Decadal Variation of Boreal Summer 30–60-day Intraseasonal Oscillation and Its Influence on Precipitation and Temperature over Eastern China

This study investigates the decadal variations in the 30–60-day boreal summer intraseasonal oscillation (BSISO1) over the Asian summer monsoon region and its influence on precipitation and temperature over eastern China using the daily high-resolution gridded station-observed precipitation and temperature as well as NECP Ⅱ atmospheric reanalysis datasets for the 1979–2020 period. On the decadal timescale, BSISO1 was strong during the 1997–2008 period (P1 episode), while the BSISO1 amplitude was relatively weak during 2009–2018 (P2 episode). During the P1 episode, BSISO1 showed considerable influence on precipitation and temperature over the Yangtze–Huai River Basin (YHRB), but it could only weakly affect those over Southeast China. However, the influences of BSISO1 on precipitation and temperature over the YHRB were substantially reduced during the P2 episode. In contrast, its precipitation and temperature influences over Southeast China became significant during this episode. During the P1 episode, BSISO1-related suppressed (active) convection anomalies propagated coherently from the equatorial western Pacific to the South China Sea‒western North Pacific, inducing a vertical cell between the South China Sea and YHRB. This caused anomalous ascents (descents) and low-level moisture convergence (divergence) over the YHRB and was favorable for the intraseasonal positive (negative) rainfall anomalies and reduction (improvement) in local temperature. Compared with P1, the seasonal mean background humidity over the YHRB reduced substantially during the P2 episode, leading to a decreased vertical transport of the BSISO1-related circulation to the seasonal mean moisture, causing smaller intraseasonal rainfall fluctuations over the YHRB. Notably, the BSISO1-related suppressed (active) convection anomalies could shift further northward to Southeast China during the P2 episode. The accompanied descents (ascents) also caused negative (positive) rainfall anomalies locally. Moreover, adiabatic heating (cooling) associated with the anomalous descents (ascents) resulted in positive (negative) temperature anomalies over Southeast China. In conclusion, compound extreme events such as persistent heavy rainfall over the YHRB and persistent heat waves over Southeast China are more likely to occur during the P2 episode.