South China Sea Monsoon Onset Research Articles

Aerosol effects on liquid cloud microphysical properties in south China: Land–ocean contrasts

The aerosol effects on liquid cloud microphysical properties over South China and adjacent ocean regions are investigated by using aerosol reanalysis data from MERRA-2, cloud property data from CLARA-A2, and reanalysis data from ERA5 from 1995 to 2015. The aerosol optical depth (AOD) in South China and adjacent ocean regions shows an upward trend with obvious land–ocean contrasts. The AOD is significantly positively correlated with the liquid cloud droplet effective radius (LREF) over both land and the ocean in summer and autumn, while increasing AOD significantly suppresses the LREF in spring and winter over land. This indicates that the thermal supply is likely the reason for the differences in aerosol effects on liquid cloud properties in different seasons. The weak correlation between the AOD and LREF over the ocean in spring may be because the South China Sea monsoon onset increases the atmospheric instability, and springtime in southeastern China is characterized by high rainfall. This indicates that the effects of aerosols on clouds are not as significant as those on large-scale systems. Further analysis shows that the aerosol effects on the liquid cloud properties are strong and positive at low liquid water paths (LWP) in summer and autumn, while the effect is weak at high LWPs. This may be due to the presence of sufficient liquid water droplets with large LWPs in summer and autumn, and few liquid water droplets in spring and winter. The aerosol effects on liquid cloud properties also significantly depend on lower-tropospheric stability (LTS).

The Principal Modes of Morning Extreme Precipitation over Inland Guangdong, China during Pre-Summer Rainy Season

The study explores the characteristics of morning extreme precipitation (MEP) during the pre-summer in inland Guangdong. Based on the principal modes, MEP events can be classified into four groups. The first group of MEP (G1) is a typical southeastward-propagating rainfall system originating from the northwestern mountains. This is caused by the strongest accelerated southwesterly winds at night, which bring abundant moist and warm air from the South China Sea (SCS) along with the shear line and the highest convective available potential energy (CAPE). The second group of MEP (G2) is warm-sector heavy rainfall with large-scale warming and higher CAPE. This local rainfall system originates in the south of Nanling mountains at night and reaches its mature stage in the morning. The rainfall system of the third group (G3) originates in central Guangxi and propagates to the southern inland region. The southeasterly winds in Guangxi intensify at night due to the anomalous cyclonic circulation. However, in the morning, the easterly winds shift to the westerlies, favoring eastward propagation. After SCS monsoon onset, cold air intrudes southward, colliding with moist warm air from the SCS, leading to heavy frontal precipitation in the inland region, classified as the fourth group MEP (G4).

Evolution of South China Sea and East Asian monsoon from spring to summer by the progression of daily weather types

AbstractThe monsoon development in the early rainy season in East Asia is analysed by the evolution of daily weather types (WTs). The WTs are classified by a k‐means clustering analysis based on 850 hPa winds from April 1 to July 31 in the 40‐year period of 1979–2018. Five WTs are identified, typifying the progression of weather regimes in the monsoonal evolution. WTs 1 and 2, with easterly winds in the South China Sea (SCS), occurs mostly in April and early May, corresponding to typical WTs before the SCS monsoon onset. In WT3, winds in the SCS turns westerly, and a rain band emerges in South China, extending east‐northeastward to the Pacific Ocean south of Japan, which signifies the May–June Mei‐yu in South China. Then, with the rain band moved to Yangtze River Valley (YRV) in WT4, this WT corresponds to the June–July Mei‐Yu in central and eastern China along the YRV. Finally, WT5 occurs more frequently towards the end of July, and corresponds to the post‐Mei‐Yu WT, indicating the ending of the early summer rainfall season in East Asia. The evolution of the WTs is gradual but intermittent, representing the weather‐within‐climate information. Each WT likely persists to itself. But WT1 and WT2 also progress to each other. The long persistence of WT3 and WT4 up to several weeks indicates the quasi‐stationary Mei‐Yu in South China and the YRV, respectively. The year‐to‐year variability of the WTs is also evident in the WTs. The El Niño–Southern Oscillation (ENSO) impact on the precipitation in the East Asian region is analysed for three types of ENSO: Eastern Pacific, Central Pacific, and mixed El Niño and La Niña. The seasonal mean precipitation anomalies can be interpreted by the frequencies of the daily WTs under different ENSO conditions.

