Ku et al. (2026) Enhanced persistence of Ural blocking under strong positive AO: the role of North Atlantic storm tracks and potential vorticity dynamics

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2026
• Date: 2026-03-24
• Authors: Ho-Young Ku, Hayeon Noh, Muyin Wang, James Overland, Seong-Joong Kim, Baek-Min Kim
• DOI: 10.1038/s41612-026-01384-x

Research Groups

• Division of Earth Environmental System Sciences Major of Environmental Atmospheric Sciences, Pukyong National University, Busan, South Korea
• Climate and Environmental Research Institute, Korea Institute of Science and Technology, Seoul, Republic of Korea
• Cooperative Institute for Climate, Ocean, and Ecosystem Studies, University of Washington, Seattle, WA, USA
• NOAA/Pacific Marine Environmental Laboratory, Seattle, WA, United States of America
• Office of Vice President, Korea Polar Research Institute, Incheon, South Korea
• KOPRI School, University of Science and Technology, Incheon, South Korea

Short Summary

This study investigates how the magnitude of a positive Arctic Oscillation (AO) influences the persistence of Ural Blocking (UB) events. It finds that strong positive AO paradoxically enhances UB longevity by organizing North Atlantic storm tracks, leading to Arctic warming, sea ice loss, and a weakened meridional potential vorticity gradient that anchors the blocking system.

Objective

• To investigate how the magnitude of the positive Arctic Oscillation (AO) phase modulates the persistence of Ural Blocking (UB) events.
• To elucidate the threshold-dependent mechanisms, specifically the role of North Atlantic storm tracks and potential vorticity (PV) dynamics, that govern UB longevity under varying positive AO amplitudes.

Study Configuration

• Spatial Scale: Northern Hemisphere, with a focus on the Ural region (30°–80°E, 30°–90°N), Barents-Kara Sea (BKS; 75°–85°N, 30°–80°E), and Central Eurasia (CEU; 40°–60°N, 60°–100°E).
• Temporal Scale: Winter (December–February) seasons from 1982 to 2023 (specifically, December 1981 to February 2023). Analysis of atmospheric and surface variables from Lag -7 to +14 days relative to UB onset.

Methodology and Data

• Models used: No climate models were used. The study employed the Regional Hybrid (RHYB) blocking detection method for Ural Blocking and a storm tracking method adapted from previous studies for North Atlantic storms.
• Data sources:
  • Japanese 55-year Reanalysis (JRA-55) daily data (2-meter temperature, sea level pressure, 500-hPa geopotential height, 850-hPa temperature, 850-hPa relative vorticity, zonal and meridional wind components, downward longwave radiation, latent heat flux, sensible heat flux, potential vorticity) at 1.25° × 1.25° resolution.
  • NOAA Optimum Interpolation (OI) Sea Surface Temperature (SST) V2 High-Resolution Dataset for daily Sea Ice Concentration (SIC), remapped to 1.0° × 1.0° resolution.
  • Climate Prediction Center (CPC) daily Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) indices.

Main Results

• Under strong positive AO (SPAO, AO > +1 standard deviation), Ural Blocking (UB) events persist significantly longer (mean 6.1 days) compared to weak positive AO (WPAO, mean 4.7 days), representing an approximately 30% increase in duration.
• Strong positive AO conditions lead to highly organized North Atlantic storm tracks, with approximately 71.6% of storms reaching the Arctic, compared to 55% under weak positive AO. This organization facilitates intense poleward heat and moisture transport into the Barents-Kara Sea (BKS).
• This enhanced transport under SPAO triggers rapid BKS warming (peaking at +8 °C) and significant sea ice loss, accompanied by a substantial increase in downward longwave radiation (DLR) of approximately +40 W/m².
• The storm-driven thermodynamic feedback and associated circulation changes under SPAO significantly weaken the meridional potential vorticity (PV) gradient (PVy) around 60°–70°N in the mid-to-upper troposphere (320–300 K), effectively anchoring the UB system.
• A statistically significant negative correlation exists between PVy anomalies at UB onset and the total duration of individual UB events under SPAO, indicating that a stronger PVy reduction leads to longer-lasting blocking.
• Central Eurasia (CEU) experiences more abrupt and dynamic cold anomalies under SPAO, reaching a minimum of −2.5 °C, reinforcing the eastward extension of the Warm Arctic–Cold Eurasia (WACE) pattern.

Contributions

• Challenges the conventional understanding that a positive Arctic Oscillation (AO) primarily suppresses high-latitude blocking, demonstrating a nonlinear, threshold-dependent mechanism where an "excessively strong" positive AO paradoxically promotes blocking persistence.
• Identifies a novel "Storm-PV coupling" mechanism, clarifying that strong internal atmospheric variability (SPAO-driven storms) acts as a trigger, while boundary forcing (sea ice loss and radiative feedback) serves as an amplifier for the Warm Arctic–Cold Eurasia (WACE) pattern.
• Provides critical insights for improving sub-seasonal to seasonal (S2S) forecasting by suggesting that monitoring the magnitude of the AO index, alongside North Atlantic storm track coherence, could serve as an early warning signal for prolonged Eurasian cold spells.
• Suggests that anticipated future climate changes, such as a poleward shift and intensification of the eddy-driven jet, might alter the nature of blocking, favoring more thermodynamically driven, persistent events initiated by intense storm intrusions.

Funding

• Republic of Korea-NOAA Joint Project Agreement (JPA) Polar Panel, Task Number P91 "Comparative understanding of Arctic-Midlatitude Weather Connections occurring over United States and Korea in recent decades".
• NOAA Global Ocean Monitoring and Observing (GOMO) office through the Cooperative Institute for Climate, Ocean, & Ecosystem Studies (CICOES) under NOAA Cooperative Agreement NA20OAR4320271, Contribution No 2025-1449, and Pacific Marine Environmental Laboratory (PMEL) Contribution No. 5768 (for M. Wang).

Citation

@article{Ku2026Enhanced,
  author = {Ku, Ho-Young and Noh, Hayeon and Wang, Muyin and Overland, James and Kim, Seong-Joong and Kim, Baek-Min},
  title = {Enhanced persistence of Ural blocking under strong positive AO: the role of North Atlantic storm tracks and potential vorticity dynamics},
  journal = {npj Climate and Atmospheric Science},
  year = {2026},
  doi = {10.1038/s41612-026-01384-x},
  url = {https://doi.org/10.1038/s41612-026-01384-x}
}