Influences of MJO-induced Tropical Cyclones on the Circulation-Convection Inconsistency for the 2021 South China Sea Summer Monsoon Onset

The South China Sea Summer Monsoon(SCSSM)onset is characterized by an apparent seasonal conversion of circulation and convection.Accordingly,various indices have been introduced to identify the SCSSM onset date.However,the onset dates as determined by various indices can be very inconsistent.It not only limits the determination of onset dates but also misleads the assessment of prediction skills.In 2021,the onset time as identified by the circulation criteria was 20 May,which is 12 days earlier than that deduced by also considering the convection criteria.The present study mainly ascribes such circulation-convection inconsistency to the activities of tropical cyclones(TCs)modulated by the Madden-Julian Oscillation(MJO).The convection of TC“Yaas”(2021)acted as an upper-level diabatic heat source to the north of the SCS,facilitating the circulation transition.Afterward,TC“Choi-wan”(2021)over the western Pacific aided the westerlies to persist at lower levels while simultaneously suppressing moist convection over the SCS.Accurate predictions using the ECMWF S2S forecast system were obtained only after the MJO formation.The skillful prediction of the MJO during late spring may provide an opportunity to accurately predict the establishment of the SCSSM several weeks in advance.

Interdecadal Change in the Interannual Variability of South China Sea Summer Monsoon Intensity

The interdecadal change in the interannual variability of the South China Sea summer monsoon(SCSSM)intensity and its mechanism are investigated in this study.The interannual variability of the low-level circulation of the SCSSM has experienced a significant interdecadal enhancement around the end of the 1980s,which may be attributed to the interdecadal changes in the evolution of the tropical Indo-Pacific sea surface temperature(SST)anomalies and their impacts on the SCSSM.From 1961 to 1989,the low-level circulation over the South China Sea is primarily affected by the SST anomalies in the tropical Indian Ocean via the mechanism of Kelvin-wave-induced Ekman divergence.While in 1990 to 2020,the impacts of the summer SST anomalies in the Maritime Continent and the equatorial central to eastern Pacific on the SCSSM are enhanced,via anomalous meridional circulation and Mastuno-Gill type Rossby wave atmospheric response,respectively.The above interdecadal changes are closely associated with the interdecadal changes in the evolution of El Niño–Southern Oscillation(ENSO)events.The interdecadal variation of the summer SST anomalies in the developing and decaying phases of ENSO events enhances the influence of the tropical Indo-Pacific SST on the SCSSM,resulting in the interdecadal change in the interannual variability of the SCSSM.

Impact of the Thermal State of the Tropical Western Pacific on Onset Date and Process of the South China Sea Summer Monsoon

Since the early or late onset of the South China Sea summer monsoon （SCSM） has a large impact on summer monsoon rainfall in East Asia, the mechanism and process of early or late onset of the SCSM are an worthy issue to study. In this paper, the results analyzed by using the observed data show that the onset date and process of the SCSM are closely associated with the thermal state of the tropical western Pacific in spring. When the tropical western Pacific is in a warming state in spring, the western Pacific subtropical high shifts eastward, and twin cyclones are early caused over the Bay of Bengal and Sumatra before the SCSM onset. In this case, the cyclonic circulation located over the Bay of Bengal can be early intensified and become into a strong trough. Thus, the westerly flow and convective activity can be intensified over Sumatra, the Indo-China Peninsula and the South China Sea （SCS） in mid-May. This leads to early onset of the SCSM. In contrast, when the tropical western Pacific is in a cooling state, the western Pacific subtropical high anomalously shifts westward, the twin cyclones located over the equatorial eastern Indian Ocean and Sumatra are weakened, and the twin anomaly anticyclones appear over these regions from late April to mid-May. Thus, the westerly flow and convective activity cannot be early intensified over the Indo-China Peninsula and the SCS. Only when the western Pacific subtropical high moves eastward, the weak trough located over the Bay of Bengal can be intensified and become into a strong trough, the strong southwesterly wind and convective activity can be intensified over the Indo-China Peninsula and the SCS in late May. Thus, this leads to late onset of the SCSM. Moreover, in this paper, the influencing mechanism of the thermal state of the tropical western Pacific on the SCSM onset is discussed further from the Walker circulation anomalies in the different thermal states of the tropical western Pacific.

