Eddy–Eddy and Eddy–Mean Flow Interactions in an Eastward Jet Flanked by Recirculation Gyres

Abstract To examine the role of the interaction of different eddies (eddy–eddy interaction) in driving recirculation gyres, we analyzed the budget of the eddy potential enstrophy (EE) in an eastward jet, dividing perturbations into different frequency bands. While the instability wave produced by jet instability is dominant in high-frequency bands, the Rossby wave radiated from the jet is dominant in low-frequency bands. The eddy potential vorticity (PV) flux is oriented in the upgradient direction of the mean PV in low-frequency bands, which is consistent with the PV flux generated by Rossby wave radiation. Although a large cancellation occurs between the changes in the EE by this upgradient PV flux and the advection of the EE, such cancellation is greatly reduced when the rotational component of the PV flux is subtracted. The change in the EE by the upgradient PV flux tends to be balanced by that produced by eddy–eddy interaction. The convergence of the upgradient PV flux balances with the change in the PV by the recirculation gyres, while the rotational component of the PV flux does not drive mean flows. Thus, it is strongly suggested that the Rossby wave is excited by eddy–eddy interaction and generates the upgradient PV flux, which drives the recirculation gyres. The substantial part of the EE is transferred from high- to low-frequency bands owing to this eddy–eddy interaction. The EE in high-frequency bands is supplied by jet instability in the upstream region of the jet and advected to the downstream region, where eddy–eddy interaction occurs. Thus, the EE produced by the jet instability contributes to exciting the Rossby wave.