Quasi-biweekly Oscillation of the 2016 Summer Rainfall over the Yangtze Basin in Relation to Intraseasonal Potential Vorticity around the Eastern Tibetan Plateau

During the summer of 2016, several heavy rainfall events over the middle and lower reaches of the Yangtze Basin (MLY) were regulated by the 10–20-day quasi-biweekly oscillation (QBWO). The characteristics and mechanisms of the QBWO associated with the MLY rainfall events were examined using ERA-Interim reanalysis data. In addition to the biweekly oscillation in the western North Pacific subtropical high in the lower troposphere, The QBWO of the MLY rainfall was closely linked with upstream intraseasonal potential vorticity (PV) anomalies generated over the eastern slope of the Tibetan Plateau (TP) due to topographic lateral friction. The PV budget analysis demonstrates that the horizontal PV advection and subsequent topographic friction with a four-day phase-lag between them dictated the QBWO of PV anomalies around the eastern TP. The TP-generated PV anomalies then migrate downstream to facilitate the development of the anomalous circulation over the MLY. Importantly,  the strongest of the six heavy rainfall events occurred in the end of June 2016, and it was attributable to a TP Vortex (TPV) in conjunction with a Southwest China Vortex (SWCV). The eastward-moving TPV merged vertically with the SWCV over the eastern Sichuan Basin due to the positive vertical gradient of the TPV-related PV advection, leading the lower-tropospheric jet associated with moisture transport to intensify greatly and converge over the downstream MLY. The merged TPV–SWCV specially facilitated the upper-tropospheric isentropic-gliding ascending motion over the MLY. With the TPV-embedded mid-tropospheric trough migrating continuously eastward, the almost stagnant SWCV was re-separated from the overlying TPV, forming a more eastward-tilted high-PV configuration to trigger stronger ascents including isentropic-gliding, isentropic-displacement, and diabatic heating-related ascending components over the MLY, thus resulting in the most intense rainfall.