Forecasting South China Sea Monsoon Onset Using Insight From Theory

AbstractMonsoon onset over the South China Sea occurs in April–May, marking the start of the wet season over East Asia. Skillful prediction of onset timing remains an open challenge. Recently, theoretical studies using idealized models have revealed feedbacks at work during the seasonal transitions of the Hadley cells and have shown that these are relevant to monsoon onset over Asia. Here, I hypothesize that monsoon onset occurs earlier in years when the atmosphere over the South China Sea is already in a state where these feedbacks are more easily triggered. I find that local anomalies in lower‐level moist static energy in the preceding January–March are well correlated with South China Sea Monsoon onset timing. This relationship remains relatively consistent on decadal timescales, while correlations with other teleconnections vary, and is used to develop a simple forecast model for onset timing that shows skill competitive with that of more complex models.

Large‐scale dynamic, heat and moisture structures of monsoon‐influenced precipitation in the East Asian monsoon rainy area

AbstractThis study investigated large‐scale dynamic structures along with heat and moisture budgets associated with monsoon‐influenced precipitation over a typical rainy domain in the East Asian monsoon region. Large‐scale dynamically and thermodynamically consistent forcing data based on multiple measurements were produced by a one‐dimensional constrained variational analysis method. Using the forcing data, distinct characteristics of large‐scale states were documented for cases of notable pre‐monsoon rainfall before the South China Sea monsoon onset (SCSMO), extreme rainfall during SCSMO, and persistent strong rainfall after SCSMO. The pre‐monsoon period mainly resulted from weak and discrete cloud regimes. The extreme‐rain period was associated with a severe deep convective system moving from the west, and the persistent‐rain period was related to moderate convective cells separated from the strong convective system over the northern South China Sea. Large‐scale features including vertical velocity, wind convergence, and diabatic heating and drying were the strongest (weakest) during the extreme‐rain (pre‐monsoon) period. For the pre‐monsoon and persistent‐rain periods, the period‐averaged profiles of vertical velocity and diabatic heating and drying showed a one‐peak structure. However, during the extreme‐rain period, a leap‐forward mutation was seen in the vertical structure and magnitude of these large‐scale states. The multiple peaks shown in the vertical profiles of vertical velocity and diabatic heating during the extreme‐rain period may indicate various convective cloud systems co‐existed. The diabatic heating profiles of extreme rainfall indicated that the first rainfall peak was related to the successive occurrence of stratiform anvils and convective clouds, while the second rainfall peak, which was most intense, was associated with shallow convective clouds, severe deep convective clouds, and detraining stratiform anvils.

Signature of the South China Sea summer monsoon onset on spring-to-summer transition of rainfall in the middle and lower reaches of the Yangtze River basin

The South China Sea (SCS) summer monsoon onset has been regarded as the beginning of the East Asian summer monsoon. In this study, we investigated the impacts of the SCS monsoon onset on the transition from the spring persistent rainfall to the summer Meiyu in the middle and lower reaches of the Yangtze River basin (MLYZB). It is found that rainfall in the MLYZB reduces after the SCS monsoon onset. This reduction in rainfall persists until the onset of the Meiyu and is accompanied by a weakening of southwesterlies to the south of the MLYZB. These features exist in both climatology and interannual variability. Rainfall increases significantly over the SCS and the subtropical western North Pacific after the SCS monsoon onset. The latent heating of the increased rainfall can excite an anomalous cyclone over the western North Pacific, which weakens the mean southwesterlies to the south of the MLYZB and decreases water vapor entering the MLYZB. It also generates descending motion over southeastern China. Thus, the SCS monsoon onset could suppress rainfall over the MLYZB by the latent heating induced changes in circulation. Compared to increased rainfall over the SCS, the latent heating of increase rainfall over the subtropical western North Pacific plays a more important role in the reduction of rainfall over the MLYZB. As the SCS monsoon onset affects the timing of the reduction of rainfall in the MLYZB, an early SCS monsoon onset is accompanied by below-normal May rainfall in the MLYZB, while a late SCS monsoon onset is accompanied by above-normal May rainfall.

