Synoptic and spatial variability of the rainfall along the northern Peruvian coast during the 1997-8 El Niño event

The recent 1997-8 El Niño event was the strongest in recent history. Because the event was anticipated in general terms some months in advance it was possible to plan additional meteorological observations to examine the regional impact along the coast of Peru and Ecuador. As part of PACS-SONET activities a special network of 7 pilot balloon stations was installed just prior to the onset of heavy rains over the region in December 1997. Additional raingauges were also established in both Ecuador and northern Peru to more accurately quantify the rainfall and its temporal variability. This talk presents the results of an examination of the synoptic-time scale variations in rainfall over northern Peru during the period January-April 1998. During this period the daily rainfall from between 60 and 110 stations was averaged to obtain a value of the daily rainfall per station. This was found to be a good indicator of wet and dry periods. A time series of this value showed large variations of roughly one week periodicity. Composites of wet and dry days were prepared from NCEP reanalysis windfields, pilot balloon observations, radiosonde and wind profiler observations made at Piura, Peru, and satellite imagery. There is a clear difference between wet and dry days from the NCEP reanalyses, with westerly wind anomalies of about 2 m/s (with respect of mean conditions) in the lower troposphere. The anomalies are strongest between the Galapagos Islands and the coast of Peru. Other results, using the observed wind and radiosonde soundings, will be presented at the conference.