Tropical to extratropical interactions in the Southern Hemisphere

Abstract

Tropical variability has long been identified as having an important influence on climate variability and change in the Southern Hemisphere (SH). In all three ocean basins, heating from tropical convection can generate stationary Rossby waves that propagate polewards and eastwards towards Antarctica, influencing temperature and rainfall patterns along the way. Recent studies have also highlighted the reverse – an influence of the polar regions on changes further north, for example, the stratospheric weakening of the SH polar vortex that contributed to the prolonged drought and extreme fire weather in Australia in the spring and summer of 2019. On longer timescales, the climate of the Southern Hemisphere has undergone significant changes over the past 30-50 years. The extratropical atmosphere has seen a shift to a more positive phase of the Southern Annular Mode and a stronger and more poleward eddy-driven jet, particularly in austral summer. While the influence of anthropogenic forcing such as ozone depletion and increasing greenhouse gases on these changes is well-established, the importance of tropical to extratropical interactions in shaping some recent events is becoming more evident. Examples include the deepening of the Amundsen Sea Low which has been associated with tropical Pacific decadal variability and the rapid decline in Antarctic sea ice in 2016 which was linked to a record positive Indian Ocean Dipole event. Recent insights into tropical to extratropical interactions, including the mechanisms through which they operate and links to observed changes on interannual to interdecadal timescales will be discussed.