Precipitation is vital in the outer tropical Andes, regulating freshwater availability, flooding, glacier mass balance, and droughts. Precipitation in the region, however, is highly seasonal and exhibits considerable interannual variability. The primary driver in interannual variability is the El Niño Southern Oscillation (ENSO), with most investigations reporting that the El Niño (La Niña) results in negative (positive) precipitation anomalies across the region. Recent investigations, however, have identified substantial spatiotemporal differences in ENSO-precipitation relationships. Motivated by the dissimilarity of these findings, this study examines a carefully selected dataset (≥ 90% completeness) of ground-based precipitation observations from 75 high-elevation (≥ 2,500 m asl) meteorological stations in the tropical Andes of southern Peru and Bolivia to identify distinct groups and associated variability in precipitation characteristics (e.g., total seasonal precipitation, wet season onset, and wet season length). Using no spatial constraints, the K-Means algorithm optimally grouped stations into five easily identifiable groups. Groups 3, 4, and 5 farthest from the Amazon basin had significant positive (negative) precipitation anomalies (p < .05) during La v Niña (El Niño), aligning with the traditional view of ENSO-precipitation relationships while Groups 1 and 2 closest to the Amazon had opposite relationships. Additionally, though studies have reported delays in the wet season, years characterized by El Niño had an earlier wet season onset in all five groups. These findings may aid in improving seasonal climate prediction and managing water resources, and could also allow for improved interpretation of tropical Andean ice cores. vi ix Dedication I dedicate this thesis to my loving and beautiful wife Jenna Rose, our baby John Casimir, and any other future children with which God may bless our family.