South China Sea Summer Monsoon Onset Research Articles

Regional Characteristics of Cloud Radiative Effects before and after the South China Sea Summer Monsoon Onset

The South China Sea summer monsoon (SCSSM) onset is characterized by rapid thermodynamical changes in the atmosphere that are critical to regional weather and climate processes. So far, few studies have focused on the changes in the associated cloud and radiative features. This study investigates spatiotemporal characteristics of top-of-atmosphere (TOA) cloud radiative effects (CREs) before and after the SCSSM onset over the South China Sea (SCS) and South China (SC), based on the 2001–2016 Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) satellite data and ERA-Interim reanalysis data. Before the SCSSM onset, strong net CRE (NCRE) dominated by its cooling shortwave component occurs over SC, while descending motion and weak NCRE prevail over the SCS. In the SCSSM onset pentad, convection, high clouds, and longwave and shortwave CREs (LWCRE and SWCRE) abruptly increase over the southern and central SCS, and their high-value centers subsequently move northeastward and are strongly affected by the western Pacific subtropical high. The strong offset between LWCRE and SWCRE enables the NCRE intensity (TOA radiation budget) to be quite small (large) between the SCS and the western North Pacific after the SCSSM onset. In contrast, low-middle-level clouds and strong cooling SWCRE remain over SC after the SCSSM onset, but the increasing high clouds and LWCRE weaken (intensify) the regional NCRE (TOA radiation budget) intensity. These marked latitudinal differences in CREs between the SCS and SC primarily arise from their respective dominant cloud types and circulation conditions, which manifest the differences between the tropical SCSSM and subtropical East Asian monsoon processes. The results indicate that regional cloud fractions and CREs before and after the SCSSM onset are strongly modulated by quickly changed large-scale circulation over the East Asian monsoon regions, and the spatiotemporal variation of CREs is a response to the monsoonal circulation adjustment to a large extent.

Revisiting the relationship between Indo‐Pacific heat content and South China Sea summer monsoon onset during 1980–2020

AbstractThe relationship of the tropical Indo‐Pacific upper ocean heat content (HC) and the South China Sea (SCS) summer monsoon (SCSSM) onset during 1980–2020 was investigated. It was found that when HC anomalies in the preceding winter–spring are positive (negative) in the eastern Indian–western Pacific Ocean or negative (positive) in the southwestern Indian Ocean and eastern Pacific Ocean, SCSSM onset is usually early (late). Empirical orthogonal function (EOF) analysis suggested that the primary EOF modes of HC anomaly in both tropical Pacific and Indian Oceans are associated with El Niño–Southern Oscillation (ENSO). It is through modulating the westerly over the north Indian Ocean–SCS and the western north Pacific subtropical high (WNPSH) via the Walker circulation over the Indo‐Pacific basin, monsoon circulation in the Indian Ocean that the ENSO related Indo‐Pacific HC variation connects with the SCSSM onset. Both the second EOF modes for the tropical Pacific and Indian Ocean also show significant correlations with the SCSSM onset. The former related vertical motion over the SCS–far northwestern Pacific can influence the WNPSH and convection, thus occurrence of the SCSSM. The latter represents the intrinsic Indian Ocean HC variability; the air temperature variation associated with it can modulate the atmospheric circulation in the Indian Ocean–SCS through thermal wind balance, and thus the SCSSM onset by acting on the westerly over the SCS and the WNPSH. A western Pacific index and an Indian Ocean dipole index were defined to represent the leading HC modes in the tropical Pacific and Indian Ocean, respectively. A binary linear regression model was constructed using these two indices to improve SCSSM onset prediction.

