Scale Selection for Tropical Bi-weekly and Intraseasonal Oscillations

The power-spectrum analysis of the observed daily OLR in boreal summer for 1979-2001 shows significant peaks on bi-weekly (10-20-day) and intraseasonal (20-70-day) bands. The main convective centers on both timescales are located in the South China Sea/western Pacific and the Bay of Bengal. The lagged correlation maps with reference to the two strong convective centers show that the propagation properties of the bi-weekly and intraseasonal modes are quite different. The 10-20 day mode shows a pronounced westward or northwestward propagation, originating from off-equatorial western/central Pacific and having a Rossby-wave type structure. The 20-70-day mode originally comes from the equatorial regions, propagating eastward along the equator and then northward. The different scale selection and propagation characteristics of the two modes bring forth a hypothesis that the two distinctive modes may involve different dynamics and physical origins at the equatorial and off-equatorial regions.

Based on a two-and-half layer atmospheric model, a simple dynamic framework is formed for the mode off the equator (at 15°N). Diabatic heating of the system is determined by low-level vorticity at top of the PBL. Through an eigenvalue analysis, the frequency and growth rate of the mode are obtained. It is found that the northwestward propagating Rossby wave with a time period of 10-20 days and a horizontal wavelength of 3,000 km is the most unstable mode in the off-equatorial region. This sheds light on the time and spatial scale selection of the off-equatorial mode. The easterly vertical shear of the mean flow plays an important role in leading to the instability.

For the equatorial mode, we examine the effect of the mean Walker cells (that have a zonal wavenumber-2 structure) and nonlinear heating on the scale selection. A series of numerical experiments with the same simple two-and-half layer atmospheric model are designed to examine the instability and propagation properties of the equatorial mode. In the presence of a linear heating, the most unstable mode prefers a short wavelength. With a nonlinear heating, no matter what wavenumber perturbations are input initially, the amplitude of the zonal wavenumber one perturbation grows fastest. Thus the nonlinear heating acts as a critical factor in the spatial scale selection in the equatorial region. The wavenumber-2 mean flow, on the other hand, shows little impact on the long wave selection. The preferred time period for the unstable equatorial mode appears on 20-70 days.