The influence of the North Atlantic Oscillation on the winter characteristics of Windermere (UK) and Pääjärvi (Finland)

The atmospheric pressure gradient known as the North Atlantic Oscillation (NAO) has been shown to influence the dynamics of lakes in a number of European regions. Here, we compare the impact of the NAO on the winter dynamics of Windermere, a lake located in the north-west of England, and Pääjärvi, a lake situated in southern Finland. At both sites, the variations in the NAO index had a significant effect on the winter weather and the physical and chemical characteristics of the lakes. At Windermere, the strongest correlations were those noted for the air temperature, precipitation, the number of days when ice was recorded on the lake and the de-trended concentration of nitrate. At Pääjärvi the strongest correlations were with the air temperature and the de-trended concentrations of nitrate. One striking difference between the sites was the sign of the correlations noted between the NAO index and the de-trended nitrate. At Windermere, this correlation was negative and appeared to be driven by the enhanced terrestrial uptake of nitrate in mild winters. At Pääjärvi, the correlation was positive and was associated with the earlier `flush' of melt water from the catchment. The results are discussed in relation to the regional effects of climate change and the sensitivity of boreal lakes to subtle changes in the `centre of action' of the NAO.