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

Abstract 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 an increase in ECDs over the southeastern TP but a 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 the Atlantic–European polar vortex, respectively, exert important impacts on the dipole mode. Correspondingly, a pair of anomalous cyclonic and anticyclonic circulations dominates over the southern and northwestern TP, respectively, and the East Asian jet stream shifts southward, leading to an increase in cold weather occurrence over the southern TP but a decrease over the north. These findings provide potential support for monitoring and prediction of the TP extreme low temperature or cold disasters under the warming trend. Significance Statement The cold extremes over the Tibetan Plateau (TP) occur significantly even though they display a declining trend in a warming climate. This study is aimed at revealing the main features of the year-to-year variability of winter extreme cold days (ECDs) over the TP, their key modulating factors, and associated physical mechanisms. A uniform increasing pattern and a northwest–southeast dipole of the ECDs dominate over the TP, respectively. Both El Niño and Northern Hemisphere polar vortexes can modulate these two modes effectively through different atmospheric teleconnections. Understanding the variations of TP cold extremes and corresponded modulating factors can provide a scientific support for meteorological disaster prevention and mitigation, improving disaster weather monitoring and early warning in the TP region.