Interannual Variability of Winter Extreme Cold Days over Tibetan Plateau: Modulations by Northern-Hemispheric Polar Vortex and ENSO

The Tibetan Plateau (TP) has experienced remarkable warming and its winter extreme cold days (ECDs) have significantly decreased during the past decades. However, the interannual variability of TP ECDs is quite evident under the warming background. The current study investigates the two dominant modes of winter ECDs over the TP and their key modulating factors on the year-to-year time scale. The first mode features an overall uniform variation pattern in winter ECDs over the main body of the plateau and the second one presents a northwest-southeast dipole pattern with increase in ECDs over the southeastern TP but decrease in the northwestern TP. It is demonstrated that an evident stationary wave with southeastward-propagating wave activity fluxes related to the intensified Northern-Hemispheric polar vortex plays a dominant role in forming the uniform increase of winter ECDs through the intensification and eastward extension of the Middle East jet stream and induced low-pressure anomalies over the TP. Two other stationary wave trains, which are forced by the weakened Indo-Pacific heating due to El Niño-Southern Oscillation (ENSO) and the intensification of Atlantic-European polar vortex, respectively, exert important impacts on the dipole mode. Correspondingly, a pair of anomalous cyclonic and anticyclonic circulation dominate over the southern and northwestern TP, respectively, and the East Asian jet stream shifts southward, leading to increase of cold weather occurrence over the southern TP but decrease over the north. These findings provide potential supports for monitoring and prediction of the TP extreme low-temperature or cold disasters under the warming trend.