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This paper investigates the impact of the South China Sea summer monsoon (SCSSM) on the Indian Ocean dipole (IOD). The results show that the SCSSM has a significant positive relationship with the IOD over the boreal summer [June–August (JJA)] and fall [September–November (SON)]. When the SCSSM is strong, the enhanced southwesterly winds that bring more water vapor to the western North Pacific (WNP) lead to surplus precipitation in the WNP, inducing anomalous ascending there. Consequently, the anomalous descending branch of the SCSSM Hadley circulation (SCSSMHC) develops over the Maritime Continent (MC), favoring deficit precipitation in situ. The precipitation dipole over the WNP and MC as well as the enhanced SCSSMHC leads to intensification of the southeasterly anomalies off Sumatra and Java, which then contributes to the negative sea surface temperature (SST) anomalies through the positive wind–evaporation–SST and wind–thermocline–SST (Bjerknes) feedbacks. Consequently, a positive IOD develops because of the increased zonal gradient of the tropical Indian Ocean SST anomalies and vice versa. The SCSSM has a peak correlation with the IOD when the former leads the latter by three months. This implies that a positive IOD can persist from JJA to SON and reach its mature phase within the frame of the positive Bjerknes feedback in SON. In addition, the local and remote SST anomalies in the tropical Indian and Pacific Oceans have a slight influence on the relationship between the SCSSM and precipitation dipole over the WNP and MC. [ABSTRACT FROM AUTHOR]

Hainan, an island province of China in the northern South China Sea, experienced two sustained rainstorms in October 2010, 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. [ABSTRACT FROM AUTHOR]

Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979–2006 period. The existing indices can be classified into five categories: the east–west thermal contrast, north–south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Niño, while the first category reflects the second mode that corresponds to the developing El Niño. The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring. The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon. [ABSTRACT FROM AUTHOR]

This paper reveals synoptic-scale atmospheric conditions over the South China Sea (SCS) that cause heavy rainfall in central Vietnam through case study and composite analyses. The heavy rainfall event discussed in this study occurred on 2–3 November 1999. Precipitation in Hue city (central Vietnam) was more than 1800 mm for these 2 days. Two atmospheric disturbances played key roles in this heavy rainfall. First, a cold surge (CS) northerly wind anomaly in the lower troposphere, originating in northern China near 40°N, propagated southward to reach the northern SCS and then lingered there for a couple of days, resulting in stronger-than-usual northeasterly winds continuously blowing into the Indochina Peninsula against the Annam Range. Second, a southerly wind anomaly over the central SCS, associated with a tropical depression–type disturbance (TDD) in southern Vietnam, seemed to prevent the CS from propagating farther southward. Over the northern SCS, the southerly wind anomaly formed a strong low-level convergence in conjunction with the CS northeasterly wind anomaly, and supplied warm and humid tropical air. These conditions induced by the CS and TDD are favorable for the occurrence of the heavy orographic rainfall in central Vietnam. The TDD can be regarded as a result of a Rossby wave response to a large-scale convective anomaly over the Maritime Continent associated with equatorial intraseasonal variability. Using a 24-yr (1979–2002) reanalysis and surface precipitation datasets, the authors confirm that the coexistence of the CS and TDD is important for the occurrence of heavy precipitation in central Vietnam. In addition, it is observed that CSs without a TDD do not lead to much precipitation. [ABSTRACT FROM AUTHOR]

