Both the climatology and the case studies were performed using the global gridded 2.5 degree NCEP/NCAR and ERA-40 reanalysis datasets. Gridded fields of sea level pressure (SLP) and 1000 hPa geopotential height were extracted from these datasets. At each grid point a counter kept track of the number of times a SLP or geopotential height threshold (e.g., 1050 hPa) was exceeded. Global, NH, and SH anticyclone annual, seasonal and monthly frequency distributions were constructed and mapped for the specified selection thresholds. The case studies and non counter-based climatology information (e.g., anticyclone month of the year) were also performed using these two datasets.
Significant analysis differences were noted over areas of elevated terrain (e.g., Greenland) where the NCEP/NCAR reanalyses showed a higher frequency of intense anticyclones than in the ERA-40 reanalyses. Intense cool-season NH anticyclones (1050+ hPa) populated mostly higher-latitude, snow-covered continental regions whereas SH intense cool-season anticyclones (1040+ hPa) dominated the midlatitude oceanic storm tracks. In the NH, intense cool-season anticyclone frequency maxima extended equatorward along the eastern margins of major topographic barriers (e.g., the Rockies). Minima in intense anticyclone frequency were found over and to the west of major topographic barriers. The frequency distributions also suggest that throughout the latter portion of the twentieth century there has been a decline in the number of intense anticyclones in the NH and SH. Time permitting, other aspects of the climatology of intense anticyclones will be presented. Case study results will be used to illustrate additional aspects of the behavior of intense anticyclones.