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HUANGFU Jing-Liang, HUANG Rong-Hui, CHEN Wen. 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 . Atmospheric and Oceanic Science Letters, 8(2): 95-99  
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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
HUANGFU Jing-Liang1,2, HUANG Rong-Hui1, CHEN Wen1
1 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
Corresponding author: HUANGFU Jing-Liang, hfjl@mail.iap.ac.cn

Citation: Huangfu, J.-L., R. H. Huang, and W. Chen, 2015: Reexamination of the interdecadal change in the onset date of the South China Sea summer monsoon during the late 1990s, Atmos. Oceanic Sci. Lett., 8, 95-99.
doi:10.3878/AOSL20150002.

Received:5 January 2015; revised:7 February 2015; accepted:20 February 2015; published:16 March 2015

Abstract

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 Reanalysis dataset. The mean onset date was brought forward by two pentads during 1999-2013 compared to that during 1979-1998. The large-scale atmospheric and oceanic change associated with this shift exhibits a significant interdecadal variation signal around 1998/1999, indicating that the shift during the late 1990s is robust. Different from the well-known mid-1990s shift, this shift carried more important systematical significance. Diagnostic analysis suggests that the earlier outbreak of the SCSSM was due to the interdecadal warming of the warm pool, which brought stronger convection anomalies and led to a weak western Pacific subtropical high (WPSH) during boreal spring (March-May). The earlier retreat of the WPSH was a direct cause of this shift.

Keyword: South China Sea summer monsoon; interdecadal change; convection; western Pacific subtropical high
1 Introduction

The South China Sea (SCS) summer monsoon (SCSSM) has been studied for the reason that it has significant influence on East Asian rainfall (Huang et al., 2003; Kim et al., 2006; Li et al., 2006; Chen et al., 2009). The outbreak of the SCSSM brings rainfall together with southwesterly winds over the SCS and can be treated as the start of the broad-scale East Asian summer monsoon. The early or late outbreak of the SCSSM may have an effect on the occurrence of drought/flood during boreal summer (Ding and Chan, 2005; Huang et al., 2006).

The variability of the SCSSM involves multiple time scales and its properties have been studied from the intraseasonal to the interdecadal scale (Chan and Zhou, 2005; Mao and Chan, 2005; Zhou and Chan, 2005; Wen et al., 2006; Zhou et al., 2006; Li and Pan, 2007; Ding et al., 2008; Wang et al., 2009; Wu, 2010). On the interdecadal timescale, it is well known that the SCSSM experienced a shift around the late 1970s, with a pronounced cooling in the warm pool (Huang et al., 2003; Ding and Chan, 2005; Li et al., 2006; Zhou et al., 2009). Recent studies (Wu et al., 2010; Kajikawa and Wang, 2011; Yuan and Chen, 2013) suggest that the SCSSM also underwent another shift around 1993. It is argued that the interdecadal warming of the tropical western Pacific is an original cause (Yuan and Chen, 2013). However, the recent interdecadal abrupt warming of the western Pacific was identified to have occurred around late 1998/1999 (McPhaden et al., 2011; Chung and Li, 2013). Therefore, it is necessary to reexamine the SCSSM onset date and the related environmental factors to seek a better understanding. This is the aim of the present study.

The data and methodology used in this study are described in section 2. The interdecadal change of the SCSSM is then investigated in section 3. A summary of the results and discussion are presented in section 4.

2 Data and methods

To obtain the SCSSM onset date series, European Centre for Medium Range Weather Forecasts Interim Reanalysis (ERA-Interim) daily 850 hPa zonal wind data (Simmons et al., 2007) are used. In addition, ERA-interim 850 hPa meridional wind and mean sea level pressure (SLP) data, National Oceanic and Atmospheric Administration daily outgoing longwave radiation (OLR) data (Liebmann, 1996), and the Met Office Hadley Centre's sea ice and sea surface temperature monthly mean data (Rayner et al., 2003) are used to investigate the process of the interdecadal shift that happened around the SCS. The resolution for the ERA-interim data and OLR data is 2.5° × 2.5° , and for the SST data it is 1° × 1° . All data are reorganized into pentad (one pentad equals five days) and monthly data, spanning from 1979 to 2013.