Impact of the March Arctic Oscillation on the South China Sea summer monsoon onset

AbstractThis study reveals that the South China Sea summer monsoon (SCSSM) onset is closely linked to the preceding March Arctic oscillation (AO), the dominant mode of atmospheric circulation over extratropical Northern Hemisphere. The physical processes for the impact of the March AO on the SCSSM onset are further examined. March AO leads to an anomalous low‐level cyclone over subtropical western North Pacific (WNP) via eddy–mean flow interaction. The southwesterly wind anomalies to the southeastern side of the anomalous cyclone weaken the climatological trade winds, which contribute to warm sea surface temperature (SST) anomalies over tropical western‐central North Pacific via reduction of surface evaporation and latent heat flux. These warm SST anomalies excite an atmospheric Rossby wave response and reinforce the anomalous cyclone over the subtropical WNP. Via the above positive atmosphere–ocean interaction, the anomalous cyclone induced by the March AO could persist through the whole spring and shift equatorward. This anomalous cyclone creates favourable environment for the SCSSM onset via weakening the WNP subtropical high and is conducive to the transition of the low‐level zonal wind. Further analysis indicates that the March AO‐SCSSM onset relationship is independent of El Niño‐Southern Oscillation (ENSO). This result suggests that, besides the dominant tropical system (i.e., ENSO), interannual variation of the SCSSM onset also has a close relation with the dominant extratropical atmospheric system (i.e., AO), which may provide additional source for the seasonal prediction of the monsoon onset.

The Record-Breaking Extreme Drought in Yunnan Province, Southwest China during Spring-Early Summer of 2019 and Possible Causes

In spring and early summer of 2019, Yunnan Province experienced the most severe seasonal drought on record, with days of extreme drought area exceeding 105 km2 far more than normal. Consistently, the precipitation in each month from February to June is over 30% less than normal, and about 50% less in the most severe drought period (April–June). The rainy season in Southwest China (SWC) started on the third pentad in June 2019, which is the second latest in history. The rainy season in Yunnan started on 24 June, which is the latest (29 days later than normal). On the contrary, the onset of the South China Sea summer monsoon (SCSSM) is abnormally early. The lag time between the start of the rainy season in SWC and the onset of the SCSSM in 2019 is 7 pentads, which is the largest since 1961, much longer than the climate mean (less than 1 pentad). The present study analyzes the possible reasons why the rainy season came extremely late and the drought disaster persisted and intensified after a much early SCSSM, at both seasonal and subseasonal timescales. The abnormally late onset of the rainy season and the second greatest potential evapotranspiration (PET) since 1981 are the direct reasons for the persistent drought. Statistical results show that the water vapor from southwest of Yunnan in April-June contributes more than that from the east at the seasonal scale. In April-June 2019, however, the southern branch trough (SBT) was abnormally weak, the large and strong anticyclonic wind anomaly prevailed over the Bay of the Bengal (BOB), and the meridional water vapor transport to Yunnan was weak. At the subseasonal scale, the weaker SBT lasted the longest, and the strong convection over the BOB came up late despite of an early onset of the SCSSM, which resulted in reduced low-level moisture convergence in Yunnan and development of drought prior to the SCSSM onset. From the onset of SCSSM to the start of rainy season in SWC, the SBT and meridional water vapor transport from the BOB were still weak, and the water vapor was mainly transported into the coastal area of South and Southeast China rather than Yunnan. After the start of the rainy season in SWC, the SBT was still weak. This led to less moisture transport in the westerlies to the west of Yunnan and the persistent extreme drought. Both the statistical results and case analysis indicate that the stronger Australian high in spring and early summer of 2019 was associated with the abnormally strong anticyclone over the BOB and the always weak SBT. In sum, the anomalous weakness of SBT played a critical role in the extreme drought occurrence and persistence in Yunnan of Southwest China in 2019.

Extremely Early Summer Monsoon Onset in the South China Sea in 2019 Following an El Niño Event

