Many studies suggested the formation mechanisms of the WNPSH including convection over the warm pool (Lu and Dong, 2001), remote El Niño forcing (Wang and Zhang, 2002), Indian ocean-western Pacific warming (Zhou et al., 2009) To date, however, there is no consensus on the key factor pertaining to WNPSH variability. Moreover, most studies focused on the summer mean (JJA) state of the WNPSH. Observational evidences, on the other hand, show that summer (JJA) WNPSH variability can be divided into two periods. Lagged correlation between the pentad 850 hPa geopotential height anomalies averaged on 110°-150°E and the DJF Nino3 index shows that the period from June 5 to July 5 is not affected by El niño, whereas the period from July 15 to August 15 shows a significant correlation with the DJF Nino3 index.
In this study, two WNPSH indices are defined as anomalous 850 hPa geopotential height averaged over the area 110°-150°E 10°-30° from 1979 to 2007; one averaged in June 5-July 5 and the other averaged in July 15-August 15. Correlation between the two is 0.21, which implies that the WNPSH variability for the two periods should be investigated separately. The DJF Nino3 index is correlated at 0.28 with the first WNPSH index while it is correlated at 0.76 with the second WNPSH index.
This study aims to find the physical mechanisms of the WNPSH variability for the two periods separately. Since WNPSH in July 15-August 15 is largely affected by DJF El Niño, mechanism of the delayed summertime circulation will be addressed. WNPSH's relationship with the East Asian summer monsoon is also investigated for the two periods using composite analysis. In the first period, Baiu-like frontal zone along East Asia is developed whether the WNPSH is developed or not; the precipitation intensity is larger when WNPSH is developed. In the second period, on the other hand, Baiu-like frontal zone appears only when the WNPSH is developed, while there is no obvious Baiu-like frontal zone when the WNPSH is weakened. This means that El-Niño affects the western north Pacific low-level circulations and the East Asian rainfall anomaly from mid July to mid August.