The North Atlantic Oscillation and its relationship with near surface temperature

A new NAO index is presented here using homogenized surface pressure data f rom Reykjavik and Gibraltar (for November to March) and Reykjavik and Ponta Delgada (for April to October). This index suggests that the positive tren d 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 temperat ures over the North Atlantic region and surrounding land masses is examined on a seasonal basis using standardized temperature anomalies since 1900. T he nearsurface temperature field separates into two independent modes, whic h we designate a "warming" mode and dynamic ("NAO") mode, with distinct sea sonal cycles.