On the Onset of the South China Sea Summer Monsoon in 1998

Through analyzing the NCEP/NCAR reanalysis data, the satellite observational data and the ATLAS-2 mooring buoy observational data, it is shown that May 21 is the onset date of the South China Sea summer monsoon in 1998. There were abrupt variations in the general circulation pattern at the lower troposphere and the upper troposphere, in upper jet stream location and in the convection and rainfall over the South China Sea region corresponding to the outbreak of the South China Sea summer monsoon. It is also indicated that there was rainfall in the southern China coastal region before onset of summer monsoon, but it resulted from the (cold) front activity and cannot be regarded as the sign of summer monsoon outbreak in the South China Sea. Key words Onset - South China Sea summer monsoon - General circulation pattern, Jet stream - Convection This work was supported by the State Key Project for Research—“ The South China Sea Monsoon Experiment”, CAS (KZ951-B1-408) and CNSF (49823002).

On the relationship between convection intensity of South China Sea summer monsoon and air-sea temperature difference in the tropical oceans

The annual, interannual and inter-decadal variability of convection intensity of South China Sea (SCS) summer monsoon and air-sea temperature difference in the tropical ocean is analyzed, and their relationship is discussed using two data sets of 48-a SODA (simple ocean data assimilation) and NCEP/NCAR. Analyses show that in wintertime Indian Ocean (WIO), springtime central tropical Pacific (SCTP) and summertime South China Sea-West Pacific (SSCSWP), air-sea temperature difference is significantly associated with the convection intensity of South China Sea summer monsoon. Correlation of the inter-decadal time scale (above 10 a) is higher and more stable. There is inter-decadal variability of correlation in scales less than 10 a and it is related with the air-sea temperature difference itself for corresponding waters. The inter-decadal variability of the convection intensity during the South China Sea summer monsoon is closely related to the inter-decadal variability of the general circulation of the atmosphere. Since the late period of the 1970s, in the lower troposphere, the cross-equatorial flow from the Southern Hemisphere has intensified. At the upper troposphere layer, the South Asian high and cross-equatorial flow from the Northern Hemisphere has intensified at the same time. Then the monsoon cell has also strengthened and resulted in the reinforcing of the convection of South China Sea summer monsoon.

Impacts of Land Surface and Sea Surface Temperatures on the Onset Date of the South China Sea Summer Monsoon

The present study analyzes the differences in spatial and temporal variations of surface temperatures between early and late onset years of the South China Sea summer monsoon （SCSSM）. It is found that when the land surface temperature north of 40°N is lower （higher） and the sea surface temperature over the South China Sea-western North Pacific （SCS-WNP） is higher （lower） in winter, the onset of the SCSSM begins earlier （later）. When the land surface temperature north of 40°N is higher （lower） and the sea surface temperature over the SCS-WNP is lower （higher） in spring, the onset of the SCSSM occurs earlier （later）. The reason why the anomalies of the land surface temperatures north of 40°N can influence the atmospheric circulation is investigated by analysis of the wind and temperature fields. In order to verify the mechanisms of influence over the land and sea surface temperature distribution patterns and test the ability of the p-σ regional climate model （p-σ RCM9） to simulate the SCSSM onset, three types of years with early, normal, and late SCSSM onset are selected and the SCSSM regimes are numerically simulated. According to the results obtained from five sensitive experiments, when the land surface temperature is higher in the eastern part, north of 40°N, and lower in the western part, north of 40°N, and it rises faster in the eastern coastal regions and the Indian Peninsula, while the sea surface temperatures over the SCS-WNP are lower, the early onset of the SCSSM can be expected.

