South China Sea Summer Monsoon Research Articles

Interdecadal change of South China Sea summer monsoon intensity in the mid‐1990s

ABSTRACTThis study analysed the interdecadal variation of the South China Sea summer (June–September) monsoon (SCSSM) in recent 32 years (1979–2010). The SCSSM showed a strengthening interdecadal variability starting from 1995, which was also verified in time series of precipitation and precipitable water averaged over the SCSSM domain. Thus, 32 years of total analysis period was divided into two: first half of past 16 years (1979–1994, hereafter 7994) and recent 16 years (1995–2010, hereafter 9510) thereby analysing a mean difference between 9510 and 7994 periods to determine causes of the strengthened SCSSM since the mid‐1990s. The spatial distribution of difference in outgoing longwave radiation, precipitable water, and precipitation between the two periods showed a typical La Niña pattern. The pattern which occurred during La Niña event has been verified through analysis on differences in 850‐ and 200‐hPa stream flows between the two periods. At 850 hPa, anomalous huge anticyclones were strengthened in the both hemispheres so that anomalous easterlies were strengthened in the equatorial central and western Pacific. As a result, the SCS was affected by anomalous southeasterlies. On the contrary, anomalous huge cyclones were strengthened in the both hemispheres at 200 hPa so that anomalous westerlies were strengthened in the equatorial western and central Pacific. This means that rising air from the tropical western Pacific since the mid‐1990s was moved to the east and then Walker circulation, which was descended at the tropical central and eastern Pacific, was strengthened. Due to the strengthened Walker circulation, La Niña events have been more dominant than before in recent years and as a result of this, SCSSM was more strengthened than before.

Has the prediction of the South China Sea summer monsoon improved since the late 1970s?

Based on the evaluation of state-of-the-art coupled ocean–atmosphere general circulation models (CGCMs) from the ENSEMBLES (Ensemble-based Predictions of Climate Changes and Their Impacts) and DEMETER (Development of a European Multimodel Ensemble System for Seasonal to Interannual Prediction) projects, it is found that the prediction of the South China Sea summer monsoon (SCSSM) has improved since the late 1970s. These CGCMs show better skills in prediction of the atmospheric circulation and precipitation within the SCSSM domain during 1979–2005 than that during 1960–1978. Possible reasons for this improvement are investigated. First, the relationship between the SSTs over the tropical Pacific, North Pacific and tropical Indian Ocean, and SCSSM has intensified since the late 1970s. Meanwhile, the SCSSM-related SSTs, with their larger amplitude of interannual variability, have been better predicted. Moreover, the larger amplitude of the interannual variability of the SCSSM and improved initializations for CGCMs after the late 1970s contribute to the better prediction of the SCSSM. In addition, considering that the CGCMs have certain limitations in SCSSM rainfall prediction, we applied the year-to-year increment approach to these CGCMs from the DEMETER and ENSEMBLES projects to improve the prediction of SCSSM rainfall before and after the late 1970s.

Pacific decadal oscillation and the decadal change in the intensity of the interannual variability of the South China Sea summer monsoon

The decadal variation of the intensity of the interannual variability (IIV) of the South China Sea summer monsoon (SCSSM) was partly modulated by the Pacific Decadal Oscillation (PDO) during the tw...

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.

Generation of near-inertial oscillations by summer monsoon onset over the South China Sea in 1998 and 1999

The summer monsoon onset over the South China Sea (SCS) is an abrupt event in May or early June every year. After the summer monsoon onset in 1998 and 1999, strong near-inertial oscillations (NIOs) in the central SCS were observed with Acoustic Doppler Current Profilers (ADCP) mooring data. The near-inertial current speed reached 0.25ms−1, comparable to that induced by tropical storms (TS) in the same area, although the wind speed (~10ms−1) of the monsoon onset was much lower than what is typical of TSs. Further analyses suggest that the shallow mixed-layer (<30m) in spring and the abrupt change in wind speed and direction resulting from the summer monsoon onset were responsible for developing the near-inertial current. The generated NIOs could be enhanced by a warm eddy appearing during the monsoon onset in the central SCS. The strong NIOs appeared in the middle of the SCS in May when the SCS summer monsoon starts to prevail, which implies that the beginning of the SCS summer monsoon may be a vital factor for generation of the strong NIOs in May.

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.

