Origins of Intraseasonal Precipitation Variability over North China in the Rainy Season

Abstract

We investigated the characteristics and mechanisms of subseasonal precipitation variability in North China during the rainy season (June–September). Two dominant intraseasonal modes with periods of 8–20 and 30–60 days were identified via spectral analysis. Together, they explain 62.8% of the total precipitation variability. Nearly all persistent heavy rainfall events in North China were observed concurrently with the enhanced positive phases of biweekly or/and 30–60-day precipitation modes. To elucidate the origins of these two intraseasonal precipitation variabilities, we performed moisture and vertical motion analyses. The moisture diagnosis results show that the anomalous lower-level southerly perturbations, which transport the background summer-mean moisture from the tropical areas toward North China, are the key process causing abundant moisture for the anomalous precipitation occurrence. The local ascending motion anomalies associated with the occurrence of intraseasonal precipitation come mainly from the anomalous vorticity advection induced by summer-mean thermal wind. Although the key processes causing the precipitation anomalies of these two intraseasonal modes are similar, the geneses and evolutions of large-scale conditions associated with them are distinct. The biweekly circulation and convective anomalies are driven by the midlatitude wave train pattern propagating southeastward across the Eurasian continent, while the 30–60-day anomalies are determined by both upper-level perturbations along 40°N and northward-propagating intraseasonal convective activities from the tropics.