The present study describes some observed surface and upper-air features of the low-level jet (LLJ) and southerly jet (SJ). Our results suggest the existence of this low-level circulation to the east of the Andes that transports moisture from tropical South America toward the south during the warm/wet season of 1999. We explore the synoptic variability, diurnal variation, and alternations between LLJ and SJ episodes by using a combination of surface and high-resolution upper-air observations (1 to 8 soundings per day) and global reanalysis. Our results show strong synoptic fluctuations; with the LLJ more frequent than SJs. The LLJ has stronger winds in the afternoon and its core of maximum winds is located between 1600 and 2000 m above the surface. Special observational efforts, such as the pilot balloon sounding network in Bolivia (Pan American Climate Studies Sounding Network program), the Large-Scale Biosphere-Atmosphere (LBA) Experiment-WET Atmospheric Mesoscale Campaign (AMC), and Tropical Rainfall Measuring Mission (TRMM)-LBA in Southwest Amazonia, have provided upper-air information with high temporal and spatial resolution to describe the structure of both the LLJ and the SJ during the January--April 1999 period.