The Relationship of Land-Ocean Thermal Anomaly Difference with Mei-yu and South China Sea Summer Monsoon

Based on the NCEP/NCAR reanalysis data for the period of 1948-2004 and the monthly rainfall data at 160 stations in China from 1951 to 2004, the relationships among the land-ocean temperature anomaly difference in the mid-lower troposphere in spring （April-May）, the mei-yu rainfall in the Yangtze River- Huaihe River basin, and the activities of the South China Sea summer monsoon （SCSSM） are analyzed by using correlation and composite analyses. Results show that a significant positive correlation exists between mei-yu rainfall and air temperature in the middle latitudes above the western Pacific, while a significant negative correlation is located to the southwest of the Baikal Lake. When the land-ocean thermal anomaly difference is stronger in spring, the western Pacific subtropical high （WPSH） will be weaker and retreat eastward in summer （June-July）, and the SCSSM will be stronger and advance further north, resulting in deficient moisture along the mei-yu front and below-normal precipitation in the mid and lower reaches of the Yangtze River, and vice versa for the weaker difference case. The effects and relative importance of the land and ocean anomalous heating on monsoon variability is also compared. It is found that the land and ocean thermal anomalies are both closely related to the summer circulation and mei-yu rainfall and SCSSM intensity, whereas the land heating anomaly is more important than ocean heating in changing the land-ocean thermal contrast and hence the summer monsoon intensity.

Relationship between Indian Ocean dipole and ENSO and their connection with the onset of South China Sea summer monsoon

Using Reynolds and Smith 1950 - 1998 re-constructed monthly-mean SST to discuss the relationship between the ENSO and Indian Ocean dipole （IOD） and their possible connection with the onset of South China Sea summer monsoon（ SCSSM）, the results are obtained as follows ： Most of IOD events have a closely positive relation to simultaneous ENSO events in summer and autumn. IOD events in autumn （ mature phase） are also closely related to ENSO events in winter （ mature phase）. When these two kinds of events happen in phase, i.e. , positive （negative） IOD events are coupled with E1 Nifío （La Nifía） events, they are always followed by late （ or early） onsets of SCSSM. On the contrary, when these two kinds of events happen out of phase, i.e. positive （negative） IOD events are coupled with La Nifia （ E1 Nifío） events, they are followed by normal onsets of SCSSM. In addition, single IOD events or single ENSO events cannot correspond well to the abnormal onset of SCSSM.

Strengthened Regulation of the Onset of the South China Sea Summer Monsoon by the Northwest Indian Ocean Warming in the Past Decade

Traditionally,a delayed(early)onset of the South China Sea summer monsoon(SCSSM)has been observed to follow a warm(cold)El Niño-Southern Oscillation(ENSO)event in winter,supporting high seasonal predictability of SCSSM onset.However,the empirical seasonal forecasting skill of the SCSSM onset,solely based on ENSO,has deteriorated since 2010.Meanwhile,unexpected delayed onsets of the SCSSM have also occurred in the past decade.We attribute these changes to the Northwest Indian Ocean(NWIO)warming of the sea surface.The NWIO warming has teleconnections related to(1)suppressing the seasonal convection over the South China Sea,which weakens the impacts of ENSO on SCSSM onset and delays the start of SCSSM,and(2)favoring more high-frequency,propagating moist convective activities,which enhances the uncertainty of the seasonal prediction of SCSSM onset date.Our results yield insight into the predictability of the SCSSM onset under the context of uneven ocean warming operating within the larger-scale background state of global climate change.

VARIATION OF AEROSOL OPTICAL CHARACTERISTICS IN GUANGZHOU DURING SOUTH CHINA SEA SUMMER MONSOON EVENTS

Variations in Guangzhou's aerosol optical characteristics and their possible causes are studied against the large-scale background of South China Sea summer monsoons(SCSSM) using aerosol data derived from Panyu Atmospheric Composition Watch Station in Guangzhou and the National Centers for Environmental Prediction/National Center for Atmospheric Research(USA). The data is reanalyzed to develop a composite analysis and perform physical diagnoses. Analysis of the results shows that aerosol extinction in Guangzhou first increases then decreases during the active period of a SCSSM, with variations in the stratification of the planetary boundary layer(PBL) and environmental winds playing important roles in affecting Guangzhou's aerosol optical characteristics. Regional diabatic heating and anomalous cyclonic circulations excited by monsoon convection induce environmental wind anomalies that significantly modify the stratification of the PBL.

