A combined barotropic-baroclinic instability study of the upper tropospheric tropical easterly jet

Abstract

A combined barotropic-baroclinic stability analysis is performed for an upper tropospheric tropical easterly jet representing the observed mean monsoon zonal flow during summer. Numerical solutions are obtained by time integration of a 20-layer linear spectral quasi-geostrophic model, which is based on truncated Fourier series representations in y. It is seen from the growth rate and phase speed spectra that the asymmetric barotropic-baroclinic preferred wave has a wavelength of 6500 km, an e-folding time of 3.3 days, a westward phase speed of 20.5 m s^- and a period of 3.8 days. The geopotential, vertical velocity and temperature fields associated with the most unstable barotropic-baroclinic wave are computed. The most unstable wave has a vertical scale of 125 mb, a meridional scale of 1650 km and a zonal scale of 2135 km. The relationship between the vertical and meridional scales of the wave with the corresponding basic zonal flow scales is discussed. The large southward easterly momentum transports associated with the unstable wave are essentially due to the antisymmetric components of the jet. The computed sensible heat transports are found to be down the basic state meridional temperature gradient. The energetics of the unstable wave is computed and it is inferred that the energy sources for the wave growth lie in a narrow vertical layer around the jet level. It is also found that the contribution of baroclinic process is larger than the contribution of barotropic in the wave growth. The contribution of different processes in the movement of the unstable wave are also investigated. The beta effect is identified as the most important physical factor responsible for the westward propagation of the wave.