Seasonality of the North Atlantic Oscillation

Monthly sea level pressure (SLP) data from the National Centers for Environmental Prediction reanalysis for 1948–99 are used to develop a seasonally and geographically varying ‘‘mobile’’ index of the North Atlantic oscillation (NAOm). NAOm is defined as the difference between normalized SLP anomalies at the locations of maximum negative correlation between the subtropical and subpolar North Atlantic SLP. The subtropical nodal point migrates westward and slightly northward into the central North Atlantic from winter to summer. The NAOm index is robust across datasets, and correlates more highly than EOF coefficients with historical measures of westerly wind intensity across North Atlantic midlatitudes. As measured by this ‘‘mobile index,’’ the NAO’s nodes maintain their correlation from winter to summer to a greater degree than traditional NAO indices based on fixed stations in the eastern North Atlantic (Azores, Lisbon, Iceland). When the NAOm index is extended back to 1873, its annual values during the late 1800s are strongly negative due to negative contributions from all seasons, amplifying fluctuations present in traditional winter-only indices. In contrast, after the mid-1950s, the values for different seasons sufficiently offset each other to make the annually averaged excursions of NAOm smaller than those of winter-only indices. Global teleconnection fields show that the wider influence of the NAO—particularly in the western North Atlantic, eastern North America, and Arctic—is more apparent during spring–summer–autumn when the NAOm is used to characterize the NAO. Thus, the mobile index should be useful in NAO investigations that involve seasonality.