Synoptic Variability of Rainfall and Cloudiness along the Coasts of Northern Peru and Ecuador during the 1997/98 El Niño Event

Douglas, Michael W.; Mejia, John; Ordinola, Norma; Boustead, Joshua
January 2009
Monthly Weather Review;Jan2009, Vol. 137 Issue 1, p116
Academic Journal
This paper describes the meteorological conditions associated with large fluctuations in rainfall over the coastal regions of northern Peru and Ecuador during the 1997/98 El Niño event. Using data from a network of routine rain gauges and special gauges established just prior to the onset of heavy rains, it is shown that large variations in the daily rainfall on quasi-weekly time scales occurred during the period January–April 1998. These rainfall fluctuations were approximately in phase along the coast from near the equator to ∼7°S. The daily rainfall data was averaged to develop a subset of wet and dry days, and then these dates were used as the basis for compositing. Special pilot balloon observations were composited with respect to the wet and dry days, showing that westerly and northerly wind anomalies are associated with wet spells. Composites of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis and outgoing longwave radiation (OLR) data support a modest association of anomalous westerly wind events with enhanced rainfall. The relationship observed between westerly zonal wind anomalies and rainfall west of the Andes during 1998 suggested using the NCEP reanalysis to develop composites based on westerly wind events observed during other years. Zonal wind anomalies at 700 hPa were used as the primary criterion for stratifying “wet” and “dry” days, despite reservations about the association between rainfall and zonal wind. Compositing Geostationary Operational Environmental Satellite (GOES) and OLR data for 220 west wind anomaly events from the months of January–April for the years 1990–2005 showed that they are associated with enhanced cloudiness that propagates eastward at ∼10 m s-1. The composites using NCEP reanalyses show the evolution of the wind field associated with the wet days and suggest a link between extratropical wave passages across North America and anomalous westerly wind events off the coast of Ecuador and northern Peru.


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