Characteristics of southern hemisphere blocking as determined from a time series of observational data

A time series of 500 mb geopotential heights has been used to assess characteristics of southern hemisphere blocking. A zonal index similar to the index used in an analogous study of northern hemisphere blocking has been used. Characteristics of blocking situations have been computed with a simple algorithm, which has been programmed, and are presented as statistics. We found one preferred region for blocking action: the Australian‐New Zealand region. Two other regions where blocks occur are the Atlantic east of South America, and the Indian Ocean south‐east of South Africa. The number of days with blocked flow is considerably less over the Atlantic and the Indian Ocean than in the Australian‐New Zealand region. The annual variation shows two maxima: one peak value of 19% in April and one extended maximum of c. 20% during June–August in the Australian‐New Zealand region. There is no clear indication of a seasonal variation in the Atlantic and the Indian Ocean region. Investigation of individual blocking episodes reveals that the majority of the blocks have a mean eastward velocity, although the longer they last the more of them are westward‐moving. Longlasting episodes are almost exclusively found in the Australian region.