2002 Annual

Tuesday, 15 January 2002: 4:00 PM
Modulation of Seasonal Precipitation in northern Venezuela and Antecedent Rinfall to the Floods and Landslides of December 1999
Bradfield Lyon, International Research Institute for Climate Prediction, Columbia Univ., Palisades, NY
Poster PDF (168.4 kB)
The devastating landslides that occurred in northern Venezuela during December 1999 were preceded by a season of greatly enhanced precipitation. Factors associated with such enhanced seasonal rainfall are the main focus of this study. Using observational data for the period 1950-1998 we investigate the seasonality of rainfall in northern Venezuela and examine specific seasons where it was unusually intense. We compare these general results to the 1999 case and include a brief description of the synoptic scale atmospheric circulation near the time that torrential rains triggered the event.

Our statistical results indicate that rainfall in northern Venezuela is influenced by both the tropical Pacific and North Atlantic basins, consistent with other studies in the region. In addition, we find that this influence varies by season with the tropical Atlantic more influential during boreal summer and the eastern Pacific during autumn and winter. Cases where rainfall was especially intense from June to August were also found to often be associated with transitions from the warm to cold phases of ENSO.

Low level moisture flux composites indicate a large scale atmospheric circulation anomaly is associated with enhanced seasonal rainfall during both northern summer and fall seasons. This anomalous circulation consists of an off-equator westerly flux extending from the eastern Pacific to the Caribbean with enhanced moisture convergence over the Caribbean. During the fall of 1999 a similar pattern was observed.

The low-level atmospheric circulation associated with the rainfall that triggered the landslides consisted of quasi-stationary confluence zone located just to the north of Venezuela over the Caribbean. An anticyclone off of the coast of the southeast United States simultaneously resulted in an anomalous low level northerly flow which appears to have lead to mechanical lifting of this humid air as it impinged on the local topography.

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