A study of quasi-millennial extratropical winter cyclone activity over the Southern Hemisphere

The winter extratropical cyclone activity in the Southern Hemisphere during the last one thousand years within a global climate simulation was analyzed by tracking cyclones, and then clustering them into ten clusters consecutively for each hundred years. There is very strong year-to-year variability for Southern Hemispheric winter extratropical cyclone numbers and larger variations on centennial time scale, more so than for its Northern Hemispherical counterparts. However, no obvious trend can be found. The mean tracks of clusters over the Southern Indian Ocean and near New Zealand shift poleward from the eleventh to the twentieth century while the clusters in the central Southern Pacific shift equatorward. Storm track clusters with largest deepening rates are found over the Southwestern Indian Ocean. In the twentieth century, rapidly deepening cyclones appear more often while long lifespan cyclones appear less frequently. The winter storm activity in the Southern Hemisphere is closely related to the Antarctic Oscillation. The cyclone frequency over the Indian Ocean and South Pacific Ocean can be associated with the Indian Ocean Dipole and El Nino-Southern Oscillation respectively.