A Southern Hemisphere tropical cyclone climatology: analysis of variability under a changing climate

Abstract

Tropical cyclones are becoming more intense and the locations at which they form and attain their lifetime maximum intensities are migrating poleward, exposing more regions to tropical cyclone impacts. This study investigated a new climatology for Southern Hemisphere tropical cyclones, aimed to update and re-examine the previous tropical cyclone climatology from the canonical works of Gray (1968, 1979). The optimal and minimum ranges of thermodynamic and dynamic climate variables during tropical cyclogenesis were determined, to define the conditions at which the majority of Southern Hemisphere tropical cyclones are generated. The change over time of tropical cyclone activity and the magnitude of the climate drivers at the 1000and 850mb levels was also examined. The sea surface temperatures of 27.5-28.5°C is observed as the optimal range at which a majority of storms undergo genesis. However, the lowest observed magnitude at which genesis was possible is 24°C. This challenges the 26.5°C threshold for genesis in the Southern Hemisphere. Based on factor analysis weightings and multivariate regression, sea surface temperature, air temperature and relative humidity also account for a relatively large amount of variability in the formation process of tropical cyclones, compared to the other climate variables examined. Over 1980-2016, an increase for Southern Hemisphere tropical cyclone maximum intensity of 2.4 m.s-1 per decade is observed. Of the climate drivers measured at the point of genesis; omega, meridional shear, sea surface temperature and solar flux display statistically significant correlations with tropical cyclone intensity. Sea surface temperature, air temperature, solar flux and vertical shear at the point of genesis for Southern Hemisphere tropical cyclones, demonstrate statistically significant increases in magnitude, while omega and relative humidity indicate a decreasing trend. The increase in thermodynamic variables and their relationship to storm intensity infers that a warming climate is capable of influencing tropical cyclone intensity. The El Niño Southern Oscillation, Indian Ocean Dipole, Southern Annular Mode and volcanic eruption events indicated a statistically significant relationship to metrics of tropical cyclone activity, mainly intensity and longitude of storm genesis. The tropical cyclone climatology established here by providing an up to date seasonality and background of Southern Hemisphere tropical cyclone activity, can assist impact and risk assessment studies in areas potentially vulnerable to tropical cyclone impacts within the Southern Hemisphere.