Abstract
Abstract A two-layer quasigeostrophic model is used to investigate whether dissipation can induce absolute instability in otherwise convectively unstable or stable background states. It is shown that dissipation of either temperature or lower-layer potential vorticity can cause absolute instabilities over a wide range of parameter values and over a wide range of positive lower-layer velocities (for positive vertical shear). It is further shown that these induced absolute instabilities can be manifested as local instabilities with similar properties. Compared to the previously known absolute instabilities, the induced absolute instabilities are characterized by larger scales, weaker absolute growth rates, and substantially weaker vertical phase tilt (typical values for subtropical states are zonal wavenumber 1–3, absolute growth rate 80–100 days, and period 7–10 days). The analysis of absolute instabilities, including the case of multiple absolute instabilities, is reviewed in an appendix. Because the disp...
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