Climate and Rainfall in Northern Australia

11 October 2017 John Macfarlane, Research Intern, Northern Australian and Land Care Research Programme

Background

A recent FDI Strategic Analysis Paper outlined how the climate and water availability presents significant challenges for any future development of northern Australia (as was advocated by the 2015 Government White Paper on developing northern Australia). While there are significant groundwater resources in northern Australia this water can be tens or hundreds of thousands of years old. Aquifer recharge events can be millennia apart and are certainly unlikely to occur within our lifetimes. In addition to these aquifers, surface water and more transient groundwater are also significant. These can be emptied and recharged entirely within the space of a year. Rainfall, therefore, is a critical component of the northern Australian water supply but inconsistent patterns of seasonal and annual rainfall require rainwater to be captured and stored. Given the potential dependence on these water sources, an understanding of the climatic patterns that bring drought and flood to northern Australia will help in planning for any potential development.

Comment

The Australian climate has many influences. The Bureau of Meteorology (BOM) lists seventeen phenomena that have various effects on rainfall and temperature across the landmass. Among these influences are water temperatures in the Pacific Ocean to the east and the Indian Ocean to the west which are, in turn, affected by weather cycles that do not necessarily occur with any long-term predictability. In the Pacific, cycles of the El Niño Southern Oscillation (ENSO) affect the distribution of water temperature within the ocean as well as strengths of trade winds and precipitation in the Americas and Australasia. To the west of the continent, the Indian Ocean Dipole (IOD) is a similarly oscillating pattern of water temperature and wind that affects the climate in eastern Africa and the Middle East as well as in Australia. Each of these phenomena trend between positive and negative phases. Cooler than usual seawater temperatures limit the amount of precipitation in the atmosphere and hence lead to drier seasons and warmer seawater brings increased rainfall. According to the BOM, a positive phase of the IOD will be more likely to coincide with an El Niño event (which is one part of the ENSO pattern) but, based on historical occurrences, there is little pattern of connection between the two. When they do coincide, however, the drying effects of an indicative El Niño event can be broadened across Australia, as the winter and spring rainfall decile charts below indicate.

EL Nino LHS and RHS CombinedComparison of rainfall deciles in Australian winter-spring months for El Niño only and El Niño plus positive IOD events. (click to enlarge).

These figures show that a combination of ENSO and IOD events do affect the Australian climate more so than independent events might, as was last observed late in 2015.

It should be noted, however, that these charts are for the Australian winter and spring months, June through to November, which do not coincide with the northern Australian wet season. The effects of the IOD tend to decay around the onset of the southern monsoon and, as such, would not seem to have a significant effect on the northern wet season. ENSO events can, however, persist over the northern wet season months and do affect northern rainfall during this time.

When assessing water resources in northern Australia, the CSIRO considers several climate scenarios that vary rates of climate change from now until 2070. When making climate predictions, the CSIRO will take into account at least fifteen global climate models and, in the case of more recent assessments, the use of Representative Concentration Pathways (RCPs). These scenarios span the range of plausible global warming scenarios. From these global climate models and RCPs, predictions for coming years will be categorised as either ‘wet’, ‘mid’ or ‘dry’ indicating the likelihood of increased or reduced rainfall.

To date, predictions are inconclusive and significant uncertainty remains as to the potential for changes in rainfall patterns in northern Australia in the decades to come. In the Pilbara region of Western Australia, for example, average annual rainfall by 2050 is predicted to be either 17 per cent lower than the current average in an extreme dry scenario or eight per cent higher in an extreme wet scenario. For the Carpentaria region of the Great Artesian Basin, rates were up to 83 per cent higher under a wet extreme scenario or eleven per cent lower under a dry extreme scenario. Perhaps the only judgement that can be confidently asserted is that, for the foreseeable future, climate and rainfall uncertainty will remain a restricting factor for northern Australian development.

Any opinions or views expressed in this paper are those of the individual author, unless stated to be those of Future Directions International.

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