Cold Air Outbreaks in Midlatitude Asia With and Without Precursory Pulse in the Stratospheric Poleward Warm Air Transport

Using ERA5 data from 1979 to 2024, this study classifies 173 wintertime Cold Air Outbreak (CAO) events in midlatitude Asia, based on their temporal phasing relative to pulse-like intensifications of warm air mass transport into the polar stratosphere above 400 K (PULSEs). Two PULSE-related types are identified: PULSE_lead (18.0%), where the PULSE precedes the CAO peak, and PULSE_lag (21.4%), where it follows. PULSE_lead events exhibit more persistent and widespread cold anomalies across Eurasia. The phasing is found to be governed by the planetary-wave driven coupling between the poleward stratospheric warm branch and equatorward tropospheric cold branch of the isentropic meridional mass circulation at 60°N, respectively dominated by warm air transport over the Northwestern Pacific and cold air transport over Asia. PULSE_lead events are preceded by rapid propagation of wavenumber-2 energy into the stratosphere, simultaneously intensify both branches. In contrast, PULSE‐lag events are triggered by a stronger Ural ridge and downstream energy dispersion, with delayed wavenumber‐1‐dominated upward wave flux strengthening the stratospheric warm branch only after the CAO. While PULSE_lag events are mainly caused by tropospheric processes, a downward impact from the stratosphere is found for PULSE_lead. The precursory PULSE induces a stratospheric mass deficit over the East Asian trough region, resulting in barotropic low anomalies, which helping maintain the trough and prolong the CAO. Furthermore, PULSE_lead events have detectable stratospheric polar vortex anomalies 2 weeks in advance. This study clarifies that though most Asian CAOs have a lagged stratospheric response, a significant subset is preceded by active stratospheric forcing.