The Potential Predictability of the South China Sea Summer Monsoon in a Dynamical Seasonal Prediction System

The potential predictability of climatological mean circulation and the interannual variation of the South China Sea summer monsoon (SCSSM) were investigated using hindcast results from the Institute of Atmospheric Physics Dynamical Seasonal Prediction System (IAP DCP),along with the National Centers for Environmental Prediction (NCEP) reanalysis data from the period of 1980-2000.The large-scale characteristics of the SCSSM monthly and seasonal mean low-level circulation have been well reproduced by IAP DCP,especially for the zonal wind at 850 hPa;furthermore,the hindcast variability also agrees quite well with observations.By introducing the South China Sea summer monsoon index,the potential predictability of IAP DCP for the intensity of the SCSSM has been evaluated.IAP DCP showed skill in predicting the interannual variation of SCSSM intensity.The result is highly encouraging;the correlation between the hindcasted and observed SCSSM Index was 0.58,which passes the 95% significance test.The result for the seasonal mean June-July-August SCSSM Index was better than that for the monthly mean,suggesting that seasonal forecasts are more reliable than monthly forecasts.

Sub-seasonal Prediction of the South China Sea Summer Monsoon Onset in the NCEP Climate Forecast System Version 2

This study depicts the sub-seasonal prediction of the South China Sea summer monsoon onset(SCSSMO)and investigates the associated oceanic and atmospheric processes,utilizing the hindcasts of the National Centers for Environmental Prediction(NCEP)Climate Forecast System version 2(CFSv2).Typically,the SCSSMO is accompanied by an eastward retreat of the western North Pacific subtropical high(WNPSH),development of the cross-equatorial flow,and an increase in the east-west sea surface temperature(SST)gradient.These features are favorable for the onset of westerlies and strengthening of convection and precipitation over the South China Sea(SCS).A more vigorous SCSSMO process shows a higher predictability,and vice versa.The NCEP CFSv2 can successfully predict the onset date and evolution of the monsoon about 4 pentads(20 days)in advance(within 1–2 pentads)for more forceful(less vigorous)SCSSMO processes.On the other hand,the climatological SCSSMO that occurs around the 27th pentad can be accurately predicted in one pentad,and the predicted SCSSMO occurs 1–2 pentads earlier than the observed with a weaker intensity at longer leadtimes.Warm SST biases appear over the western equatorial Pacific preceding the SCSSMO.These biases induce a weaker-thanobserved WNPSH as a Gill-type response,leading to weakened low-level easterlies over the SCS and hence an earlier and less vigorous SCSSMO.In addition,after the SCSSMO,remarkable warm biases over the eastern Indian Ocean and the SCS and cold biases over the WNP induce weaker-than-observed westerlies over the SCS,thus also contributing to the less vigorous SCSSMO.

Relative Roles of Intraseasonal and Above-seasonal Components in the South China Sea Summer Monsoon Onset

Multi-scale contributions are involved in the South China Sea(SCS)summer monsoon(SCSSM)onset process.The relative roles of intraseasonal oscillation and above-seasonal component in the year-to-year variation of the SCSSM onset are evaluated in this study.The 30-90-day and above-90-day components are major contributors to the year-to-year variation of the SCSSM onset,and the former contributes greater portion,while the 8-30-day component has little contribution to the onset.In the early onset cases,the 30-90-day westerly winds move and extend eastward from the tropical Indian Ocean(TIO)to the SCS monsoon region relatively earlier,and replace the easterly winds over the SCS with the cooperation of the 30-90-day cyclone moving southward from northern East Asia.The westerly anomalies of the above-90-day component in spring jointly contribute to the early SCSSM onset.In the late onset cases,the late eastward expansion of 30-90-day westerly wind over the TIO,accompanied by the late occurrence and weakening of the 30-90-day anticyclone over the SCS,and its late withdraw from the SCS,as well as the persistent easterly anomalies of above-90-day component,suppress the SCSSM onset.However,the SCSSM outbreaks in the obvious weakening stage of 30-90-day easterly anomalies.The easterlies-to-westerlies transition of the 30-90-day 850-hPa zonal wind over the SCS in spring is closely associated with sea surface temperature in the tropical western Pacific in preceding winter and spring,while the interannual variation of the above-90-day zonal wind in April-May is closely related to the decaying stage of the El Niño-Southern Oscillation events.

