Interannual Variation of Global Surface Potential Vorticity Forcing in Boreal January and Its Link with Long-Persisting Droughts in Southwest China

Abstract

Abstract An increasing number of studies have recognized the essential influence of surface potential vorticity (PV) forcing on atmospheric circulation. In this study, we investigated the temporal characteristics of global surface PV forcing in January and its associated climate anomalies. The global surface PV forcing exhibited a pronounced decreasing trend, implying reduced forcing on atmospheric PV. Its interannual component was accompanied by cold winters on the Eurasian and North American continents and long-persisting droughts in southwest China (SWC), consistent with the coexistence of these extreme events. Based on the global surface PV forcing index, the mechanism underlying long-persisting droughts, which lasted from October to January, was investigated. The formation mechanisms of persistent drought varied monthly. Specifically, the occurrence of drought in October was closely related to Rossby wave activity over Eurasia, which enhanced the anomalous anticyclone over the Tibetan Plateau and subsequently induced air descent over SWC. In contrast, drought in November and December could be ascribed to La Niña events in the central Pacific, which facilitated subsidence over SWC through local meridional circulation anomalies. Distinct from other months, the combined effects of La Niña events and circulation anomalies over northern Eurasia caused the drought in January. The former reduced precipitation over southern SWC, whereas the latter influenced precipitation over central SWC. The present study provides novel insights into simultaneous extreme events in the Northern Hemisphere. Significance Statement Long-persisting droughts in southwest China (SWC) usually coexist with cold winters in the Eurasian and North American continents. Here, we suggested that the coexistence of these extremes was closely linked to interannual variability in global surface potential vorticity forcing. Based on this relationship, the mechanism underlying long-persisting droughts in SWC, lasting from October to January, was investigated. The formation of these droughts could be ascribed to Rossby wave activity over Eurasia in October and to La Niña events in the central Pacific in November and December. The drought in January was ascribed to the combined effects of La Niña and circulation anomalies over northern Eurasia. This study provides a broader view for understanding extremes that occurred simultaneously in the Northern Hemisphere.