Tropical And Stratospheric Influences On Winter Atmospheric Circulation Patterns In The North Atlantic Sector

Seasonal forecast systems have demonstrated significant skill in forecasting the winter North Atlantic Oscillation (NAO) in recent years. This skill is a result of influences that have remote origins, such as the El Nino-Southern Oscillation or the stratospheric Quasi-Biennial Oscillation. Here we assess the role of the tropical troposphere and the extra-tropical stratosphere in driving North Atlantic-European wintertime sea level pressure, using numerical experiments in which conditions in these regions are constrained to be similar to observational reanalyses. We show that both of these remote regions play a role in influencing surface circulation. In particular, the East Atlantic Pattern-the second most important mode of regional winter variability after the NAO-is linked to convective anomalies in the tropical Atlantic. Tropical and stratospheric influences are shown to lead to the reproduction of observed large-scale pressure patterns in the majority of winters, but often by a single driver rather than a blend of both. We argue that while stratospheric influences are well represented, tropical influences are underrepresented in winter predictions. These results highlight tropical Atlantic predictability as an important focus for further research.