The North Atlantic Oscillation and its relationship with near surface temperature

A new NAO index is presented here using homogenized surface pressure data from Reykjavik and Gibraltar (for November to March) and Reykjavik and Ponta Delgada (for April to October). This index suggests that the positive trend in recent years is not unprecedented, as the early 20th century was also a period of persistently positive NAO values. The relationship between the secular warming trend and the influence of the NAO on near-surface temperatures over the North Atlantic region and surrounding land masses is examined on a seasonal basis using standardized temperature anomalies since 1900. The nearsurface temperature field separates into two independent modes, which we designate a warming mode and dynamic (NAO) mode, with distinct seasonal cycles.