The onset of the SCSSM can be depicted as a combination of the eastern retreat of the western Pacific subtropical high (WPSH), the intrusion of low-level southwesterly winds into the SCS, and the abrupt enhanced rainfall. This process has been depicted by various definitions using different datasets, including precipitation, wind, geopotential height and radiation fields (Tao and Chen, 1987; Xie et al., 1998; Lu and Chan, 1999; Wang et al., 2004). According to Wang et al. (2004), it is simple yet effective to describe the outbreak of the SCSSM with the 850 hPa zonal wind around the central SCS (5-15° N, 110-120° E). The index used in this study follows their work and a more detailed description can be found in their paper.

The moving T test is employed as the abrupt change detection method, and we use the absolute values for unity. A nine-year lowpass filter is adopted to isolate the interdecadal variability signal. Common statistical techniques such as the mean bias and Student’ s t-test are also applied.

3 Results
3.1 Interdecadal change of the SCSSM during the late 1990s

Based on the definition of Wang et al. (2004), Fig. 1a shows the time series of the SCSSM onset date from 1979 to 2013. A noticeable feature is the early onset in the latter years. The mean onset pentad is about 29 during 1979-1993 and 27 during 1999-2013, with the years during 1994-1998 appearing to be a transition period. Figure 1b shows the moving T test result, which exhibits two significant interdecadal change years. One is the best-known 1993/1994 shift and another is apparent during the late 1990s. Supposing the latter to be robust, then what is the relationship between these two shifts? Previous studies have debated the possible causes for the interdecadal change of the SCSSM around 1993/1994. It is suggested that this shift was affected by enhanced tropical disturbances after 1994 from the equatorial western Pacific (Kajikawa and Wang, 2011). Yuan and Chen (2013) further examined the roles of tropical convective activity over different regions and found that the tropical western Pacific was the most influential area. Hence, it is necessary to reexamine the interdecadal shifts of environmental factors associated with the SCSSM, which might be the key to proving the existence of the latter shift and revealing the relationship between the two shifts.

The three most related environmental factors during spring are reexamined, including mean SLP, SST, and OLR. Based on correlation analysis between the SCSSM onset date and the factors (figures not shown), the key area for SLP is selected as (10-25° N, 110-140° E). For SST and OLR, the area is (0-15° N, 120-140° E). Figure 2a shows the standardized SCSSM onset date and its three influential factors’ interannual variations. Obviously, the SCSSM onset date is changing in phase with the SLP in the southwest of the WPSH, and OLR around the Philippine Sea, while being out of phase with the SST around the Philippine Sea. Figure 2b shows the interdecadal change years of SLP, SST, and OLR, with all three factors sharing a noticeable change point around 1998/1999. There is also a relatively weak interdecadal variation signal around 1993/1994. Therefore, we may conclude that the shift in the SCSSM onset date during the late 1990s is authentic, and more importantly, it carried more systematical significance reflecting the climate regime shift. The interdecadal warming of the northwestern Pacific warm pool may be the original cause for this turning. The shift around 1993/1994 is suggested to be a presage of the latter.

Figure 1 (a) The interannual variations of the SCSSM onset date from 1979 to 2013 (units: pentad). The dashed line denotes the mean onset pentad of 1979-2013. (b) Moving T test of the SCSSM onset date. The asterisks represent significance at the 90% confidence level.

Figure 2 (a) The time series of the normalized OLR around the Philippine Sea (green solid line), SLP in the southwest of the WPSH (red dashed line), and SST around the Philippine Sea (blue dashed line) during the study period (March-May), with the normalized SCSSM onset date shown by the histogram. (b) Moving T tests of the OLR, SLP, and SST. The asterisks represent significance at the 95% confidence level.

