ReviewMulti-scale climate variability of the South China Sea monsoon: A review
Introduction
The South China Sea (SCS) is a marginal sea located in Southeast Asia roughly between the equator and 22°N and from 110°E to 120°E (Fig. 1). Geographically, the SCS resides at the center of the Asian-Australian monsoon (30°S–40°N, 40°E–170°E) and joins four monsoon subsystems: the subtropical East Asian (EA) monsoon, the tropical Indian monsoon, the western North Pacific (WNP) monsoon, and the Australian monsoon. Fig. 1 presents the differential precipitation pattern between June/July/August (JJA) and December/January/February (DJF); this underlines the differential latent heating between the Northern Hemisphere (NH) and Southern Hemisphere (SH), which drives the annual cycle of the Asian-Australian monsoon.
While the SCS summer monsoon (SCSSM) has been regarded as a part of the EA summer monsoon (EASM; e.g., Zhu et al., 1986, Tao and Chen, 1987, Ding, 1992), it is a typical tropical monsoon and is more closely linked to the tropical WNP monsoon (Murakami and Matsumoto, 1994, Wang, 1994). Because of its special geographic location and unique monsoon characteristics, which will be discussed shortly, the SCS monsoon has been one of the foci of monsoon research, especially after the SCS Monsoon Experiment (SCSMEX) in 1998 (Lau, 1995, Lau et al., 2000, Ding et al., 2004).
Of great scientific importance is the prominent climate variability of the SCS monsoon on intraseasonal to geological timescales. On the intraseasonal timescale, the SCS exhibits the largest intraseasonal (10–100-day) variability in the Asia-Pacific region during boreal summer (Kemball-Cook and Wang, 2001). The westward-propagating quasi-biweekly (QBW) mode originating from the SCS and the Philippines have significant influences on Indochina, the Bay of Bengal, and India (Chen and Chen, 1993). The northward-propagating 30–50-day mode from the SCS seems to be linked to the occurrence of extreme rainfall events in subtropical East Asia (Zhu et al., 2003). During northern summer, convective bursts over the northern SCS and the Philippines extend their influences all the way to North America through the establishment of a circum-Pacific Rossby wave train (Kawamura et al., 1996, Fukutomi and Yasunari, 2002). On the annual timescale, the onset of the SCS summer monsoon (SCSSM) signifies the onset of the large-scale summer monsoon over EA and the WNP (Tao and Chen, 1987). The SCS also acts as a water vapor pathway connecting the Indian and EA-WNP monsoon during boreal summer and connecting the most powerful EA winter monsoon with the Australian summer monsoon (Fig. 1). During boreal winter, the SCS encounters the strongest tropical–extratropical interaction, hemispheric interaction, and multi-scale interaction. The year-to-year variability of SCSSM precipitation acts as an anomalous heat source, further influencing EA, India, and Australia (e.g., Tao and Chen, 1987, Ding, 1992, Lau and Yang, 1997, Wang et al., 2004). On the orbital and geological timescales, sediment recorded in SCS monsoon upwelling regions provides valuable information about the variability of the EASM (Wang, 1999).
Understanding of the SCS monsoon's climate variability is a great challenge because the sources of variability are complicated due to influences from the four adjacent monsoon subsystems. The equatorial Madden and Julian, 1971, Madden and Julian, 1972, Madden and Julian, 1994 Oscillation (MJO) has a significant influence on the SCS. Cold surges and baroclinic waves from the north or west and tropical storms and disturbances from the east also propagate into the SCS and cause synoptic and intraseasonal fluctuations. As such, the summer monsoon onset and winter monsoon multi-scale interaction have attracted extensive attention in previous studies (Chang et al., 2006). The SCS is also a region of tropical cyclogenesis, hosting most typhoons or tropical storms that pass through the Philippines and make landfall in southern China and Vietnam. The tropical cyclone (TC) activity is significantly modulated by intraseasonal to interdecadal climate variations.
The principal goal of this review is to provide a concise synopsis of the distinct multi-scale climate variability of the SCS monsoon and to discuss the physical processes that give rise to this variability. We first review unique features of the seasonal march in Section 2. For dynamic consistency, we propose a unified multi-scale circulation index to describe the climate variability on timescales ranging from intraseasonal to interdecadal (Section 3). An account is then given to intraseasonal variations (Section 4), interannual variations (Section 5), interdecadal variability (Section 6), and the long-term trend over the past 60 years with instrumental data (Section 7). The last section discusses challenges in understanding numerical modeling and climate prediction of the SCS monsoon.
