Abstract
The observed multi-annual variability of streamflow for three rivers (Paraná River (PR), Uruguay River (UR) and Negro River (NR)) in La Plata Basin (LPB) during the twentieth century is analysed. Several spectral methods (singular spectrum analysis, maximum entropy method and multi-taper method) are applied to annual and seasonal run-off time series in order to capture low frequency variability (LFV) modes and pseudo-periodic patterns. Very robust quasi-periods in the three to six years band are detected for the three rivers, suggesting a strong link with El Niño-Southern Oscillation. Pseudo-cycles of eight to nine years appear in NR and UR. No quasi-periods above 10 years are obtained for any of the rivers. The three rivers exhibit significant LFV components associated with increasing trends that show a clear seasonality. For PR, LFV modes appear between May and December while for UR this occurs during December to April. Both periods cover the whole annual cycle, giving rise to complementary behaviours in the timing of trends of these two rivers. Remarkably, for the three rivers, the most intense increasing trend and the minimum streamflow occur at the same time of the annual cycle. Six global climate indices are selected in order to analyse the relationship between the multi-year streamflow variability modes in LPB and those of global climate: Niño 3.4 (N3.4), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO), Southern Annular Mode (SAM) and Global Temperature Trend (GTT). Similar to the rivers, almost all these indices present very robust modes of variability in the 3–4.5 years band. Except for N3.4 and PDO, the indices show significant LFV modes that exhibit a pronounced increase at the common end of their records. The NAO, AMO and SAM seem to behave in a nearly cyclic way with periods longer than 60 years while the GTT increases steadily along the whole register.
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