Influence of Winter North Atlantic Oscillation Index (NAO) on Climate and Snow Accumulation in the Mediterranean Mountains

This work analyses the influence of NAO on the interannual evolution of winter temperature, precipitation and snowpack in the Mediterranean mountains. Due to lack of snow data in many mountain areas, the occurrence of four different winter modes are used as a proxy of the amount of accumulated snow. Winter modes are defined on the basis of combined precipitation and temperature thresholds: warm and wet (WW), warm and dry (WD), cold and wet (CW), and cold and dry (CD). The study focuses on 15 relevant mountain areas located in the Mediterranean Europe, Morocco, Turkey and Lebanon. Moreover, we present the relationship between winter NAO and snow depth data in the Swiss Alps and the Spanish Pyrenees. It has been demonstrated that snowpack accumulation in a given year is closely related to the occurrence of these winter modes. However, such relationship is variable among mountain areas and also there are differences depending on elevation in a particular mountain range. Results show that occurrence of different winter modes is strongly related to the winter NAO for the majority of the mountain chains under study, although these relationships are weaker in the easternmost part of the Mediterranean basin. Moreover, it has also been proven that the snow cover response to winter NAO may differ spatially as a consequence of the different influence of winter NAO on precipitation and temperature. In Switzerland, NAO is correlated more with temperature than with precipitation. Therefore, the influence of NAO on snow is significant at the lowest elevation areas, where temperature is the main control on snowpack accumulation. On the other hand, over the Spanish Pyrenees the NAO mainly controls the interannual variability of precipitation. In this region, the highest correlation with snow is found at high elevations where the interannual variability of temperature does not significantly influences snowpack, whereas precipitation controls mainly the accumulation of snow.