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

Abstract

This 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.

An interdecadal extension of the Indo-Pacific warm pool and its strengthened influence on the South China Sea summer monsoon since the late 1980s

“…To verify the change in the connection between the IPWP and the SCSSM, figure 3 shows the normalized WPSI and SCSSM intensity index during May (figure 3(a)) and the sliding correlation coefficients between the two indices with a window of 31 years ( figure 3(b)). The strength of the SCSSM (figure 3(a)) has increased since the 1990s, which is consistent with the findings of previous studies (Kwon et al 2007, Wang et al 2009, Choi et al 2017. The 31 year sliding correlation coefficients between the SCSSM intensity index and the WPSI has experienced a remarkable shift since the late 1980s ( figure 3(b)).…”

“…Moreover, (Huangfu et al 2015) suggested that the interdecadal warming of the western Pacific warm pool results in an earlier outbreak of the SCSSM in the late 1990s by inducing stronger convection anomalies and weak WPSH in the early spring. Compared to the SCSSM onset date, relatively few studies have focused on the relation between the variability in the SCSSM intensity (Wu et al 2003, Wang et al 2009, Choi et al 2017 and the thermal state of the IPWP. (Choi et al 2017) found an enhancement in the SCSSM intensity during the mid-1990s and attributed it to strengthening of the Walker circulation and La Niña events.…”

“…Compared to the SCSSM onset date, relatively few studies have focused on the relation between the variability in the SCSSM intensity (Wu et al 2003, Wang et al 2009, Choi et al 2017 and the thermal state of the IPWP. (Choi et al 2017) found an enhancement in the SCSSM intensity during the mid-1990s and attributed it to strengthening of the Walker circulation and La Niña events. Whether the interdecadal change in the thermal state of the IPWP, especially its size variation, plays a role in strengthening the SCSSM intensity remains unclear.…”

“…(Chen et al 2011) indicated that when the 30-60 d oscillation is in an active state, southwesterlies prevail over the SCS, and cyclonic circulation appears in the northern SCS, which leads to a strengthened SCSSM trough and SCSSM. In recent years, the interdecadal changes in the SCSSM have attracted extensive attention (Wang et al 2009, Choi et al 2017, while few of these studies have focused on the contribution from the change in ISO activities. This issue is also investigated in the present study.…”

Seasonal, Interannual, and Interdecadal Variations of the East Asian Summer Monsoon: A Diurnal-Cycle Perspective

“…The Q4-related rainfall over SEC is small in 1960s-1980s and it becomes most pronounced during 1993-2002 with a maximum in 1998. Such an interdecadal trend coincide with the increasing occurrences of severe flooding over SEC since mid-1990s (e.g., Kajikawa and Wang 2012;Choi et al 2017). Overall, the rainfall amount related to Q4 occurrence (~20% summer days) contributes ~30% of the summer total rainfall at 27-31°N (Fig.…”