Climatology and changes in extratropical cyclone activity in the Southern Hemisphere during austral winters from 1948 to 2017

Abstract

An objective detection and tracking algorithm based on relative vorticity at 850 hPa using National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis I data was applied to track cyclones in the Southern Hemisphere during austral winters from 1948 to 2017. The climatological characteristics of extratropical cyclones, including track density, frequency, intensity, lifetime, and their related variabilities, are discussed. The frequency and average lifetime of cyclones have substantially decreased. The average maximum intensity of cyclones has shown an increasing trend over the 70 year study period. The cyclone track density shows a decreasing trend in lower latitudes, consistent with the region where the upper troposphere zonal wind weakens. Baroclinicity can explain the increase in cyclone intensity: when a cyclone moves to higher latitudes and enters the region with greater baroclinicity, it strengthens. As there is no discernible increase in cyclogenesis in the medium latitudes (45°–70°S), but significantly less cyclogenesis in lower and higher latitudes, it is hypothesized that there is no clear poleward cyclogenesis shift over the Southern Hemisphere.