AbstractThe El Niño–Southern Oscillation (ENSO) is regarded as one of the most important factors for onset of the South China Sea summer monsoon (SCSSM). Previous studies generally indicated that an El Niño event tends to result in a late onset of the SCSSM monsoon. However, this relationship has not been true in recent years, particularly when an extremely early SCSSM onset (1 May 2019) occurred following the 2018/19 El Niño event in the preceding winter. The processes of the second earliest SCSSM onset in the past 41 years were investigated using NCEP–DOE reanalysis, OLR data, and ERSST. A negative sea surface temperature and associated anticyclonic anomalies were absent over the western North Pacific in the late spring of 2019 following an El Niño event in the preceding winter. Thus, the mean circulation in the late spring of 2019 does not prevent SCSSM onset, which is in sharp contrast to the composited spring of the El Niño decaying years. The convective active and westerly phases of a 30–60-day oscillation originating from the Indian Ocean provided a favorable background for the SCSSM onset in 2019. In addition, the monsoon onset vortex over the Bay of Bengal and the cold front associated with a midlatitude trough over East Asia also played important roles in triggering the early onset of the SCSSM in 2019. No tropical cyclone appeared over the western North Pacific during April and May, and the enhancement of quasi-biweekly oscillation mainly occurs after the SCSSM onset; thus, these two factors contribute little to the SCSSM onset in 2019.

Record-breaking heat wave in southern China and delayed onset of South China Sea summer monsoon driven by the Pacific subtropical high

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

Climate shift of the South China Sea summer monsoon onset in 1993/1994 and its physical causes

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

Signals of Spring Thermal Contrast Related to the Interannual Variations in the Onset of the South China Sea Summer Monsoon

AbstractLand–sea thermal contrast is one of the key factors modulating the onset and strength of summer monsoons. This study investigates the precursory thermal-contrast signal of the onset of the South China Sea summer monsoon (SCSSM) and reveals that the land–sea thermal contrastDT(ToceanminusTlandat the surface) in March tracks the date of the monsoon onset very well. In an energy-balance framework, the monthly anomalies in this thermal contrast are decomposed into different components associated with the anomalies of various radiative and nonradiative dynamical processes over the South China Sea in early spring. It is found that the interannual variations ofDT, and thus the onset date of monsoon, are mainly tied to the anomalies in surface heat fluxes, ocean–land heat storage rate, and oceanic dynamical processes. The variation of ocean–land heat storage rate and ocean dynamics is the largest positive contributor to the change inDT. Specifically, an early SCSSM onset tends to be associated with an amplified “cold-land, warm-ocean” pattern that can be further attributed to increased soil moisture content in March, which enhances land surface cooling and weakens oceanic heat storage rate that benefits ocean surface warming. Furthermore, the variations ofDTand soil moisture content in March are positively related with the late-February precipitation over the Indochina Peninsula, which could therefore be regarded as a precursory signal for both the land–sea thermal contrast in March and the onset of the SCSSM.

Asymmetric effects of atmospheric circulation on the South China Sea summer monsoon onset

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

Extremely Late Onset of the 2018 South China Sea Summer Monsoon Following a La Niña Event: Effects of Triple SST Anomaly Mode in the North Atlantic and a Weaker Mongolian Cyclone

AbstractThe onset of South China Sea (SCS) summer monsoon (SCSSM) in June 2018 was extremely late following a La Niña event, indicating a breakdown in the traditional positive relationship between ENSO and SCSSM onset. Although the weaker western Pacific subtropical high in early May facilitated the SCSSM establishment, the persistent weaker Mongolian cyclone featured by an anomalous barotropic anticyclone over Northeast Asia during mid–late May dramatically postponed the SCSSM onset in 2018. This not only reduced upper‐tropospheric divergence over the SCS, which suppressed monsoon convection by attenuating the South Asian High, but cooled the mid–upper troposphere to decrease the in‐situ vertical easterly shear. The convection over the Maritime Continent was therefore trapped in the tropics, which further delayed the SCSSM onset via an enhanced local Hadley cell. It suggests a potential effect of triple SSTA mode in North Atlantic on the SCSSM onset by changing the mid‐latitudinal circulation.

