Laterally Forced Equatorial Perturbations in a Linear Model. Part I: Stationary Transient Forcing

Abstract

Atmospheric equatorial perturbations generated by extratropical transient forcing are simulated numerically in a linear model. Impacts of mean zonal wind on such lateral forcing processes are emphasized. The extratropical forcing source is stationary and localized, with its amplitude varying with time. This forcing configuration simulates atmospheric events that may intrude into the tropics from the extratropics, such as the cold surges during the Asian winter monsoons and the deep midocean troughs. For forcing of large time scales (≥4 day), the equatorial responses are completely dominated by the Rossby mode component. When the forcing time wale is one day or smaller, the major components in the equatorial perturbations are the Rossby, westward mixed Rossby-gravity, and Kelvin modes. The amplitudes of these modes are discriminatingly modulated by the mean zonal wind. Whereas both the Rossby and mixed Rossby-gravity modes exhibit larger amplitudes in mean westerlies than in mean easterlies, the latter is more sensitive to the mean zonal wind. The Kelvin mode, on the other hand, is stronger in mean easterlies than in mean westerlies. Therefore, the, horizontal structures, as well as the amplitudes of the laterally forced equatorial perturbations, substantially depend upon the position of the extratropical forcing source relative to the tropical mean zonal wind, which varies in both latitude and longitude (especially in the upper troposphere). This study suggests that, in the presence of strong influences originated from higher latitudes, the observed spatial and temporal variability of equatorial perturbations in the atmosphere can be attributed to those of the tropical mean zonal wind.