Intraseasonal variability of western North Pacific subtropical high based on the El Nio influence and its relationship with East Asian summer monsoon

The boreal summer season could be divided into two periods in terms of the variability of western North Pacific subtropical high (WNPSH) based on the El Nio influence. The correlation analysis indicates that the WNPSH in the period of pentad 32-37 (June 5 to July 4, first period) is not affected by El Nio, while that in the period of pentad 40-45 (July 15 to August 13, second period) is strongly affected by sea surface temperature in the equatorial eastern Pacific in the previous winter. The different response of low-level circulation over the western North Pacific (WNP) to the El Nio forcing between two periods seems to be due to the difference of mean climatological fields over the WNP and the East Asian regions. The WNPSH in the first period is closely connected to the variability of North Pacific subtropical High. In the second period, on the other hand, the WNPSH variability is dominantly controlled by the convective activity over the WNPSH region and it is related with the El Nio forcing. The composite analysis on the relationship between the WNPSH and the East Asian summer monsoon exhibits distinct contrasts between two periods. In the first period, the East Asian stationary front exists all the time regardless of the strength of the WNPSH. On the other hand, in the second period the East Asian stationary front appears only when the WNPSH is strong, while there is no obvious East Asian frontal zone when it is weak.