Summer monsoon indices on June rainfall in Hong Kong and its vicinity

Climatologically, June is usually the wettest month in Hong Kong. With significant interannual variation of the summer monsoon, the rainfall variability in June is also large. As Hong Kong is in close proximity to the peripheries of different monsoon regions, the variability of June rainfall largely depends on the relative strength of various monsoon systems. In the present study, a new index comparing the relative condition of the western North Pacific summer monsoon and the South China Sea summer monsoon is developed based on the difference between the respective monsoon indices WNPMI (western North Pacific summer monsoon index) and UMI (unified monsoon index). It is shown that June rainfall in Hong Kong and its vicinity is better correlated with this new index than either WNPMI or UMI alone. Based on the signs of the new index in conjunction with those of WNPMI and UMI, a framework to stratify the monsoon conditions into different configurations together with a simple scheme to summarize the associated rainfall responses is formulated. This study highlights how the rainfall variability on a local or regional scale can be quantified by the broad-scale conditions of different monsoon systems.

Empirical prediction of the onset dates of South China Sea summer monsoon

The onset of South China Sea summer monsoon (SCSSM) signifies the commencement of the wet season over East Asia. Predicting the SCSSM onset date is of significant importance. In this study, we establish two different statistical models, namely the physical–empirical model (PEM) and the spatial–temporal projection model (STPM) to predict the SCSSM onset. The PEM is constructed from the seasonal prediction perspective. Observational diagnoses reveal that the early onset of the SCSSM is preceded by (a) a warming tendency in middle and lower troposphere (850–500 hPa) over central Siberia from January to March, (b) a La Nina-like zonal dipole sea surface temperature pattern over the tropical Pacific in March, and (c) a dipole sea level pressure pattern with negative center in subtropics and positive center over high latitude of Southern Hemisphere in January. The PEM built on these predictors achieves a cross-validated reforecast temporal correlation coefficient (TCC) skill of 0.84 for the period of 1979–2004, and an independent forecast TCC skill of 0.72 for the period 2005–2014. The STPM is built on the extended-range forecast perspective. Pentad data are used to predict a zonal wind index over the South China Sea region. Similar to PEM, the STPM is constructed using 1979–2004 data. Based on the forecasted zonal wind index, the independent forecast of the SCSSM onset dates achieves a TCC skill of 0.90 for 2005–2014. The STPM provides more detailed information for the intraseasonal evolution during the period of the SCSSM onset (pentad 25–35). The two models proposed herein are expected to facilitate the real-time prediction of the SCSSM onset.

Interdecadal increase of tropical cyclone genesis frequency over the western north Pacific in May

ABSTRACTThe interdecadal changes in the frequency of tropical cyclone (TC) genesis in the western North Pacific (WNP) were examined for the epoch 1979–2013. There was an interdecadal increase in the frequency of TC genesis in the WNP in May during 1999–2013 than during 1979–1998, and such changes differed from those occurring between June and December. This interdecadal change is attributed to the early onset of the South China Sea summer monsoon (SCSSM) during 1999–2013, when the southwesterly winds spread eastwards and the monsoon trough (MT) moved into the WNP earlier. The MT favoured enhanced convection and anomalous low‐level cyclonic vorticity, facilitating more TC genesis. During 1979–1998, in contrast, the onset of the SCSSM was delayed and the MT was confined to the west of the South China Sea. As a result, weakened convection and anomalous anticyclonic low‐level vorticity are observed over the WNP, facilitating less TC genesis.

The East Asian subtropical summer monsoon: Recent progress

The East Asian subtropical summer monsoon (EASSM) is one component of the East Asian summer monsoon system, and its evolution determines the weather and climate over East China. In the present paper, we firstly demonstrate the formation and advancement of the EASSM rainbelt and its associated circulation and precipitation patterns through reviewing recent studies and our own analysis based on JRA-55 (Japanese 55-yr Reanalysis) data and CMAP (CPC Merged Analysis of Precipitation), GPCP (Global Precipitation Climatology Project), and TRMM (Tropical Rainfall Measuring Mission) precipitation data. The results show that the rainy season of the EASSM starts over the region to the south of the Yangtze River in early April, with the establishment of strong southerly wind in situ. The EASSM rainfall, which is composed of dominant convective and minor stratiform precipitation, is always accompanied by a frontal system and separated from the tropical summer monsoon system. It moves northward following the onset of the South China Sea summer monsoon. Moreover, the role of the land–sea thermal contrast in the formation and maintenance of the EASSM is illustrated, including in particular the effect of the seasonal transition of the zonal land–sea thermal contrast and the influences from the Tibetan Plateau and midlatitudes. In addition, we reveal a possible reason for the subtropical climate difference between East Asia and East America. Finally, the multi-scale variability of the EASSM and its influential factors are summarized to uncover possible reasons for the intraseasonal, interannual, and interdecadal variability of the EASSM and their importance in climate prediction.