Warm Sector Torrential Rains (WSTRs) occurring during the outbreak of the monsoon in May of 2015 in South China were studied using surface automatic weather observational data, sounding, European Centre for Medium-Range Weather Forecasts Reanalysis interim Data (ERA-interim), satellite and radar data, and a four-level nested grid simulation with the finest grid spacing of 1 km using the Weather Research and Forecasting model (WRF). The results show that the extreme precipitation event, which had maximum rainfall amounts of 406.3 mm in 10 h and 542.2 mm in 24 h on 20 May 2015, and was characterized by its rapid development and its highly concentrated and long duration of heavy rainfall, occurred over the trumpet-shaped topography of Haifeng. The simulation results indicated that the South China Sea (SCS) atmospheric moisture transportation (AMT) was crucial in triggering the precipitation of the WSTR over South China. The simulation of the WSTR was conducted by using the total energy-mass flux scheme (TEMF), which provided a reasonable simulation of the circulation and the vertical profile in the Planetary Boundary Layer (PBL) as well as the estimation of the precipitation. The AMT, which extends from the Beibu Gulf and the South China Sea to the coastal areas and provides Shanwei with a considerable amount of moisture in the boundary layer, and the effects within the PBL, which include orographic effects, an extra low-level jet, and a high-energy tongue characterized by a high-potential pseudo-equivalent temperature tongue with a warm and moist southwesterly wind, were the important large-scale factors causing the WSTR. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of a heavy rain event that occurred in southern Taiwan in June 2005. From 11 - 13 June 2005, a weak Mei-yu front moved southeastward from China to Taiwan, while mesoscale convective systems (MCSs) were developing and moving northward over the northern South China Sea (SCS). During the first day of the event the southwesterly flow intensified when a ridge associated with the Pacific high extended northwestward from the Philippines to the southern Taiwan Strait (TS). This pressure pattern produced a large northwestward pressure gradient force that created a southeasterly wind speed increase followed by intensification of the southwesterly flow through Coriolis acceleration. An low-level jet (LLJ) formed consequently and transported moisture and unstable air toward the southwestern coast of Taiwan. MCSs were triggered in the southwesterly flow because the potentially unstable air was lifted in a low-level convergence and shearing vorticity environment. They intensified, became organized, and moved northeastward overland, resulting in heavy rainfall in southern Taiwan. On the second day, low pressure formed near the southern TS because of the combined effect of a travelling short-wave trough and a pressure reduction resulting from the latent heat release by the evolving MCSs. This pressure change produced down-gradient acceleration in the northeastward direction, resulting in southwesterly flow strengthening. The local wind acceleration was smaller than that of the first day because the dominant pressure system was local scale, while that of the first day was synoptic scale. [ABSTRACT FROM AUTHOR]

We have investigated how the Borneo vortex develops over the equatorial South China Sea under cold surge conditions in December during the Asian winter monsoon. Composite analysis using reanalysis and satellite data sets has revealed that absolute vorticity and water vapour are transported by strong cold surges from upstream of the South China Sea to around the Equator. Rainfall is correspondingly enhanced over the equatorial South China Sea. A semi-idealized experiment reproduced the Borneo vortex over the equatorial South China Sea during a "perpetual" cold surge. The Borneo vortex is manifested as a meso- α cyclone with a comma-shaped rainband in the northeast sector of the cyclone. Vorticity budget analysis showed that the growth/maintenance of the meso-α cyclone was achieved mainly by the vortex stretching. This vortex stretching is due to the upward motion forced by the latent heat release around the cyclone centre. The comma-shaped rainband consists of clusters of meso-β-scale rainfall cells. The intense rainfall in the comma head (comma tail) is generated by the confluence of the warmer and wetter cyclonic easterly flow (cyclonic southeasterly flow) and the cooler and drier northeasterly surge in the northwestern (northeastern) sector of the cyclone. Intense upward motion and heavy rainfall resulted due to the low-level convergence and the favourable thermodynamic profile at the confluence zone. In particular, the convergence in the northwestern sector is responsible for maintenance of the meso-α cyclone system. At both meso-α and meso-β scales, the convergence is ultimately caused by the deviatoric strain in the confluence wind pattern but is significantly self-enhanced by the nonlinear dynamics. [ABSTRACT FROM AUTHOR]

During May and June, the monsoon rainfall in northern Southeast Asia is primarily produced by rainstorms. At the mature stage, these storms, coupled with a midtropospheric subsynoptic-scale trough, produce rainfall ≥50 mm(6h)-1 and exhibit a cyclonic surface vortex. With a scale ~O(10²) km, rainstorms during the period of 1979-2016 are identified with station and satellite observations, along with assimilation data. Several dynamic processes of rainstorm geneses are disclosed by an extensive analysis. 1) Maximum occurrence of rainstorm geneses is located in the midtroposphere of two regions (northern Vietnam-southwestern China and the northern South China Sea), but eventually penetrates downward to the surface. 2) The environment favorable for rainstorm genesis is a southwest-northeast-oriented narrow trough formed by the confluence of the midtropospheric northeasterly around the eastern Tibetan Plateau and the lower-tropospheric monsoon southwesterlies. Because the criterion for Charney--Stern instability is met by the shear flow of this narrow trough, rainstorms are likely initiated by this instability. 3) The majority of rainstorm geneses occurs during the evening over the land and the morning at sea. This timing preference is caused by the modulation of the clockwise rotation of the East Asia continent circulation in response to the diurnal variation of the land-sea thermal contrast. These new findings from this study offer not only a new perspective for the genesis mechanism of the late spring-early summer rainstorms in northern Southeast Asia, but also a new initiative to develop medium-range forecasts for these rainstorms. [ABSTRACT FROM AUTHOR]

