Abstract
The Year-to-year variability of the western Pacific subtropical high (WPSH) is primarily controlled by atmosphere–ocean interaction (AOI) between the WPSH and the Indo-Pacific warm pool dipole SST anomalies (AOI mode) and the anomalous SST forcing from the equatorial central Pacific (the CP forcing mode). In this study, we show that the impacts of the WPSH variability on Asian summer monsoon rainfall have changed after the late 1990s. Before the late 1990s (the PRE epoch), the WPSH primarily affects East Asian summer monsoon (EASM) and had little influence on Indian summer monsoon (ISM), whereas after the late 1990s (the POST epoch), the WPSH has strengthened its linkage to the ISM while weakened its relationship with the EASM. This epochal change is associated with a change in the leading circulation mode in the Asia-WP region. During the PRE (POST) epoch the WPSH variation is mainly controlled by the AOI (CP forcing) that mainly affects EASM (ISM). The epochal change of the leading mode may be attributed to the change of the ENSO properties in late 1990s: the CP types of El Nino become a leading ENSO mode in the POST epoch. This work provides a new perspective for understanding decadal changes of the ENSO-monsoon relationship through subtropical dynamics.
Highlights
The important role of the western Pacific subtropical high (WPSH) in affecting Asian summer climate has been noticed for many years
The WPSH is closely related with Yangtze River rainfall index (YRRI) with a CC of 0.67 during 1979–1999; but after the late 1990s, the CC between WPSH index (WPSHI) and YRRI decreases to 0.40, which is apparently lower than the PRE epoch CC, and the statistical significance drops below the 90% confidence level
During the POST epoch, the relationship between WPSHI and EAJS index (EAJSI) weakens, no significant anticyclone circulation anomaly is observed over western north Pacific (WNP) (Fig. 8b), and the connection between the East Asian jet stream (EAJS) and the precipitation over the WNP-EA weakens, no remarkable precipitation anomalies are observed over the middle and lower reach of the Yangtze River Valley and WNP (Fig. 8d)
Summary
The important role of the western Pacific subtropical high (WPSH) in affecting Asian summer climate has been noticed for many years. Based upon previous work and their own numerical experiments, Wang et al (2013) (hereafter W13) have identified two different major SSTA mechanisms governing the two-leading empirical orthogonal function (EOF) modes of the interannual variability of low-level circulation in the Asian-western Pacific domain. The equatorial central Pacific SSTA has become a major forcing to the WPSH after the late 1990s, causing the change of the leading mode These findings have important implications for Asian summer monsoon prediction. It should be noted that the influence of the autocorrelation on the freedom degree has not been considered, because of the short record in this study
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