The relation of South China Sea monsoon onset with the subsequent rainfall over the subtropical East Asia

ABSTRACTThe relationship between the South China Sea summer monsoon (SCSSM) onset and the subsequent subtropical East Asian monsoon rainfall from May to September (SEAMR) is investigated. The SCSSM onset is found to have a significant positive correlation with the SEAMR stretching from the lower reaches of the Yangtze River valley (LYR) to the southern Japan; e.g. the early onset of the SCSSM tends to be succeeded by less SEAMR, whereas the late onset is likely to be followed by more SEAMR. The positive relationship between the SCSSM onset and the SEAMR is directly controlled by the persistent western North Pacific anti‐cyclonic/cyclonic (WNPAC/WNPC) circulation anomaly from boreal spring to summer. The persistence of the large‐scale circulation anomalies are accompanied by the decaying phase of El Niño (La Niña) with the pronounced sea‐surface temperature anomalies (SSTA) over the tropical Indian Ocean (IO), the western North Pacific and the topical central and eastern Pacific Ocean (PO), respectively. Further numerical experiments indicate that the topical IO SSTA governs the positive relationship between the SCSSM onset and the subsequent SEAMR. On one hand, the wavelength of the Kevin‐wave response to tropical IO SSTA forcing is three times that of the Rossby‐wave response to the western North Pacific SSTA forcing. The Kevin‐wave response directly influences the low level circulation anomalies field throughout the South China Sea region, determining the onset date of the SCSSM. On the other hand, the horizontal wind shear that induced by the persistent Kevin‐wave response would also maintain the WNPAC/WNPC and further control the SEAMR from May to September. Thus, it is the topical IO SSTA forcing that consolidates the positive relationship between the SCSSM onset and the SEAMR.

The effects of asymmetric potential vorticity forcing on the instability of South Asia High and Indian summer monsoon onset

Based on the theory of potential vorticity (PV), the unstable development of the South Asia High (SAH) due to diabatic heating and its impacts on the Indian Summer Monsoon (ISM) onset are studied via a case diagnosis of 1998. The Indian Summer Monsoon onset in 1998 is related to the rapidly strengthening and northward moving of a tropical cyclone originally located in the south of Arabian Sea. It is demonstrated that the rapid enhancement of the cyclone is a consequence of a baroclinic development characterized by the phase-lock of high PV systems in the upper and lower troposphere. Both the intensification of the SAH and the development of the zonal asymmetric PV forcing are forced by the rapidly increasing latent heat released from the heavy rainfall in East Asia and South East Asia after the onsets of the Bay of Bengal (BOB) monsoon and the South China Sea (SCS) monsoon. High PV moves southwards along the intensified northerlies on the eastern side of the SAH and travels westwards on its south side, which can reach its northwest. Such a series of high PV eddies are transported to the west of the SAH continuously, which is the main source of PV anomalies in the upper troposphere over the Arabian Sea from late spring to early summer. A cyclonic curvature on the southwest of the SAH associated with increasing divergence, which forms a strong upper tropospheric pumping, is generated by the anomalous positive PV over the Arabian Sea on 355 K. The cyclone in the lower troposphere moves northwards from low latitudes of the Arabian Sea, and the upper-layer high PV extends downwards and southwards. Baroclinic development thus occurs and the tropical low-pressure system develops into an explosive vortex of the ISM, which leads to the onset of the ISM. In addition, evolution of subtropical anticyclone over the Arabian Peninsula is another important factor contributing to the onset of the ISM. Before the onset, the surface sensible heating on the Arabian Peninsula is very strong. Consequently the subtropical anticyclone which dominated the Arabian Sea in spring retreats westwards to the Arabian Peninsula and intensifies rapidly. The zonal asymmetric PV forcing develops gradually with high PV eddies moving southwards along northerlies on the eastern side of the anticyclone, and a high PV trough is formed in the middle troposphere over the Arabian Sea, which is favorable to the explosive barotropic development of the tropical cyclone into the vortex. Results from this study demonstrate that the ISM onset, which is different from the BOB and the SCS monsoon onset, is a special dynamical as well as thermodynamic process occurring under the condition of fully coupling of the upper, middle, and lower tropospheric circulations.

An Analysis of the Characteristics of Monsoon Onset over the Bay of Bengal and the South China Sea in 2010

AbstractBased on NCEP/NCAR daily reanalysis and the Tropical Rainfall Measuring Mission data, the back-ground atmospheric circulation and the characteristics of meteorological elements during the period of the Bay of Bengal monsoon (BOBM) and the South China Sea (SCS) monsoon (SCSM) in 2010 are studied. The impacts of the BOBM onset on the SCSM onset and the relationship between the two monsoons are also analyzed. The two main results are as follows: (1) The BOBM onset obviously occurs earlier than the SCSM onset in 2010, which is a typical onset process of the Asian monsoon. During the BOBM–s onset, northward jump, and eastward expansion, convective precipitation and southwest winds occurred over the SCS, which resulted in the onset of the SCSM. (2) The relationship among strong convection, heavy rainfall, and vertical circulation configuration is obtained during the monsoon onsets over the BOB and SCS, and it is concluded that the South Asian High plays an important role in this period.

