A strong stratospheric harbinger for cold extremes: Weak polar vortex transition from displacement to split pattern

Weak stratospheric polar vortex (WSPV) events are dynamically connected with the variations in the tropospheric circulation, serving as crucial harbingers for surface cold extremes in the Northern Hemisphere. Although WSPV events are usually featured with either displaced or split stratospheric polar vortex pattern, a notable portion of WSPV events experiences both patterns successively, leading to inconclusive surface impacts of different WSPV events. Here, we propose a novel method to quantitatively identify WSPV events with vortex transition (namely, mixed-type WSPV events) by performing clustering analysis on WSPV days based on 42-yr ERA5 reanalysis, and further examine their climatological features, surface impacts and tropospheric precursors. Results show that the mixed-type WSPV events are usually featured with a routine vortex evolution from displacement to split. In contrast to comparatively weak tropospheric response to pure displaced- and split-type events, the mixed-type WSPV events feature the longer persistence of stratospheric circulation anomalies and are followed by stronger negative Arctic Oscillation-like surface signatures, further contributing to more robust cold anomalies over northern Eurasia and the central U.S. 10–39 days after event onset. Moreover, mixed-type events are typically induced by upward propagated wave activity flux into the stratosphere contributed by the synergistic enhancement of tropospheric planetary wavenumbers 1 and 2. The enhancement of tropospheric planetary wavenumbers 1 and 2 is associated with deepening of the Aleutian Low and strengthening of the dipole over northern Scandinavia-eastern Siberia, respectively. This tropospheric configuration can sevrve as a vital precursor pattern for mixed-type WSPV events, hinting at extreme cold events with far-reaching societal impacts.