Detection and attribution of human-induced winter rainfall decline in Australia using a high-resolution global climate model

Recent studies have shown clear evidence of a decline in winter rainfall over Southern Australia over the last several decades. Observational and modeling studies have suggested that at least part of this decline is associated with an upward trend in the Southern Annular Mode (SAM), a pattern of atmospheric circulation in the extratropical Southern Hemisphere. The increase in the SAM is linked to increasing greenhouse gases and reductions in stratospheric ozone over Antarctica. The rainfall trend is also linked to increasing atmospheric surface pressure over Australia associated with a strengthening subtropical ridge. In this study we use a newly developed high-resolution global climate model to simulate the regional-scale aspects of this rainfall decline, assess the physical factors contributing to the decline, and provide projections of regional-sale rainfall changes over the coming decades in response to a plausible scenario of changing radiative forcing.

This new global climate model (GFDL CM2.5) model has resolution of 50Km in the atmosphere, allowing a much improved simulation of many aspects of regional climate and hydrology. There is also fine resolution in the ocean, varying from 27Km at the Equator to 8 Km at high latitudes. Using multiple ensembles of simulations covering the period 1861-2100, our model results are able to reproduce many aspects of the observed winter rainfall decline over southern Australia with remarkable fidelity, including regional features. Further, this decline is only simulated when we include changes in radiative forcing from increasing greenhouse gases and aerosols, thereby demonstrating a significant human component in the recent decline in Australian winter rainfall. We use this model to project winter rainfall changes over the rest of the 21st century; the model results suggest that the pattern of winter rainfall decline observed over the last several decades will continue and intensify over the course of the century, leading to increased drought and related precipitation extremes. The ability of this model to simulate observed changes over the last several decades increases our confidence in its ability to project future changes. Models with coarser resolution are able to simulate some of the broad scale features but without the regional-scale details that may be of importance.