Seasonal and regional signature of the projected southern Australian rainfall reduction

Abstract

A projected drying of the extra-tropics under enhanced levels of atmospheric greenhouse gases has large implications for natural systems and water security across southern Australia. The drying is driven by well studied changes to the atmospheric circulation and is consistent across climate models, providing a strong basis from which adaptation planners can make decisions. However, the magnitude and seasonal expression of the drying is expected to vary across the region. Here we describe the spatial signature of the projected change from the new CMIP5 climate models and downscaling of those models, and review various lines of evidence about the seasonal expression.Winter rainfall is projected to decline across much of southern Australia with the exception of Tasmania, which is projected to experience little change or a rainfall increase in association with projected increases in the strength of the westerlies. Projected winter decrease is greatest in southwest Western Australia. A 'seasonal paradox' between observations and CMIP5 model projections in the shoulder seasons is evident, with strong and consistent drying projected for spring and less drying projected for autumn, the reverse of the observed trends over the last 50 years. The models have some biases in the simulation of certain synoptic types (e.g. cutoff lows), the rainfall brought by those synoptic types, and the mechanism of rainfall production. Rainfall projections based on statistical downscaling are examined in relation to some of these biases, projecting stronger future declines in some regions of southeast Australia in autumn than indicated by the host models, as well as little change to the magnitude of the projected declines in spring. Apart from Tasmania in winter, the decline of rainfall in southern Australia during the cool season remains a confident projection but the seasonal expression of change remains an ongoing research topic.

The Effect of Increasing CO2 on the Extreme September 2016 Rainfall Across Southeastern Australia

“…A drier future is projected for the MDB in September at the end of the century by most climate models (CSIRO and Bureau of Meteorology, 2015;Hope et al 2015), and results suggest that increasing levels of CO 2 over the last 55 years have already led to circulation and stability changes that would promote slightly drier conditions than in the 1960s, even for an extreme, two-week rainfall event. This study highlights the influence of changing levels of CO 2 alone, perhaps thus aligning more strongly with expected future changes and historical trends in models forced only with greenhouse gases compared to those attribution methods or models with all forcings (Bindoff et al 2013).…”

Long-term fluctuations in distribution and populations of a threatened rodent (Pseudomys novaehollandiae) in coastal woodlands of the Otway Ranges, Victoria: a regional decline or extinction?

“…In south-west Victoria the wet decades of the 1950s and 1970s were followed by the significant 'millennium drought ' (1996-2010) (CSIRO and Bureau of Meteorology 2015) and rainfall is projected to decrease by 25-45% under high-carbonemission scenarios by 2090 (Hope et al 2015). This vulnerable species thus faces significant threats from climate change and modelling of rainfall changes on the distribution of the New Holland mouse predicted a decline of up to 50% (Brereton et al 1995).…”

Extreme monthly rainfall over Australia in a changing climate

“…It allows coastal and island land data to be represented, but some of the grid boxes are partly ocean. As shown in Figure 2.3 of CB15 and Figure 1 of Hope et al (2015) the marine boundaries of the super-cluster regions were defined so as to encompass offshore islands. Regional averages for each model were calculated by weighting values on the model data grids by their area within the boundary, but including only grid boxes that were treated as land points, or were more than 70 per cent land if fractional land coverage was used.…”

“…While changes in the mean are of interest, for rainfall in particular, it is important to assess potential changes in its intensity. This may follow from an increase in the moisture-carrying capacity of the atmosphere under warming, or through changes in circulation and weather systems (Kunkel et al 2013, Hope et al 2015, Otto et al 2016. The high extremes of daily events have been the focus of many studies (e.g., Alexander and Arblaster, 2013), and CB15 show that 20-year extreme daily rainfall tends to increase, even for regions with declines in the 20-year means.…”