Review
Multi-scale climate variability of the South China Sea monsoon: A review

https://doi.org/10.1016/j.dynatmoce.2008.09.004Get rights and content

Abstract

This review recapitulates climate variations of the South China Sea (SCS) monsoon and our current understanding of the important physical processes responsible for the SCS summer monsoon's intraseasonal to interannual variations. We demonstrate that the 850 hPa meridional shear vorticity index (SCSMI) can conveniently measure and monitor SCS monsoon variations on a timescale ranging from intraseasonal to interdecadal. Analyses with this multi-scale index reveal that the two principal modes of intraseasonal variation, the quasi-biweekly and 30–60-day modes, have different source regions and lifecycles, and both may be potentially predicted at a lead time longer than one-half of their corresponding lifecycles. The leading mode of interannual variation is seasonally dependent: the seasonal precipitation anomaly suddenly reverses the sign from summer to fall, and the reversed anomaly then persists through the next summer. Since the late 1970s, the relationship between the SCS summer monsoon and El Niño-Southern Oscillation (ENSO) has significantly strengthened. Before the late 1970s, the SCS summer monsoon was primarily influenced by ENSO development, while after the late 1970s, it has been affected mainly in the decaying phase of ENSO. The year of 1993 marked a sudden interdecadal change in precipitation and circulation in the SCS and its surrounding region. Over the past 60 years, the SCS summer monsoon's strength shows no significant trend, but the SCS winter monsoon displays a significant strengthening tendency (mainly in its easterly component and its total wind speed). A number of outstanding issues are raised for future studies.

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

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