Potential Vorticity Source, Potential Vorticity Circulation, and Their Weather and Climate Significances

Based on a brief review of the research progress on surface potential vorticity (PV), this study introduces the calculation of PV and its generation on complex terrain and the research progress on the source of PV and PV circulation (PVC) in recent years, focusing on the particularity of the surface PV on the Tibetan Plateau (TP) and its important influence on weather and climate. For adiabatic and frictionless atmospheric motion, the structural recombination of the PV (i.e., PV reconstruction) can cause the development of vertical vorticity, which can cause the formation of a plateau vortex in summer and make the eastern part of the plateau an important source of surface vorticity in winter. Based on the derived equation for the vertical motion associated with isentropic displacement (\omega _\rmI\textD), which includes the impact of diabatic heating, this study shows that the eastward propagation of the positive vorticity generated on the TP along the westerly wind will cause the development of cyclonic vorticity in the downstream area, southerly wind, and upward motion in the lower troposphere, resulting in the increase in PV advection with altitude, which stimulates the development of extreme weather and climate events. Notably, the diurnal variations of surface heating and latent heat release at the cloud bottom over the TP significantly affect the diurnal variation of the PV near the surface, resulting in the development of the low vortex and precipitation system over the TP from late afternoon to night. Compared with the traditional surface sensible heating index, the surface PV index of the TP can better characterize the seasonal changes of local precipitation and is more closely related to the Asian summer monsoon precipitation.　The concept of PVC is also briefly introduced. Because changes in convergence of PVC across the close boundary of a region are directly related to changes in PV of the region, to maintain the relative stability of the total PV in the Northern Hemisphere, changes in PVC on the trans-equatorial plane and those in surface PVC must complement each other. Thus, changes in PVC on the trans-equatorial plane can be considered a window for monitoring near-surface climate change. The near-equatorial air–sea interaction can directly cause changes in the vertical shear of the zonal wind on the vertical plane along the equator, stimulating the trans-equatorial PVC anomaly, thereby affecting climate change near the surface of the Northern Hemisphere through the change of PVC in the atmosphere and the regulation of the TP. This study shows that PVC analysis opens up a new way for establishing the link between tropical and extratropical atmospheric circulation changes and has broad application prospects.