Influence of Tropical Western Pacific Warm Pool Thermal State on the Interdecadal Change of the Onset of the South China Sea Summer Monsoon in the Late-1990s

An interdecadal shift in the onset date of the South China Sea summer monsoon(SCSSM) is identified during the late 1990 s by using the European Centre for Medium-Range Weather Forecasts Interim Reanalysis dataset. The mean onset date was brought forward by two pentads during 1999–2013 compared to that during 1979–1998. The large-scale atmospheric and oceanic change associated with this shift exhibits a significant interdecadal variation signal around 1998/1999, indicating that the shift during the late 1990 s is robust. Different from the well-known mid-1990 s shift, this shift carried more important systematical significance. Diagnostic analysis suggests that the earlier outbreak of the SCSSM was due to the interdecadal warming of the warm pool, which brought stronger convection anomalies and led to a weak western Pacific subtropical high(WPSH) during boreal spring(March–May). The earlier retreat of the WPSH was a direct cause of this shift.

DETERMINATION OF ONSET DATE OF THE SOUTH CHINA SEA SUMMER MONSOON IN 2006 USING LARGE-SCALE CIRCULATIONS

Since the South China Sea (SCS) summer monsoon (SCSSM) is pronouncedly featured by abruptly intensified southwesterly and obviously increased precipitation over the SCS,the lower-tropospheric winds and/or convection intensities are widely used to determine the SCSSM onset.The methods can be used successfully in most of the years but not in 2006.Due to the intrusion of Typhoon Chanchu(0601)that year,the usual method of determining SCSSM onset date by utilizing the SCS regional indices is less capable of pinpointing the real onset date.In order to solve the problem,larger-scale situations have to be taken into account.Zonal and meridional circulations would be better to determine the break-out date of SCSSM in 2006.The result indicates that its onset date is May 16.Moreover,similar onset dates for other years can be obtained using various methods,implying that large-scale zonal and meridional circulations can be used as an alternative method for determining the SCSSM onset date.

Interannual variability in the onset of the South China Sea summer monsoon from 1997 to 2014

本文基于南海地区850 hPa风场,降水以及海温定义了南海夏季风爆发指数,将南海季风爆发过程分为季节转换和季风爆发两个过程来进行研究。对18年的观测分析发现,南海季风爆发可归纳为三种情况:第一种是季风正常爆发,随着季节转换结束后,西南季风和降水在南海地区有明显增强;第二种是间接性爆发,在季节转换结束后,西南季风和降水的建立不是特别明显;第三种是推迟爆发,在季节转换结束后,南海地区没有建立西南季风也没有降水产生。进一步研究发现,西太副高异常西伸是导致南海季风延迟爆发的重要因素之一。此外,大尺度环流背景ENSO的影响也对南海季风爆发时间的早晚有重要影响,但并不是唯一决定性因素,印度洋和亚洲大地形的局地热力差异变化是影响季风爆发的另一重要因素。

