VARIABILITY OF PRECIPITATION IN THE ARGENTINIAN ALTIPLANO. INCIDENCE OF THE 1976/1977 CLIMATE TRANSITION AND THE ENSO PHENOMENON IN NORTHWESTERN ARGENTINA.
Daniel Barrera y Alejandro Maggi
Universidad de Buenos Aires, CONICET, FAUBA, Buenos Aires, Argentina
Departamento de Ingeniería Agrícola y Uso de la Tierra, FAUBA UBA
Departamento de Ciencias de la Atmósfera y los Océanos, FCEN UBA
Manuscript received on 2nd November 2015, in final form on 19th April 2017
From monthly rainfall series in six locations in Northwest Argentina (NOA) in the period 1935/36 – 2015/16 we built series of cumulative rainfall for the monsoonal period October-April (O-A). We verified the occurrence of climatic jumps in precipitation, which are in harmony with the Pacific Climate Transition (TCP) 1976/77. We showed that subsequent to the TCP occurred a significant change in rainfall in the semiarid NOA sub-region. Decreasing precipitation trends from the 1990s in the NOA region were found. Three cumulative O-A ENSO indices associated to the monsoonal period were built from monthly indexes: the Southern Oscillation Index (SOI), the Oceanic Niño Index (ONI) and the Multivariate ENSO Index (MEI). They showed high cross-correlations. We also proposed three new O-A ENSO indices (location dependent) based on SOI, ONI and MEI and weighted with the average monthly rainfall at a given location. We performed polynomial regressions relating them with O-A precipitation. These indices explained relevant percentages of O-A precipitation variability only in Abra Pampa (AP) and La Quiaca (LQ) (the Altiplano sub-region). We found that in LQ there is a significant higher proportion of accumulated rainfall in October, November and December compared to AP. We developed one criteria to classify, from the monthly ONI index, the O-A periods as Neutral (NE), Niña (NA) and Niño (NO) phases. When splitting the record data into two sub-series (before and after the TCP) and discriminating the monsoonal periods according to ENSO phases, samples became small so the following conclusions are preliminary. Shifts to higher values of precipitation were found in all three phases. It was found that after climate shifts in AP and LQ there is less interannual variability of precipitation in the NA phases with respect to NE and NO phases. Most precipitation maximums occur in NE phase. Mean O-A precipitation in NO phase before and after the TCP were significantly lower than in NE and NA phases in both locations. Increased rainfall variability in the NO phase after the TCP was also found.