A climatological study on the role of the South China Sea monsoon onset in the development of the East Asian summer monsoon

The unique role of the South China Sea summer monsoon (SCSSM) onset process in the development of the East Asian summer monsoon (EASM) is demonstrated in this study. The SCSSM onset process is examined in terms of the vertical linkage between the Western Pacific subtropical high (WPSH) and the South Asian high (SAH). A composite analysis is performed in order to adequately describe the vertical linkage in a synoptic timescale. The South China Sea (SCS) is a key region for the seasonal migrations of the WPSH and the SAH, with the former retreating northeastward, the latter advancing northwestward, and both taking place over the SCS during the SCSSM onset period. The SCSSM onset process is characterized by a significant change in the relative configuration of the ridge lines of the WPSH and the SAH. Just prior to the onset period, the ridge lines intersect vertically over the SCS, thus prohibiting convective activities. During the onset period, the ridge line intersection moves away from the SCS due to the retreating WPSH and the northward shift of the SAH ridge line. This coincides with the emergence of monsoonal convective activities over the SCS and the establishment of a moisture channel from the tropics, which in turn provides favorable conditions for the development of deep convective activity. The northeastward intrusion of the lower level southwesterlies and the moisture supplying channel are closely related to the development of a preexisting twin cyclone in the Bay of Bengal. The northeastward lower level southwesterlies form a monsoonal ascending motion in the SCS, which further merges upward into the northeasterlies to the south of the SAH ridge line. This is a signature of the establishment of the local Hadley circulation, which marks the beginning of the EASM. The frontal system is the most frequent attendant synoptic event during the SCSSM onset. From the viewpoint of synoptic process, the SCSSM undergoes a two-stage onset process which is characterized by the southward intrusion of the frontal system in the earlier stage and the outbreak of the tropical convection in the later stage. The frontal system may act as a trigger for the outbreak of the tropical convection in the later stage. The burst out of the monsoonal convection over the SCS is essential for the breakdown of the vertical intersection between the WPSH and the SAH therein.

Impacts of the basin‐wide Indian Ocean SSTA on the South China Sea summer monsoon onset

AbstractThis article explores the impacts of the Indian Ocean basin‐scale sea surface temperature anomaly (SSTA) on the South China Sea (SCS) summer monsoon onset. Basin‐wide warming in the tropical Indian Ocean (TIO) is found to occur in the spring following an El Niño event, and the opposite occurs for a La Niña event. Such changes of the Indian Ocean SSTA apparently prolong the El Niño‐Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the Philippine Sea anti‐cyclone.Warming in the TIO induces an anomalous reversed Walker circulation over the tropical Indo–Pacific Ocean, which leads to descending motion, and hence suppressed convection in the western Pacific. The intensified Philippine Sea anti‐cyclone in May and June advances more westward and prevents the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late SCS monsoon onset. The case is opposite for the TIO cooling such that the Philippine Sea anti‐cyclone weakens and retreats eastward, thus favouring an early onset of the SCS monsoon. Copyright © 2008 Royal Meteorological Society

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

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

Air-Sea Flux Exchange over the Xisha Sea Area Before and After the Onset of Southwest Monsoon in 2002

The third South China Sea (SCS) air-sea flux experiment was conducted in Xisha sea area from April 24 to June 22, 2002. The heat flux (latent and sensible heat) exchange and momentum exchange were computed using eddy-correlation method and TOGA COARE2.5b algorithms. Variation of radiation components, heat and moisture flux exchange and their relations with synoptic condition were discussed. The results indicate that heat and moisture fluxes, especially short-wave radiation, net radiation budget of ocean surface, latent heat flux and net oceanic heat flux, changed significantly before and after the onset of SCS monsoon in that year. Relationships of sensible heat flux, latent heat flux and momentum fluxes with different environment elements are different. Sensible heat flux has positive correlation with air-sea temperature difference Tw – Ta. Latent heat flux has positive correlation with wind speed V,Tw–Ta and SST, especially with wind speed. Momentum fluxes vary with wind speed (V) as a function of = 0.00185V 2 – 0.00559V + 0.01248.

