A study on possible solar and geomagnetic effects on the precipitation over northwestern Argentina Teresita Heredia a,⇑ , Ana G. Elias a,b a Universidad Nacional de Tucuma ´ n, Facultad de Ciencias Exactas y Tecnologı ´a, Dpto. de Fı ´sica, Tucuma ´ n, Argentina b CONICET – Universidad Nacional de Tucuma ´ n, Facultad de Ciencias Exactas y Tecnologı ´a, Dpto. de Fı ´sica, Tucuma ´ n, Argentina Available online 29 January 2013 Abstract The precipitation over Tucuman (26.8°S; 65.2°W), which is representative of the Northwestern region of Argentina, is analyzed in search of an association with solar and geomagnetic activity, with the purpose of contributing to the controversial issue on the connec- tion between climate variation and anthropogenic vs. natural forcing. Monthly time series of precipitation, sunspot number (Rz), and aa index were used for the period 1884–2010. A wavelet analysis was performed first which, due to the time series length, shows significant results only for periodicities lower than 32 years. Due to the transient character and non-constant phase of the results, any sustained wavelet coherence between precipitation and either sunspots or aa could be noticed. Moving averages and correlations were also assessed. The 11 and 22-year running mean of precipitation is positively correlated to Rz and aa when the whole period of analysis is considered. However, a shift in the long-term behavior of precipitation is noticed around 1940, which implies different correlation values with Rz and aa when the period before or after this year are considered. The solar cycle length is also considered for this statistical study and partly confirms the results obtained with Rz and aa. We propose plausible physical explanations based on geomagnetic activity and total solar irradiance effects over atmospheric circulation that could support the statistical result. A deeper analysis and broader geo- graphical coverage is needed in order to detect a connection between precipitation and solar variability discernible from greenhouse gases effects. We emphasize the idea of the importance of recognizing and quantifying the different forcing acting on precipitation (or any other climate parameter), which sometimes can be barely evident from a solely statistical analysis. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Precipitation; Solar activity; Geomagnetic activity; Wavelet 1. Introduction The study of periodicities in solar and terrestrial atmo- sphere data has long been of interest, being important for understanding solar variability and solar-terrestrial rela- tionships (Hoyt and Schatten, 1997; Le Treut et al., 2007). Specially now, for global warming studies, it is essential to elucidate the natural variability in climate parameters (Solomon et al., 2007). Among the earliest hints of sun-climate connection is that by Antonio Maria Schyrleo de Rheita in 1645 (Hoyt and Schatten, 1997). Since then on there were successive studies which analyze and suggest an association between climate and solar variability (for good reviews see Hoyt and Schatten, 1997 and Brunetti, 2003). Particularly, in the 1990s, a controversial paper by Friis-Christensen and Lassen (1991) intensified the debate on the role of the sun in climate change. Their achievement was to use the solar cycle length (SCL) instead of the sunspot number (Rz), the 10.7 cm solar radio flux (F10.7) or any geomag- netic activity index. SCL lags ahead Rz around 20 years, and this was enough to match almost exactly the time series of the Northern Hemisphere temperature. Although there are many papers showing statistically significant correlations between climate parameters and solar variations, there is not a convincing physical 0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.01.020 ⇑ Corresponding author. Tel./fax: +54 381 4365710. E-mail addresses: theredia@herrera.unt.edu.ar (T. Heredia), aelias@ herrera.unt.edu.ar (A.G. Elias). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 51 (2013) 1883–1892