Spatial and temporal variability of winter snow and precipitation days in the western and central Spanish Pyrenees

In this study we analysed the spatial distribution of the long-term average and interannual variability of the number of snow days (NSD) and the number of precipitation days (NPD) in winter (DJFM) in the Spanish Pyrenees, using data from 38 meteorological stations for the period 1981-2010. The interannual variability of the NSD and the NPD in winter was related to the frequency of weather types over the Iberian Peninsula. Data from six stations were also used to analyse a longer time period (1961-2013) to confirm the consistency of the results obtained during the main study period (1981-2010). The results indicated that the NPD is only influenced by the distance to sea whereas the NSD is determined by elevation and distance to the sea. A high frequency of west (W), northwest (NW) and cyclonic (C) weather systems led to a high NPD in winter across the entire study area, whereas the frequency of north (N) weather types was only correlated with the NPD at the most westerly stations. For the NSD there was a gradient from the Western Pyrenees to eastern areas, mainly explained by the frequency of N weather types in the former area, and high frequencies of NW and W weather types associated with the latter. For most stations there was no significant trend found in the NPD or the NSD for the 1981-2010 period. However, a slight decrease was found for stations strongly correlated with NW weather types, and a slight increase was found for stations strongly correlated with the C weather type, which was related to a decreasing (increasing) frequency of NW (C) weather types during the same period. Analysis of the 1961-2013 and 1971-2000 time slices using a smaller subset of stations revealed a similar relationship between weather types and the NSD. This indicates that the 1981-2010 period is sufficiently representative to describe the relationship of the NSD and the NPD to weather type frequency. However, the study period chosen can markedly influence the trends observed, as the results showed a statistically significant decrease in the NSD for the 1971-2000 period, but no significant trends for the 1961-2013 and 1980-2010 periods.