Intraseasonal large‐scale circulations and mesoscale convective activity in tropical South America during the TRMM‐LBA campaign

Abstract

An investigation linking the large‐scale intraseasonal variability in low‐level wind regimes and mesoscale convective activity in tropical South America (TSA) is presented. Observational evidence is gathered to support the hypothesis that periods of active and break phases in the South American Monsoon System (SAMS) are directly related to persistent westerly and easterly intraseasonal low‐level wind‐regime anomalies (10–70 days). The large‐scale circulation and tropical convection features are examined in this study with 19 austral summer seasons. The pattern of significant anomalies of the low‐level wind (850 hPa) and outgoing long‐wave radiation (10–70 days) evidenced the existence of an enhancement/suppression dipole of large‐scale features of convective activity over the northwest and southeast of TSA. Regional impacts upon the mesoscale activity during the Tropical Rainfall Measuring Mission (TRMM)/Large‐Scale Biosphere Atmosphere (LBA) Experiment campaign (23 January to 28 February 1999) are investigated. The mesoscale convective activity is described with the observation of the spatial and temporal characteristics of evolving convective systems in the infrared Geostationary Operational Environmental Satellite (GOES) images. The results indicated that the northwest TSA region was subjected to the most significant contrasts in the characteristics of convective systems as the wind regime changed. During easterly regimes (or break periods), the enhancement of convective systems in the northwest TSA and decrease in the southeast TSA resembled the autumn season. Properties such as number of convective systems, diurnal distribution, radius, fraction of cold tops, and fragmentation were modulated by wind anomalies in the southeast TSA as well.