Possible Feedback Path for Low-Frequency Atmospheric Oscillations

Abstract

Equatorial low-frequency oscillations with periods of 1–2 months are being intensively studied by many investigators. A strong equatorial “dipole” pattern is observed in which atmospheric variables such as temperate, wind, and pressure are out of phase between the Indian Ocean-Indonesia region and the western Pacific. While it is generally thought that the oscillations make a complete circuit around the earth from their excitation region in the equatorial Indian Ocean-western Pacific, the signal is more difficult to observe over the South America-Atlantic-Africa sector. Using analyses of four year of satellite-derived microwave radiance data, evidence is presented here for the possibility of a feedback route in the Southern Hemisphere. The observed propagation path extends from the central equatorial Pacific across lower South America and heads equatorward after passing south of Africa. The response route finally reenters the equatorial Indian Ocean with the correct phase to enhance the primary equatorial dipole structure. The Southern Hemisphere propagation path migrates northward in April-September and southward in October-March. The largest correlation with the equatorial dateline region occurs at the turning point of the feedback route, in the South Atlantic. This propagation path appears to constitute a feedback mechanism which could aid in stabilizing the low frequency oscillations through positive feedback. The correlations are shown to be statistically significant by several methods, including Monte Carlo simulations.