Troposphere–Stratosphere–Mesosphere Coupling and Ozone Response to Arctic Stratospheric Variability during the 2024/25 Winter Season

The Arctic stratosphere during the first half of the 2024/25 winter was characterized by an exceptionally strong polar vortex, comparable to the coldest winter of 2019/20 with record ozone depletion. In mid-February 2025, enhanced propagation of planetary wave activity over northeastern Eurasia led to a deceleration of the wind in the upper stratosphere, followed by downward wave reflection into the troposphere over Canada, northern USA, and northwestern Eurasia. This stratosphere–troposphere interaction resulted in significant surface cooling in these regions and drove the Arctic Oscillation (AO) index to a winter minimum of −5. Such strong AO anomalies, exceeding −2σ, have been observed in February over the past 25 years only in 2010, 2021, and 2025. A major sudden stratospheric warming (SSW) event in early March was preceded by a prolonged preconditioning stage of the polar vortex, characterized by intensified stratospheric vacillations between zonal winds and planetary waves. During the SSW, enhanced wave activity propagation into the stratosphere was identified over northeastern Eurasia and Europe. The amplified upward wave flux over Europe was linked to the eastward redistribution of wave activity fluxes in the upper troposphere over the North Atlantic, originating from the wave reflection region over North America. Using lidar sounding and spectral measurements of excited hydroxyl molecules OH* temperature, the stratosphere and upper mesosphere temperature variations in February–March 2025 were analyzed. Simulation with the CCM (chemistry–climate model) SOCOLv3 estimated the total chemical ozone loss in the Arctic stratosphere in winter 2024/25 as ~50% of the record maximum in 2019/20.