Areal distribution of the influence of ENSO on the annual rainfall in Ecuador
The influence of ENSO (El Niño/Southern Oscillation) on rainfall has been studied in numerous regions of the world and especially in South America. Ecuador, located between Peru and Colombia, is one of the regions most affected by this phenomenon. This country has been little studied and this part of the world remains undefined in global studies due to lack of data. We propose to fill this gap by homogenization of rainfall series from 210 stations spread between the coastal regions, the Andes and the Amazonian slope of Ecuador. This study was carried out with the regional vector method of Brunet-Moret (1979), based on the hypothesis of pseudo-proportionality between the total rainfall of the stations located in a homogeneous area. This method also allowed us to regionalize the annual rainfall and thus to define 18 homogeneous areas in which the inter-annual variations are represented by a rainfall index. This zoning differs from previous ones (Pourrut, 1994) in that the parameter determining the regionalization is the chronological evolution of total rainfall and not the mean rainfall values. This approach is the most appropriate for the analysis of annual rainfall evolution. The available data permit the calculation of these indices in a continuous way for the 30-year period between 1964 and 1993, the annual cycle considered being the hydrological year that begins in September and finishes in August of the following year. We propose a simple numeric method to identify the various El Niño events considered as the regional consequence in South America of the global ENSO phenomenon. We consider that an El Niño event corresponds to a sequence of at least three consecutive months during which the monthly sea surface temperature (SST) of a Niño 1+2 block is superior to 23°C and presents a positive anomaly of more than 1°C (Rossel, 1997). In the period 1964-93, the years 1965, 1969, 1972, 1973, 1976, 1983, 1987 and 1992 correspond to this definition. The observed rainfalls during the El Niño of 1983 are exceptional, and taking them into account would mask the influence of the other events. We thus studied the year 1983 separately and removed it from the global analysis. We analyzed and quantified the influence of El Niño on rainfall using two simple methods and two statistical tests that allowed us to demonstrate the link between the annual rainfalls and El Niño or the SST of the eastern Pacific. For this, we first examined the coincidence between the Niño years and years with rainfall surpluses. Then we determined the differences between the average annual rainfall totals of the Niño and normal years within the 18 zones. This difference, expressed in millimeters, affords an appreciation of the quantity of rainfall associated on average with El Niño. Expressed in percentage, it yields an estimate of the importance of the event compared to the normal rainfall. Using an ascending classification of the rainfall indices and of the SST of the Niño 1+2 block, we grouped the rainfall zones according to the similarities between their inter-annual variation and determined which zones were best related to the SST variation of eastern Pacific and thus linked to El Niño. We finished by a principal component analysis of the annual rainfall indices in order to group the indices with respect to the similarity between their inter-annual variability and their sensitivity to ENSO. The comparison and synthesis of these analyses allowed us to divide Ecuador into three regions. ENSO has a strong influence on the annual rainfalls in the majority of the Ecuador coastal zone included between Esmeraldas in the north (1°N) and Peru in the south. On the other hand, there is no direct link between ENSO and total rainfall in the inter-Andean and Amazonian slope regions. Finally, the western slopes of the Andes, the northern coastal regions and the eastern slope of the coastal range represent a transition area influenced by ENSO, but to a lesser extent than the rest of the coastal regions.