3.2 The interdecadal difference in environmental factors associated with the SCSSM

It is certainly worth investigating the interdecadal difference in the environmental factors in spring (March- May) to investigate the background for the shift during the late 1990s. As discussed in section 3.1, the interdecadal change is defined as the mean state difference (P2 minus P1) between 1979 and 1998 (P1) and 1999-2013 (P2). Interdecadal differences in SST, OLR, 850 hPa vector, and SLP are studied in this section. As depicted in Fig. 3a, the northwestern Pacific warm pool exhibits a significant interdecadal warming to the east of the SCS. Accordingly, there is a significant band representative of stronger convection during the period after the late 1990s, which locates along the WPSH’ s southern part (Fig. 3b). This convection change favors the northeast retreat of the WPSH. The weaker SLP in the latter period further proves it. Besides, the anomalous 850 hPa wind field exhibits a significant cyclonic flow to the west of the WPSH. Based on a previous study (Huang et al., 2006), all these environmental performances would lead to the earlier onset of the SCSSM.

We further analyzed the pentad data from pentad 22 to 27 averaged for each epoch and obtained their interdecadal difference. The evolution process characteristics of the SCSSM onset are shown in Fig. 4. It is found that the stronger convection in the latter epoch had already shown up around the Philippine Sea in pentad 22. This signal became enhanced and extended to the west after that time. During pentad 23, the cross-equatorial flows showed their commencement, with which the WPSH’ s southern part became much flatter. The central part of the WPSH turned to become weaker and there was anomalous cyclonic flow surrounding it in pentad 25. The salient phase came in pentad 26, when the WPSH shrank significantly and anomalous westerly winds intruded in the southern part of the SCS. Combined with the original wind field, it is found that the westerly winds had entered the SCS in pentad 26 and then occupied it in pentad 27.

Figure 3 Difference between decadal performances during the study period (March-May) (1999-2013 minus 1979-1998) of (a) SST (red solid line; units: ° C) and (b) OLR (shading; units: W m-2), SLP (contours; units: hPa), and 850 hPa wind field (vectors; units: m s-1). The blue box denotes the SCS region (5-15° N, 110-120° E) selected by Wang et al. (2004). All quantities and shading shown are tested at the 95% confidence level.

Figure 4 Difference between the decadal performances of successive pentads from 22 to 27 during the study period (March-May) (1999-2013 minus 1979-1998) of OLR (shading; units: W m-2), SLP (contours; units: hPa), and 850 hPa wind field (vectors; units: m s-1). The blue box denotes the SCS region (5-15° N, 110-120° E) selected by Wang et al. (2004). All quantities and shading shown are tested at the 95% confidence level.

In addition, compared with the interdecadal change of the mid-1990s (figure not shown), the late 1990s shows a more significant degree of variability. It is speculated that these two interdecadal shifts of the SCSSM were brought about by different background scales. The late-1990s interdecadal regime shift was of basin-wide scale, while the mid-1990s is suggested to be more local. The mid-1990s interdecadal shift presaged the latter, with the years during 1994-1998 appearing to be a transition period.

4 Summary

This work reexamines the interdecadal change in the SCSSM onset date and its climatological background. A new interdecadal shift during the late 1990s is identified and its relationship with the 1993/1994 shift is further investigated. Based on the analysis of interdecadal shifts of environmental factors associated with the SCSSM, the shift during the late 1990s is proven to be authentic and carries more systematical significance than the mid-1990s shift. The temporal and spatial characteristics of the related environmental factors are then investigated to reveal the background for the shift during the late 1990s. It is suggested that this earlier outbreak of the SCSSM was due to the interdecadal warming of the warm pool during spring, which brought stronger convection anomalies and a weaker WPSH. Different to the mid-1990s shift, it is speculated that the mid-1990s interdecadal shift presaged the later shift, with the years during 1994-1998 appearing to be a transition period.

This study represents an attempt to investigate the interdecadal change of the SCSSM during the late 1990s and its relationship with the mid-1990s shift. More associated factors still need to be studied. Furthermore, it would be meaningful to conduct an intensive study to examine the climate impact of this systematical shift.

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