Vacillation cycles and blocking in a channel

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.