An approach to prediction of the South China Sea summer monsoon onset

在现在的纸，在发作和热在西方的太平洋在温暖的水池的上面的层满足的华南海夏天季风( SCSSM )之间的关联为时期用海洋学数据集的 Scripps 机构被检验 1955 1998 并且预言的一条途径 SCSSM 发作被建议。关联嘘在那里欠那存在内部 SCSSM 发作的十的可变性温暖的水池而非在它的中心附近的中心向西在区域与最大的关联系数在 1970 界定，从另外的因素暗示某些效果而且包含了 ENSO。因为关联为在 1970 前的时期是差的，在 1970 以后的温暖的水池的热内容异例被用来预先显示 SCSSM 的早或迟了的发作。一个理想的代表性的区域(为温暖的水池热内容的 1 °× 1 °) 在 3 ° N /138 ° E 与它的中心被决定。系在到中心的最近的道(道热带的空气海洋数组) 在 2 ° N /137 ° E，并且选择为预言计算热内容。在 2 ° N /137 ° E 的 TAO 绳索能被用来在上面的层与热内容预言 SCSSM 发作，这被建议，如果在 SCSSM 发作和温暖的水池的热内容之间的关联象的一样跑在 1970 以后。在另一方面，如果状况做，在 1970 前喜欢，代表性的车站与相对差的关联在 13 ° S /74 ° E 被决定，意味着在西方的太平洋的温暖的水池比印度洋在 SCSSM 发作起更多的重要作用。

Predictability of South China Sea Summer Monsoon Onset

Predicting monsoon onset is crucial for agriculture and socioeconomic planning in countries where millions rely on the timely arrival of monsoon rains for their livelihoods. In this study we demonstrate useful skill in predicting year-to-year variations in South China Sea summer monsoon onset at up to a three-month lead time using the GloSea5 seasonal forecasting system. The main source of predictability comes from skillful prediction of Pacific sea surface temperatures associated with El Ni?no and La Ni?na. The South China Sea summer monsoon onset is a known indicator of the broadscale seasonal transition that represents the first stage of the onset of the Asian summer monsoon as a whole. Subsequent development of rainfall across East Asia is influenced by subseasonal variability and synoptic events that reduce predictability, but interannual variability in the broadscale monsoon onset for East Asian summer monsoon still provides potentially useful information for users about possible delays or early occurrence of the onset of rainfall over East Asia.

Characteristics of Tropical Sea Surface Temperature Anomalies and Their Influences on the Onset of South China Sea Summer Monsoon

The characteristics of sea surface temperature anomalies(SSTAs)in the tropical oceans and their influences on the onset of South China Sea summer monsoon(SCSSM)have been studied.The anomaly of SST in tropical Pacific Ocean exerts persistence impact for one to three months on atmospheric circulations.If the warm pool becomes anomalously warmer during an earlier period from February to April,the SCSSM breaks out earlier,and vice versa.Singular value decomposition(SVD)and composite analysis have shown that,in La Ni a pattern,the convection over Western Pacific will occur earlier and be stronger than normal,which favors the convergence at a lower layer over Western Pacific,as well as the strengthening of upwelling branch of Walker circulation,leading to an earlier burst of westerly in the southern South China Sea.Moreover,the convection in Sumatra appears earlier than normal and favors the westerly evolution in eastern Indian Ocean,resulting in the splitting of the subtropical high belt and an early onset of SCSSM.However,the atmospheric circulation anomaly is reversed in El Nio pattern.

REASONS FOR THE LATE ONSET AND ANOMALOUS SOUTHWARD PERSISTENCE OF THE SOUTH CHINA SEA SUMMER MONSOON IN 2005

Features of atmospheric circulation and thermal structures are discussed using the NCAR/NCEP data to reveal the reasons for the late onset and anomalous southward persistence of the South China Sea Summer Monsoon(SCSSM) in 2005.The results show that three factors are crucial.First,a strong Arabian High overlaps with a high-latitude blocking high and channels strong cold air to southern Asia.Second,the Tibetan Plateau has a bigger snow cover than usual in spring and the melting of snow cools down the surface.Third,the Somali Jet breaks out at a much later date,being not conducive to convection over Indochina.The former two factors restrict atmospheric sensible heating over the Tibetan Plateau and nearby regions while the third one limits latent heating over Indochina.All of the factors slow down atmospheric warming and postpone the onset of SCSSM.Long after the onset of SCSSM,strong cold air over India advances the Southwest Monsoon northward slowly,resulting in weaker convection and latent heating over the Tibetan Plateau and nearby areas.The negative feedback conversely inhibits further northward movement of Southwest Monsoon.