Enso and rainfall along the Zongo valley (Bolivia) from the Altiplano to the Amazon basin

The location of eight rainfall stations in the Zongo valley (Eastern Cordillera - Bolivia) between 1195 and 4750 meters above sea level (masl) gives a unique opportunity to study the time-space rainfall variability between two regions, i.e. the Altiplano and the Amazonian lowlands of Bolivia, that are differently related to El Nino-Southern Oscillation (ENSO) events in the Pacific. In the central part of the lowlands, a La Nina/dry signal is observed during the peak of the rainy season (February) and around wintertime (May and September). Going toward the Andes and on the foothills up to 1500 masl, no ENSO signal has been found. On the median slopes of the Zongo valley, between 1800 and 3500 m, a strong Nino/dry signal is depicted on an annual timescale and during summer. In February, a Nina/wet signal is also significant. Going westward through the high Andes and the eastern Altiplano, this signal weakens. On the western edge of the Altiplano, an El Nino/dry and La Nina/wet signals are clearly identified. In the central lowlands and on the western Altiplano, ENSO indexes account for nearly 25% of rainfall variability, and for as much as 50% along the median slopes of the eastern Cordillera, in the Zongo valley. This finding deserves further study as previous researches have shown that in the Andes the best relationship between rainfall and equatorial Pacific sea-surface temperature (SST) is in the western Altiplano, far from the Amazonian source of moisture. Copyright (C) 2006 Royal Meteorological Society.