Connection between NAO, weather types and precipitation in León, Spain (1948–2008)

S. Fernández‐González,A. Castro,R. Fraile,A. Penas,M. Fernández‐Raga,A. I. Calvo,S. del Río

doi:10.1002/joc.2431

Abstract

AbstractThe variability of winter precipitation in the west of the Iberian Peninsula is strongly affected by the North Atlantic Oscillation (NAO). This study focuses on identifying the relationship that exists between precipitation registered in the city of León (in northwestern Spain), the NAO index and the associated weather type during the same period. In order to achieve this objective, the prevailing weather type has been calculated for each day in León from January 1948 to March 2009, using the objective Lamb Weather Types classification method. The most significant results appear in winter (from December to March). During these months an increase has been observed in the frequency of ‘anticyclonic’ weather type (A) (very dry), and a decrease in the ‘cyclonic’ (C), ‘south‐westerly’ (SW) and ‘westerly’ (W) types (the three rainiest weather types). The positive trend in the NAO index could be the main cause of the decrease in the frequency of the three rainiest weather types (C, SW and W) and therefore, responsible for the relevant decline in winter precipitation observed in the city of León. The high correlation coefficients between the NAO index, the frequency of the three rainiest weather types and winter precipitation suggests an interesting method to forecast rainfall. Using a binary logistic regression model, a downscaling model for daily precipitation has been obtained based on the weather types and the NAO index. The daily results obtained for the winter months are good (TSS = 0.64) bearing in mind that only sea level pressure data were used in the logistic model. Copyright © 2011 Royal Meteorological Society

Highlights

The rainfall regime in the Iberian Peninsula is highly irregular, on both the spatial and the temporal scale (Esteban-Parra et al, 1998)
The limit is frequently set at f (z) = 0.5, in this study we have done the opposite: Huntrieser et al (1997) used the values of True Skill Statistic (TSS) to determine the optimum orientation of their different storm predictors, and in this study, we have looked for the threshold value of f (z) so that the skill scores have their maximum value
The main aim of this article was to obtain the daily classification of weather types for the city of Leon from January 1948 until March 2009, based on the classification of weather types developed by Lamb (1972) and improved and adapted for Iberia by Trigo and DaCamara (2000)

Summary

Introduction

The rainfall regime in the Iberian Peninsula is highly irregular, on both the spatial and the temporal scale (Esteban-Parra et al, 1998). del Rıo et al (2011) carried out a study on precipitation trends for the whole of Spain between 1961 and 2006, and found negative trends in more than 75% of the weather stations analysed. This decrease is significant in 18% of mainland Spain

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