Study On The Dynamic Process Of The Quasi-biweekly Oscillation Of The Qinghai-Tibet Plateau Summer Monsoon And Its Impact On Extreme Weather Events In Southwest Chin

The Tibetan Plateau(TP)summer monsoon(TPSM)is a unique monsoon system generated by the uplifted thermal forcing over the large-scale terrain.The TPSM shows a significant 10–20-day quasi-biweekly oscillation.Using the high-resolution precipitation,temperature gridded data and reanalysis datasets from 1979–2018,we considered both the dynamic and thermodynamic features of TPSM and identified two distinct quasi-biweekly modes based on multivariate empirical orthogonal function analysis.The three-dimensional structures,propagation and origin mechanisms of the two 10–20-day TPSM modes have been examined and clarified in this paper.The modulating effects and physical processes responsible for the extreme rainfall and heatwaves in Southwest China by the two distinct quasi-biweekly modes are discussed in details.The first mode of 10–20‐day TPSM variability is related to the tropics–mid‐latitude coupled system.The low‐level circulation and diabatic heating perturbations over TP reveal a quasi‐stationary oscillatory feature that is closely linked with the southward-propagating anomalous vorticity wave train in the upper troposphere.In contrast,the second mode is generated by a southeastward-propagating wave train crossing over Eurasia to form a west-east dipole distribution over the TP.The diagnostic results of a vorticity budget analysis and Rossby wave dynamics suggest that the first mode originates from the Mediterranean and propagates eastward along the westerly jet.As the upper-level vorticity anomaly approaches the northern TP,the vorticity advection induced by zonal background winds shows a leading phase to the southern sector of the TP,inducing southward movement.Once the southward-moving upper‐level vorticity arrives over the Indian peninsula,it is coupled with tropical convection disturbances in the low level,whereupon they together propagate northwestward and dissipate in the Arabian Sea.The wave train associated with the second mode originates from the North Atlantic.Its southeastward propagation is also related to the advection of vorticity perturbations caused by background westerly mean flows.Before reaching the western TP,the vorticity anomalies of the wave train are well-organized and quasi-barotropic in structure.The blocking effect of the TP topography seems to disturb the wave structure.The movements of lower‐level vorticity and diabatic heating slow down and form a baroclinic structure over the TP.The two distinct quasi-biweekly modes of TPSM both exert evident modulations on the extreme rainfall and heatwaves in Southwest China,while the physical mechanisms causing these modulations are various.The probability of extreme rainfall events increases by 30%relative to the climate states when upper‐level anticyclonic disturbances of the two types of quasi-biweekly oscillation pass through the TP‐Southwest China.The moisture diagnosis results show that the strong biweekly westerly anomaly of the first mode,observed in the north of the anomalous upper‐level anticyclone,transports moisture from the Bay of Bengal and South China Sea toward Southwest China,favoring the extreme rainfall events.In contrast,for the effect of second mode,the associated water vapour convergence plays a major role.The modulations of heatwave occurrence probability by the two quasi-beweekly modes are smaller than those for extreme rainfall events.As the upper‐level cyclonic anomaly propagates over the TP‐Southwest China,the probability of heatwave occurrences increases by over 20%.The key processes controlling the heatwaves in Southwest China of these two quasi‐biweekly modes are similar.The descending anomalies enhanced adiabatic heating around the areas.Meanwhile,the diabatic heating also contributes to the heatwave occurrences,mainly due to the increased upward surface net thermal radiation,sensible and latent heat flux as the surface net shortwave radiation is increased in a clear‐sky condition.