Search

In this paper a composite analysis was used to assess the influence of the North Atlantic Oscillation (NAO) on the winter daily rainfall and seasonal runoff at 28 stations of the Abruzzo region (Central Italy) during the period 1951-2012. Compositing was based on NAO and NAO phases, identified by mean winter values of the normalized NAO index (NAOI) ≤−0.75 and ≥+0.75, respectively. In accordance with previous studies, it was found that NAO phases determine, in general, a greater number of wet days ( N ) and (consequently) higher seasonal rainfall amounts in comparison to NAO phases. However, the NAO influence is characterized by a certain spatial variability, that can mostly be explained by orographic effects due to the Apennine Mountains. This is particularly evident for the mean rainfall depth per event ( P ) that, during NAO phases, increases for the stations to the west of the Apennines, while it decreases for most of the stations to the east. The structure of winter daily rainfall of NAO and NAO type, was described by a simple but effective first-order Markov process, determining the transition probabilities P01 (dry to wet) and P10 (wet to dry) and modelling the rainfall depth on wet days by a Weibull distribution. The most significant influence of NAO concerns P01 and the shape parameter of the Weibull distribution that are both higher during the NAO phase. This means that NAO phases are characterized by less persistent dry periods and less variable daily rainfall depths, in comparison to NAO phases. The effect of these differences on the winter seasonal runoff was explored by applying a Curve Number rainfall-runoff model. Significant increments of the mean seasonal runoff during NAO phases were observed only for few stations (mainly on the west), characterized by corresponding increments of N , P and P .). NAO phases, instead, are characterized by relevant increments of the seasonal runoff variability, particularly on the eastern areas. In this context, the important regulating function of the watershed conditions was also discussed. [ABSTRACT FROM AUTHOR]