北半球夏季30~60天季节内振荡年代际变化及其对中国东部降水和气温的影响

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

本文利用1979~2020年基于中国台站观测的高分辨格点化逐日降水和气温资料以及NCEPⅡ大气再分析资料,探讨了亚洲季风区夏季30~60 d大气季节内振荡(BSISO1)的年代际变化及其对中国东部降水和气温的影响。在1997~2008年(P1阶段),BSISO1年代际偏强,而在2009~2018年(P2阶段),BSISO1年代际偏弱。在P1阶段,BSISO1仅对江淮流域的降水和气温具有显著的调控作用,对华南地区的影响较弱;而在P2阶段,BSISO1对江淮流域的降水和气温的调控作用减弱,但对华南地区的降水和温度具有显著的调控作用。在P1阶段,BSISO1有关的异常抑制(活跃)对流可从赤道西太平洋北传至南海—西北太平洋,激发出一个连接南海和江淮流域的经向垂直次级环流圈,引起江淮流域强烈的异常上升(下沉)运动和低层水汽辐合(辐散),造成局地降水的持续性偏多(偏少),气温的持续性偏低(偏高)。相比P1阶段,在P2阶段江淮流域的季节平均水汽显著减少,BSISO1有关的水汽垂直输送减弱,削弱了江淮流域季节内降水变化。但在P2阶段,BSISO1相关的异常抑制(活跃)对流可进一步北传到达相对偏北的华南地区,其伴随的异常下沉(上升)运动造成局地降水减少(增加);同时绝热增温(冷却),使得局地出现持续性气温偏高(偏低)。因此,在P2阶段,更容易出现江淮流域持续性强降水和华南地区持续高温热浪同时发生的复合型极端事件。

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 NECPⅡatmospheric 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.