Regional Synchronies and Amplitude Changes in the January 500-Mb Middle Latitude Geopotential Height Record

The relationship between middle tropospheric long wave circulation and surface weather is recognized as an important component in the middle latitude climate system. Regional variations in long wave position and temporal variations in long wave amplitude exert a considerable influence on surface temperature and precipitation. This study seeks to organize the spatial and temporal structure of the long wave system within a climate regionalization that is based upon associated trends in the middle tropospheric geopotential height record for the purpose of assessing wave amplitude variations within these regions. This regionalization is performed using January 500-mb geopotential height data over the northern hemispheric middle latitudes for the period 1946–1987 and is constructed using a cluster analysis, in which the resultant clusters represent regions of synchronous height behavior during the data record. The resultant clusters are related spatially to long wave ridge and trough positions, and are characterized temporally by changes in wave amplitude. The regionalization is best developed over the Pacific Ocean, North America, and the Atlantic Ocean. Before 1960, geopotential heights were higher within both the ridge and trough clusters, suggesting that the overall 500-mb surface may have been higher, similar to that occurring when the circumpolar vortex is contracted. After a period of variable heights during the early 1960s, the flow became more zonal during the late 1960s and early 1970s. After 1975, Pacific, North American, and Atlantic flow became more amplified. [Key words: climatic regionalization, teleconnection, long waves, cluster analysis.]