Transitions between baroclinic modon equilibria in terms of hetonic quartets

Abstract

A new concept, a hetonic quartet, is presented. A hetonic quartet is a pair of two aligned synchronously translating hetons. Hetons and hetonic quartets offer a finite-dimensional model for exploring the modon stability and transitions. Baroclinic modons, i.e., localized steady-state solutions to the nonlinear equations of potential vorticity (PV) conservation in a (differentially) rotating stratified fluid, represent a paradigm for coherent structures in geophysical flows. A baroclinic modon appears as two PV chunks of opposite signs, which residing at different depths and shifted relative to each other in the north-south direction. A hetonic quartet represents a discrete model for a two-layer modon with a considerable overlap of the upper- and lower-layer PV chunks, while a heton models a non-overlapping modon. The modon transitions from overlapping to non-overlapping states are explained in terms of stability of hetonic quartets and their breakout into two non-interacting hetons.