The interannual variability of blocking on a global scale

Blocking anticyclone are large-scale phenomena, which have long been of interest to the meteorological community. Using the NCEP re-analyses archived at the National Center for Atmospheric Research (NCAR), a comprehensive 25-year global climatology extending from January 1974 through December 1998 was completed. In order to understand thoroughly global seasonal and interannual variations of blocking anticyclone occurrences and characteristics, as well as long term trends, a study of adequate length for both the Northern and Southern Hemispheres must be conducted. Until recently, a long-term comprehensive climatology of blocking anticyclones in the Southern Hemisphere has not been developed. The variability of blocking with respect to the El Nino and Southern Oscillation (ENSO) has been examined recently. However, variability with respect to longer-term oscillations such as the North Atlantic Oscillation (NAO) and the Pacific Decadal Oscillation (PDO). In addition to developing a 25-year comprehensive climatology for the Southern Hemisphere, a Northern Hemispheric counterpart was developed in order that a globally consistent climatology can be used to examine the issue of long-term trends and interannual variability. This climatology will also be compared to similar climatologies to extend the global record, and this work endeavors to eventually derive a 40-year climatology. The initial results demonstrate that in both the Northern and Southern Hemisphere, blocking characteristics do vary with ENSO phase. During El Nino years, there are fewer blocking events and these events tend to be less persistent and weaker than during La Nina years. These relative differences in the frequency of blocking activity among each region are more pronounced in the 1990's than during the earlier period. For both the Northern and Southern Hemisphere climatologies, characteristics such as intensity, duration, preferred formation regions, and seasonal variations are examined for three regions within each hemisphere.