A northwest-southeast-oriented summer monsoon trough exists between northern Indochina and northwestern Borneo. Ahead of this the South China Sea (SCS) trough is located at a convergent center west of the Philippines, which provides an environment favorable for rain-producing synoptic systems to produce rainfall over this center and form the SCS summer rainfall center. Revealed from the x- t diagram for rainfall, this rainfall center is developed by multiple-scale processes involved with the SCS trough (TR), tropical depression (TY), interaction of the SCS trough with the easterly wave/tropical depression (EI), and easterly wave (EW). It is found that 56% of this rainfall center is produced by the SCS trough, while 41% is generated by the other three synoptic systems combined. Apparently, the formation of the SCS summer monsoon rainfall center is contributed to by these four rain-producing synoptic systems from the SCS and the Philippines Sea. The Southeast Asian summer monsoon undergoes an interannual variation and exhibits an east-west-oriented cyclonic (anticyclonic) anomalous circulation centered at the western tropical Pacific east of the Luzon Strait. This circulation change is reflected by the deepening (filling) of the SCS summer monsoon trough, when the monsoon westerlies south of 15°N intensify (weaken). This interannual variation of the monsoon westerlies leads to the interannual variation of the SCS summer monsoon rainfall center to follow the Pacific-Japan oscillation of rainfall. The rainfall amount produced over this rainfall center during the weak monsoon season is about two-thirds of that produced during the strong monsoon season. The rain-production ratio between TR and TY + EI + EW is 60:38 during the strong monsoon season and 47:49 during the weak monsoon season. [ABSTRACT FROM AUTHOR]

The peak intensity occurrence frequency over the life cycles of parent cold-surge vortices (CSVs) for heavy rainfall/flood (HRF) events is classified into two types depending on their life cycles having two or three peak intensities, denoted as HRF2 or HRF3, respectively. The formation of an HRF2 event from its parent CSV(HRF2) formation is ≤5 days, while the formation of an HRF3 event is ≥6 days. The latter group contributes ~57% of the total number of HRF events. As a result of some model constraints, the formation and development of HRF3 events are not well forecasted by the Global Forecast System (GFS) and regional forecast models. The life cycle and second peak intensity for CSV(HRF3) allow for the introduction of a forecast advisory for HRF3 events. Identification of CSVs and two sufficient requirements for the formation and occurrence of HRF events were developed by previous studies. Nevertheless, two new necessary steps are now included in the proposed forecast advisory. The population ratio for CSV(HRF3) and the regular CSV is only about 15%. The occurrence optimum time to for the CSV(HRF3) second peak intensity from this vortex formation is about 3 days 6 h. The GFS forecast over to is utilized to identify CSV(HRF3). Then, the relay of the GFS forecast from the occurrence time of the CSV(HRF3) second peak is used to predict the formation/occurrence of HRF3 events. Six HRF3 events during cold seasons for 2013–16 are used to test the feasibility of this forecast advisory. Results clearly demonstrate this advisory is a success for the forecast of HRF3 events over the entire life cycles of their parent CSV(HRF3)s. [ABSTRACT FROM AUTHOR]

About 44% of the cold-season heavy rainfall/flood (HRF) events around the South China Sea require six days or longer to develop from the formation time of their parent cold surge vortices (CSVs). Formations for both the parent CSV and HRF event are involved with interactions of the concerned vortices with two different cold surge flows. The occurrence frequency of the East Asian cold surge flow varies from 4.5 to 6 days. The longevous CSVs enable their developments to interact with the second cold surge flows between formations of these CSVs and HRF events. Two requirements for the formation of HRF events are 1) synchronized occurrence of the HRF event and the northwestern Pacific explosive cyclone and 2) simultaneous occurrence of the maximum speeds among westerlies of the northwestern Pacific explosive cyclone and easterlies of the tropical trade winds and the HRF event. These requirements cannot be met by the CSV at its second maximum peak intensity, but the CSV at this stage plays an indispensible role for the formation of the HRF event to make its intensity and rainfall amount larger than those HRF events without this relay intensification. The development of an HRF event through multiple interactions of CSVs with sequential cold surge flows may pose difficulties to numerically simulate/predict the occurrence of these HRF events over the cold-season rainfall centers around the South China Sea. [ABSTRACT FROM AUTHOR]

