Vol.:(0123456789) 1 3 Climate Dynamics (2019) 53:405–426 https://doi.org/10.1007/s00382-018-4590-8 New insights into the rainfall variability in the tropical Andes on seasonal and interannual time scales Hans Segura 1  · Clementine Junquas 1  · Jhan Carlo Espinoza 1,2  · Mathias Vuille 3  · Yakelyn R. Jauregui 4  · Antoine Rabatel 1  · Thomas Condom 1  · Thierry Lebel 1 Received: 27 February 2018 / Accepted: 17 December 2018 / Published online: 2 January 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract 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 specifc 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 eleva- tion 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 Pacifc. In addi- tion, EMWE (EMDE) in the EA are associated with an anomalous southward (northward) displaced eastern Pacifc ITCZ. Moreover, we fnd 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 tem- perature anomalies in the western (central) equatorial Pacifc. Keywords Tropical Andes · Precipitation · EOF analysis · Atmospheric mechanisms · Seasonal and interannual variability 1 Introduction The Andes are the largest mountain chain in the world, extending South–North over 7240 km from Patagonia to Venezuela, and the highest mountain chain situated in the inner tropics. Over the south tropical and equatorial region the Andes act as a topographic barrier between the cold and dry eastern Pacifc and the warm and moist Amazon region, which produces a strong zonal gradient of precipitation (Fig. 1a, b; e.g. Bendix and Lauer 1992; Garreaud 1999; Houston and Hartley 2003; Garreaud 2009; Espinoza et al. 2015). The complexity of the topography and the low density of the rainfall network over these Andean regions have com- plicated the proper characterization of the spatio-temporal Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00382-018-4590-8) contains supplementary material, which is available to authorized users. * Hans Segura hans.segura@univ-grenoble-alpes.fr 1 Univ. Grenoble Alpes, IRD, CNRS, G-INP, IGE (UMR 5001), 38000 Grenoble, France 2 Instituto Geofísico del Perú (IGP), Lima, Peru 3 Department of Atmospheric and Environmental Sciences, State University of New York at Albany, Albany, NY, USA 4 Department of Atmospheric Sciences, University of Washington, Seattle, USA