Section snippets
Seasonal march
One of the unique and spectacular features of the SCS monsoon is its abrupt climatological onset occurring in mid-May around Julian Pentad 28 (Fig. 2). The abrupt burst of monsoon rains takes place across a large latitudinal range from 5°N to 22°N with a complete reversal of lower tropospheric zonal wind (from easterly to westerly) between the equator and 18°N. Although the transition from the Asian winter to summer monsoon is in general discontinuous (Meehl, 1987, Yasunari, 1991, Matsumoto and
A multi-timescale South China Sea monsoon index
One of the major roadblocks in the current study of SCS climate variability is the lack of a generally recognized measure of summer monsoon intensity, especially on the interannual and interdecadal timescales. In this section, we explore the possibility of defining a simple, objective circulation index that can apply to a variety of timescales. Ideally, precipitation is the best measure because it depicts heat source-driving monsoon circulation and is the most important variable, practically
Intraseasonal variations
During northern summer from May to October, intraseasonal variation (ISV) over the SCS is concentrated on two frequency bands: 12–25 days and 30–60 days (e.g., Chen and Chen, 1995, Fukutomi and Yasunari, 1999, Annamalai and Slingo, 2001, Chan et al., 2002). A spectral analysis of the daily SCS meridional shear vorticity index confirms that the vorticity variability indeed has two major peaks: one on the QBW (12–25 days) timescale and the other on the 30–50-day timescale (figure not shown). The
Interannual variations
The SCS summer monsoon exhibits large year-to-year variations, which can be clearly seen from the time series of the SCSMI (Fig. 8a). The dominant spectrum peak seems different between the two reanalysis datasets; that is, a dominant 4–5-year period appears in NCEP–NCAR data (Fig. 8d) and two significant peaks are shown (3–4-year and 5–6-year periods) by ERA-40 data (Fig. 8c). This is mainly due to the different periods examined and the nonstationary nature of the SCS summer monsoon. The
A sudden change around 1993 in the past 30 years
The interdecadal variability appears to be seasonally dependent. We found that the second S-EOF, as shown in Fig. 10, registers a sudden change around 1993, representing the major mode of interdecadal variation in the SCS monsoon in the past 30 years. Before 1993, a cyclonic circulation anomaly and enhanced convection prevailed in the SCS during JJA and SON; the anomalies sudden reverse their signs from SON to DJF. An anticyclonic circulation anomaly and suppressed convection then dominate the
Strengthening trend of the SCS winter monsoon
Various indices have been used to quantify EA winter monsoon variability (e. g., Zhang et al., 1997, Jhun and Lee, 2004). These indices mainly reflect the activity of mid-latitude cold surges. The SCS is the southernmost part of the EA monsoon system, and as such, its variability is not only affected by the mid-latitude cold surge but also by changes in tropical convections. Climatologically, the maximum northeasterly wind speed in winter is located in the central SCS (Lu and Chan, 1999). Thus,
Challenging issues
It has been increasingly recognized that the SCS monsoon variability has large-scale implications for adjacent regions, including the WNP, EA, and the Maritime Continent. An improved seasonal prediction of the SCSSM may add predictability to prediction of the EA subtropical monsoon. Study of the SCS monsoon has received greater-than-ever attention since the SCS Monsoon Experiment in 1998. Remarkable progress has been made in the last decade in studying climate variations of the SCS monsoon. The
Acknowledgements
This research is supported by NSF Climate Dynamics Program (Grant ATM-0647995) and by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), NASA, and NOAA through their sponsorship of the IPRC. Fei Huang and Zhiwei Wu acknowledge the support of the National Natural Science Foundation of China (Grant Nos. 40775042 and 40605022) and the National Basic Research Program “973” (Grant No. 2006CB403600). Jing Yang acknowledges the funding from the CAS International Partnership Project
References (109)
- et al.
Coral delta O-18 records as an indicator of winter monsoon intensity in the South China Sea
Quart. Res.
(2003) - et al.
The global monsoon: major modes of annual variations in the tropics
Dyn. Atmos. Ocean
(2008) - et al.
Interdecadal change of the structure of ENSO mode and its impact on the ENSO frequency
J. Climate
(2000) - et al.
Active/break cycles: diagnosis of the intraseasonal variability of the Asian summer monsoon
Climate Dyn.
(2001) - et al.
Mechanisms responsible for the maintenance of the 1998 South China Sea summer monsoon
J. Meteorol. Soc. Jpn.
(2002) - et al.
A statistical study of winter monsoon cold surges over the South China Sea and the large-scale equatorial divergence
J. Meteorol. Soc. Jpn.
(1992) - et al.
Annual cycle of Southeast Asia—Maritime Continent rainfall and the asymmetric monsoon transition
J. Climate
(2005) - et al.
Synoptic disturbances over the equatorial South China Sea and western Maritime Continent during boreal winter
Mon. Weather Rev.
(2005) - et al.
The Asian Winter monsoon
- et al.
