Mechanisms of Meridional-Propagating High-Frequency Intraseasonal Oscillation Associated with a Persistent Rainfall over South China

Abstract

AbstractThe present study divides the intraseasonal variations into high-frequency intraseasonal oscillation (HF-ISO) with a 5?20-day period and low-frequency ISO (LF-ISO) with a 30?60-day period. Here, defined HF-ISO is heavily different from some previous work, because, over south China, persistent rainfall (PR) is not only related to the quasi-biweekly oscillation (10?20-day period), but also connected with the quasi-weekly oscillation (5?10-day period). Associated with a PR over south China during the first half of June 2016 (PR1606), the propagating components of the convections are largely attributed to the HF-ISOs. Moreover, both southward- and northward-propagating HF-ISOs are found in this case. Over southern China, the moisture advection dominated by anomalous flow and mean water vapor plays an important role in the southward propagation of the HF-ISO, and the cloud?radiation effect may be also, at least partially, responsible for the southward shift of the associated convective zone. Nevertheless, two other possible mechanisms are introduced to explain the cause of the northward propagation of the HF-ISO over southern China during the PR1606 period. The first is the vorticity advection, which is a dominant factor. The second mechanism is the wind?evaporation effect that plays a minor role. Over the South China Sea, the northward propagation of the HF-ISO is mainly attributed to the vertical wind shear effect and the vorticity advection effect, and the latter is relatively more important than the former in this case. The moisture advection is a supplementary effect caused by inducing a weak positive moisture tendency north of the convection center.