Asymmetric Boreal Summer Intraseasonal Oscillation Events over the Western North Pacific and Their Impacts on East Asian Precipitation

The boreal summer intraseasonal oscillation (BSISO) is the most prominent tropical subseasonal signature. Due to the restriction of methodology used to extract BSISO, most of the previous studies ignored its asymmetry. This study reexamines the BSISO events over the western North Pacific (WNP) for 1985-2010 with a hierarchical cluster analy-sis. Two categories of BSISO events are classified, the long-period (20-60 day) and short-period (10-20 day) events. The long-period BSISO events manifest as a northward-propagating mode with a significant phase asymmetry characterized by a fast development, but a slow decay of the intraseasonal convection. The phase asymmetry is found to be determined by the BSISO-induced amplitude-asymmetric sea surface temperature (SST) anomalies, in which the suppressed convection -induced positive SST anomalies are stronger than the active convection-induced negative ones. Such amplitude-asymmetric SST anomalies result from the nonlinear relationship between convection and surface downward shortwave radiation flux anomalies caused by the cloud transmission effect. The stronger positive SST anomalies-induced turbulence flux anomalies act as a negative feedback onto the atmosphere, making the transition from the convection suppressed phase to the active phase earlier and faster. The fast-developing convection tends to cause a fast northeastward retreat of the preceding en-hanced western North Pacific subtropical high. Accordingly, the middle and lower reaches of Yangtze River valley experi-ence a rapid reversal from the increased precipitation to the decreased. The asymmetric BSISO events over WNP and their impacts revealed in this study would provide a new potential for subseasonal-to-seasonal forecast of the East Asian summer monsoon precipitation.