Instability of a coastal jet in a two-layer model

Abstract

The instability of a shelf front, with the characteristics of the Ushant front (steep density gradient and narrow jet), is investigated on the f-plane within the framework of a two-layer shallow-water model. Linear stability analysis and nonlinear numerical simulations show that baroclinic waves, with 20 and 40~km wavelengths, are the most unstable ones, with growth rates on the order of 1/1 day, which is comparable to what is observed on SST images. The analysis of growing perturbations in a two-layer primitive equation model (MICOM) shows that the latter are vertically shifted, which corroborates that the governing mechanism for the generation asymmetric meanders and eddies in our simulations, is baroclinic instability. Then, in order to take into account tidal effects, we add a time-periodic barotropic mean flow. Although we succeed observing some characteristics of parametric resonance (frequency selection, stepwise growth), baroclinic instability remains the main mechanism to explain the growth of meanders.