Abstract
AbstractFlow on a beta‐plane driven by a steady localised anticyclonic forcing of potential vorticity (or equivalently a mass source) is considered as a simple model of the Asian monsoon flow in the upper troposphere. Previous authors have noted that the response may be steady, or unsteady, according to the magnitude of the forcing, with the unsteadiness manifested as westward eddy shedding. A detailed study of the transition between steady and eddy‐shedding regimes reveals a third regime ('break up'), for intermediate forcing magnitude, where the flow is steady in the neighbourhood of the forcing, but the westward extending plume of low potential vorticity breaks up into isolated anticyclonic vortices some distance away from the forcing region. A related spatio‐temporal instability problem for flow on a beta plane is specified and analysed. The flow can be stable, convectively unstable or absolutely unstable. It is argued that these three stability regimes correspond to the steady, break‐up and eddy‐shedding regimes for the forced flow and good quantitative correspondence between the regimes is demonstrated by explicit solution of the spatio‐temporal stability problem.
Highlights
This paper considers the behaviour of an idealised flow configuration motivated by the Asian monsoon anticyclone
We presented evidence for the existence of a third flow state, in addition to the two known states, which is characterised by a steady flow in the neighbourhood of the mass source, but with temporal variability increasing in magnitude to the west, away from the source region
This third state, which we describe as a ‘break-up state’, had the characteristics of convective instability and motivated a new approach to the problem in the form of a spatio-temporal stability analysis
Summary
This paper considers the behaviour of an idealised flow configuration motivated by the Asian monsoon anticyclone. Hsu and Plumb (2000) showed in numerical simulations that the phenomenon of westward eddy shedding can spontaneously emerge as response to a steady localised forcing and provide a possible mechanism for time dependence that does not rely on the variability of the forcing. The numerical and laboratory experiments in the studies by DK89, Cenedese and Linden (1999) and Hsu and Plumb (2000) were all performed on a domain with relatively small zonal scale This restriction on the domain size may seem consistent with the spatial scales of the problem (motivated by the monsoon anticyclone flow), but can make it difficult to observe the full range of behaviours within the system since in certain situations important features of the forced response only develop a large distance to the west of the forcing region (Section 3). Further details on the nature of the forcing will be given in Sections 3 and 5
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