On the mechanism underlying the spontaneous emergence of barotropic zonal jets

Turbulent flows are often observed to be organized into large-spatial-scale coherent jets that are maintained by the very eddies they support. Spontaneous emergence of these jets in a wide variety of geophysical settings implies that the mechanism for jet formation and maintenance is generic. In this work Stochastic Structural Stability Theory (SSST) is used to achieve a comprehensive understanding of this physical mechanism. According to SSST, the distribution of momentum fluxes arising from the eddy field associated with a given jet structure is obtained using a linear model of stochastic turbulence that was extensively verified in GCM studies. The resulting momentum flux distribution is then coupled with the mean zonal momentum equation to produce a closed set of wave-mean flow equations. We apply the tools of SSST in a barotropic model of a fluid subjected to homogeneous stochastic forcing. The goal is to investigate the role of the eddy-mean flow feedbacks in the structural instability of the eddy-mean flow equilibria giving rise to zonal jets. We show that the structural instability of the equilibrium state with no mean flow is governed by two competing mechanisms: shearing of the eddies that produces upgradient, anti-diffusive fluxes and is jet forming and advection of the vorticity gradient of the infinitesimal jet by the eddies that produces downgradient, diffusive fluxes and hinders the formation of the mean flow. For stochastic forcing with small zonal scale and amplitude larger than a certain threshold, the upgradient fluxes prevail leading to the emergence of a zonal jet. 7.202.69.61 on 2-9-2011-->