Trends in the North Atlantic Oscillation–Northern Hemisphere Annular Mode during the Twentieth Century*

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Abstract
The North Atlantic Oscillation–Northern Hemisphere annular mode (NAO–NAM) has exhibited a trend over recent decades toward high index values. It has been argued that this trend is unprecedented in the historical record and that it may be attributable to anthropogenic forcing. This study compares and contrasts wintertime trends during the recent period of rising NAO–NAM indices with trends earlier in the century when the indices declined. This analysis finds that the spatial patterns of the trends in sea level pressure (SLP) and surface air temperature (SAT) are markedly different in the two periods. The prior SLP trends were large only in the Atlantic sector, whereas the recent trends of the last few decades were more hemispheric in extent. As a result, trends in a more localized NAO index are roughly comparable for those two intervals, but the recent upward trend in the hemispheric NAM index is more prominent than the earlier downward trend.

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