The role of synoptic‐scale waves in the onset of the South China Sea summer monsoon

The role of premonsoon synoptic‐scale waves (2‐ to 10‐day oscillation) in the onset of the South China Sea summer monsoon (SCSSM) is investigated in this study. The results indicated that the onset date of the SCSSM from 1979–2016 was related to the intensity of premonsoon synoptic‐scale waves. The more active the premonsoon synoptic‐scale waves, the earlier the SCSSM tended to be established. This study focused on the accumulative effects of premonsoon synoptic‐scale waves on the SCSSM onset because active years often included more than one process. According to the regression analysis results, three premonsoon synoptic‐scale waves influenced the onset of the SCSSM. First, waves over the southern Bay of Bengal (BOB) were related to Kelvin waves, which accelerated the intrusion of southwesterly winds over the BOB into the South China Sea (SCS). Second, waves over the eastern SCS were related to tropical depression (TD)‐type waves. Westerly anomalies south of this type of synoptic‐scale wave were favorable for the onset of the SCSSM. Third, waves originating from the north were hypothesized to be related to the mid‐latitude fronts. The mid‐latitude vortex moved northeast of the SCS and induced the intrusion of southwesterlies over the SCS. All three waves continuously enabled the westerly anomalies and affected the SCSSM. In addition, the combined effects of these synoptic‐scale waves were investigated. The results showed that active waves led to significant anomalous westerlies along the boreal tropical region and cyclonic circulation over the Philippine Sea. Anomalous convection was enhanced over the BOB and SCS, and the western Pacific subtropical high was suppressed. The combined intensities of these premonsoon synoptic‐scale waves indicated their potential for use as a new predictor of the onset of the SCSSM.

Relationship between South China Sea summer monsoon onset and landfalling tropical cyclone frequency in China

Between 1979 and 2015, the anticorrelation between the frequency of landfalling tropical cyclones (LTCs) in mainland China from June to October and the South China Sea summer monsoon (SCSSM) onset date can be attributed to variations in the western North Pacific subtropical high (WNPSH) and monsoon trough (MT) that correspond to the SCSSM onset. When the SCSSM onset is early, the WNPSH shrinks northeastwards and the MT becomes increasingly strong and exhibits a northwards shift. These features lead to anomalous lower‐level cyclonic circulation around Luzon Island and a mid‐tropospheric easterly anomaly over the Chinese coastal regions, which favour the generation of TCs in the western part of the North Pacific that makes landfall on the southeastern coast of China. In contrast, the circulation anomaly prohibits landfalling TCs in mainland China when the SCSSM onset is delayed. The SCSSM onset has the potential to be an indicator of TC landfall frequency in mainland China.

On the weakening relationship between the South China Sea summer monsoon onset and cross‐equatorial flow after the late 1990s

The weakening in the relationship between the South China Sea summer monsoon onset (SCSSMO) and the low‐level cross‐equatorial flow (CEF) in May is investigated using the ERA‐Interim reanalysis data sets during 1979–2016. The SCSSMO–SCSCEF relationship has experienced a significant inter‐decadal change, and the correlation coefficient becomes weaker after the late 1990s. The correlation has shifted from the significant negative value in the earlier decade to insignificant in the later decade. This inter‐decadal change is robust under several sensitive tests and largely independent of the El Niño‐Southern Oscillation signal. One possible explanation is the change in mechanisms driving the SCSSMO over the course of the two periods. Before the late 1990s, the northwards march of the intertropical convergence zone, which has a close relationship with the SCSCEF, is mainly responsible for the SCSSMO. After the late 1990s, warming of the western North Pacific favours more tropical cyclones and disturbances during May. The westwards movement of these tropical disturbances would affect the SCSSMO and help explain why the SCSSMO–SCSCEF relationship became weaker in the later decade.

Effects of intraseasonal oscillation on South China Sea summer monsoon onset

Understanding of South China Sea (SCS) summer monsoon (SCSSM) onset response to tropical intraseasonal oscillation (ISO) is critical for extended-range prediction of SCSSM onset. In this study, we investigate the effect of ISO on SCSSM onset for the period of 1980–2013. The 34 onset cases are classified into three groups, early onsets around May 6th, normal onsets around May 21st and late onsets around June 8th, and the late onsets are even later than the Indian monsoon onsets. Before each onset, the SCS experiences a dry ISO phase to precondition the convective energy due to the easterly wind anomalies of the wet ISO phase over the tropical Indian Ocean for the group of early onsets, over the southern Bay of Bengal monsoon region for the group of normal onsets and over the southern Indian monsoon region for the group of late onsets. After each onset, the SCSSM is supported by the westerly wind anomalies of the dry ISO phase over these associated regions. Each early SCSSM onset is triggered by the northwestward propagating Rossby wave of the wet ISO in the western Pacific which comes from the Indian Ocean. For each normal (late) onset, the SCSSM is triggered by synoptic-scale low-level westerlies in conjunction with seasonal low-level westerlies when the wet ISO moves to the northern Bay of Bengal region (Indian monsoon region), since this convection to the north of 10°N cannot excite the easterly wind anomalies associated with Kelvin wave responses over the SCS to suppress the convection. The mechanisms explaining the mean state-controlled ISO-SCSSM onset relationship are also discussed.

