Spatial variability of annual precipitation and ENSO events in western Peru

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Abstract
The probability distributions of annual rainfall totals at 18 stations from north coastal Peru to the southern highlands and Altiplano are examined. Most stations exhibit high interannual variability which cannot be satisfactorily modelled by a single lognormal distribution. Indices of the El Niño-Southern Oscillation (ENSO) suggest that two precipitation regimes may be identified, the ENSO regime and the non-ENSO regime. Simple descriptive statistics of rainfalls within the two sub-samples indicate that during ENSO years the Altiplano and southern highland regions experience drought, with an average reduction of mean annual precipitation of 18%, while the north coastal region is receiving over 88% above non-ENSO rainfall. A mixed lognormal model of annual precipitation is successfully fitted at all stations. The estimated parameters reflect the contrasting response of the two regions to ENSO events. Pearson product-moment correlation matrices generated between stations for each subsample further support the hypothesized negative relationship between conditions on the northern coast and the Altiplano. The resulting model describes the stochastic nature of annual precipitation in the area and illustrates regional differences in the influence of ENSO.