, Volume 61, Issue 1-2, pp 19-28

Low Frequency Variability of Rainfall in Southeastern South America

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Summary

The climatic variations of precipitation, within the period range of 10 and 100 years, in the Southeastern South America (SSA) region are investigated. A gaussian low-pass filter with a cut-off period at 30 years, was applied to annual precipitation records, in order to smooth high frequencies and to make evident the variability within the searched low frequency ranges (longer-than-30 years period).

The principal components analysis (PCA) applied to filtered series allowed the determination of two climatic signals, defined by the first and second principal components (PC 1 and PC 2). Both signals reflect remarkable changes in the annual precipitation values (a sustained increase in precipitation), over a large region of SSA (great part of Argentine pampas and Uruguay).

The PCA applied to the filtered precipitation records for each quarter, which we have associated to seasonal rainfall, allowed the determination of the incidence of PC 1 and PC 2 on the variability of the series. In particular, it is in the summer rainfall where the two first seasonal components, (PC11 and PC12), show a high degree of similarity (correlations >0.8) with the respective annual components PC 1 and PC 2. During the spring season only the first component, (PC41), shows high correlation with PC 1; the rest of the seasonal components do not show correlations exceeding the established values of 0.8.

Received May 5, 1997 Revised April 26, 1998