Hybrid Transition of Subseasonal Atmospheric Regimes and Its Impact on the Extreme ‘West Cold-East Warm’ Mode in North America

On February 22, 2023, North America experienced an unusual weather phenomenon characterised by extreme cold in the west and extreme warmth in the east, reflecting a surface air temperature (SAT) seesaw pattern that significantly impacted human activities in the region. Utilising reanalysis data, we analysed this case as well as similar events in long-term climate records. The results indicate that this extreme temperature mode is caused by a hybrid evolution of atypical North Pacific Oscillation (NPO) and Pacific-North American (PNA) phase relationships, transitioning from a dominant positive NPO phase to a strong negative PNA phase, while maintaining and developing Pacific blocking, which we refer to as a hybrid transition of atmospheric regimes. Further analysis revealed that the anomalously weak meridional potential vorticity gradient (PVy) in southern Alaska prior to the event provided favourable conditions for the northward movement and intensification of the anticyclone in the central North Pacific, serving as a key driving factor for the transition of circulation mode from positive NPO to negative PNA. Additionally, significant anomalous outgoing longwave radiation (OLR) in the tropical Indian Ocean prior to the event indicated enhanced convective activity in that region, which may have triggered meridional Rossby waves, thereby influencing the transition between NPO and PNA in mid to high latitudes. Thus, the anomalous PVy in southern Alaska and the OLR from the tropical Indian Ocean may serve as potential precursor indicators for such extreme weather events.