Atmospheric circulation over the Bolivian Altiplano during dry and wet periods and extreme phases of the Southern Oscillation

The atmospheric circulation over the Bolivian Altiplano during composite WE T and DRY periods and during HIGH and LOW index phases of the Southern Osci llation was investigated. using daily radiosonde data from Antofagasta (Chi le), Salta (Argentina), Lima (Peru) and La Pat (Bolivia), daily precipitati on data from the Bolivian/Chilean border between 18 degrees and 19 degrees S and monthly NCEP (National Centers for Environmental Prediction) reanalys is data between 1960 and 1998. In austral summer (DJF) the atmosphere durin g WET periods is characterized by easterly wind anomalies in the middle and upper troposphere over the Altiplano, resulting in increased moisture infl ux from the interior of the continent near the Altiplano surface. The Boliv ian High is intensified and displaced southward. On the other hand, westerl y winds usually prevail during DRY summer periods, preventing the moisture transport from the east from reaching the western Altiplano. Precipitation tends to be deficient over the western Bolivian Altiplano during LOW index summers and above average during HIGH and LOW +1 summers, but the relation is weak and statistically insignificant. LOW summers feature broadly simila r atmospheric circulation anomalies as DRY periods and can be regarded as a n extended DRY period or as a summer with increased occurrence of DRY episo des. HIGH summers, and to a lesser degree LOW +1 summers, are characterized by broadly opposite atmospheric characteristics, featuring a more pronounc ed Bolivian High located significantly further south, and easterly wind ano malies over the Altiplano. In winter (JJA) precipitation events are rare; t hese are associated with increased northerly and westerly wind components, reduced pressure and temperature, and increased specific humidity over the entire Altiplano. Atmospheric circulation anomalies during LOW periods are less pronounced in austral winter (JJA) than in summer, but generally featu re similar changes (increased temperatures and a vertically expanded tropos phere). However, the significance of these anomalies, especially with regar d to the wind pattern, varies depending on station and pressure level. Acco rdingly, precipitation during austral winter shows no relationship with the extremes of the Southern Oscillation. Copyright (C) 1999 Royal Meteorologi cal Society.