Considerations for the Structure of the Tropical Intraseasonal Oscillation

Abstract

The 30-60 day tropical intraseasonal oscillation (ISO) has a quite complicated structure. For example, different behavior is observed between the wavenumber-one zonal wind and the wavenumberzero temperature (or geopotential). An attempt is made to understand such a structure from a unified view, using a linear response model based on the primitive equations. The thermal forcing, which is externally prescribed, has a 40 day period with variable amplitudes and moves eastward over a distance of 120° of longitude. This model simulates well the observed structure.
The key to understanding the structure of the ISO lies in the contrast between the slow moving speed of the forcing and the fast group velocity of Kelvin modes. Regions with upward motion move slowly with the forcing, while areas with downward motion promptly extend to the entire longitude owing to the Kelvin modes. Thus, the vertical velocity and divergence fields have the same wavenumber-one structure as the heating. The zonal wind is the integral of the divergence, so that it must exhibit a wavenumber-one structure. On the other hand, the temperature must have a wavenumber-zero structure. The reason is that high temperatures result from diabatic heating in the forced region and adiabatic heating due to downward motion in the unforced region, and the maximum of diabatic heating and that of downward motion in the unforced region occur almost simultaneously.
The geopotential shows similar behavior via the hydrostatic equation. Thus, the wavenumber-one structure of the zonal wind and the wavenumber-zero structure of the temperature (geopotential) necessarily correspond to each other.
Marked structural changes occur in one cycle of the ISO. The slowly moving heat source excites a Matsuno-Gill (MG) pattern. When the heating amplitudes become small in the eastern portion of the forced region, a free Kelvin mode separates and moves eastward. Thus, a MG pattern having a slow speed is observed in the forced region, while a fast Kelvin mode is formed in the unforced region. Further, it can be shown that the vertical structure of the geopotential should be different from that of the zonal wind.
The effects of the values of the imposed parameter on the structure of the model ISO are also examined. Moreover, five time scales which characterize the ISO are proposed, and the relationships which they must satisfy in order to reproduce the observed structure are discussed.