THE NORTH-ATLANTIC SUBTROPICAL ANTICYCLONE

The semipermanent subtropical anticyclone over the North Atlantic basi n (the ''Azores high'') has a major influence on the weather and clima te of much of North America, western Europe, and northwestern Africa. The authors develop a climatology of the Azores high by examining its spatial and temporal changes since 1899. Using gridded surface pressur e values, anticyclones are identified when the daily pressure is great er than or equal to 1020 mb and frequencies are tabulated for each hal f month from 1898 to 1990. Principal components analysis is applied to analyze the anticyclone's spatial variance structure. The Azores high is dominated by two spatial modes: a summer pattern, in which high pr essure dominates the Atlantic basin, and a winter pattern, in which an ticyclones are present over eastern North America and northwestern Afr ica. Century-long declines in these two modes indicate that there has been a net removal of atmospheric mass over the subtropical Atlantic. Other modes include a meridional versus zonal circulation pattern and omega blocks. Time series of the mean annual principal component score s indicate that meridional Row has been increasing over the Atlantic a nd that blocking anticyclones have become more prevalent over west-cen tral Europe and less common over the northeastern Atlantic and the Bri tish Isles.