The impact of anthropogenic forcing and natural processes on past, present and future rainfall over Victoria, Australia
Journal of Climate
Cool season (April to October) rainfall dominates the annual average rainfall over Victoria, Australia, and is important for agriculture and replenishing reservoirs. Rainfall during the cool season has been unusually low since the beginning of the Millennium Drought in 1997 (∼12% below the 20th century average). In this study, 24 CMIP5 climate models are used to estimate (i) the extent to which this drying is driven by external forcing, and (ii) future rainfall, taking both external forcing and
... xternal forcing and internal natural climate variability into account. All models have preindustrial, historical, and 21st century (RCP2.6, RCP4.5 and RCP8.5) simulations. It is found that rainfall in the past two decades is below the preindustrial average in two-thirds or more of model simulations. However, the magnitude of the multi-model median externally-forced drying is equivalent to only 20% of the observed drying (inter-quartile range of 40% to -4%), suggesting that the drying is dominated by internally-generated rainfall variability. Underestimation of internal variability of rainfall by the models, however, increases the uncertainties in these estimates. According to models the anthropogenically-forced drying becomes dominant from 2010-2029, when drying is evident in over 90% of the model simulations. For 2018 – 2037 period, it is found that there is only a ∼12% chance that internal rainfall variability could completely offset the anthropogenically-forced drying. By the late 21st century the anthropogenically-forced drying under RCP8.5 is so large that internal variability appears too small to be able to offset it. Confidence in the projections is lowered because models have difficulty in simulating the magnitude of the observed decline in rainfall.