Examination of the development of cold season heavy rainfall/flood (HRF) events around the South China Sea (SCS) from their parent cold surge vortices (CSVs) shows three new development processes. First, the formation mechanism of the parent CSV of an HRF event [CSV(HRF)] has a preference as to geographic location, flow type of the cold surge inside the SCS, and time of day. The surface trough east of the Philippines, Taiwan, and southern Japan island chain in late fall and the near-equator trough across Borneo in winter facilitate the CSV(HRF) formation in two regions-the vicinity of the Philippines and Borneo. The formation of the Philippine (Borneo) CSV(HRF) occurs at 0600 UTC (0000 UTC) with involvement from the Philippine Sea (PHS)-type (SCS type) of cold surge flow. Second, the flow type of the cold surge determines the CSV(HRF) propagation across the South China Sea. The PHS-type (SCS type) facilitates (hinders) the CSV(HRF) westward propagation. This occurs because the easterly (northerly) flow is greater than (less than) the northerly (easterly) flow at the maximum isotach location of the cold surge flow associated with CSV(HRF) and is centered east of the demarcation line for propagation. This flow-type contrast is substantiated by the vorticity budget analysis for CSV(HRF). The positive 925-hPa vorticity tendency is located west of (coincident with) the 925-hPa vorticity center for the PHS-type (SCS type) of cold surge. Third, the CSV(HRF) development into a HRF event is achieved through multiple interactions of former vortices with sequential cold surges across the South China Sea. The first two CSV(HRF) development processes are reported herein; the last process is presented in Part II. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) moving northwestward/westward across northern Taiwan are defined to have a type-2 track. This study aims to analyze heavy rainfall associated with type-2 TCs in Taiwan, focusing on the modulation processes of the intraseasonal oscillation (ISO). During 1958-2011, 21 summer type-2 TCs are separated into three rainfall types: strong, moderate, and weak. For the strong rainfall type, both 30-60-day and 10-24-day ISOs propagate northwestward across Taiwan. During landfall and the ensuing two days, both ISOs exhibit a cyclonic anomaly centering northwest of Taiwan that causes anomalous westerly flows (or enhance seasonal southwesterly flows) from the South China Sea (SCS) onto Taiwan. Persistent moisture supply and strong moisture convergence result in prolonged heavy rainfall on the windward side over western Taiwan. TCs with weak rainfall are accompanied by a northward-propagating 30-60-day ISO from the tropical western Pacific toward Japan and a westward-propagating 10-24-day ISO along 20°N latitude. During the landfall stage both ISOs have a cyclonic anomaly with its center south of Taiwan. Major anomalous westerly flows are displaced southward across the central SCS, leading to a weak moisture supply and rainfall in Taiwan. The moderate rainfall type features a 30-60-day (10-24 day) ISO resembling that of the weak (strong) rainfall type. The amount of rainfall thus ranges between the strong and weak rainfall types. Major processes regulating the rainfall of type-2 TCs relate to the intensity of the moisture supply associated with anomalous westerly flows from the SCS onto Taiwan, which is jointly modulated by 30-60-day and 10-24-day ISOs. [ABSTRACT FROM AUTHOR]

The Asian summer monsoon (ASM) begins firstly over the Indo-China Peninsula in early May and over the South China Sea (SCS) in mid-May. The different monsoon onset dates can exert distinct effects on the summer rainfall in Asia. Statistical results indicate that the Antarctic Oscillation (AAO) in the boreal winter has a significant precursory influence on the ASM onset dates. In stronger AAO years, both the Mascarene high and the Australia high in March are stronger owing to the 'see-saw' structure of atmospheric circulation over the subtropics and higher latitudes in the Southern Hemisphere, and the tropical intertropical convergence zone (ITCZ) is deeper. Thus, the pressure gradient between the subtropical and tropical regions increases in spring. As a result, the Somalia cross-equatorial flow (SCEF) occurs earlier, strengthens, and enhances the westerlies over the tropical Indian Ocean. The enhanced westerlies impel an eastward withdrawal of the western Pacific subtropical high and intensify the convergence and rising motion at the lower troposphere, accelerating the burst of ASM. Differently, weaker AAO weakens the pressure gradient between the tropical and subtropical regions and delays the establishment of SCEF, resulting in a delayed onset of ASM. This study extends the leading time of seasonal forecast of ASM onset from the previous spring to winter and provides useful information about precursory signals in climate prediction operation. [ABSTRACT FROM AUTHOR]

