Atmospheric blocking is a key component of extratropical weather variability1 and can contribute to various types of extreme weather events2,3,4,5. Changes in blocking frequencies due to Arctic amplification and sea ice loss may enhance extreme events6,7, but the mechanisms potentially involved in such changes are under discussion8,9,10,11. Current theories for blocking are essentially based on dry dynamics and do not directly take moist processes into account12,13,14,15,16,17. Here we analyse a 21-year climatology of blocking from reanalysis data with a Lagrangian approach, to quantify the release of latent heat in clouds along the trajectories that enter the blocking systems. We show that 30 to 45% of the air masses involved in Northern Hemisphere blocking are heated by more than 2 K—with a median heating of more than 7 K—in the three days before their arrival in the blocking system. This number increases to 60 to 70% when considering a seven-day period. Our analysis reveals that, in addition to quasi-horizontal advection of air with low potential vorticity12,13,14,15, ascent from lower levels associated with latent heating in clouds is of first-order importance for the formation and maintenance of blocking. We suggest that this process should be accounted for when investigating future changes in atmospheric blocking.
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We thank MeteoSwiss for providing access to ECMWF analysis data. Discussions with H. Davies have been very helpful in designing this study. C.M.G. acknowledges support from the Swiss National Science Foundation (Grant PZ00P2_148177/1).
The authors declare no competing financial interests.
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Pfahl, S., Schwierz, C., Croci-Maspoli, M. et al. Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geosci 8, 610–614 (2015). https://doi.org/10.1038/ngeo2487
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