Objective determination of the onset and withdrawal of the South China Sea summer monsoon

AbstractDetermining the timing (i.e. onset and withdrawal) of monsoon season precisely and objectively is an important yet difficult task. Conventional methods mainly define the monsoon timing as the date when the selected atmospheric variables (e.g. rainfall and wind) exceed an arbitrary threshold. These methods present little explicit justification and are subjective and sensitive to the fluctuation of the selected series. In this study, we propose an objective method to determine the onset and withdrawal of the South China Sea summer monsoon (SCSSM) using the cumulative low‐level zonal wind. Our proposed approach provides an easy, objective, and applicable method that is recommended for the detection of the timing of monsoon season. On the basis of the proposed definition, the SCSSM onset and withdrawal are determined, and their accompanying processes are also examined in this paper. Both onset and withdrawal of SCSSM exhibit strong variability, and the El Niño–Southern Oscillation plays an important role. Via the modification of the western North Pacific subtropical high, a preceding El Niño (La Niña) event can delay (advance) the monsoon onset and advance (delay) the monsoon withdrawal.

The relation of cross‐equatorial flow during winter and spring with South China Sea summer monsoon onset

ABSTRACTThe relationship between cross‐equatorial flow (CEF) during winter and spring and the South China Sea summer monsoon (SCSSM) onset is investigated by data diagnoses for the period of 1979–2013 and numerical experiments. The SCSSM onset is found to have a significant negative correlation with the CEF nearby the Philippines in preceding January, February and March prior to SCSSM onset. A strong CEF during January, February and March tends to be succeeded by an early onset of SCSSM, whereas a weak CEF is likely to be followed by a late onset. The CEF in preceding months links with the SCSSM onset through the subtropical high over South China Sea and western North Pacific in April. The subtropical high in April is weak and eastward shifted (strong and westward shifted) when the CEF in preceding months is strong (weak). The weak and eastward shifted (strong and westward shifted) subtropical high favours (does not favour) the convection developing (depressing) over South China Sea, resulting in an early (late) onset of SCSSM. The anomalies of CEF nearby the Philippines in preceding months and its close relationship with subsequent subtropical high are caused by the sea surface temperature anomalies (SSTA) and atmospheric heating source over tropical western Pacific. The difference of SSTA between south and north of equator in tropical western Pacific and the corresponding atmospheric diabatic heating result in the anomalies of CEF in January, February and March, and impact the activity of subtropical high in April. The positive SSTA in tropical western Pacific, combining with the wet and warm climatic background over northern South China Sea and coastal areas of Southern China in April are favourable for a cyclonic circulation anomaly occurring over northern South China Sea. As a result, the subtropical high is weak and eastward shifted, and the SCSSM onset is early.

A possible precursor of the South China Sea summer monsoon onset: Effect of the South Asian High

AbstractIn climatology, the South China Sea (SCS) summer monsoon (SCSSM) generally onsets in Pentad 28 (16–20 May). The eastward extension of the South Asian High into the SCS in Pentad 27 is a possible precursor to the SCSSM onset. In the upper troposphere, the South Asian High (SAH) warms the air by inducing the positive potential vorticity advection, which strengthens precipitation over the southern SCS. When local convection becomes strong enough to decrease the vertical temperature gradient in Pentads 28–29, the upper level warming center overlies the subcloud warm region over the SCS to satisfy the requirement of the angular momentum conservation. A cross‐equatorial flow then forms, together with strong vertical easterly wind shear and deep monsoon convection over the SCS, marking the full establishment of the SCSSM. The abrupt change in the SAH in May could therefore be an early indicator of the onset of the SCSSM.