Malaysia is geographically separated into Peninsular Malaysia and west Borneo. The rainfall maximum in the former region occurs during November-December, whereas that in the latter region occurs during December-February. This difference of maximum rainfall period indicates that the formation mechanism is different for the rainfall centers in these two parts of Malaysia. Since rainfall is primarily produced by severe weather systems, the formation of a climatological rainfall center is explored through synoptic activity and the rainfall amount of this center is estimated through contributions by rain-producing disturbances. The major cause of the rainfall maximum of Peninsular Malaysia is cold surge vortices (CSVs) and heavy rainfall/flood (HRF) events propagating from the Philippine area and Borneo. In contrast, the major cause of the rainfall maximum of Borneo is these rain-producing disturbances trapped in Borneo. Disturbances of the former group are formed by the cold surge flows of the Philippine Sea type, whereas disturbances of the latter group are formed by cold surge flows of the South China Sea (SCS) type. The population of HRF events is about one-fourth of the rain-producing disturbances in both Peninsular Malaysia and Borneo, but they produce less than ~60% rainfall for these two regions. It is revealed from the synoptic and dynamic analyses that the major Borneo rain-producing disturbances propagate westward before December by strong tropical easterlies, but they are trapped after December by strong northeasterlies of the SCS-type cold surge flow. [ABSTRACT FROM AUTHOR]

Long-term changes in rainfall and associated tropical cyclone (TC) activity in transition seasons between the wet and dry seasons in South and Southeast Asia, namely boreal spring and fall, were examined, using gridded rainfall, TC tracks, and reanalysis datasets, the focus of discussion being the long-term changes in coastal regions. It was found that long-term changes in rainfall during the transition seasons in South and Southeast Asia were closely associated with those in TC activity over the north Indian Ocean and South China Sea. Rainfall in May increased in the last 40 years over and around Myanmar, which indicates an earlier monsoon onset over the western Indochina Peninsula. Rainfall over and around northern Vietnam also increased in the last 40 years during October, which is known as the monsoon retreat period. These increases were associated with the long-term changes in TC activity. Furthermore, although linear trends have been previously suggested, monotonically increasing or decreasing trends in these long-term changes were not found over the last 60 years. [ABSTRACT FROM AUTHOR]

A fuzzy c-means clustering method (FCM) is applied to cluster tropical cyclone (TC) tracks. FCM is suitable for the data where cluster boundaries are ambiguous, such as a group of TC tracks. This study introduces the feasibility of a straightforward metric to incorporate the entire shapes of all tracks into the FCM, that is, the interpolation of all tracks into equal number of segments. Four validity measures (e.g., partition coefficient, partition index, separation index, and Dunn index) are used objectively to determine the optimum number of clusters. This results in seven clusters from 855 TCs over the western North Pacific (WNP) from June through October during 1965--2006. The seven clusters are characterized by 1) TCs striking the Korean Peninsula and Japan with north-oriented tracks, 2) TCs affecting Japan with long trajectories, 3) TCs hitting Taiwan and eastern China with west-oriented tracks, 4) TCs passing the east of Japan with early recurving tracks, 5) TCs traveling the easternmost region over the WNP, 6) TCs over the South China Sea, and 7) TCs moving straight across the Philippines. Each cluster shows distinctive characteristics in its lifetime, traveling distance, intensity, seasonal variation, landfall region, and distribution of TC-induced rainfall. The roles of large-scale environments (e.g., sea surface temperatures, low-level relative vorticity, and steering flows) on cluster-dependent genesis locations and tracks are also discussed. [ABSTRACT FROM AUTHOR]

