Abstract By analyzing the 15-yr (1979–93) reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF), it has been found that the seasonal and synoptic time-scale variations of the South American low-level jets (LLJs) are largely controlled by an upper-level trough and associated low-level zonal flow, rather than by horizontal temperature gradients along the slope of the Andes. The northerly LLJs are maintained by strong zonal pressure gradients caused by the upstream trough and westerly flow crossing the Andes through lee cyclogenesis. The process involves both baroclinic development of the upper-level trough and mechanical deflection of the westerly flow by the Andes. When an anticyclonic circulation replaces the trough and westerly flow over the eastern South Pacific, the northerly LLJs tend to diminish or reverse into southerly LLJs. The dependence of the LLJs upon the upstream wind pattern helps to explain how the seasonal variation of the South American LLJs is related to the s...