Precipitation variability in the Andes of Ecuador and its relation to tropical Pacific and Atlantic sea surface temperature anomalies

The main modes of precipitation variability in the Andes of Ecuador (1°N - 4°S) between 1963-92 are identified, based on a principal component analysis (PCA) of monthly station data. The score time series of the main modes are correlated with tropical Pacific and Atlantic Sea Surface Temperature Anomalies (SSTA) to detect areas of significant oceanic forcing. To confirm the obtained results, a reverse procedure is applied, by extracting the main modes of tropical Pacific and Atlantic SSTA, again by means of PCA. The score time series of these main SSTA modes are then correlated against station precipitation data to see whether similar coherency patterns emerged. In most cases, this two-way approach strengthened the findings obtained and results compared very favorably. In addition, composite analysis of Outgoing Longwave Radiation (OLR) data during periods of strong oceanic forcing helped to gain a better understanding of the physical mechanisms involved.

Despite the close proximity to the Pacific, precipitation variability in the Andes of Ecuador is not related to SSTA in the tropical Pacific domain alone. The El Niño-Southern Oscillation (ENSO) influence is most dominant in the northwestern part of the Andes and associated with below (above) average precipitation during El Niño (La Niña) years. Correlations are highest during DJF, but also significant during SON and MAM. Precipitation along the eastern Andean slope is affected by ENSO mainly during the dry season (JJA). During most of the year however, precipitation variability on the eastern slopes is related to a dipole-like correlation structure in the tropical Atlantic, featuring positive correlations with SSTA to the south of the ITCZ, and negative correlations to the north. The proposed mechanism involves positive SSTA in the tropical South Atlantic and contemporaneous negative SSTA in the tropical North Atlantic, resulting in increased rainfall on the eastern Andean slopes. The only region with slightly increased precipitation during El Niño events is confined to a narrow area along the western Andean slope between 1°-3°S in close proximity to the Pacific. However, this relationship is weak and only apparent in DJF.