CSIRO PUBLISHING Journal of Southern Hemisphere Earth Systems Science, 2020, 70, 328–352 Seasonal Climate Summary https://doi.org/10.1071/ES19039 Seasonal climate summary for the southern hemisphere (autumn 2018): a weak La Nin˜a fades, the austral autumn remains warmer and drier Bernard ChapmanA,B and Katie RosemondA,B ABureau of Meteorology, GPO Box 413, Brisbane, Qld 4001, Australia. BCorresponding authors. Email:
[email protected];
[email protected] Abstract. This is a summary of the austral autumn 2018 atmospheric circulation patterns and meteorological indices for the southern hemisphere, including an exploration of the season’s rainfall and temperature for the Australian region. The weak La Nin˜a event during summer 2017–18 was in retreat as the southern hemisphere welcomed the austral autumn, and before midseason, it had faded. With the El Nin˜o Southern Oscillation and the Indian Ocean Dipole in neutral phases, their influence on the climate was weakened. Warmer than average sea surface temperatures dominated much of the subtropical South Pacific Ocean and provided favourable conditions for the formation of a rare subtropical cyclone over the southeast Pacific Ocean in May. The southern hemisphere sea ice extent was slightly below the autumn seasonal average. The southern hemisphere overall during autumn was drier and warmer than the seasonal average. The season brought warmer than average temperatures and average rains to parts of the continents of Africa and South America. Australia recorded its fourth-warmest autumn, partly due to an intense, extensive and persistent heatwave, which occurred during the midseason. An extraordinary and record-breaking rainfall event occurred over Tasmania’s southeast, under the influence of a negative Southern Annular Mode.