The link between interannual variation of the South China Sea summer monsoon onset and summer precipitation in Shandong Province

Relationship between the onset data of South China Sea (SCS) summer monsoon and the summer rainfall in Shandong Province was examined by comprehensive analysis to establish a conceptual model of the link. If the summer monsoon occurs earlier, the 500 hPa level would induce the teleconnection of Eurasian pattern in the summer (June–August), which indicates that the western Pacific subtropical high is displaced northward further than usual, the Siberian high is intensified and the Okhotsk low is deepened. Under such circumstance, Shandong, Iocated in the west side of the subtropical high and in front of the mid-Siberia high, would be expected to have a wet summer because it is quite possible for cold and warm air to meet and interact with each other in Shandong. Statistical analysis revealed that the 500 hPa anomalies over Korea and Japan were sensitive to the SCS monsoon onset date and very important to precipitation in Shandong, and that the convective activities over the deep water basin in the SCS in 24–26 pentads significantly influenced the position of the ridge line of the western Pacific subtropical high. These findings yielded better understanding of the causative mechanisms involved in the precipitation generation, so that the knowledge gained can possibly be applied for long-lead forecast.

Interannual variability of the 10–25‐ and 30–60‐day variation over the South China Sea during boreal summer

The interannual variability (IAV) of the intraseasonal variation (ISV) over the South China Sea (SCS) was investigated, focusing on the relationship between the 10–25‐ and 30–60‐day variation. Both ISV activities over the SCS had larger IAV than those of the Bay of Bengal and Arabian Sea. The 10–25‐ (30–60‐) day variation was relatively active when the 30–60‐ (10–25‐) day variation was suppressed in June, July and September. The IAV of the ISV during early summer (June–July) was related to the IAV of the SCS monsoon onset. A late (early) onset was accompanied by active 10–25‐ (30–60‐) day variation. The IAV of the ISV could be divided into early and late summer (September), because there was little persistence between these two periods. Moreover, large‐scale circulation anomalies associated with the IAV had different spatial structures in these two periods.

Overview of the South China sea monsoon experiment

The present paper gives an overview of the key project “ South China Sea Monsoon Experiment (SCSMEX)” operated by the Ministry of Science and Technology of China during the period of 1996–2001. The SCSMEX is a joint atmospheric and oceanic field experiment which aims to better understand the onset, maintenance, and variability of the summer monsoon over the South China Sea (SCS). It is a large-scale international effort with many participating countries and regions cooperatively involved in this experiment. With the field observation in May–August 1998, a large amount of meteorological and oceanic data was acquired, which provides excellent datasets for the study of the SCS monsoon and the East Asian monsoon and their interaction with the ocean. The preliminary research achievements are as follows. (1) The earliest onset of the Asian monsoon over the SCS and Indo-China Peninsula has been well documented. From the viewpoint of the synoptic process, its onset is closely related to the early rapid development of a twin cyclone to the east of Sri Lanka. The conceptual model of the SCS monsoon onset in 1998 was put forward. The 50-year time series of the SCS monsoon onset date was also made. (2) Two major modes, namely the 30–60-day and 10–20-day oscillations were ascertained. The influences of the abnormal SCS monsoon on the precipitation over eastern China and its modes were identified. A strong (weak) monsoon over the SCS usually leads to less (more) precipitation over the middle and lower reaches of the Yangtze River basin, and more (less) precipitation in North China. (3) During the monsoon onset over the SCS, a wide variety of organized mesoscale convective systems (MCSs) were observed by a Doppler radar array deployed over the northern SCS. The relationship between large-scale circulations and MCSs during the monsoon onset process in 1998 was clearly revealed. It was suggested that there is a kind of positive feedback mechanism between large-scale circulations and MCSs. (4) The SST over the SCS during the early period influences the timing of the monsoon onset date and the monsoon’s intensity. During the monsoon onset, the ocean undergoes a process of energy release through air-sea interaction. During the break phase of the SCS monsoon, the ocean demonstrates the process of energy re-accumulation. Obvious differences in the air-sea turbulent flux exchange between the southern and northern parts of the SCS due to different characteristic features of the atmosphere and sea structure were observed in those regions. (5) The verification of impact of intensive observations on the predictive performance is made by the use of regional models. The air-sea coupled regional climate model (CRCM) was also developed under the SCSMEX Project. The simulation of the oceanic circulation in 1998 produced with the model was well compared with the observations.

