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

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.