10–30-Day Subseasonal Features Associated with Multiple and Isolated Persistent Rainfall Events over South China

Abstract

Abstract In this study, persistent rainfall (PR) over South China (SC) is divided into two types. One type occurs multiple times in succession [defined as multiple PR (MPR)]; another type represents isolated PR (IPR), for which no new PR occurs for 10 days after the previous PR. The spatiotemporal structures of the 10–30-day intraseasonal oscillations (ISOs) associated with the two types of PR are compared and analyzed. The results reveal that the low-level moisture and air temperature perturbations always have a leading phase relative to the anomalous precipitation. In addition, the positive low-level moisture tendency appears in the MPR ending phase, whereas that in the IPR is close to zero. This difference results in convective development after the MPR ending phase, though not after the IPR. The moisture budget shows that the difference in moisture tendency between MPR and IPR is mainly due to meridional advection, including advections by the mean meridional flow across the perturbation moisture gradient and by the perturbation meridional flow across the mean moisture gradient. For the former, the difference is attributed to the perturbation moisture gradients, while the mean moisture gradients are responsible for the difference of the latter. Furthermore, an essential cause of the difference is the influence of higher-latitude disturbances that affect the IPR more significantly than the MPR. Two associated mechanisms are proposed. One is the perturbation stacking effect, and the other is the effect of angular momentum conservation. By contrast, the low-level temperature anomalies are not the key factor causing the difference between MPR and IPR. Significance Statement The persistent rainfall over South China sometimes occurs multiple times in succession, though most are in isolation. We want to understand the structures of the related 10–30-day ISOs and then find the possible cause of why convection develops after the ending stage of the multiple persistent rainfall events but not after the isolated rainfall events. We reveal that the effect of higher-latitude disturbances is an essential factor for the development of the two types of persistent rainfall over South China, with two proposed mechanisms of the perturbation stacking effect and angular momentum conservation. This is helpful for predicting persistent rainfall over South China. Future work should examine the findings by numerical experiments with a climate model.