Dept. of Geosciences, University of Massachusetts, USA

For paleoclimatic reconstruction and interpretation of proxy records obtained from an ice core on Nevado Sajama, Bolivia, the knowledge of atmospheric circulation anomalies during extreme climatic events is crucial. Hence the atmospheric circulation over the Bolivian Altiplano during composite WET and DRY periods and during HIGH and LOW index phases of the Southern Oscillation was investigated using daily radiosonde data from Antofagasta (Chile), Salta (Argentina), Lima (Peru) and La Paz (Bolivia), daily precipitation data from climatic stations surrounding Nevado Sajama and monthly gridded NCEP (National Centers for Environmental Prediction)-Reanalysis data between 1960 and 1998. Results are compared with the climate at the summit of Nevado Sajama as recorded by an automatic weather station near the drilling site.
In austral summer (DJF) the atmosphere during WET periods is characterized by easterly wind anomalies in the middle and upper troposphere over the Altiplano, resulting in increased moisture influx from the interior of the continent. The Bolivian High is intensified and displaced southward. On the other hand, westerly winds usually prevail during DRY summer periods, preventing the moisture transport from the east from reaching the western Altiplano. Precipitation tends to be deficient in the Nevado Sajama region 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 similar atmospheric circulation anomalies as DRY periods and can be regarded as an extended DRY period or as a summer with increased occurrence of DRY episodes. HIGH summers, and to a lesser degree LOW+1 summers, are characterized by broadly opposite atmospheric characteristics, featuring a more pronounced Bolivian High located significantly further south, and easterly wind anomalies over the Altiplano. In winter (JJA) precipitation events are rare near Nevado Sajama; these 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 index winters (JJA) are less pronounced and generally feature the same changes as in summer (increased temperatures and a vertically expanded troposphere). However the significance of these changes, especially with regard to the wind pattern, varies depending on station and pressure level. Accordingly, precipitation during austral winter shows no relationship with the extremes of the Southern Oscillation.

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Document maintained by Doug Hardy (dhardy@geo.umass.edu)
Last updated: 1 October 1998
http://www.geo.umass.edu/climate/bolivia/agu98mv.html