Spatial and temporal variations of the precipitation regime and trend of the last century in mainland Spain

The evolution of the seasonal rainfall regime and trend in mainland Spain (western Mediterranean basin) in the period 1916-2015 has been analyzed with the new high spatial resolution grid of the MOPREDAS_century (10 km²) database, included in CLICES project.

Seasonal rainfall regime changes have been analysed in mainland Spain. Comparison of the seasonal precipitation values in four different periods of 25 years in length were done. The spatial distribution of seasonal rainfall highlights a winter regime to the north and west, autumn to the eastern Mediterranean coastland, and spring predominates in between the aforementioned areas, but the analysis shows that the seasonal distribution of precipitation has undergone remarkable changes between 1916 and 2015. During the first 25 years’ period (1916-1940) winter predominates in 44% of grid, increasing to 55% in 1941-1965 and to 60% in 1966-1990 to decrease in the most recent period (1991-2015) to 38%; in the meantime, spring remain around 30% until 1991-2015 when decrease to 16%, and autumn, initially occupying 33%, decrease in the 2^nd and 3rth period to increase in the most recent ones to 50% of grid. These changes have been produce by the different behaviour of rainfall trends, particularly in spring (mostly related to march) and autumn (particularly October). Global spatial changes show the substitution of areas of winter regime by spring, and on the other hand spring substitution by autumn regime, but the analyses of detailed period discover a more complex pattern.

In general, the seasonal rainfall trend present changes throughout the analyzed period, with clear decreases in spring and increases in autumn in the final decades. However, the analysis of temporal scale allows us to observe that since the mid-1970s the seasonal precipitation trends are not statistically significant in the study area. The analysis in four periods of 25 years of the seasonal average values shows that the dominant seasonal regime of precipitations has undergone changes. Between 1916-2015, a replacement of the winter and spring regime for the autumn regime has been detected in extensive areas of the central western peninsular. The results are in line with previous analyzes carried out in the whole of the Mediterranean basin and especially in its western sector and suggest that they may be partly related to the variations in time of the NAO and WeMO atmospheric variability modes. Possible effects on natural systems and human activities are discussed, as a step prior to the adoption of mitigating measures.

The objectives and results obtained in the CLICES project are available on the website www.CLICES.unizar.es.