September 4, 2020 | Published by | ,

Picture: Rainbow. Credit: Binyamin Mellish (Pexels).

Where does our rain come from? For a drought-prone continent like Australia, and a country with communities and industries affected by drought and flooding rains, the answer to that question is of vital importance. 

Broadly, we know that rainfall derives from two primary evaporation sources: the oceans or other large water bodies and nearby land. Surprisingly, we don’t know exactly where the primary sources of evaporation are and whether or not they change from year to year, seasonally or barely at all. Understanding the origins of the evaporation that drives our rainfall and the atmospheric processes that bring rainfall to different parts of Australia matters because it has the potential to improve short-term weather forecasts, medium-term seasonal forecasts and the long-term rainfall impacts of climate change.

We explored this problem using meteorological simulations to effectively travel backwards in time. We traced the path of water that fell as rainfall back through the atmosphere to discover where that water originally evaporated. We did this for every rainfall event that occurred over Australia between 1979 and 2013.

Figure: Mean moisture contribution to Australian precipitation (a) – (d) in each season [mm], 4 with mean climatological low-level wind vectors; (e) annually [mm], and (f) as a percent of 5 annual Australian precipitation [%]. Note the non-linear colour scale. 

Not surprisingly, over three-quarters of rainfall falling in Australia comes from evaporation that occurred over the surrounding oceans. But there were some very interesting nuances in this simple result.

One of those was for the southwest of Western Australia, which has already seen large reductions in rainfall that led Perth to introduce desalination plants. We have now found that over 90% of the moisture that brings rain to this part of the country comes from one small region over the Southern and Indian Oceans. This means southwest Western Australia is particularly vulnerable to any future changes in evaporation or atmospheric circulation which affect its supply of moisture from that part of the ocean.

While most of Australia’s rainfall comes from ocean moisture, it’s not the whole story. Some regions are more reliant on moisture supplied by land than others. North and northwest Australia gets 25% of its rainfall from water that has been evaporated from soils and plants. In southeast Australia, 18% of the rainfall originally came from plants and soils. This tells us that if we change the vegetation cover in these regions, there is a risk that we could impact rainfall.

This evaporative relationship that recycles water between the land and the atmosphere in these two regions has an amplifying effect. If the soil is damp from previous rainfall, the high level of evaporation returns it quickly to the atmosphere where it can sometimes help produce more rain. This is particularly true in spring and summer where around 9% of all rainfall gets recycled each month.

All these are examples of what is happening now. But we also found something else – the origin of our rainfall is changing.

Over the past 35 years northwest Australia has received more ocean and land evaporation in spring. The upward trend in moisture supply to the northwest coincides with increased evaporation in its ocean source regions, suggesting observed rainfall increases may be a result of intensified evaporation in the tropical oceans.

Meanwhile, the southeast has received less moisture in winter but more in summer. Intriguingly, the processes behind the seasonal shift in moisture supply to southeast Australia were not associated with a change in ocean evaporation. Instead, it appears rainfall in southeast Australia is being affected by increased easterly flows of moisture from the Tasman Sea in summer, and by reduced westerly flows of moisture from the Southern Ocean in winter. This changing seasonality of moisture supply and rainfall in the southeast has important implications, particularly for agriculture and water resource management.

For example, more rainfall in summer can be problematic for horticultural farming operations, as it can make crops more susceptible to fungal diseases due to a wet canopy, decreases the quality of wine grape crops and affects harvest scheduling.

On the other hand, summer rain can contribute less to soil moisture stores and runoff into rivers and dams, creating uncertainty for dam operators and water resource managers more broadly.

While this is fascinating and already has implications for Australia, it is only the first step in the process of understanding the origins of our rainfall and what changes to it means. Now that we know where the rainfall usually comes from, and how it is changing with the seasons, we can investigate the causes behind important rainfall changes like droughts and floods. It is hoped that through developing a better understanding of the causes behind these extreme events, we will ultimately help improve their prediction, allowing farmers and water resource managers to make better and more confident planning decisions.

  • Paper: Holgate, C. M., J. P. Evans, A. I. J. M. Van Dijk, A. J. Pitman, and G. Di Virgilio, 2020: Australian precipitation recycling and evaporative source regions. Journal of Climate, https://doi.org/10.1175JCLI-D-19-0926.1.