Influence of the boreal summer intraseasonal oscillation on subseasonal wet-dry rapid transition over the middle and low reaches of the Yangtze River Valley

Abstract

This study identified two distinct categories of subseasonal precipitation events (SPEs) over the middle and low reaches of the Yangtze River Valley (MLYRV) during boreal summer based on the wet-dry rapid transition (WDRT) phenomenon, namely: subseasonal WDRT precipitation events (SRTPEs) and non-WDRT precipitation event (SNPEs). Although after cessations of these two events, area-averaged negative intraseasonal precipitation anomalies over the MLYRV both reach their peaks at pentad 2, the amplitude is greater for SRTPEs, in contrast to SNPEs. In conjunction with the stronger negative precipitation anomaly, the intensity of the anomalous descending motion at 500 hPa and the moisture flux divergence anomaly at 700 hPa averaged over the MLYRV are larger for SRTPEs. Furthermore, observational and model results reveal that the boreal summer intraseasonal oscillation (BSISO) activities over the western North Pacific (WNP) are more vigorous for SRTPEs and the BSISO variations serve as a pivotal origin for discrepancies in these two dynamic factors at pentad 2 through the linear dry dynamic process. In order to elucidate underlying mechanisms of the area-averaged BSISO convection change, moisture budget and multi-scale interaction diagnoses are conducted and results underscore the crucial role of the difference in regionally averaged vertical gradient of low-frequency background moisture over the WNP in driving such BSISO convection variation. Additionally, some individual cases of SRTPEs and SNPEs are also examined. These discoveries provide us with a novel perspective to comprehend diversities of SPEs over the MLYRV and may contribute potentially to the subseasonal forecasting for this region.