Work

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

Year: 2026

Type: article

Abstract: Ural blocking (UB) and the associated Warm Arctic–Cold Eurasia (WACE) pattern are typically linked to the negative phase of the 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. Specifically, how variations in positive AO magnitude—from weak to strong regimes—differentially modulate UB persistence and Arctic-midlatitude linkages remain poorly understood. This study investigates how different positive AO amplitudes affect UB characteristics through analysis of atmospheric circulation, storm tracks, surface processes, and potential vorticity dynamics. We find that under strong positive AO conditions (AO > + 1), UB events persist significantly longer (6.1 vs 4.7 days) and occur at higher latitudes (60°–70°N) compared to weak positive AO conditions (0 < AO ≤ + 1). This enhanced persistence results from organized North Atlantic storm tracks that facilitate intense poleward heat transport, triggering rapid Arctic warming over the Barents–Kara Sea. The warming is sustained through enhanced downward longwave radiation, reduced surface heat fluxes, and accelerated sea ice loss, creating positive feedback that precondition the Arctic for prolonged blocking. Additionally, significant weakening of the meridional potential vorticity gradient in the mid-to-upper troposphere dynamically reinforces UB persistence by reducing Rossby wave restoring forces. These processes collectively produce a pronounced WACE pattern with concentrated Eurasian cooling. Conversely, weak positive AO conditions generate dispersed storm tracks, gradual Arctic warming, weaker radiative feedback, and minimal potential vorticity modifications, resulting in shorter-lived UB and a spatially diffuse WACE pattern. Our findings reveal that the magnitude of positive AO phase acts as a critical modulator of Arctic–midlatitude teleconnections, with implications for improving sub-seasonal prediction of Eurasian winter extremes by revealing threshold-dependent AO-storm interactions.

Source: npj Climate and Atmospheric Science

DOI: 10.1038/s41612-026-01384-x

Authors: Ho-Young Ku, Hayeon Noh, Muyin Wang, James Overland, Seong-Joong Kim, Baek-Min Kim