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Vacillation cycles and blocking in a channel
Author(s) -
Haines K.,
Holland A. J.
Publication year - 1998
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712454711
Subject(s) - baroclinity , eddy , instability , amplitude , zonal and meridional , mechanics , physics , zonal flow (plasma) , forcing (mathematics) , jet (fluid) , channel (broadcasting) , blocking (statistics) , geostrophic wind , geology , turbulence , atmospheric sciences , climatology , mathematics , optics , tokamak , statistics , plasma , electrical engineering , quantum mechanics , engineering
Abstract The response to a low‐level high‐frequency wavemaker forcing in a two‐layer, β‐plane, quasi‐geostrophic channel model is examined. The wavemaker simulates regular baroclinic instability which then propagates to upper atmospheric levels to excite blocking. By altering the meridional shear in the upper layer, the large‐scale response can vary from a steady large‐amplitude split jet, very similar to observed blocks, to a weaker split with a low‐frequency vacillation cycle. The eddies will always resonantly excite the split flow, but a mixed instability process is responsible for the breakdown in cases which oscillate, and this is demonstrated using a simplified zonal stability analysis and energy‐tendency diagnostics. The re‐excitement by the eddies continues the cycle. This model provides a theory of how the meridional structure of the upper‐level winds may determine whether a large‐amplitude block can be excited or persist in the presence of similar high‐frequency eddy activity propagating up from below.