A heavy rainfall event associated with the passage of Tropical Storm Rachel (1999) over southern Taiwan was studied in which a conceptual model was proposed. In the model, Tropical Storm Paul (1999) plays an important role in impeding the movement of Rachel, thus becoming one of the key factors in enhancing the rainfall amount in southern Taiwan. To further quantify the above concept, a mesoscale numerical model is used to evaluate the influence of Paul on the simulated rainfall associated with Rachel near Taiwan. Sensitivity experiments are performed by removing the circulation of Paul, and/or the large-scale monsoon trough system, where Paul is imbedded. The potential vorticity diagnosis shows that the movement of Rachel is indeed affected by the presence of Paul. Nevertheless, a more detailed analysis shows that it is the presence of the entire monsoon trough that impedes the movement of Rachel and steers the storm toward southwestern Taiwan especially before its landfall. In all, these results generally support the conceptual model with regard to the heavy rainfall mechanism proposed in a previous study. Moreover, this study further points out that it is the circulation associated with both Paul and the entire monsoon trough that affects the movement of Rachel. In addition, the analyses based on the no-terrain simulation depict the relationships among the moisture-rich air from the South China Sea associated with Rachel, relatively dry air from South China, and the mechanism of forming a warm and dry region to the eastern side of the Taiwan terrain, which greatly influences the heavy rainfall distribution in the event. [ABSTRACT FROM AUTHOR]

Analysis of the NCEP and 40-yr ECMWF Re-Analysis (ERA-40) data and the Xisha Island station observation indicates that the low-level meridional wind (LLMW) over the South China Sea (SCS) experienced an interdecadal variation since the late 1970s. The LLMW change is associated with the reduction of tropospheric temperature in midlatitude East Asia. A mechanism is put forward to explain the triggering and maintenance of the tropospheric cooling. The enhanced convective heating over the southern South China Sea results in a meridional vertical overturning circulation, with anomalous descending motion appearing over continental East Asia. The anomalous descending motion reduces the local humidity through both anomalous low-level divergence and dry vertical advection. The decrease of the local tropospheric humidity leads to the enhanced outgoing longwave radiation into space and thus cold temperature anomalies. The decrease of the temperature and thickness leads to anomalous low (high) pressure and convergent (divergent) flows at upper (lower) levels. This further enhances the descending motion and leads to a positive feedback loop. The fall in tropospheric temperature over continental East Asia reduces the land–sea thermal contrast and leads to the weakening of cross-equatorial flows and the LLMW over SCS. A further diagnosis indicates that the LLMW is closely linked to the summer precipitation and temperature variations in China on interdecadal time scales. A weakening of the LLMW after 1976 is associated with a “-, +, -” meridional rainfall pattern, with less rain in Guangdong Province and north China but more rain in the Yangtze and Huaihe River basins and northeast China, and a “+, -, +” temperature pattern, with increased (decreased) surface temperature in the south and north (central) China. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Rainfall characteristics and mesoscale properties of precipitation systems in mei-yu seasons over South China, Taiwan, and the South China Sea (SCS) during 1998–2007 are investigated in this study. Mei-yu rainbands are defined using the Tropical Rainfall Measuring Mission 3B42 rainfall product and then applied to divide the mei-yu season into the mei-yu and break periods. In the 10-yr “climatology,” on average, the mei-yu rainbands have a lifetime of 4–5 days and most frequently occur near the South China coast. During the mei-yu periods, rainfall maxima are found over the Pearl River Delta, the foothills of the Yun-Gui Plateau, and Wuyi Mountain, with the first two maxima corresponding to especially heavy rainfall. Intraseasonal variations on the convective structures, especially over land, are distinct among the mei-yu, break, pre-mei-yu, and post-mei-yu, based on analysis of convection intensity proxies and vertical radar reflectivity profiles of precipitation features. Lightning flash rates are consistent with the convective structure. The most frequent lightning over South China and Taiwan is in the pre-mei-yu and the least is during the mei-yu, which suggests different microphysical structures. Therefore, the discrimination of intraseasonal transitions on convective vertical structures may have important implications to the problems of cumulus parameterization, model validation, rainfall estimation, and latent heat retrievals. Intraseasonal variations of convective structures over the SCS are less evident than those over land. Storms over the SCS during the mei-yu are slightly convectively stronger than those in the break. Oceanic features with strong ice scattering have much lower lightning flash rates than their counterparts over land. [ABSTRACT FROM AUTHOR]

