THE DYNAMICS OF EURO-ATLANTIC BLOCKING ONSETS

The mechanisms leading to Euro-Atlantic blocking are investigated usin g 31 winters of 500 MPa geopotential height from NMC and ECMWF daily a nalyses, and a 30 000-day perpetual winter run from a simple 3-level q uasi-geostropic model. A composite analysis of the atmospheric flow du ring 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 Atlan tic 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 block ing anomaly, are examined by a budget analysis of the quasi-geostrophi c potential-vorticity equation. These diagnostics demonstrate that wav e-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 pre cursors are necessary and sufficient conditions for blocking onsets by performing various sets of initial-value experiments performed with t he quasi-geostrophic model. These experiments show that the two precur sors occur simultaneously but only the synoptic-scale baroclinic wavet rain is necessary to trigger Euro-Atlantic blocking.