Atmospheric circulation anomalies associated with 1996/1997 summer precipitation events on Sajama ice cap, Bolivia

The analysis of atmospheric circulation anomalies related to snowfall events on Sajama volcano (Bolivian Andes) provides important information for the calibration of an ice core, recently recovered from the summit. Seventeen precipitation episodes were recorded on Sajama volcano during the 1996/1997 summer season (November 1996 to March 1997) by snow depth sensors and additional measurements of an automatic weather station located on the summit. The analysis of atmospheric circulation patterns during these events is based on zonal and meridional wind, air temperature, relative humidity, geopotential height and horizontal divergence at three pressure levels (400, 500, and 700 hPa levels), atmospheric thickness (700 hPa-400 hPa), and precipitable water (vertically integrated), all extracted from the National Centers for Environmental Prediction (NCEP) data set. Highly convective situations prevailed through most of December and January, with strong vertical motion over the Bolivian Altiplano. In February and March, increased moisture advection from the east occurred in midtropospheric levels. These results are confirmed by isobaric 5-day back trajectories and transit time analysis at the 400 hPa level. The extremely southern position of the upper air high-pressure system ("Bolivian High") in February and March is the main reason for the unusually high precipitation amounts on the Altiplano in 1996/1997. Highly variable patterns of atmospheric circulation can lead to snowfall on Sajama during the summer months.