Climate Variability in the Andes of Ecuador and Its Relation to Tropical Pacific and Atlantic Sea Surface Temperature Anomalies

The main spatiotemporal modes of seasonal precipitation and temperature variability in the Andes of Ecuador (18N-48S) and their relation to tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs) between 1963-92 are identified based on rotated principal component analysis and cross-correlation techniques. Outgoing longwave radiation composites are analyzed during periods of strong oceanic forcing to confirm the proposed physical mechanisms. 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 Nino-Southern Oscillation influence is most dominant in the northwestern part of the Andes during December-February (DJF) and in the eastern Cordillera during June-August (JJA) and in both cases associated with below- (above-) average precipitation during El Nino (La Nina) years. During most of the year precipitation variability over the eastern Andes is related to a dipolelike 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 over the eastern Cordillera. The only region with slightly increased precipitation during El Nino events is confined to a narrow area along the western Andean slope between 18 and 38S in close proximity to the Pacific. However, this relationship is weak and only apparent in DJF. Temperature variability in the Andes can largely be explained by SSTA in the tropical Pacific domain. The temperature response closely follows SSTA in the Nino-3 and Nino-3.4 regions with approximately one-month lag. The northernmost part of the Andes (north of 0.58N) is the only region where temperatures are significantly correlated with tropical North Atlantic SSTA.

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