Journal of the Atmospheric Sciences, 65, 470-489.

Poleward-propagating intraseasonal monsoon disturbances in an intermediate-complexity axisymmetric model

Gilles Bellon
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY.

Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.

Abstract

A model of intermediate complexity based on quasi-equilibrium theory -- a version of the Quasi-equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature -- is used in a zonally-symmetric configuration to simulate aspects of the South Asian monsoon and its variability. Both the mean state and the 30-60-day mode of the intraseasonal variability are simulated satisfactorily. The model has two limit cycles of similar period and structure that can account for this mode. Both feature northward propagation of the Tropical Convergence Zone from 5\dg S to 25\dg N with a period of about 50 days. The structures of the modes are qualitatively similar to those of the composite of observed variability in terms of divergence and vorticity.

The dynamics of the oscillations is essentially linear. Both wind-induced surface heat fluxes and the prognostic boundary layer humidity and dynamics are necessary to obtain the instability of the mean monsoon flow.