17.4
Instability of the axisymmetric monsoon flow and intraseasonal oscillation
Gilles Bellon, Columbia University, New York, NY; and A. H. Sobel
A model of intermediate complexity based on quasi-equilibrium theory - a version of the Quasi-equilibrium Tropical Circulation Model - is used in a zonally-symmetric aquaplanet 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 oscillation appears to result from a Hopf bifurcation that occurs when the asymmetry of the sea surface temperature forcing is increased. Beyond the bifurcation, the mean flow is linearly unstable, and the one linearly unstable mode is similar in structure and period to the non-linear mode.
The linear mode is further studied to understand the fundamentals of the oscillation. It appears that the mean meridional shear and the wind-induced surface fluxes associated with surface westerlies cause the instability, and the former controls the period of the oscillation. Extreme simplification of the model allows one to identify a mechanism of northward propagation and a mechanism of equatorial reinitiation.
Session 17, Monsoons and Easterly Waves
Friday, 29 June 2007, 10:30 AM-12:00 PM, Ballroom South
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