Structure, genesis and scale selection of the tropical quasi-biweekly mode
Quart. J. R. Meteorol. Soc.
(2004)
The 10–20-day mode of the 1979 Indian monsoon: its relation with the time variation of monsoon rainfall
Mon. Weather Rev.
An observational study of the South China Sea monsoon during the 1979 summer—onset and life-cycle
Mon. Weather Rev.
The 30–50 day variation of convective activity over the Western Pacific-Ocean with Emphasis on the Northwestern Region
Mon. Weather Rev.
Global land precipitation: a 50-yr monthly analysis based on Gauge observations
J. Hydrometeor.
Interannual variation in Indochina summer monsoon rainfall: possible mechanism
J. Climate
Interannual and interdecadal variations of summer monsoon activities over South China Sea
Climatic Environ. Res.
Summer monsoon rainfall in China
J. Meteorol. Soc. Jpn.
Overview of the South China Sea monsoon experiment (SCSMEX)
Adv. Atmos. Sci.
Mechanism of the northward propagating intraseasonal oscillation in the south Asian monsoon region: insights from a zonally averaged model
J. Climate
A note on the South China Sea shallow interocean circulation
Adv. Atmos. Sci.
Relations between interannual and intraseasonal monsoon variability diagnosed from AMIP integrations
Quart. J. R. Meteorol. Soc.
Differences of boreal summer intraseasonal oscillations simulated in an atmosphere–ocean coupled model and an atmosphere-only model
J. Climate
Coupling between northward-propagating. intraseasonal oscillations and sea surface temperature in the Indian Ocean
J. Atmos. Sci.
10–25-day intraseasonal variations of convection and circulation over East Asia and western North Pacific during early summer
J. Meteorol. Soc. Jpn.
Tropical–extratropical interaction associated with the 10–25-day oscillation over the western Pacific during the Northern summer
J. Meteorol. Soc. Jpn.
A mechanism of scale selection in tropical circulation at observed intraseasonal frequencies
J. Atmos. Sci.
Some simple solutions for heat-induced tropical circulation
Quart. J. R. Meteorol. Soc.
Secular changes of annual and interannual variability in the tropics during the past century
J. Climate
The Definition of Onset Date of South China Sea Summer Monsoon and the Monsoon Indices
Northwestward propagation of the intraseasonal oscillation in the western North Pacific during the boreal summer: structure and mechanism
J. Climate
Contrasting characteristics between the northward and eastward propagation of the intraseasonal oscillation during the boreal summer
J. Climate
Spatial-temporal characters of the monsoon-ocean coupled mode over the South China Sea and its relation with summer precipitation of China
Periodical Ocean Univ. China
Factors contributing to the onset of the Australian summer monsoon
Quart. J. R. Meteorol. Soc.
A new East Asian winter monsoon index and associated characteristics of the winter monsoon
J. Climate
Structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation
J. Climate
Interannual variability of the 10–25- and 30–60-day variation over the South China Sea during boreal summer
Geophys. Res. Lett.
NCEP–DOE AMIP-II reanalysis (R-2)
Bull. Am. Meteorol. Soc.
Principal modes of climatological seasonal and intraseasonal variations of the Asian summer monsoon
Mon. Weather Rev.
Tropical and mid-latitude 45-day perturbations over the Western Pacific during the northern summer
J. Meteorol. Soc. Jpn.
Equatorial waves and air–sea interaction in the boreal summer intraseasonal oscillation
J. Climate
Rainfall variability over South-east Asia–connections with Indian monsoon and enso extremes: new perspectives
Int. J. Climatol.
Oscillations of a monsoon system. Part I. Observational aspects
J. Atmos. Sci.
The 10 to 20 day westward propagating modes and ‘breaks in the monsoons’
Tellus
Decadal change in relationship between East Asian and WNP summer monsoons
Geophys. Res. Lett.
Decadal change in East Asian summer monsoon circulation in the mid-1990s
Geophys. Res. Lett.
A report of field operations and early results of the South China Sea monsoon experiment (SCSMEX)
Bull. Am. Meteorol. Soc.
Aspects of the 40–50 day oscillation during the northern summer as inferred from outgoing longwave radiation
Mon. Weather Rev.
Tropical intraseasonal oscillation and its prediction by the NMC operation model
J. Climate
Cited by (306)
Size-dependent zoogeographical distribution of gelatinous thaliaceans associated with current velocity and temperature
2024, Science of the Total EnvironmentInfluences of tides on shelf circulation in the northeastern South China Sea during summer
2024, Journal of Marine SystemsSurface floating objects moving from the Pearl River Estuary to Hainan Island: An observational and model study
2024, Journal of Marine SystemsCold-water coral diversity along the continental shelf margin of northwestern South China Sea
2023, Marine Environmental Research