Decadal changes in two types of the western North Pacific subtropical high in boreal summer associated with Asian summer monsoon/El Nino-Southern Oscillation connections

Abstract

The decadal variability of boreal summertime western North Pacific subtropical high (WNPSH) associated with Asian summer monsoon/El Nino-Southern Oscillation is investigated through the two types of WNPSH that are identified from the empirical orthogonal function of 850 hPa horizontal wind anomalies. These two types of WNPSH are associated with two different connections between the summer mean rainfall variability over the East Asian-western North Pacific (EA-WNP) region and the sea surface temperature (SST) variability over the equatorial Pacific. The contribution of each type to these connections has been changed after the mid-1990s. Both types of WNPSH are predominantly associated with the local subsidence compensating the enhanced convection over equatorial regions and exhibit two significant periodicities, 2.5 and 4-5 years. However, the first type accompanies a fast decay of El Nino-like warming centered in the eastern Pacific from preceding winter to concurrent summer, while the second type features the persistent SST warming shifted to the central Pacific. The relative dominance of type 1 after the mid-1990s may be one of possible causes for the increase of August rainfall over northeast Asia. Moreover, temporal variations of two types of WNPSH are characterized by the 1 year lagged relationship, which has been significantly weakened after the mid-1990s, which may partly explain the decadal change in the relationship between EA and WNP summer monsoons. This result suggests the close linkage between WNPSH and EA-WNP summer monsoon on the long-term variability.