Excitation of European Transient Warming on the Caspian Sea Vortex

Central Asian low vortex (CAV) is the dominant synoptic circulation, resulting in frequent heavy snowfall over Central Asia during 1983-2018. The present study investigates winter CAVs to explore their possible causes. The results suggest that the CAVs produced near the Caspian Sea (named as Caspian Sea Vortices, CSVs) are the main component of January CAVs, which account for more than 80%, exhibiting both interannual and decadal variabilities and causing stronger heavy snowfall over Central Asia since 2004. The formation of CSV is usually due to the joint effect of both low-frequency waves and transient waves/vortices, in which the development of the latter is prior to the former, being the prophase signal of CSV enhancement and related heavy snowfall occurrences. The dominant transient waves/vortices related to CSVs not only originate from North Atlantic but also from European continent, which respectively correspond to the decadal and interannual variabilities of CSV frequency. In terms of European continent, the transient surface warming over eastern Europe enhances the atmospheric baroclinicity at lower troposphere, which on one hand excites vertical wave activity flux transporting to the upper troposphere and disturbs the transient vortex, on the other hand causes southeastward movement of North Atlantic jet. Consequently, wave energy guided by North Atlantic jet and excited over eastern Europe both disperses downstream and enhanced transient vortex evolves into the wavelike pattern, favoring the CSV formation on the next day. The transient surface warming over eastern Europe could be thereby a reference for the predictions of CAV heavy snowfall.