Climate Regime Transitions in Australia

Open Access Editor’s Choice Article

by Jorgen S. Frederiksen 1,2,* and Stacey L. Osbrough 1,2

1 CSIRO Oceans and Atmosphere, Aspendale, Melbourne 3195, Australia

2 School of Earth, Atmosphere and Environment, Monash University, Clayton, Melbourne 3800, Australia

* Author to whom correspondence should be addressed.

Submission received: 14 April 2022 / Revised: 16 May 2022 / Accepted: 17 May 2022 / Published: 19 May 2022

Abstract

Systematic changes, since the beginning of the 20th century, in average and extreme Australian rainfall and temperatures indicate that Southern Australian climate has undergone regime transitions into a drier and warmer state. South-west Western Australia (SWWA) experienced the most dramatic drying trend with average streamflow into Perth dams, in the last decade, just 20% of that before the 1960s and extreme, decile 10, rainfall reduced to near zero. In south-eastern Australia (SEA) systematic decreases in average and extreme cool season rainfall became evident in the late 1990s with a halving of the area experiencing average decile 10 rainfall in the early 21st century compared with that for the 20th century. The shift in annual surface temperatures over SWWA and SEA, and indeed for Australia as a whole, has occurred primarily over the last 20 years with the percentage area experiencing extreme maximum temperatures in decile 10 increasing to an average of more than 45% since the start of the 21st century compared with less than 3% for the 20th century mean. Average maximum temperatures have also increased by circa 1 °C for SWWA and SEA over the last 20 years. The climate changes in rainfall and temperatures are associated with atmospheric circulation shifts.

1. Introduction

Over the last seventy years, since the middle of the 20th century, aspects of Australian climate, particularly rainfall and temperatures, have undergone significant changes. The notable rainfall deficits in southern Australia have been linked to declines in extra-tropical storminess and the intensity of explosive storms. Some of those changes have been quasi-cyclical due, for example, to variability associated with the El Niño-Southern Oscillation, the Interdecadal Pacific Oscillation, the Walker and Hadley Circulations, or the Indian Ocean Dipole. On the other hand, there is also compelling evidence for systematic climate shifts in both hemispheres due to global warming.

Frederiksen, et al., Figure 7, consider the systematic or secular trends in Southern Hemisphere winter baroclinicity over the second half of the 20th century based on reanalysis data and the interannual and decadal variability about the trend. Frederiksen, et al. made similar determinations for each season and studied the roles of externally forced and internal covariability of rainfall and baroclinicity in a suite of 12 comprehensive coupled ocean atmosphere climate models. These models were chosen on their ability to produce similar trends in baroclinicity over the second half of the 20th century as found in reanalysis data. They also examined these model trends for the second half of the 21st century under conditions of strong radiative forcing by increasing carbon dioxide with no stabilization and, as well, with stabilization. In this study, and in a similar study by Frederiksen and Grainger on the covariability of rainfall and 500 hPa geopotential height, it was concluded that the secular trends, in rainfall and circulation over the second half of the 20th century by the ensembles of skilful models, which are similar to those from reanalyses, are closely reproduced by the externally forced modes of covariability. Moreover, the continuing similar secular trends into the 21st century are conditional on the continuing increasing trend in equivalent carbon dioxide without stabilization. The attribution study of Franzke, et al. led to their conclusion that anthropogenic carbon dioxide is the dominant cause of secular changes in the Southern Hemisphere circulation in recent decades with a lesser contribution from stratospheric ozone depletion. They also noted the consistency with the model-based study of Freitas, et al..

Our particular interest in this article is whether the changes that have occurred in Australian climate and climate extremes over the last seventy years are indicative of regime transitions in a noisy environment. There has been a long history of studies examining the possibility of regime transitions in various aspects of the climate system. The early simple energy balance models (EBMs) of the earth’s climate exhibited thermodynamical regime transitions in the mean temperature between several states as the order parameter, the solar constant, is varied. Indeed, as shown in Figures 1 and 3 of Frederiksen, the number of stable states and the number of bifurcation points (or critical or tipping points) may vary depending on the form of the thermodynamical functions, such as the effective albedo, and lead to the possibility of closely spaced tipping points.

Charney and Devore and Wiin-Nielsen studied low order dynamical models of the atmospheric circulation and found multiple equilibrium states dominated by either strong zonal flow and weak wave structure or weak zonal flow and strong wave structure that they interpreted as a blocking state. Charney and Devore found that regime transitions between the zonal and blocking states occurred as the order parameter, the height of the topography, varied through the bifurcation point. Similar regime transitions were also found in baroclinic models by Charney and Straus. Frederiksen and Frederiksen reviewed subsequent developments in the theory of multiple equilibria and the role of topographic instability in regime transitions.

Frederiksen examined regime transitions of inviscid barotropic and baroclinic zonal flows over topography in high dimensional systems using methods of equilibrium statistical mechanics. The critical points for barotropic flow and critical lines and triple points for baroclinic flows were determined and the similarities and differences with magnetic phase transitions were examined. Zidikheri, et al. studied the interaction of barotropic zonal flows with topography in high resolution forced dissipative numerical simulations and established the phase diagram (their Figure 2) for regime transitions. They found hysteresis effects in transitions between strong and weak zonal flow states with qualitative similarities to those for magnetic phase transitions (e.g., Figure 3 of Saghayezhian, et al. and references therein). The regime transitions between strong zonal states and blocking found in simple models have also been found in comprehensive weather prediction models (e.g., Frederiksen, et al.) and associated with observed climate shifts by O’Kane, et al..

Further developments in the role of regime transitions and tipping points in various aspects of the climate system, including under global warming, have been considered by Franzke, et al., Freitas, et al., Dijkstra, Kypke, et al., Lenton, et al., Yan, et al., Fabiano, et al., Jones and Ricketts and Australian Academy of Science. It is clear from all the studies mentioned in this Introduction that there are dynamical and thermodynamical processes of the climate system that can result in regime transitions. However, the methodologies for analysing components and simplifications of the climate system are not easily applied to the full system given its complex equations and interactions over vast scales. This is clearly the case for the analytical and semi-analytical bifurcation methods, including singularity theory, for analysing low order systems and for the equilibrium statistical mechanics methods. Renormalization group methods and renormalized perturbation theory are more generally applicable to the statistical dynamics of phase transitions of high dimensional systems. However, they are most suited to systems described by a few mathematically elegant equations such as the Navier–Stokes equations or quasigeostrophic equations. The complex equations, some of which include discontinuous processes such as convection, and vastly different time scales of interactions, of the climate system again make these approaches unfeasible for computational as well as theoretical reasons. In this study we therefore take an approach based on the general characteristics of phase transitions which involve a discontinuity in the dependent variable (first order phase transition) or its derivative (second order phase transition) as the order parameter transits through a critical point.

The paper is structured as follows. Section 2 outlines the data and sources as well as the methodology for their analysis used in this study. The mean and extreme rainfall, streamflow into Perth dams, the mean and extreme surface temperature data sets, and the reanalysis data determining atmospheric flow fields, are described in this Section.