M. Pavec
Publications:
Pavec M., Carton X.
Parametric instability of a twolayer wall jet
2004, vol. 9, no. 4, pp. 499507
Abstract
The parametric instability of a wall jet, with timevarying potential vorticity or transport of the baroclinic mean flow, is studied in a twolayer quasigeostrophic model. This wall jet is composed of two superimposed strips of uniform potential vorticity, and the layer thicknesses are equal. The steady flow is stable with respect to short waves and its domain of linear instability grows with stratification. The timedependent flow evolution is governed by a Hill equation which allows parametric instability. This instability indeed appears in numerical flow calculations. It is favored near the marginal stability curve of the steady flow. Near that curve, the evolution equation of the flow is calculated with a multiple timescale expansion. This equation shows that for zero baroclinic transport of the mean flow, subcritical steady flows can be destabilized by flow oscillation, and supercritical steady flows can be stabilized by medium frequency oscillations. For finite baroclinic transport, this parametric instability vanishes in the limit of short waves or of long waves and narrow potential vorticity strips. Consequences for coastal flows in the ocean are drawn.