The spectral latent heating (SLH) algorithm was developed to estimate latent heating profiles for the Tropical Rainfall Measuring Mission Precipitation Radar (TRMM PR). The method uses TRMM PR information (precipitation-top height, precipitation rates at the surface and melting level, and rain type) to select heating profiles from lookup tables (LUTs). LUTs for the three rain types—convective, shallow stratiform, and anvil rain (deep stratiform with a melting level)—were derived from numerical simulations of tropical cloud systems from the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) using a cloud-resolving model (CRM). The two-dimensional (2D) CRM was used in previous studies. The availability of exponentially increasing computer capabilities has resulted in three-dimensional (3D) CRM simulations for multiday periods becoming increasingly prevalent. In this study, LUTs from the 2D and 3D simulations are compared. Using the LUTs from 3D simulations results in less agreement between the SLH-retrieved heating and sounding-based heating for the South China Sea Monsoon Experiment (SCSMEX). The level of SLH-estimated maximum heating is lower than that of the sounding-derived maximum heating. This is explained by the fact that using the 3D LUTs results in stronger convective heating and weaker stratiform heating above the melting level than is the case if using the 2D LUTs. More condensate is generated in and carried from the convective region in the 3D model than in the 2D model, and less condensate is produced by the stratiform region’s own upward motion. [ABSTRACT FROM AUTHOR]

This study reveals the close relationship between the first transition of the Asian summer monsoon (ASM), the tropical intraseasonal oscillation (TISO), and the mei-yu in Taiwan, which occurs climatologically between mid-May and mid-June. For about half of the years in 1958–2002, the first transition of the Asian summer monsoon can be classified as a sharp onset, which is characterized by an abrupt reversal of the monsoon flow from northeasterly to southwesterly. The evolution of the large-scale monsoon circulation and convection in the sharp-onset years is characterized by an eastward-propagating TISO from eastern Africa and the western Indian Ocean to the Maritime Continent. Upon the arrival of the TISO in the Maritime Continent, a sharp onset of the ASM occurs, and a channel supplying moist air in the lower troposphere is well established across the Indian Ocean to the South China Sea (SCS). This channel consists of the Somali jet, transporting the moisture from the Southern Hemisphere to the Northern Hemisphere, and the southwesterly monsoon, delivering the moisture across the Indian Ocean to the SCS and the western North Pacific. This efficient and persistent transport of moisture to the SCS and surrounding areas presumably provides a favorable condition for the maintenance of the mei-yu front and the development of convective systems. This also marks the onset of the Taiwan mei-yu season. Because a strong TISO signal, which tends to occur concurrently with the sharp onset of the ASM, is often observed prior to the onset of the first transition and Taiwan mei-yu, a close monitoring of the TISO can be informative for the weather forecasters in Taiwan to project the initiation of the Taiwan mei-yu. [ABSTRACT FROM AUTHOR]

Observations from two enhanced sounding arrays during the May–June 1998 South China Sea Monsoon Experiment (SCSMEX) are used to determine and contrast the properties of convection over the northern and southern South China Sea (SCS). A regression analysis between SST data and monthly rainfall indicates that the ENSO signal exerted a strong influence on the rainfall distribution over the SCS during SCSMEX. This resulted in wetter-than-normal conditions along the south China coast and northern SCS, and generally drier-than-average conditions elsewhere, particularly over the Philippine Islands. The monsoon onset as determined by a shift in the low-level winds from easterly to southwesterly over the SCS occurred around mid-May. Over the southern enhanced sounding array (SESA), the onset was characterized by a rainy period associated with the passage of a convectively coupled Kelvin wave. This was followed by a weeklong break and then several episodic rain events with increasingly higher rain rates. Rainfall over the northern enhanced sounding array (NESA), which was largely out of phase with SESA rainfall events, occurred primarily during two 10-day periods separated by a weeklong break. Convective characteristics over the SESA, deduced primarily from heat and moisture budget profiles, indicate a high stratiform rain fraction consisting of alternating periods with decaying mesoscale systems that organized near the western Borneo coastline and shallower convective clouds. In contrast, NESA-averaged profiles were indicative of deep convection with a relatively small stratiform rain fraction, which was confirmed with radar analyses during the onset convective period. The diurnal cycle of convection is a dominant feature throughout much of the SCS. Over both budget regions, early morning (0500–0800 LT) convective systems were frequently initiated near the coasts, then gradually dissipated during the course of the day as the midlevel steering currents moved the systems away from the coastline. These decaying convective systems resulted in an early afternoon (1400 LT) rainfall peak over both sonde arrays. [ABSTRACT FROM AUTHOR]