THE DYNAMICS OF EURO-ATLANTIC BLOCKING ONSETS

Citation
Pa. Michelangeli et R. Vautard, THE DYNAMICS OF EURO-ATLANTIC BLOCKING ONSETS, Quarterly Journal of the Royal Meteorological Society, 124(548), 1998, pp. 1045-1070
Citations number
37
Categorie Soggetti
Metereology & Atmospheric Sciences
ISSN journal
00359009
Volume
124
Issue
548
Year of publication
1998
Part
B
Pages
1045 - 1070
Database
ISI
SICI code
0035-9009(1998)124:548<1045:TDOEBO>2.0.ZU;2-J
Abstract
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.