Long Term Variability of Surface Solar Radiation, Sunshine Duration and Diurnal Temperature Range at São Paulo, Brazil

Fifty-six years of daily downward solar irradiation or simply surface solar radiation (SSR), diurnal temperature range (DTR), sunshine duration (SD) and sky cover fraction (SCF) observed at São Paulo, Brazil, were analyzed in order to verify if, similar to other parts of the globe, dimming and brightening of global solar radiation periods can be identified. The second objective was to analyze the contribution of cloud cover to the observed SSR variability. Finally, we aimed at analyzing how well SD and DTR at São Paulo perform as proxy data to assess SSR variability. Results showed a clear change in the variables trend as of 1983, when SSR, SD and DTR reached their lowest annual mean values and SCF presented a peak, what motivated us to separate the time series in two periods, one from 1961 to 1983 and the second from 1983 up to 2016. It is worth to mention that 1983 was characterized by a strong El Niño event. Applying the modified Mann-Kendall trend test, we found that, in the first period, a 95% statistically significant positive trend for SCF was observed, with an increasing trend of about 2.9 %/decade. Other variables presenting statistically significant, but negative trend, were SD with approximately -0.43 hours/decade and DTR of -0.57 °C/decade. SSR also presented negative trend, but at a 90% level, of about -47 kJ m^-2/decade. Thus, the first period trend coincides with the dimming observed in other parts of the globe and, at São Paulo, it can be attributed mainly to cloud cover variability. During the second period, cloud fraction presented no significant trend, while SD and DTR presented statistically significant positive trend of around 0.18 hours/decade and 0.21 °C/decade, respectively. SSR, by contrast, presented a significant negative trend of about -28 kJ m^-2/decade, at 95% significance level, in spite of its positive correlation with SD and DTR of around 0.60 and 0.52, respectively. A preliminary hypothesis for the observed trend discrepancies in the second period is related to the aerosol direct effect being more significant on SSR than on SD and DTR. This hypothesis is a matter of ongoing investigation.