Solar Forcing of the Stream Flow of a Continental Scale South American River

Solar forcing on climate has been reported in several studies although the evidence so far remains inconclusive. Here, we analyze the stream flow of one of the largest rivers in the world, the Paraná in southeastern South America. For the last century, we find a strong correlation with the sunspot number, in multidecadal time scales, and with larger solar activity corresponding to larger stream flow. The correlation coefficient is $r=0.78$, significant to a 99% level. In shorter time scales we find a strong correlation with El Niño. These results are a step toward flood prediction, which might have great social and economic impacts.

Figure 1

(a) Paraná’s annual stream flow at the Corrientes gauging station. (b) Yearly international sunspot number ($S_N$). (c) Solar irradiance reconstruction [22]. The secular trends, obtained with a low-pass Fourier filter with a 50-yr cutoff, are shown as thick lines.

Figure 2

The detrended time series for the Paraná’s stream flow (full line), the sunspot number (dashed line), and the irradiance reconstruction (dotted line). The detrended series were obtained by subtracting from each data series the corresponding trend, shown as a thick line in Fig. 1, and were smoothed by an 11-yr running mean to smooth out the solar cycle. The series were normalized by subtracting the mean and dividing by the standard deviation, to avoid introducing arbitrary free parameters. The Pearson’s correlation coefficient of the stream flow data with the sunspot number is $r=0.78$, and with the irradiance reconstruction is $r=0.69$.

Figure 3

Paraná’s stream flow minus the multidecadal component (full line) compared with the Niño1+2 index (dotted line), which is available from 1950. Both series were normalized by subtracting the mean and dividing by the standard deviation, to avoid introducing arbitrary free parameters. The Pearson’s correlation coefficient for the whole series is $r=0.65$.