Non-linear trends and low frequency oscillations in annual precipitation over Argentina and Chile, 1931-1999

Abstract

Since the 1950’s-1960’s a large portion of Argentina has experienced a long period of high precipitation with important changes a 30-year period annual averages. Opposite conditions have taken place throughout the twentieth century in a large portion of Chile, with declining tendencies in annual precipitations. If such conditions prevail, the socio-economic implications of these phenomena could eventually gain importance due to the expansion of grain crops in the semi-arid borders of Argentina and a decline in the availability of reservoir water used for irrigation or energy generation in the central region of Chile and western Argentina. An accurate diagnosis of these phenomena is very important for applied purposes in medium term planning that should be undertaken by private and state-owned companies. These diagnoses are also important from a methodological point of view, in order to find out whether these changes in long-term averages are constant and represent a climatic change or if they are low frequency interdecadal fluctuations that would mean a return to opposite conditions. We suggest that the two intense La Ni˜na events that took place during 1988- 1989 and 1995-1996 caused a decline in the precipitation trends over a large region of semi-arid Argentina and intensified droughts in Central Chile. A regionalized local precipitation analysis of the 1931-1932/1998-1999 agricultural periods over large areas was performed. Afterwards, long-term variations for homogeneous areas were analysed through their nonlinear trends and low frequency fluctuations in a regional scale. Seven inter-annual patterns with three different types of tendencies have been found. One of them shows an increasing linear trend throughout the analysed period; another one shows an increasing trend, but also a decrease after the 1980’s, and a third one shows a continuous decrease. Regions with continuous trends include medium latitude semi-arid regions in Argentina and Chile, where an intense human impact is observed. An 18-26-year-quasi-period that affects four of the selected regions and has a clearer signal in the Cuyo region has been found. This long period signal is in phase with the oscillations of other atmospheric variables in South America and with precipitation in South Africa.