Climatology of 500 mb Cyclones and Anticyclones, 1950–85

Abstract

Abstract The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the n...