Impact of Potential Vorticity Anomalies around the Tibetan Plateau on Quasi-biweekly Oscillations of Summer Rainfall over Hunan-Hubei-Jiangxi Provinces south of Yangtze Basin in 2016

This study uses the ERA-Interim reanalysis data to examine the characteristics and mechanisms for intraseasonal oscillations of persistent heavy rainfall events over Hunan-Hubei-Jiangxi (HHJ) provinces south of Yangtze Basin during the 2016 summer (May-August) from the perspective of atmospheric potential vorticity (PV) anomalies. Wavelet analysis suggests that the summer rainfall over HHJ in 2016 exhibited a distinct 10-20-day quasi-biweekly oscillation (QBWO). Composite and time-lag correlation analyses in terms of low-level isentropic PV demonstrate that the wet (dry) phases of the 10-20-day HHJ rainfall in the 2016 summer depended on the alternations of the local positive (negative) PV anomalies. Such local PV anomalies over HHJ are not only affected by the tropical QBWO modifying the intensity and location of western North Pacific subtropical high, but also modulated strongly by the upstream PV anomalies around the Tibetan Plateau (TP). Eastward-propagating PV anomalies associated with anomalous southerlies (northerlies) east of TP tended to strengthen (weaken) the lower tropospheric southwesterly jet stream and moisture transportation toward HHJ. Except the poleward propagating energy from the tropics to the TP, in the middle and upper troposphere, an intraseasonal wave train existed in the mid-high latitudes, which originated from the Atlantic and then propagated southeastward through Eurasian continent to East Asia. This wave train triggered a divergent (convergent) saddle-shaped flow pattern over East Asia, facilitating ascending (descending) motion over HHJ through the dynamical forcing of PV advection. The quantitative diagnosis for PV density (PVD) budgets indicates that the QBWO of the PVD over HHJ depended largely on the contributions of the PVD advection and the diabatic heating terms. The PV anomalies around the eastern TP due to condensation latent heating also affect downstream PV anomalies over HHJ through subsequent advection.