Shear flow Instability

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Instability of shear flows is one of the main sources of disturbances in the Atmosphere and oceans, giving rise to synoptic weather systems patterns and ocean eddies.

Fundamental mechanism: There are two basic theories of the mechanism behind Rossby-wave based shear flow instability – a mutual amplification of counter propagating Rossby waves, and Overreflection of Rossby waves. In Harnik and Heifetz (2007) we related these two fundamental theories, in terms of Kernel Rossby Waves – the fundamental building blocks of shear instability. This framework also explains stratified shear flow instabilities which involve gravity rather than Rossby waves (Harnik et al, 2008; Rabinovich et al, 2011). 

Nonlinear equilibration: We have examined the nonlinear equilibration of shear-flow instability in various contexts.

The global circulation - the equilibration of baroclinic instability, in a forced-dissipative system,  gives rise to different jet regimes, governed by different dynamical balances. The different balances lead, correspondingly, to different variability behaviors (Lachmy and Harnik, 2014, 2016, 2020). Recently we are examining how midlatitude diabatic heating, similar to that due to latent heat release by storm tracks, affects this equilibration (White et al, 2024).

Barotropic instability - In Harnik et al (2014) we examine the equilibration of an initially barotropically unstable mean flow in an inviscid unforced model run. We combine constraints of linear stability with conservation of wave activity and circulation, to obtain a theory for the equilibrated mean flow and wave amplitudes. See a beautiful movie of this nonlinear evolution in which dragon-head structures form in the equilibrated stage.