Persistent Multiple Jets and PV Staircase

Abstract

The persistence of multiple jets is investigated with a quasigeostrophic, two-layer, ?-plane channel model. Linearly unstable normal modes are found to be capable of qualitatively describing the eddy fluxes of the nonlinear model. For a persistent double jet (PDJ) state, the most unstable normal mode has its largest amplitude located between the two jets, with a downshear tilt that acts to keep the jets separated. The opposite tilt occurs for a double jet state that is intermittent. An analysis of these normal modes, which utilized the concept of counterpropagating Rossby waves (CRWs), suggests that the downshear tilt in the interjet region hinges on the presence of critical latitudes only in the lower layer. This conclusion in turn suggests that the initial generation of the persistent jets requires L/Cgy < r?1, where L is the distance between the wave source (jet) and sink (interjet), Cgy is the meridional group velocity, and r is the linear damping rate. Similar CRW analysis for a conventional normal mode, which has its largest amplitude at the jet centers, suggests that the downshear tilt adjacent to the jet maxima is associated with the presence of critical latitudes only in the upper layer. The PDJ is found to be accompanied by potential vorticity (PV) staircases in the upper layer, characterized by a strong PV gradient at the jet centers and a broad region of homogenized PV between the jets. This PV mixing is realized through baroclinic waves that propagate slowly westward in the interjet region. Nonlinear evolution of the most unstable normal mode of the PDJ shows that northward heat flux by these waves is crucial for broadening the interjet PV mixing zone necessary for producing the PV staircase.