Seasonal and Geographical Variation of Blocking

Abstract

We have calculated the seasonal variation of frequency of blocking and its geographical location by examining the grid point values of daily 500 mb geopotential height over the Northern Hemisphere for 15 consecutive years (1963?77). Blocking events are objectively identified by requiring that a large positive anomaly of a specified magnitude persist for seven days or more. The magnitude of the threshold anomaly is assumed to be 200 gpm for winter, 100 gpm for summer, and 150 gpm for fall and spring. It is found that the geographical locations of the maximum frequency, characterized by three distinctly different maxima, remain nearly the same in all the four seasons. These maxima which occur in the Pacific to the west of the Rockies, in the Atlantic to the west of the Alps, and over land to the west of the Ural mountains, coincide with the maxima of the low frequency and total variance. If the persistence criterion is changed to 1?3 days, the geographical distribution of frequency for winter is very similar to the 2?6 day band-pass variance, showing maximum values in the areas of storm tracks. Large persistent negative anomalies during the winter season are found to be mostly associated with local high index flow, and in a few instances with a neighboring blocking ridge. We have also examined the seasonal variability of persistent characteristics of wave numbers 1, 2, 3, 4 for 500 mb geopotential height between 50?70°N. It is found that the large scale planetary waves have preferred phase locations for persistence beyond seven days. It is also found, from the grid point analysis, as well as from the wave number analysis, that seasonal mean anomalies account for let than 25% of blocking events.