Abstract
The IBTrACS global best track data set endorsed by the World Meteorological Organization provides a valuable global record of tropical cyclone genesis, track and intensity, and spans 1842 to the present. The record is significantly more robust from the late 1970s onwards, as it is supported by satellite imagery. These records indicate that the first tropical cyclone in the South Indian Ocean to intensify to CAT5 status did so in 1994. This date is significantly later than the first CAT5 storms recorded in the IBTrACS database for the Atlantic Ocean (1924) and the North Pacific (1951) recorded from ship records, and half a decade later than those of the North Indian Ocean (1989) and South Pacific (1988), captured from satellite imagery. Following this late emergence, in the period 1990–2000, eight CAT5 tropical cyclones were recorded for the South Indian Ocean. A further four have been recorded for the period 2010–2015. This recent emergence of tropical cyclones attaining category five intensity in the South Indian Ocean is of significance for the forecasting of tropical cyclone landfall and the anticipation of storm damage for the developing economies that characterise the region. Although an increase in tropical cyclone intensity is frequently projected under global climate change scenarios, the dynamics for the South Indian Ocean have remained poorly understood. Notable are early results indicating an increased frequency and poleward migration of these CAT5 storms, concurrent with a poleward migration in the position of the 26.5 °C, 28 °C and 29 °C sea surface temperature isotherms in the South Indian Ocean.Significance:
 
 Category 5 tropical cyclones, the strongest category of storms, have only recently emerged in the South Indian Ocean. Since 1989, their frequency of occurrence has increased. This increase poses a heightened risk of storm damage for the South Indian Ocean Island States and the countries of the southern African subcontinent as a result of the strong winds, heavy rainfall and storm surges associated with these storms, and the large radial extent at category 5 strength.
Highlights
An increased frequency and intensity of extreme weather events is cited often as one of the most dangerous impacts projected under anthropogenic-induced climate change.[1,2,3] Increases in the intensity and frequency of heatwaves[4], precipitation[5] in flood events[2], and tornadoes[6] have been recorded over the past five decades, and are projected to continue under even modest emission scenarios
It could be argued that CAT5 tropical cyclones in regions of lower population density may have gone unnoticed in the early 19th century, the latter two records would have detected them by satellite imagery that provides a more globally equivalent database for all ocean basins from the 1970s onwards
This study reveals two important findings regarding the changing characteristics of intense tropical cyclones in the South Indian Ocean
Summary
An increased frequency and intensity of extreme weather events is cited often as one of the most dangerous impacts projected under anthropogenic-induced climate change.[1,2,3] Increases in the intensity and frequency of heatwaves[4], precipitation[5] in flood events[2], and tornadoes[6] have been recorded over the past five decades, and are projected to continue under even modest emission scenarios Despite both atmospheric and sea surface warming over the past century[7], historical analysis has demonstrated that tropical cyclone numbers have not increased for many regions in the world[8,9]. A poleward shift in tropical cyclones, and in particular their landfall, has been observed, and a number of climate models forecast a continued poleward displacement of these systems under even the lowest carbon emission scenarios.[8,10,13] these results are contested by subsets of the historical record that would indicate an equatorward trajectory of the genesis latitude of these storms.[9]
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