Terence O'Kane, Jorgen S Frederiksen, Carsten S Frederiksen, Illia Horenko

Analysis of observations, reanalysis and model simulations, including via machine learning methods specifically designed for regime identification, have revealed changes in aspects of the Southern Hemisphere (SH) circulation and Australian climate and extremes over the last half century that point to transitions to new states. In particular, our analysis shows a dramatic shift in the metastability of the SH climate occurred in the late 1970s, associated with a large scale regime transition in the SH atmospheric circulation with systematic changes to the subtropical jet, blocking, zonal winds and storm tracks. Analysis via nonstationary clustering reveals a regime shift was coincident with a sharp transition to warmer oceanic sea surface temperatures and increased baroclinicity in the large scales of the Antarctic Circumpolar Circulation (ACC), extending across the whole hemisphere. At the same time, the background state of the tropical Pacific thermocline shoaled, leading to increased likelihood of El Ni\~{n}o events. These changes in the dynamics have preceded additional regional tipping points associated with reductions in mean and extreme rainfall in South-west Western Australia (SWWA) and streamflow into Perth dams, and also with increases in mean and extreme rainfall over Northern Australia since the late 1970s. The drying of South-eastern Australia (SEA) occurred against a background of accelerating increases in average and extreme temperatures across the whole continent since the 1990s, implying further inflection points may have occurred. Through climate model simulations capturing the essence of these observed shifts our analysis indicates these systematic changes will continue into the late 21st century under high greenhouse gas emission scenarios. Here we review two decades of work, revealing for the first time, that tipping points characteristic of first and second order regime transitions are inferred to have already occurred in the SH climate system.