The North Atlantic Oscillation (NAO) is a large-scale atmospheric circulation pattern with marked effects on Northern Hemisphere climate. A positive phase of the NAO (NAO+), is associated with increased westerlies. Over Northwest Europe, NAO+ in winter coincides mild temperatures and above average precipitation. In summer, however, NAO+ leads to rather cool and unsettled conditions over the same region.
Even though the NAO is one of the most prominent modes of atmospheric variability during all seasons, its relative importance in regulating the variability of the European climate during non-winter months has been less studied and is less well understood than for the winter season. In particular, there is a lack of studies exploring the impact of wintertime NAO on subsequent seasons.
In this study, we show that a persistent wintertime NAO+ can interact with slower components of the climate system (e.g., ocean, land surface, soil, sea ice, and snow), inducing persistent surface anomalies that may affect the atmosphere via surface feedback mechanisms resulting in a lagged atmospheric response. We show that the major Central European heat waves since 1980 were preceded by a prolonged positive phase in the NAO during the preceding winter /spring months.
This suggests that persistent wintertime NAO+ might be a necessary condition to the development of such extreme events.
Given the increased capability to predict winter NAO events in the latest generation of seasonal forecasting systems, the outcome of this study could be a step towards forecasting Central European heat waves from one year to the next.