Persistent multiple jets and the PV staircase

The persistence of multiple jets in a baroclinic flow, together with the associated potential vorticity (PV) staircase, is investigated with a quasi-geostrophic, two-layer, beta-plane channel model. Two types of zonal jet states, persistent double jets (PDJ) and intermittent double jets (IDJ), are investigated. The PDJ is found to be accompanied by a nearly perfect PV staircase, represented by a strong PV gradient at the jet centers and a broad region of homogenized PV between the jets, whereas evidence of a PV staircase is weak for the IDJ. Linearly unstable normal modes are found to be capable of describing the qualitative behavior of these jets, including the transition between the jet states, and the persistency (intermittency) of the PDJ (IDJ). For the PDJ, the normal mode tilts downshear between the jets, acting to keep the jets separated. The opposite is the case for the IDJ. The relationship between the normal mode structure and the background flow is investigated using counter–propagating Rossby waves (CRWs). It was found that to generate a normal mode with downshear tilt, a critical latitude must be present in the lower layer. However, we also find that the normal mode cannot explain the meridional scale of the homogenized PV region, and nonlinearity is necessary.