Monitoring early-flood season intraseasonal oscillations and persistent heavy rainfall in South China

Abstract

Rainfall variability during the early-flood season (April–June) in South China is largely controlled by both the 10–20 and 20–70-day intraseasonal oscillations (ISO). In this study, a method is described to monitor the ISO and persistent heavy rainfall in South China. Three existing daily real-time 20–70-day ISO indices are compared. It is found that the regional East Asia–western North Pacific (EAWNP) ISO index best represents the early-flood season 20–70-day ISO in South China. A new bivariate boreal summer ISO index is designed to describe the 10–20-day ISO in the EAWNP region. Composite analysis shows that the rainfall anomaly in South China is well captured by the northward propagation of both the 10–20 and 20–70-day ISO. With different phase combinations of the 10–20 and 20–70-day EAWNP ISO, nine conditions are defined ranging from those favorable to those unfavorable to heavy rainfall in South China that can be used to effectively monitor the early-flood season ISO and persistent heavy rainfall in South China.