The dynamics of Euro‐Atlantic blocking onsets

Abstract

Abstract The mechanisms leading to Euro‐Atlantic blocking are investigated using 31 winters of 500 hPa geopotential height from NMC and ECMWF daily analyses, and a 30 000‐day perpetual winter run from a simple 3‐level quasi‐geostropic model. A composite analysis of the atmospheric flow during days preceding blocking events is carried out. It indicates that Euro‐Atlantic blocking events have two simultaneous main precursors: a high‐latitude retrograding wave‐number 1 pattern at planetary scale, and an enhanced baroclinic wavetrain travelling across the North Atlantic at synoptic scale. These phenomena are identified both in observed and in simulated data. The nonlinear interactions between small scale, large scale and the mean flow, during the establishment of the blocking anomaly, are examined by a budget analysis of the quasi‐geostrophic potential‐vorticity equation. These diagnostics demonstrate that wave‐wave interactions contribute to about 50% of the local amplification of the blocking anomaly while wave/mean‐flow interactions contribute to propagation of the large‐ and small‐scale anomalies and their phase locking. Finally, the question is addressed as to whether the two precursors are necessary and sufficient conditions for blocking onsets by performing various sets of initial‐value experiments performed with the quasi‐geostrophic model. These experiments show that the two precursors occur simultaneously but only the synoptic‐scale baroclinic wavetrain is necessary to trigger Euro‐Atlantic blocking.