Search

Persistent shifts in mean rainfall have wide-ranging impacts to hydrology and water availability, and a reliable set of climate projections of change to mean rainfall is a useful tool for future planning. Most climate models project a decrease in winter rainfall in southern Australia, however there is a wide model range and there is not yet a robust assessment of underlying physical processes that can inform and constrain projections. Here, a multiple linear regression model between indices of atmospheric circulation and gridded rainfall in observations and in CMIP5 climate models is developed for July, representing the peak of winter. The regression is used as an evaluation tool for models and a basis to select models. Spatial distributions of the coefficients from the regression illustrate the relative important of different circulation features for rainfall across the region, and illustrate where climate models have deficiencies. As an additional check of projections, historical years that are an analogue for the projected future mean state of the atmospheric circulation are identified and the rainfall anomaly during those years is examined. Both approaches broadly agree and support previous work in suggesting a constraint on rainfall change to a decrease only. The regression analysis also suggests that the median projection for southwest Western Australia should be revised lower than the median of all climate models. The results demonstrate value in applying statistical techniques to understand relationships of rainfall to circulation and to refine confidence in regional climate projections.

At the global scale, reconstructions of cool season temperature over past centuries are relatively rare. Here we present 277-year reconstructions of cool season (July–October) temperatures for southern Australia based on three different data sets: a spatial field reconstruction based on highly resolved temperature data from the Australian Water Availability Product data; reconstructions for the four southeast Australian states based on the Berkeley Earth mean temperature data for each state; and reconstructions for individual stations in southeastern Australia from the Australian Bureau of Meteorology’s Australian Climate Observations Reference Network–Surface Air Temperature data. Our reconstructions typically capture 25–50% of the variation over the late twentieth Century calibration period and are strongest for the southern state of Tasmania and the southeastern part of mainland Australia. All three use Tasmanian tree-rings sensitive to cool season temperatures and display similar variability. In the context of our reconstructions, the persistent warming in the observed record since ~ 1950 is unprecedented. While the low frequency variability of winter temperatures is generally in step with that in summer (December–February) temperatures, high frequency variability is not, illustrating the need for seasonal reconstructions to help improve understanding of variability in inter-seasonal dynamics and the historical importance of this on the environment. The reconstructions covary with changes in the Southern Annular Mode and may be useful for future reconstructions of this phenomenon.

We identify the dynamical drivers of systematic changes in persistent quasi-stationary states (regimes) of the Southern Hemisphere troposphere and their secular trends. We apply a purely data-driven approach, whereby a multiscale approximation to nonstationary dynamical processes is achieved through optimal sequences of locally stationary fast vector autoregressive factor processes, to examine a high resolution atmospheric reanalysis over the period encompassing 1958–2013. This approach identifies regimes and their secular trends in terms of the predictability of the flow and is Granger causal. A comprehensive set of diagnostics on both isentropic and isobaric surfaces is employed to examine teleconnections over the full hemisphere and for a set of regional domains. Composite states for the hemisphere obtained from nonstationary nonparametric cluster analysis reveal patterns consistent with a circumglobal wave 3 (polar)–wave 5 (subtropical) pattern, while regional composites reveal the Pacific South American pattern and blocking modes. The respective roles of potential vorticity sources, stationary Rossby waves and baroclinic instability on the dynamics of these circulation modes are shown to be reflected by the seasonal variations of the waveguides, where Rossby wave sources and baroclinic disturbances are largely contained within the waveguides and with little direct evidence of sustained remote tropical influences on persistent synoptic features. Warm surface temperature anomalies are strongly connected with regions of upper level divergence and anticyclonic Rossby wave sources. The persistent states identified reveal significant variability on interannual to decadal time scales with large secular trends identified in all sectors apart from a region close to South America.

We assess the occurrence and probability of extreme heat over Australia in association with the Southern Annular Mode (SAM), persistent anticyclones over the Tasman Sea, and the Madden–Julian Oscillation (MJO), which have previously been shown to be key drivers of intra-seasonal variations of Australian climate. In this study, extreme heat events are defined as occurring when weekly-mean maximum temperature anomalies exceed the 90th percentile. The observed probability of exceedance is reduced during the positive phase of the SAM and enhanced during the negative phase of the SAM over most of Australia. Persistent anticyclones over the Tasman Sea are described in terms of (1) split-flow blocking at 160°E and (2) high pressure systems located in the vicinity of the subtropical ridge (STRHs), about 10° north of the split-flow blocking region, for which we devise a simple index. Split-flow blocks and STRHs have contrasting impacts on the occurrence of extreme heat over Australia, with STRHs showing enhanced probability of upper decile heat events over southern Australia in all seasons. The observed probability of an upper decile heat event varies according to MJO phase and time of year, with the greatest impact of the MJO on extreme heat occurring over southern Australia (including the Mallee agricultural region) in spring during phases 2–3. We show that this modulation of the probability of extreme heat by the SAM, persistent anticyclones over the Tasman Sea, and the MJO is well simulated in the Bureau of Meteorology dynamical intra-seasonal/seasonal forecast model POAMA-2 at lead times of 2–3 weeks. We further show that predictability of heat extremes increases in association with the negative SAM phase, STRH and MJO, thus providing a basis for skilful intra-seasonal prediction of heat extremes.