Subtropical and Extratropical South American Intraseasonal Variability:
A Normal-Mode Approach
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. Intraseasonal
variability was separate into the 30-90-day Low-Frequency Intraseasonal
(LFI) and 10-30-day High-Frequency Intraseasonal (HFI) variability, and
analyzed the contribution of the rotational (ROT) and inertio-gravity
(IGW) components to the observed convective and circulation features.
The seasonal cycle of the LFI and HFI convective and dynamical structure
is well-described by the first leading pattern (EOF1). The LFI EOF1
spatial structure during the rainy season is the dipole-like between the
South Atlantic Convergence Zone (SACZ) and southeastern South America
(SESA), influenced by the large-scale Madden-Julian Oscillation (MJO).
During the dry season, alternating periods of enhanced and suppressed
convection over South America are primarily controlled by extratropical
wave disturbances. The HFI spatial pattern also resembles the SESA–SACZ
structure, in response to the Rossby wave trains. Results based on
normal-mode decomposition of reanalysis data and the LFI and HFI indices
show that the tropospheric circulation and SESA–SACZ convective
structure observed over South America are dominated by ROT modes (e.g.,
Rossby). A considerable portion of the LFI variability is also
associated with the inertio-gravity (IGW) modes (e.g., Kelvin mode),
prevailing mainly during the rainy season. The proposed decomposition
methodology provides new insights into the dynamics of the South
American intraseasonal variability, giving a powerful tool for
diagnosing circulation model issues in order to improve the prediction
of precipitation.