Enhanced persistence of Ural blocking under strong positive AO: the role of North Atlantic storm tracks and potential vorticity dynamics

Ural blocking (UB) and the associated Warm Arctic–Cold Eurasia (WACE) pattern are typically linked to the negative Arctic Oscillation (AO). However, robust UB events are surprisingly observed even during the positive AO phase, a condition generally expected to suppress blocking due to enhanced zonal flow. This study investigates how positive AO magnitude modulates UB persistence. We find that under strong positive AO conditions (AO > +1), UB events persist significantly longer (6.1 days) than weak positive AO (4.7 days). This enhanced persistence results from organized North Atlantic storm tracks that facilitate intense heat and moisture into the Barents–Kara Sea. The resulting Arctic warming and sea ice loss trigger a thermodynamic feedback loop that weakens the meridional potential vorticity (PV) gradient, effectively anchoring the UB system. Our findings reveal that strong positive AO paradoxically promotes persistent blocking through storm-PV coupling, offering critical insights for improving sub-seasonal predictions of Eurasian winter extremes.