An alternative approach to assess the South America intraseasonal
variability is presented. In this study, we use a normal-mode
decomposition method to decompose the South American 30-90-day
Low-Frequency Intraseasonal (LFI) and 10-30-day High-Frequency
Intraseasonal (HFI) variability systematically into rotational (ROT) and
inertio-gravity (IGW) components in the reanalysis data.
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 is 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
(Rossby). A considerable portion of the LFI variability is also
associated with the inertio-gravity (IGW) modes (Kelvin mode),
prevailing mainly during the wet season.
The proposed decomposition methodology provides insights into the
dynamic of the South America intraseasonal variability, giving a
powerful tool for diagnosing circulation model issues in order to
improve the prediction of precipitation.