Self-amplifying hetons in vertically sheared geostrophic turbulence

Abstract

This paper examines baroclinic turbulence in the local approximation using a quasi-geostrophic beta-plane numerical model with bottom friction. New coherent vortex structures are found to emerge and self-amplify in a horizontally homogeneous mean flow directed westward in the upper layer where the potential vorticity gradient (PVG) is negative. Opposite sign eddies shifted eastward are formed in the lower layer with positive PVG owing to the Rossby wave radiation. Such baroclinic structures can be viewed as self-amplifying hetons supported by the energy transfer from the mean flow. Their growth and saturation are controlled by the competition between vortex amplification associated with the crossflow drift and viscous decay enhanced by core deformations. The presented numerical simulations indicate that subtropical regions with westward flows in the upper layer favor eddy growth and long-distance propagation.