Influence of the South China Sea Biweekly Sea Surface Temperature on the South China Sea Summer Monsoon Especially during the Indian Ocean Dipole

ABSTRACTThe influence of the biweekly sea surface temperature (SST) in the South China Sea (SCS) on the SCS summer monsoon, especially during the Indian Ocean Dipole (IOD) is presented using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST and rainfall data for April to June from 1999 to 2013. During positive IOD (PIOD) years the biweekly SST anomalies over the SCS lead the rain anomalies by three days, with a significant correlation (r = 0.8, at the 99% confidence level), whereas during negative IOD (NIOD) years, the correlation is only 0.2. The biweekly SST is observed to influence the westward and northward propagating rainfall anomalies over the SCS and, hence, affect the SCS summer monsoon, especially during PIOD years. No such propagation was seen during NIOD years. The biweekly intraseasonal oscillation of SST in the SCS results in enhanced sea level pressure and surface shortwave radiation, especially during PIOD years. The potential findings here indicate that the biweekly SST in the SCS is strongly (weakly) influenced during PIOD (NIOD) years. Further, it is observed that SST in the SCS has a strong (weak) effect on the SCS summer monsoon by westward and northward propagation of rainfall, especially during PIOD (NIOD) years. When a PIOD or NIOD exists over the tropical Indian Ocean, the SCS SST will be strongly (r = 0.6, at the 99% confidence level) or weakly correlated with the residual index, respectively.

Comments on “Interdecadal Change of the South China Sea Summer Monsoon Onset”

Abstract In a recent paper, Kajikawa and Wang detected the interdecadal shift of the South China Sea summer monsoon (SCSSM) onset with a late SCSSM onset in an earlier epoch (1979–93) and an early SCSSM onset in a later epoch (1994–2008) and attributed this change to enhanced tropical cyclone (TC) activity and intraseasonal variability (ISV) related to 30–80-day and 10–25-day anomalies in the second epoch. This comment assesses the individual impact of TCs and ISV on the interdecadal change of the SCSSM onset by means of the removal of anomalies associated with TCs and ISV. Results herein show that TCs have no significant impact on the SCSSM onset in all years, except 2006 in which a strong and long-lived TC occurred over the South China Sea. After removing the 30–80-day anomaly, the difference in the mean SCSSM onset date in the two epochs decreases to some extent, implying that the 30–80-day anomaly can, in part, play a role in the interdecadal shift of the SCSSM onset. In contrast, the 10–25-day anomaly has an insignificant contribution to the interdecadal shift of the SCSSM onset. The discrepancy of ISV contribution results from the SCSSM background state, the magnitude and spatiotemporal scale of ISV, and the phase relationship between ISV and SCSSM transition from easterly to westerly.

Reply to “Comments on ‘Interdecadal Change of the South China Sea Summer Monsoon Onset’”*

Understanding the variability and change of monsoon onset is of the utmost importance for agriculture planning and water management. The sudden onset of the South China Sea summer monsoon (SCSSM) is one of the most spectacular phenomena and has a profound societal impact. Kajikawa and Wang (2012, hereinafter KW12) detected a significant change in the mean onset date of the SCSSM around 1993/94: the epochal mean onset date is 30 May for 1979‐93 and 14 May for 1994‐2008, so the onset date in the post-1994 epoch has advanced by 16 days. Chen (2015, hereinafter C15) has carefully checked the KW12 finding by using a different reanalysis dataset and extending the analysis period from 1979‐2008 to 1979‐2012. The result of C15 confirms that the KW12 finding is robust even during the extended period after 2008. We appreciate this contribution from C15. The fundamental science issue hereis why the SCSSM onset date advanced remarkably after 1993/94. The comment of C15 states in its abstract that Kajikawa and Wang (2012) ‘‘attributed this [onset] change to enhanced tropical cyclone (TC) activity and intraseasonal variability (ISV) related to 30‐80-day and 10‐25-day anomalies in the second epoch.’’ This misleading statement motivated C15 to assess the individual impact of TCs and ISV on the change of the SCSSM onset. We would like to start our reply by correcting C15’s misunderstanding of the work of KW12. The fact is that KW12 attributed the root cause of the interdecadal change of the SCSSM onset to decadal changes in the lower boundary anomalies. They pointed out in their abstract that ‘‘the advanced SCSSM onset is rooted in the decadal change of the SST over the equatorial western Pacific.’’ KW12 further indicated that ‘‘the advanced onset during the second epoch is affected by the enhanced activity of northwestward-moving tropical disturbances from the equatorial western Pacific’’(italicsaddedforemphasis).Heretheyhighlighted

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

An interdecadal shift in the onset date of the South China Sea summer monsoon (SCSSM) is identified during the late 1990s by using the European Centre for Medium-Range Weather Forecasts Interim Rea...

