| Summary: | Under atmospheric blocking conditions, the normal passage of storms is interrupted by a region of high-pressure which remains lodged at the end of the storm tracks for periods of a week or more, causing the jet to split and the storms forced to pass around to the north and south, causing anomalous weather conditions over this high-pressure region. Being able to predict when these events occur, how long they will persist, and their eventual decay would be of value to improve weather prediction. This work looks at a simplified idealisation of this situation, with a view to improve understanding of any precursors to such events occurring and their subsequent behaviour. A 2-layer, <I>β</I>-plane, quasi-geostrophic channel model is used to examine the interactions between an upper-layer jet and high-frequency eddies supplied from a wavemaker in the lower layer. For certain initial jets, a dipole similar to an atmospheric block is formed, which remains stable to large-amplitude. By adding a shear to the upper-layer jet, a low-frequency vacillation cycle is induced, whereby the high-frequencies excite a split in the jet, which breaks down due to instability. This instability is demonstrated using a local instability analysis technique, and is also reflected in energy diagnostics. The role of the high-frequency eddies through the various phases of the cycle is also examined. A spherical-geometry model is also used with an aim to help bridge the gap between this highly-simplified model and the real atmosphere. These results suggest that the meridional shear in the upper-level atmospheric jetstream may determine whether blocking would develop, persist or breakdown. The structure of the upper-level jet could be controlled by seasonal variations or large-scale teleconnection patterns.
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