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 var iability in the Andes of Ecuador (1 degrees N-4 degrees S) and their relati on to tropical Pacific and Atlantic sea surface temperature anomalies (SSTA s) between 1963-92 are identified based on rotated principal component anal ysis and cross-correlation techniques. Outgoing longwave radiation composit es are analyzed during periods of strong oceanic forcing to confirm the pro posed physical mechanisms. Despite the close proximity to the Pacific, prec ipitation 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 parr of the Andes during December-Febr uary (DJF) and in the eastern Cordillera during June-August (JJA) and in bo th cases associated with below- (above-) average precipitation during El Ni no (La Nina) years. During most of the year precipitation variability over the eastern Andes is related to a dipolelike correlation structure in the t ropical Atlantic. featuring positive correlations with SSTA to the south of the ITCZ and negative correlations to the north. The proposed mechanism in volves positive SSTA in the tropical South Atlantic and contemporaneous neg ative SSTA in the tropical North Atlantic, resulting in increased rainfall over the eastern Cordillera. The only region with slightly increased precip itation during El Nino events is confined to a narrow area along the wester n Andean slope between 1 degrees and 3 degrees S in close proximity to the Pacific. However, this relationship is weak. and only apparent in DJF Tempe rature variability in the Andes can largely be explained by SSTA in the tro pical Pacific domain. The temperature response closely follows SSTA in the Nino-3 and Nino-3.4 regions with approximately one-month lag. The northernm ost part of the Andes (north of 0.5 degrees N) is the only region where tem peratures are significantly correlated with tropical North Atlantic SSTA.