The Interannual Variability of Blocking on a Global-Scale: An Update

Blocking anticyclones 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 30-year global climatology extending from January 1970 through December 1999 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 (NH) and Southern Hemispheres (SH) must be conducted. Until recently, a long-term comprehensive climatology of blocking anticyclones in the SH has not been developed. The variability of blocking with respect to the El Nino and Southern Oscillation (ENSO) has also been examined recently. However, variability with respect to longer-term oscillations such as the North Atlantic Oscillation (NAO) and the Pacific Decadal Oscillation (PDO) will also be examined. For both the NH and SH climatologies, characteristics such as intensity, duration, preferred formation regions, and seasonal variations are examined for three regions within each hemisphere. The initial results demonstrate that in both the NH and SH, blocking characteristics do vary with ENSO phase. During El Nino years, there are fewer (more) blocking events and these events tend to be less (more) persistent and weaker (no change in intensity) than during La Nina years in the NH (SH). These relative differences in the frequency of blocking activity among each region are more pronounced during the 1980 - 1999 period than during the earlier period. Additionally, there is a trend toward fewer blocking event in the SH, a result significant at the 95% confidence interval. These results are similar over all regions of the SH. Finally, it was found that a correlation (significant at the 99% confidence interval) between block duration and intensity in the NH in the total sample and when the data are stratified by ENSO phase. There is no correlation between these variables in the SH.