A multicentury perspective on the summer North Atlantic Oscillation (SNAO) and drought in the eastern Atlantic Region

The summer North Atlantic Oscillation (SNAO) is strongly associated with July-August climate variability over Europe, especially in northern regions. This association includes drought, where a positive SNAO corresponds to dry conditions over much of northern Europe and wet conditions in Southern Europe, but the SNAO/climate association is weaker and less homogeneous in the South. Here we use a dendroclimatological reconstruction of the SNAO for the last 550 a to investigate the SNAO/drought relationship in the past. An association between the SNAO and a regional summer drought index from Sweden Suggests that the northern European drought relationship holds back to 1700. In the last 550 a, the relationship between SNAO and drought in the Mediterranean region as a whole is weak, but over the Eastern Mediterranean the relationship is clearer and statistically significant (P < 0.05 level). The Mediterranean relationship is clearest at century scales. An association between the SNAO and Sahel rainfall can clearly be seen on interannual as well as longer timescales in the 20th century. Past droughts in the Sahel, as inferred from historical data, correspond quite well with positive phases of the SNAO on multidecadal timescales back to 1500, the phase expected from instrumental data. The physical reasons for the relationship between Sahel rainfall and the SNAO are, however, not yet understood. This research is a first step towards understanding how the atmospheric circulation over the North Atlantic region affects drought, necessary for forecast future droughts. Copyright (C) 2009 John Wiley & Sons, Ltd. and (C) Crown Copyright 2009.