Dynamics of the tropical intraseasonal oscillation and its interaction with the synoptic variability

Numerical experiments with a 2.5-layer and a 2-level model are conducted to study the mechanism for the planetary scale selection of the Madden-Julian Oscillation (MJO), in which the nonlinear heating leads to the interference between the dry and wet Kelvin waves and eventually dissipates and "filters out" shorter wavelength perturbations, leading to a longwave selection. The boundary layer plays a role in destabilizing the MJO through frictional moisture convergence and in retaining the in-phase zonal wind-pressure structure. Using the same 2.5 layer model the origin of the off-equatorial 10-20-day mode is studied and it turns out to be an off-equatorial Rossby mode intrinsic of the atmosphere and the boreal summertime mean flow plays an important role in this scale selection.;The interactions between the tropical intraseasonal oscillation (ISO) and the synoptic variability are examined. The synoptic variability is much stronger during the ISO active phase than during the break phase in the northwestern Pacific. Northwestward propagating synoptic wavetrains are associated with the active ISO and other variables also show much stronger synoptic variability during the ISO active phase. The easterly vertical shear during the active phase of the ISO might favor the enhancement of the synoptic disturbances and the mean cyclonic shear and convergence may also contribute to the strong high-frequency variabilities. The latent heat flux calculated based on the bulk formula shows a large proportion from the synoptic contribution, in which the synoptic winds seem to be more important than the air-sea humidity difference. The main synoptic contributions to the intraseasonal latent heat flux are located in the tropical Indian Ocean, western Pacific and the northeastern Pacific. In the northwestern Pacific, the larger latent heat flux contributed by the synoptic motion during the ISO active phase might favor the growth and northwestward propagation of the ISO convection.