Tropical and Subtropical South American Intraseasonal Variability: A Normal-Mode Approach

Abstract

Abstract Instead of using the traditional space-time Fourier analysis of filtered specific atmospheric fields, a normal-mode decomposition method is used to analyze the South American intraseasonal variability (ISV). Intraseasonal variability is examined separately in the 30-90-day band, 20-30-day band, and 10-20-day band. The most characteristic structure in the intraseasonal time-scale, in the three bands, is the dipole-like of convection between the South Atlantic Convergence Zone (SACZ) and the central-east South America (CESA) region. In the 30-90-day band, the convective and circulation patterns are modulated by the large-scale Madden-Julian Oscillation (MJO). In the 20-30-days and 10-20-day bands, the convection structures are primarily controlled by extratropical Rossby wave trains. The normal-mode decomposition of reanalysis data based on 30-90-day, 20-30-day, and 10-20-day ISV show that the tropospheric circulation and CESA--SACZ convective structure observed over South America are dominated by rotational modes (i.e., Rossby waves, mixed Rossby-gravity waves). A considerable portion of the 30-90-day ISV has also been associated with the inertio-gravity (IGW) modes (e.g., Kelvin waves), prevailing mainly during the austral rainy season.The proposed decomposition methodology demonstrated that a realistic circulation can be reproduced, giving a powerful tool for diagnosing and studying the dynamics of waves and the interactions between them in terms of their ability to provide causal accounts of the features seen in observations.