While not all persistent closed anticyclones (CAs) produce extreme weather, a subset of CAs that occur during the warm season over continental regions can be associated with severe heat waves. In order the better understand the seasonally varying distribution and hemispheric differences of these features, the results of an objective climatology of CAs at 200, 500 and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented. Three case studies of warm season continental CAs associated with severe heat waves will also be presented to supplement the climatological results. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP/NCAR reanalysis 200, 500 and 850 hPa geopotential height fields for the period 1950-2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once.

The climatology results show that 200 hPa CAs occur preferentially during summer over subtropical continental regions, while 500 hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850 hPa CAs occur preferentially over oceanic regions beneath upper-level mid-ocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter.

Three case studies of objectively identified CAs that produced heat waves over the United States, Europe and Australia in 1995, 2003 and 2004, respectively, illustrate how climatologically hot continental tropical air masses produced over arid and semi-arid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets. Furthermore, upper-tropospheric mesoscale disturbances evident on the dynamic tropopause, known as coherent tropopause disturbances (CTDs), moved along the periphery of these CAs. These CTDs provided triggering mechanisms for organized mesoscale convective systems in the US and Australian cases.