Influence of the convection over the South China Sea on the summer precipitation of Shandong Province

The pentad average minimum outgoing longwave radiation (OLR) data over the northern South China Sea (SCS) are selected as indexes to analyze the intensity of the convection connected with the SCS monsoon onset. Statistic analysis demonstrates that the index can account for the intensity of the SCS monsoon about, at least, 75%. A significant negative correlation (confident level over 90%) between Shandong’s summer rainfall and the index is found only in the period of 24–26 pentads and limited to the area above the deeper water basin of the SCS (10°–20°N, 110°–117.5°E). Thus the minimum OLR over the deeper water basin during 24–26 pentads can be used as a valuable predictor for the long-lead forecast of the precipitation. The 500hPa geopotential height data in the Northern Hemisphere for the period from 1951 to 2000 are used in order to characterize the physical mechanisms involved. The composite anomalies of the 500hPa level allow for the identification and detection of the teleconnection of the East Asia-North America (EAP) pattern that is responsible to some extent for the interannual variability of the precipitation of Shandong Province. Besides, the interannual differences of the intraseasonal variations (ISV) of OLR and their northward transmission probably make a contribution to the position of the subtropical high which is vital for the summer rainfall in the province.

Definition of South China Sea Monsoon Onset and Commencement of the East Asia Summer Monsoon*

Abstract The climatological mean summer monsoon onset in the South China Sea (SCS) is a remarkably abrupt event. However, defining onset dates for individual years is noticeably controversial. The controversies and complications arise primarily from a number of factors: the intermittent southward intrusion of cold fronts into the northern SCS, the bogus onset in late April before the establishment of tropical monsoons over Indochina, and active intraseasonal oscillation. In this paper, a simple yet effective index, USCS, the 850-hPa zonal winds averaged over the central SCS (5°–15°N and 110°–120°E), is proposed for objectively defining the SCS monsoon onset. This onset index depicts not only the sudden establishment of the tropical southwesterly monsoon over the SCS but also the outbreak of the rainy season in the central-northern SCS. In this paper the East Asian summer monsoon (EASM) is defined as the broadscale summer monsoon over East Asia and the western North Pacific region (0°–40°N, 100°–140°E). It is shown that the seasonal transition of EASM can be objectively determined by the principal component of the dominant empirical orthogonal mode of the 850-hPa zonal winds, UEOF1. It is found that the local index USCS represents UEOF1 extremely well; thus, it can be used to determine both the SCS onset and the commencement of the broadscale EASM. The result suggests that the SCS summer monsoon onset indeed signifies the beginning of the summer monsoon over East Asia and the adjacent western Pacific Ocean. Evidence is also provided to show the linkage between the two salient phases of EASM: the local onset of the SCS monsoon and the local onset of the mei-yu (the rainy season in the Yangtze River and Huai River basin and southern Japan).

Definition of South China Sea Monsoon Onset and Commencement of the East Asia Summer Monsoon*

Definition of South China Sea Monsoon Onset and Commencement of the East Asia Summer Monsoon*

Mechanism of thermal features over the Indo-China peninsula and possible effects on the onset of the South China sea monsoon

The thermal characteristics during the South China Sea (SCS) summer monsoon onset period near the Indo-China Peninsula are analyzed by using the South China Sea Monsoon Experiment (SCSMEX) reanalysis data from 1 May to 31 August 1998 and the NCEP/ NCAR pentad-mean reanalysis data from January 1980 to December 1995. The possible relationships between the anomaly of thermal features near the Indo-China Peninsula and the SCS monsoon onset are investigated, and the mechanism causing the SCS summer monsoon onset is also discussed. Results from the 1998 SCSMEX reanalysis data show that there exists a strong persistent surface sensible heating near the Indo-China Peninsula prior to the SCS monsoon onset, which has apparent low frequency oscillation features. This sensible healing leads lo a warmer center in the lower atmosphere near the Indo-China Peninsula and strong local horizontal temperature and geopotential height gradients which are favorable to strengthening the southwest wind over the Indo-China Peninsula. It is also found that stronger convergent winds at lower levels and stronger divergent winds at high levels appear, which provide a favorable configuration for the development of vertical motion, enhancement of precipitation, and onset of the SCS monsoon. These results can be verified by analysis of the multi-year mean data. Additionally, it is found that the temperature at 850 hPa increases more rapidly over the Indo-China Peninsula than the South China Sea prior to the SCS monsoon onset, which leads to a strengthening of the temperature difference between the Indo-China Peninsula and the South China Sea. Moreover, results from the analysis of the longitudinal temperature and geopotential height differences show that the eastern retreat of the subtropical high over the Indo-China Peninsula during the period of SCS monsoon onset is associated with the temperature increase over the Indo-China Peninsula and the eastern extension of low trough over the Bay of Bengal.