Influence of Cross-Andes Flow on the South American Low-Level Jet

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 seasonal changes of the large-scale circulation pattern over the eastern South Pacific. On synoptic time scales, the relation between LLJs and cross-Andes zonal flow is strong in austral winter, spring, and fall. This relation weakens somewhat in summer, when Amazon convection is strongest. The analysis also demonstrated strong connections of the LLJs with South American precipitation, intensity of the South Atlantic convergence zone (SACZ), and low-level cross-equatorial flow. A method for up to 5-day forecasts of the LLJs based on 700-hPa zonal winds over the subtropical eastern South Pacific was also introduced. A cross validation indicates a certain degree of predictability for South American LLJs. The results further suggest that the upstream flow pattern over the South Pacific should be closely monitored to determine the variability of the South American LLJs.