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.