Nonlinear Barotropic Instability of Upper-Tropospheric Tropical Easterly Jet on the Sphere

Abstract

Abstract The nonlinear evolution of perturbation superimposed on the barotropic, unstable observed mean easterly jet at 100 hPa is studied over the sphere. The nondivergent barotropic nonlinear global spectral model with rhomboidal truncation at zonal wavenumber 21 is integrated for 120 days for initial random and linear unstable perturbations. The model includes a Rayleigh friction and restoring mechanism for zonal wind to its initial distribution. Time variations of eddy and zonal kinetic energy, zonal-wave and wave-wave interactions, and eddy and zonal kinetic energy dissipations are examined. The growth of perturbation begins with exponential increase in its kinetic energy for a short period, followed by a linear increase. The perturbations undergo oscillations before approaching a steady state. During the initial exponential phase, the nonlinear interactions further destabilize the jet, and this effect is more pronounced for the random initial perturbation. It is found that oscillations in kinetic en...