The effects of the NAO on the ice phenology of Spanish alpine lakes

Three Spanish alpine lakes located in the Central Range (Pealara Lake and Cimera Lake) and the Pyrenees (Redon Lake) are selected to understand the effects of the North Atlantic Oscillation (NAO) on ice phenology. A conceptual lake model is formulated based on Pearson's correlation coefficients obtained between season-scale time series of the NAO index, climatic data (i.e., precipitation, air temperature and snow data) and limnological variables (ice phenology records). The results suggest that the effects of the NAO are only reflected in the thawing process via the air temperature and the insulating effect of snow accumulation on the ice cover. An altitude component is evident in our survey because the effects of the NAO on Pealara Lake (the lowest altitude studied lake) are restricted to winter, whereas for Redon Lake (the highest altitude studied lake), the effects extend into spring. A latitudinal component is also clear when comparing our data with northern European lakes. Snow accumulation primarily depends on the air temperature at high latitudes, and both precipitation and the air temperature control snow accumulation at lower latitudes. Consequently, in northern Europe, the NAO signal is primarily reflected in lake ice phenology via the air temperature, whereas our results confirm that in southern Europe, the strong dependence of precipitation on the NAO determines the importance of the NAO for lake ice cover.