Anticyclones cause high-impact weather too: A global anticyclone climatology and case studies

Intense anticyclones have historically been less studied than intense cyclones, a situation that likely reflects the perception that these features are associated with less widespread hazardous weather. However, intense cool-season anticyclones are often associated with the strongest continental cold surges,and widespread severe cold, snow and ice in midlatitudes. The purpose of this presentation is to present a global climatology and selected case studies of intense cool-season anticyclones. The case studies will show the structure and evolution of strong anticyclones. The anticyclone climatology will be shown on annual, seasonal and monthly time scales for the Northern and Southern Hemispheres (NH AND SH) with emphasis on continental vs. oceanic distributions.

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.