K. Haines et Aj. Holland, VACILLATION CYCLES AND BLOCKING IN A CHANNEL, Quarterly Journal of the Royal Meteorological Society, 124(547), 1998, pp. 873-895
The response to a low-level high-frequency wavemaker forcing in a two-
layer, beta-plane, quasi-geostrophic channel model is examined. The wa
vemaker simulates regular baroclinic instability which then propagates
to upper atmospheric levels to excite blocking. By altering the merid
ional 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 w
ill always resonantly excite the split flow, but a mixed instability p
rocess is responsible for the breakdown in cases which oscillate, and
this is demonstrated using a simplified zonal stability analysis and e
nergy-tendency diagnostics. The re-excitement by the eddies continues
the cycle. This model provides a theory of how the meridional structur
e of the upper-level winds may determine whether a large-amplitude blo
ck can be excited or persist in the presence of similar high-frequency
eddy activity propagating up from below.