Feature analysis and primary causes of pre-flood season “cumulative effect” of torrential rain over South China

When persistent rainfall occurs frequently over South China, meso-scale and micro-scale synoptic systems persist and expand in space and time and eventually form meso-scale and long-scale weather processes. The accumulation of multiple torrential rain processes is defined as a “cumulative effect” of torrential rain (CETR) event. In this paper, daily reanalysis datasets collected by the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) during 1979–2014 are used to study the anomalous features and causes of heavy CETR events over South China. The results show that there is a significant difference in the spatial distribution of the heavy CETR events. Based on the center position of the CETR, the middle region displayed middle-region-heavy CETR events while the western region displayed west-region-heavy CETR events. El Nino events in the previous period (December, January, February, March (DJFM)) are major extra-forcing factors of middle-region-heavy CETR events, which is beneficial for the continuous, anomalous Philippine Sea anticyclone and strengthens the West Pacific Subtropical High (WPSH), extending it more westward than normal. The primary water vapor source for precipitation in middle-region-heavy CETR events is the Tropical Western Pacific Ocean. The major extra-forcing factor of a west-region-heavy CETR is the negative anomaly in the southern Tropical Indian Ocean (TIO) during the previous period (DJFM). This factor is beneficial for strengthening the cross-equatorial flow and westerly winds from the Bay of Bengal to the South China Sea (SCS) and early SCS summer monsoon onset. The primary water vapor source of precipitation in the west-region-heavy CETR is the southern TIO.

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

AbstractUsing observed and reanalysis datasets, the South China Sea summer monsoon (SCSSM) onset process is analyzed in each year from 1997 to 2014. Regional mean (5–20°N, 110–120°E) 850 hPa zonal wind, precipitation, and SST are used as indices to describe SCSSM onset. Three distinct onset types are identified: among the 18 years studied, nine are normal onset years, which are characterized by a well-established westerly wind and associated precipitation over the South China Sea (SCS); eight are intermittent onset years, in which monsoon precipitation does not occur continuously following the establishment of the westerly wind over the SCS; and one year, 2014, is a delayed onset year, in which the western Pacific subtropical high dominates over the SCS after the seasonal transition and prevents the monsoon onset. A comparison of the first two types suggests that a positive SST gradient in the northern Indian Ocean and local SST warming in the SCS are two key factors in the normal SCSSM onset type. With r...

Two Types of Interannual Variability of South China Sea Summer Monsoon Onset Related to the SST Anomalies before and after 1993/94

Abstract An advance in the timing of the onset of the South China Sea (SCS) summer monsoon (SCSSM) during the period 1980–2014 can be detected after 1993/94. In the present study, the interannual variability of the SCSSM onset is classified into two types for the periods before and after 1993/94, based on their different characteristics of vertical coupling between the upper- and lower-tropospheric circulation and the differences in their related sea surface temperature anomalies (SSTAs). On the interannual time scale, type-I SCSSM onset is characterized by anomalous low-level circulation over the northern SCS during 1980–93, whereas type-II SCSSM onset is associated with anomalies of upper-level circulation in the tropics during 1994–2014. The upper-tropospheric thermodynamic field and circulation structures over the SCS are distinct between the two types of SCSSM onset, and this investigation shows the importance of the role played by the spring SSTAs in the southern Indian Ocean (SIO) and that of ENSO events in type-I and type-II SCSSM onset, respectively. In the early episode, the warming SIO SSTAs can induce an anomalous low-level anticyclone over the northern SCS that affects local monsoonal convection and rainfall over land to its north, demonstrating a high sensitivity of subtropical systems in type-I SCSSM onset. However, in type-II SCSSM onset during the later episode, the winter warm ENSO events and subsequent warming in the tropical Indian Ocean can influence the SCSSM onset by modulating the spring tropical temperature and upper-level pumping effect over the SCS.