*Journal of Geophysical Research*,
submitted 8/07.

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 linear model of intermediate complexity based on quasi-equilibrium theory is
used in a zonally-symmetric aquaplanet configuration to investigate the
stability or instability of the mean summer monsoon flow in the South Asian region.
In the control case, the mean state has one linearly unstable mode that
corresponds in period and structure to the mode of the non-linear model
described in a previous paper (Bellon and Sobel, 2007) and to the observed
30-60-day mode of the monsoon intraseasonal variability. This mode features
propagation of the precipitation anomalies from the equator to the monsoon
trough around 20\dg N.
The period of this mode and the direction of propagation appear to be determined
from the direction and magnitude of the mean meridional baroclinic flow. The
wind-induced surface heat fluxes associated with the surface westerlies in the
northern tropics are essential to its instability.

The mechanisms of propagation and reinitiation of convection are further
described. The off-equatorial mechanism responsible for the northward
propagation involves the creation of barotropic vorticity north of the
convection center by baroclinic advection of baroclinic vorticity. This
barotropic vorticity in turn creates low-level convergence via Coriolis
acceleration, and subsequent convection. The mechanism responsible for the
reinitiation of the convection at the equator involves the creation of
barotropic easterlies by baroclinic advection of baroclinic zonal momentum.
These barotropic easterlies in turn generate low-level convergence via the
$\beta$ term of the Coriolis acceleration, and subsequent convection.