Changes in summer rainfall and implications for agriculture

Annual and seasonal rainfall are important drivers of agricultural productivity and profitability in Australian agriculture and various climatological and synoptic drivers influence rainfall patterns in Australia’s diverse climate. In some parts of Australia’s important agricultural production zones producers have experienced summer rainfall in the previous decade as largely being absent, for example in northern New South Wales.

It is important to understand whether these are only local trends or can be observed on a larger spatial scale and whether recent changes are part of a longer-term trend requiring transformational changes to the farming systems or merely attributed to natural variability.

By conducting a thorough review of relevant literature and analysing past and current rainfall data for selected locations and larger production areas, we investigate whether there has been a specific shift in summer rainfall in northern New South Wales, southern Queensland and at similar latitudes in Western Australia.

By analysing future projections and changes in the main climate processes, we examine how the historic trends might continue and how this may impact on agriculture in the region. The characteristics of the climate in the study area in Queensland and New South Wales are distinct as the regions are situated between the tropical and the temperate climate zone with associated complexities in the climate system.

We find that there are robust trends in summer rainfall for two out of the fourteen stations studied and for the Western Australia wheat belt overall. Although some locations in Eastern Australia experienced below average rainfall in seven consecutive summers in the recent decade, this does not constitute part of a long-term trend and our current results suggest that they are part of natural variability in seasonal rainfall. The Western Australia wheat belt is the only region with significant and long-term changes in summer and winter rainfall. Seasonal rainfall has increased by 0.18-0.21 mm per year in summer and has decreased by 0.42-0.43 mm per year in winter.

Among the large-scale climate influences on seasonal rainfall, the El Nino-Southern Oscillation, the Southern Annual Mode and atmospheric blocking are the main influences on warm season rainfall the Western Australia wheat belt. The decline in winter rainfall is expected to continue as there is high agreement in climate models for further declines in winter rainfall, while changes in rainfall in Eastern Australia are less certain.

The outputs from this research can be further explored in the Australian Rainfall Trend Explorer application and can assist producers in developing transformation processes and coping strategies in industries affected by changes in seasonal rainfall.

Our results do not suggest a shift in the rainfall distribution or long-term trend in seasonal rainfall in the Northern Murray Darling basin and the Coastal mid-latitudes of New South Wales and Queensland that would justify the need for transformational changes in the types of commodities produced.

However, such transformations might be required in the Western Australia wheat belt as there is a robust decline in winter rainfall observed and projected to continue. Any follow up climate change impact assessments for specific commodities and regions can build on the findings of this study by using the information provided on model consensus for directions of change in rainfall and the climate models that are representative of these changes and by using the rainfall indicators that were found to have had a positive or negative trend in the past as this continuing into the future will be relevant for agricultural impact studies.

This project is funded by the Managing Climate Variability Research and Development Program and is carried out in a collaboration with CSIROs Oceans and Atmosphere Regional Projections team, and with the help of researchers at the Bureau of Meteorology and University of New South Wales.

Contact Katharina Waha for more information.