Identification of drought phases in a 110-year record from Western Mediterranean basin: Trends, anomalies and periodicity analysis for Iberian Peninsula

The Iberian Peninsula (IP), located in a transition area between temperate and subtropical latitudes, has marked temporal and spatial variability of precipitation between its various climatic zones. Therefore, the peninsula is particularly sensitive to climate change owing to frequent periods of precipitation deficit together with societal dependence on water. In this study, we address meteorological droughts of the peninsula between 1901 and 2010, using the Global Precipitation Climatology Center database. First, we carried out a statistical classification of the peninsula according to annual precipitation cycle. The spatiotemporal variability of droughts was evaluated using the Standardized Precipitation Index (SPI) by determining the evolution of areas affected by severe and extreme drought. Subsequently, we calculated the overall trend of precipitation and anomalies to investigate whether an observed drought increase is attributable to a decrease of precipitation or an increase of precipitation extremes. Finally, we examined periodicities of precipitation anomalies via wavelet analysis, relating these to large-scale modes such as the North Atlantic Oscillation (NAO). The results show an increase in the area affected by severe and extreme drought, particularly on Atlantic-facing slopes. The mainland areas have longer and more intense periods of drought, with an average SPI of -0.52 and 4-month duration. Over the 110 years analyzed, we observed weak trends of annual precipitation in the extreme northwest and southern IP. However, in the last decades of the series, long periods of strong negative anomalies are seen, interspersed with years of strong positive anomalies, suggesting an increase in precipitation pattern extremes. Finally, the relationship between the NAO index and precipitation anomalies on the Atlantic seaboard is highlighted. Thus, in both series we found a 5-6-year periodicity in their first years, with a diminishing cycle period reaching a periodicity of 2-3 years in the final years. This confirms the existence of regular temporal cycles in which droughts alternate with wet periods modulated by large-scale modes. (C) 2015 Elsevier B.V. All rights reserved.