Interannual climate variability in the Central Andes and its relation to tropical Pacific and Atlantic forcing

The main spatiotemporal modes of interannual temperature and austral summer (DJF) precipitation variability in the Central Andes are identified based on a two-way principal component analysis (PCA) of 30-year (1961-1990) monthly station data and related to contemporaneous tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs). In addition, various meteorological fields, based on National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, NOAA-Outgoing Longwave Radiation (OLR) and station data, are analyzed during periods of strong positive and negative SSTA and the respective composites tested for local significance using a Student's t-test approach. Temperature variability in the Central Andes is primarily related to El Nino - Southern Oscillation (ENSO) and closely follows SSTA in the central equatorial Pacific with a lag of 1-2 months. In the southern Altiplano, temperatures have significantly increased since the late 1970s. DJF precipitation is also primarily related to ENSO, featuring below (above) average precipitation during El Nino (La Nina). Precipitation over the dry western part of the Altiplano shows the closest relationship with ENSO, due to ENSO-induced atmospheric circulation anomalies. Precipitation variability over the western Altiplano features a decadal-scale oscillation, related to a similar climatic shift in the tropical Pacific domain in the late 1970s. Over the northern Altiplano the precipitation signal is reversed in the austral summer following the peak phase of ENSO, presumably due to the temporal evolution of tropical Pacific SSTA, rapidly switching from one state to the other. No evidence for a tropical Atlantic influence on DJF precipitation was found. SSTAs in the tropical NE Atlantic, however, presumably are influenced by heating and convection over the Altiplano through an upper air monsoon return flow, altering the strength of the NE trades that emanate from the Sahara High.