New insights into the rainfall variability in the tropical Andes on seasonal and interannual time scales

In this study, we analyze the atmospheric mechanisms associated with the main rainfall patterns in the tropical Andes (20 ^∘S –1 ^∘N) on seasonal and interannual time scales. Using a homogeneous and high spatial resolution precipitation data set (0. 05 ^∘× 0. 05 ^∘) at monthly time step (CHIRPS; 1981–2016), in-situ precipitation from 206 rain-gauge stations, power spectrum and EOF analysis, we identify three Andean regions characterized by specific seasonal and interannual rainfall modes: the equatorial Andes (EA, 5 ^∘S –1 ^∘N), the transition zone (TZ, 8 ^∘S –5 ^∘S) and the southern tropical Andes (STA, 20 ^∘S –8 ^∘S). On seasonal time scales, the main mode of precipitation in the EA and STA are characterized by a unimodal regime, while the TZ is represented by a bimodal regime. The EA and the TZ share the same wet season in the February–April period, which is associated with a weakened Walker Cell, the southerly position of the Intertropical Convergence Zone (ITCZ) and a strong westward humidity transport from the equatorial Amazon. This latter mechanism and a reduced elevation of the Andes are associated with the October–November wet season in the TZ. The presence of the Bolivian High and the northward extension of the Low Level Jet are associated with the precipitation over Andean regions between 20^∘S and 8^∘S in the December–March period. On interannual time scales, extreme monthly wet events (EMWE) in the STA (TZ) are related to convection over the western (equatorial) Amazon during the December–March (February–April) period, showing an atmospheric relationship between the Amazon and the Andes. Extreme monthly dry events (EMDE) in the TZ and in the EA during the February–April period are related to a strengthened Walker Cell, especially in the eastern Pacific. In addition, EMWE (EMDE) in the EA are associated with an anomalous southward (northward) displaced eastern Pacific ITCZ. Moreover, we find a relationship between precipitation at higher elevations in the Andes north of 10 ^∘S and easterly winds at 200 hPa during February–April EMWE. Finally, extreme monthly events in the EA (STA) are related to sea surface temperature anomalies in the western (central) equatorial Pacific.