Detection of statistically significant trends in the summer precipitation of mainland Spain

Abstract

In the context of global warming, it is a well-known fact that there is an increase in precipitation in middle latitudes. Convective phenomena have also been on the increase in the past few decades. It is gradually becoming more difficult to characterize climatic trends in middle latitudes. Data show that there may be different results depending on the study zones. Thus, it is interesting to study climatic changes in recent precipitation series, especially if we can have at our disposal a dense network of rainfall stations with reliable precipitation series.

The analysis of precipitation series in 333 rainfall stations from 1961 to 1990 has revealed that summer precipitation shows a statistically significant trend in wide areas of Mainland Spain. In the months of July and August, the precipitation is exclusively convective. In those months, the precipitation increased in 20% of the southern areas, that is, in those areas where temperatures are on average higher. June and September present different trends showing a decrease in precipitation in 10% of the territory.

Introduction

Over the 20th century, annual zonally averaged precipitation increased about 7% to 12% for the zones 30°N to 85°N in emerged landmasses in the Northern hemisphere, except in the Far East (IPCC, 2001). There is a general consensus about a number of changes in precipitation during the 20th century. It has also been noted that in middle latitudes—where most of the world's population lives—changes occur on a regional scale. However, in the zone between 10°N and 30°N the precipitation over emerged landmasses has decreased in 0.3%. In tropical latitudes, there seems to be an increase of approximately 0.2% to 0.3%. The data illustrate a number of changes that seem to be taking place. These changes are usually put down to global warming, which favors convectivity and distributes precipitation differently Woodward and Gray, 1993, Hansen and Lebedeff, 1987.

Other authors such as Wetherald and Manabe (1995) present models pointing towards the existence of certain regions with summer droughts, where the drought may extend over other seasons of the year. This fact is especially important in those territories that are characteristic for their aridity.

In the Mediterranean area, precipitation is scarce and water requirements are increasing. Consequently, the amount of precipitated water is a very important variable within the sustainable development framework in the area. Some authors have found (Piervitali et al., 1998) a negative trend in precipitation from the 1950s in the Mediterranean Basin. The summer period is of special relevance because the precipitation is convective and the water requirements of the population and of agricultural and industrial activities are constantly increasing. The problem is more serious in the southwest of Europe, since the area is visited every summer by millions of tourists demanding large rates of water. Hence, it is important to establish trends of precipitation series during the summer months in Spain.

There has been a pattern of continued aridity since the late 1960s trough North Africa south of Sahara. This effect is most persistent in the western region (Nicholson, 1993). Is this signal detected in the western Mediterranean region of Europe? The Iberian Peninsula is separated from the North of Africa by only a few kilometers and the question is: are there noticeable changes in the trends of summer precipitation in the Iberian Peninsula?

This paper presents an analysis of the trends of precipitation series in Mainland Spain between 1961 and 1990. In Spain, a vast network of rainfall stations records precipitation data on a daily basis. The orography varies a lot from one region to another and the mainland is influenced by either the Atlantic climate or by the Mediterranean climate. Taking into account this variability, the analysis was carried out using the data from as many different stations as possible. The aim is to study zones with/without climatic changes in summer precipitation. The paper describes the methodology used for the detection of the trends in the summer precipitation series and presents the results found. In addition, the paper includes the methodology employed to locate the regions with clear trends in monthly precipitation series in the period when the precipitation is convective.