Investigating the December 2023 warm-to-cold weather whiplash in China using a novel flash-cold metric

Abstract In mid-December 2023, China experienced an unprecedented cold wave that marked a critical stage in a negative temperature-related weather whiplash event (WWE), which was characterized by a rapid shift from extreme warmth to extreme cold. The traditional metric—temperature variance—is insufficient to reflect the temporal evolution of this WWE. Therefore, we introduce a novel metric termed ‘flash-cold’ to identify the sharp and significant decrease in surface air temperature. This new metric not only effectively captures recent WWEs trend in line with variance-based metric but also allows daily tracking of temperature shifts. Using the flash-cold metric, we delineate the evolution of the WWE case in China into four phases: first warm spell, first flash-cold, second warm spell, and second flash-cold (corresponding to the unprecedented cold wave). A recently proposed Lagrangian temperature budget analysis highlights the crucial role of high-latitude blocking circulation in both flash-cold phases through cold advection. The first flash-cold phase was triggered by backward cold air masses driven by upstream systems associated with Okhotsk blocking, while the second flash-cold phase developed when the trough downstream of Ural blocking moved eastward and rotated counterclockwise. In contrast, subtropical systems moderated the decreasing temperature during the first flash-cold through diabatic processes but had minimal impact on the second flash-cold, helping to explain why the second flash-cold was more intense. From a multiscale interaction perspective, the development of these blocking circulations was facilitated by a background of exceptionally weak meridional gradient of potential vorticity, the smallest recorded since 1979.