Abstract

The structure and evolution characteristics of the tropical atmospheric intraseasonal oscillations (ISO) are distinctively different in boreal summer and winter. What is the major cause of this difference? We propose a thermal equator hypothesis in which the asymmetry of the mean state is essential to control the instability of equatorial Kelvin and Rossby waves. An atmospheric GCM is used to study the effect of the maritime continent in the northward bifurcation of ISO convection in boreal summer.

The intraseasonal variability of SST in the tropical Indian Ocean also exhibits a strong seasonality. We propose a background wind-ocean thermocline dome control hypothesis. Whereas the thermocline dome south of the equator determines where the largest SST variability might occur, it is the mean state of zonal surface wind, along with the atmospheric forcing strength and annual cycle of the ocean mixed layer depth, determines when the strongest variability occurs. The result also suggests a possible ocean feedback on the atmospheric ISO.

The atmospheric ISO exhibits strong bi-weekly (BW, 10-20-day) and lower-frequency (LF, 20-70-day) variabilities. Diagnosis of the NCEP/NCAR reanalysis and observed OLR/rainfall data shows that the physical origins and propagation characters of the BW and LF ISOs are significantly different. While the BW mode is characterized primarily by westward propagation, originated from the off-equatorial central Pacific, the LF mode originates from the equatorial Indian Ocean, migrating first eastward along the equator Indian Ocean and then shifting northward after reaching to the maritime continent. A simple 2.5-layer tropical atmospheric model is constructed to understand the fundamental dynamics and the geographic dependence of preferred frequency and length scales. This dynamic framework consists of a 2-level free atmosphere and a planetary boundary layer under an idealized or realistic mean flow. Our eigenvalue and numerical results show that while the most unstable mode off the equator is characterized by the coupling between the rotational component of the flow and convective heating through Ekman pumping and is favorable for a period of 10-20 days and zonal wavelength of 3000km, the equatorial mode is greatly modulated by nonlinear heating, with a dominant divergent wind component and a preferred 20-70-day period and zonal wavenumber 1 or 2 structure.