Characteristics of tropical sea surface temperature anomalies and their influences on the onset of South China Sea summer monsoon

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

남중국해 여름몬순과 ENSO와의 가능한 상관관계

This study analyzed a correlation between South China Sea summer (June to September) monsoon (SCSSM) and the ENSO for the last 32 years (1979 to 2010). There was a correlation that the higher (lower) the SST in the <TEX>$Ni{\tilde{n}}o-3.4$</TEX> region was, the weaker (stronger) the SCSSM intensity was. To identify the reason for this correlation, a difference of means between 8 El <TEX>$Ni{\tilde{n}}o$</TEX> years and 8 La <TEX>$Ni{\tilde{n}}a$</TEX> years (June to September). The analysis on the difference between two groups with respect to the 850 hPa stream flows showed that there were anomalous huge cyclones in the subtropical Pacific in the both hemispheres so that cold and dry anomalous northerlies were strengthened in the South China Sea relatively while anomalous westerlies were strengthened from the Maritime Continent to the off the coast of Chile. The analysis on the difference between two groups with respect to the 200 hPa stream flows showed that the opposite anomalous pressure system pattern to that in the 850 hPa stream flows were shown. In the subtropical Pacific of the both hemispheres, anomalous anticyclones existed so that anomalous easterlies were strengthened from the Maritime Continent to the equatorial central Pacific. Considering the anomalous atmospheric circulations in the upper and lower layers of the troposphere, upward airflows from the equatorial central and eastern Pacific were downward in the South China Sea and the Maritime Continent, which was a structure of anomalous atmospheric circulations. This means that the Walker Circulation was weakened and it was a typical structure of atmospheric circulations revealed in El <TEX>$Ni{\tilde{n}}o$</TEX> years.

Interannual variability of the onset of the South China Sea summer monsoon

ABSTRACTThis article investigates the year‐to‐year variability of the onset of the South China Sea summer monsoon (SCSSM) and the possible influences exerted by the surface temperature anomalies over land and sea. Early and late monsoon onsets are related to the temperature anomalies in different regions. It is found that an early onset follows negative sea surface temperature (SST) anomalies in the central tropical Pacific (CP) Ocean during the preceding winter and spring, corresponding to a CP La Niña. In contrast, a late onset is preceded by the negative surface air temperature anomalies over land in the central Asian continent. Negative SST anomalies in the central‐eastern equatorial Pacific Ocean and the associated warming in the western Pacific induce an anomalously enhanced Walker circulation. This anomalous Walker cell leads to an increase in convection, causing more latent heat release and a subsequent decrease of surface pressure. The anomalous Walker cell and the enhanced latent heat release weaken the Western North Pacific subtropical high and the Philippine Sea anticyclone, favouring a westerly flow from the Indian Ocean, resulting in an early SCSSM onset. On the other hand, negative land surface temperature anomalies cool the atmosphere over land, and locally modify the Hadley circulation, accompanied by the anomalous divergence in the low‐level atmosphere over the western equatorial Pacific. This divergence anomaly reduces the latent heat release and strengthens the anticyclone in the Philippine Sea, thus preventing the westward extension of the westerlies from the Indian Ocean and causing a late SCSSM onset.

Spatiotemporal analysis of precipitation trends during 1961–2010 in Hubei province, central China

Precipitation is an important climatic parameter, and its variability severely affects regional hydrological processes and water resource management. In order to explore the changing spatial and temporal characteristics of precipitation-related indices, including precipitation amounts indices, extreme precipitation indices, and precipitation concentration indices, in Hubei province, central China during 1961–2010, several precipitation-related indices series were analyzed using the Mann-Kendall test, Pettitt test, and inverse distance weighted interpolation method in this paper. The results are as follows: (1) A clear south–north gradient is apparent in the spatial distribution of the majority of precipitation-related indices, while the distribution of other indices (i.e., CDD, PCD, and PCP) is just the opposite. Most part of Hubei province can experience mixed positive and negative trends in precipitation-related indices. (2) The majority of precipitation-related indices exhibit an increasing trend, but most of which are not significant. July has the largest positive trend, with a regional average of 14.76 mm/decade. As for extreme precipitation indices, there exist large variations in Hubei, especially in R95pTOT. Besides, no obvious trends are shown in precipitation concentration indices. (3) With the exception of consecutive dry days (CDD), most of extreme precipitation indices are strongly correlated with annual mean precipitation (AMP) in Hubei. And the extreme precipitation indices are well correlated with each other except CDD and consecutive wet days (CWD). For six precipitation-related indices (i.e., SDII, RX1day, RX5day, R95pTOT R99pTOT, and PCD), there are significant positive relationships with Northern Hemisphere Subtropical High (NHSH) but significant negative relationships with Northern Hemisphere Polar Vortex (NHPV). Furthermore, the majority of precipitation-related indices have significant negative correlations with East Asian summer monsoon index (EASMI) and South China Sea summer monsoon index (SCSSMI), and the correlation coefficient with EASMI is higher than that with SCSSMI.