improve the intra-seasonal predictions of SC heat wave and SCSSM onset.

利用1961-2017年共57年中国地面观测站日降水资料,采用滑动平均、百分位和点面相关分析等方法,根据延伸期预报特点以及监测预报和研究需求,着眼于体现强降水过程的区域性、持续性和致灾性,并兼顾区域气候特征和普适性,建立区域持续性强降水过程定义指标.根据该指标查找判断我国东部四个关键区域(华南、长江、黄淮、华北)的历史降水过程,1961—2017年期间共有557次区域持续性强降水过程,平均每年约10次,其中华南、长江、黄淮、华北分别有267、155、78、49次,平均每年各有4.7次、2.7次、1.4次和0.9次,呈由南向北递减的分布.统计评估结果表明,该指标能客观地判断出持续影响同一区域的相对稳定类型的大尺度持续性强降水过程,适用于延伸期业务服务和研究.

The characteristics of anomalous circulations during spring associated with the climate shift of the South China Sea summer monsoon (SCSSM) onset in 1993/1994 and its physical causes are investigated. It is found that the interdecadal shift of SCSSM onset happened in 1993/1994 is related closely to the 850 hPa zonal wind anomalies over the area around Kalimantan Island. Easterly (westerly) anomalies over Kalimantan Island enhance (weaken) subtropical high over the western North Pacific, leading to the late (early) onset of SCSSM in 1979–1993 (1994–2013). The sea surface temperature anomalies (SSTAs) in the key region 140°–150° E, 5° S–2.5° N influence the interdecadal change of zonal winds over Kalimantan Island. The positive SSTAs over this key region in 1994–2013 force convergence toward the region at low-level and form significant westerly anomalies near Kalimantan Island located to the west of the key region. The negative anomalies of meridional gradient of zonal winds over the South China Sea region increase the atmospheric vorticity over there significantly and result in the weakening and retreating eastward of the subtropical high over the western North Pacific, which is conducive to the early onset of SCSSM.

In the present study, the northward-propagating intraseasonal oscillations (ISOs) over the South China Sea (SCS) during the summer monsoon season are divided into two types. One is the northward propagation of ISOs confined in the SCS (defined as P1-ISO), another for ISOs moving from the SCS to Southern China (SC) defined as P2-ISO. The spatial and temporal structures of the two types of ISOs are compared based on the analysis of the NCEP–NCAR reanalysis datasets. It is found that a significant difference appears in the heat flux anomalies over SC that is attributed to the cloud-radiation effect. A mechanism associated with the changes in the convective instability is proposed to understand the cause of the northward propagation of the ISO from the SCS to SC. For the P1-ISO, a more precipitation over SC leads to a less surface solar radiation and then a cold land surface temperature (LST), which induces an increase of static stability via surface longwave radiation and sensible heat flux. Whereas the total contribution to the convective instability changes does create an unfavorable for ISOs propagating northward, the moisture convergence, mainly controlled by the baroclinic vorticity advection effect, may contribute to an increase of convective instability that favors the ISOs moving northward. For the P2-ISO, a less precipitation over SC results in a more shortwave radiation reaching the ground that induces a warm LST, and thus leading to a warm low-level air temperature and destabilizing the atmosphere ahead of the convection. Moreover, the moisture convergence, dominated by the warm low-level air temperature, also contributes to the increase of convective instability. Thus the convective instability increment should favor the ISOs moving northward. It is worth noting that the baroclinic vorticity advection and moisture advection are not essential for ISOs propagating from the SCS to SC.

Observational analyses suggest a significant positive correlation between the year-to-year convection over the South China Sea (SCS)/western Pacific (WP) and the SCS summer monsoon (SCSSM) onset date. An investigation shows that there is an asymmetric relationship between the area-mean outgoing longwave radiation and the SCSSM onset date index. The analysis found that the influence of the intraseasonal scale circulation is the main cause of this asymmetric relationship. On the interannual scale, circulation and sea surface temperature (SST) anomalies are distributed symmetrically. During the convection active (inactive) years, the SST anomaly field indicates a La Ni n a (El Ni n o)-like pattern. The SCS-WP low-level westerly (easterly) anomaly is enhanced, leading to an increase (decrease) in moisture and ascending (descending) motion. The enhanced convection results in weakening (enhancement) of the subtropical high and provides a favorable condition for early (late) SCSSM establishment. On the intraseasonal time scale, the convection intensity is stronger during the active years than during the inactive years. The northwestward propagation is significant during the active years. This feature is not observed during inactive years. This difference is attributed to the asymmetric meridional distribution of the atmospheric convective instability and moisture disturbance during active years, not during inactive years. In active years, the atmosphere is potentially more (less) unstable and the moisture is to the north (south) of the intraseasonal oscillation (ISO) convective center, which is conducive to generating a favorable unstable environment for the development of new convection north of the ISO convection center, leading to northward convection propagation. The moistening in the SCS-WP is primarily attributed to the phase leading horizontal advection term. An ISO moisture budget analysis reveals that the largest positive contribution is the vertical moisture advection term.