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 northeastern Atlantic Ocean while relatively high 500 mb cyclone frequency is found over the central subtropics Pacific Ocean and near Alaska during summer, southwestern North America during winter, and near southwestern Europe throughout the year. Abnormally strong diffluent flow over southwestern North America is suggested as an antecedent condition for 500 mb cyclogenesis in this same region. The correlation between 500 mb cyclone frequencies and 300 mb westerly momentum transports is also investigated, revealing that 500 mb cyclones may be associated with the convergence of westerly momentum into the 300 mb westerly jet. Finally, temporal trends in the frequencies indicate that 500 mb cyclone frequencies declined from 1950 through 1970 but increased from 1971 through 1985, while 500 mb anticyclone frequencies declined from 1950 through 1985.
