THE INFLUENCE OF THE ANDES CORDILLERA ON TRANSIENT DISTURBANCES

The influence of the Andes Cordillera on transient disturbances is investigated in this study using a lag-correlation analysis. This analysis shows that the unfiltered geopotential height data have a wavelike pattern moving to the east while tilting to the west in the vertical. When the wave approaches the Andes Cordillera, it exhibits orographic effects such as anticyclonic turning of a low-level disturbance trajectory, a zonal trajectory in the upper levels, distortions of the isolines of correlation, and an elongation of maximum correlation on the lee side of the Andes. The anticyclonic turning of the trajectory in the low-altitude levels and a zonal trajectory in upper levels implies a decrease in the vertical tilt of the system on the windward side and an increase in the tilt on the lee side. The increase of baroclinicity on the lee side results in baroclinic development as predicted from a linearly obtained normal-mode solution in the presence of mountains. A cross-correlation analysis of the high-pass-filtered disturbances shows an eastward phase propagation and a westward vertical tilt with height on the order of one-quarter wavelength between 1000- and 300-hPa levels. The horizontal structure and phase propagation show characteristics similar to the fastest-growing baroclinic normal mode in a two-layer, quasigeostrophic, beta-plane, linear model with a mountain placed in the north-south direction. This shows that the high-pass-filtered anomalies over the South American region are associated with baroclinic disturbances influenced by the Andes Cordillera. The results further show that the interaction of these anomalies with the Andes Cordillera is responsible for lee cyclogenesis. The composite maps show that the positive and negative high-pass-filtered anomalies have the same structure and paths of phase propagation. These anomalies intensify over the Pacific Ocean near the South American continent.