Seasonal and interannual variations of blocking in the Southern Hemisphere (SH) are discussed using 25 years of NCEP and 14 years of ECMWF data sets. Both the data sets show that the highest blocking frequency occurs near New Zealand, but ECMWF data seem to show higher blocking frequency. There are large interannual variations in blocking frequency over east and west Pacific, the principal regions of blocking in the SH. Linear correlation coefficients (CC) between the frequency of blocking and the Southern Oscillation Index show mostly negative values, indicating that blocking increases during the El Ni¿o events. A significant value (at 95% level) of CC is found only for the west Pacific in the austral autumn season. Two sets of three high and low blocking frequency years are selected to explore the possible causes for interannual variations. Mak's energy equations are used to examine the differences in energetics. To maximize the differences, two extreme years of blocking, a year of high blocking frequency, 1984, and a year of low blocking frequency, 1990, are considered. It is found that the generation of kinetic energy by barotropic processes on intraseasonal timescales is higher in 1984 than in 1990. This shows that the configuration of seasonal mean (deformation) field supports higher incidence of blocking in 1984 than in 1990. ¿ 2000 American Geophysical Union