Intraseasonal modes of variability affecting the South Atlantic Convergence Zone

The intraseasonal variability of the South Atlantic Convergence Zone is investigated applying statistical techniques to the daily NCEP/NCAR reanalysis data set and the daily-interpolated NOAA satellites OLR, both ranging from 1979 to 1999. The data were filtered to retain frequency fluctuations between 30 and 90 days. The intrinsic connection among tropical convection in the Indonesian region, the subtropical flow and the convection in the South Atlantic Convergence Zone (SACZ) is discussed. The analyses are consistent with previous suggestions of the influence of Madden-Julian Oscillation (MJO) on tropical South American convection and studies of the Pacific South American (PSA) wavetrain role on SACZ. In the present study, the two dominant intraseasonal modes of variability affecting the SACZ are discussed on the basis of a zonal mode (the Madden-Julian Oscillation) and a tropical-extratropical mode (Pacific South American Pattern). The opposite convective behaviour between Indonesia and tropical South America is discussed, as well as the connection between SACZ convection and PSA-like pattern. It was noticed that an SACZ episode occurring in the northernmost position can be influenced by the MJO, and can trigger a wavetrain over the South Atlantic and Indian Ocean, while positions further south can be influenced by a tropical-extratropical PSA-like wavetrain. High-frequency (2-10 days) analysis displayed dominant wavetrain patterns of shorter wavelength than the intraseasonal wavetrain, but with similar characteristics, over South America, indicating the influence of synoptic systems, like frontal zones, over the continent. It is suggested that when these two frequencies, high (frontal systems) and intraseasonal (MJO or PSA), are in phase, they are able to establish appropriate conditions for an SACZ episode development. Copyright (C) 2006 Royal Meteorological Society.