Impact of Tropopause Folds on Regional Extreme Cold Events in Yunnan of China

The impact of tropopause folds (TFs) on the regional extreme cold events (RECEs) in Yunnan Province is examined. This province is situated in China's Low-Latitude Highlands (CLLH). In the 60-year period 1961–2020, 140 RECEs in Yunnan are identified from the air temperature measurements at 125 meteorological observation sites. Among these RECEs, 115 events are accompanied by TFs in the CLLH and referred to as TF-type events and the remaining 25 are not accompanied by TFs and termed as NTF-type events. In comparison with the NTF-type events, TF-type RECEs are generally more severe and last longer with wider areas impacted over southern China. On average, TF-type (NTF-type) events last about 2.5 (1.3) days, have a daily intensity of about 1.7 (1.2)°C, a maximum daily intensity of about 4.2 (3.2)°C and affect 54 (37) sites. For both types of RECEs, the tropopause generally exhibits a lower altitude over northeastern China, showing an intensified East Asian trough and an anomalous cyclonic circulation in this region. During the TF-type events, however, cold air is transported downward from the stratosphere to the troposphere over southwestern China. The anomalous cyclonic circulation over East Asia extends farther to the southwest, accompanied by an anomalous anticyclonic circulation over India. Thus, compared to NTF-type events, the northerly wind anomalies over northeast Yunnan tend to be stronger in TF-type events, enhancing the cold air advection in the region. Our results show that the RECEs in Yunnan are significantly affected by TFs.