Statistical relationships between large-scale circulation patterns and local-scale effects: NAO and rainfall regime in a key area of the Mediterranean basin

Abstract

In this work, we investigated the correlation between the North Atlantic Oscillation (NAO) index and the rainfall trend in Tuscany (Italy) by using a large number of rain gauges for a high-resolution spatial scale study in a region characterized by significant morphological and climatic variability and equipped with an efficient measurement network. The relationship between NAO and rainfall was calculated by the Spearman's correlation coefficient. Our study shows that the correlation between NAO and precipitation has two types of oscillations as functions of the time scale. During the year the correlation is negative in winter and positive in summer, and this is due to global atmospheric circulation, which causes the area to be affected by humid air masses from the Atlantic Ocean during the winter. It is difficult to understand these experimental observations for the summer period because the correlation is influenced by the low rain levels of this area and the results could be easily influenced by other global patterns. Investigating in particular the period from December to March, we observed a variation in the NAO precipitation correlation also over time, with periods characterized by an increase in anti-correlation. The results obtained for Tuscany were contextualized and compared with other areas of Europe and of the Mediterranean basin by applying the same methodology. Spatial analysis showed that the trend of the NAO and rainfall correlation depends on latitude. In northern Europe, the behaviour of the correlation between NAO and rainfall over time is very similar to the temporal trend of NAO itself; instead, in the southern Mediterranean area this correlation has a trend over time that is very similar to the one of the Atlantic Multidecadal Oscillation (AMO) Index. This allowed us to observe a different regulation in the circulation of the humid air masses coming from the Atlantic Ocean, which induced rainfall in the Mediterranean and in northern Europe. The circulation of humid air masses in Northern Europe is linked to the pressure difference between high and low latitudes (represented by the NAO index), whereas in the Mediterranean it is linked to the temperature of the Atlantic Ocean (represented by the AMO index). The areas between high and low latitudes, characterized by a mixed behaviour, are regulated by both pressure difference and ocean temperature. This study has made it possible to investigate a specific area of the Mediterranean and then to extend and contextualize to more geographical locations, highlighting the fact that simple linear regression models can help to investigate the role of global patterns on the local effects.