Synoptic Climatology of 500 mb Circulation Changes during Explosive Cyclogenesis

Abstract

Changes in the 500 mb circulation are examined near and during each of 141 explosive surface cyclogenesis cases during the period 1 September 1980 through 31 May 1987 (June through August excluded) over the western half of the Northern Hemisphere. Two types of circulation changes evident in the geopotential height fields are investigated: 1) amplification of 500 mb troughs and/or ridges in which the explosively deepening cyclones (bombs) are imbedded and 2) formation of 500 mb cyclones (closed lows) or anticyclones (closed highs) near the bombs. By either stratification, most bombs in the sample are found not to be associated with nearby 500 mb circulation changes on the time scale of the explosive deepening. Relatively weak bombs are more likely to be associated with identifiable circulation changes than those which are moderate or strong. Antecedent conditions for the bomb-related circulation changes are examined. For example, bombs associated with 500 mb trough (ridge) amplifications are connected with southeastward (northeastward) moving disturbances at 500 mb imbedded in the larger scale 500 mb trough (ridge). Conversely, all persistent 500 mb cyclone-anticyclone pairs (blocking patterns) identified in the sample were preceded in their formations by nearby explosive surface cyclogenesis.