Large-Scale Features of Monthly Mean Northern Hemisphere Anomaly Maps of Sea-Level Pressure

Spatial patterns of circulation variability over the Northern Hemisphere and their changes during the past 70 yr (1899–1969) are examined using eigenvector analyses of mean January and July sea-level pressure maps. The first several eigenvectors display variability associated with the major centers of action (the subpolar oceanic Lows, the subtropical oceanic Highs, the winter Siberian High, and the summer Asiatic Low). The pattern of the first eigenvector of January suggests that the intensity and latitudinal position of the major circulation features over the North Atlantic are associated with the intensity and position of the Aleutian Low over the North Pacific. The time series of the coefficients of the hemispheric eigenvectors are used to identify intervals of change in the hemispheric circulation associated with features on the scale of thc major centers of action. These time series provide a more general description of circulation change than that obtained from local or regional indices; but at the same time, they provide more detailed information than time series of hemispherically averaged indices. Two intervals of change in the large-scale hemispheric circulation are identified: the early to mid-1920s and the early to mid-1950s. Of the three periods separated by these two intervals of change, maximum contrast is noted between the first and third. For January, the strongest circulation features are found in the North Atlantic and European sectors in the first (earliest) period and in the North Pacific and Asian sectors in the third (latest) period.