Seasonality of the North Atlantic Oscillation

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Abstract
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 ... Abstract 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 ...

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