February 25, 2020 | Published by | ,

Paper: King, A.D., Pitman, A.J., Henley, B.J. et al. The role of climate variability in Australian droughtNat. Clim. Chang. (2020). https://doi.org/10.1038/s41558-020-0718-z
Click here to read the full paper.

The Conversation article: Why drought-busting rain depends on tropical oceans.

Key points

  • To understand droughts in Australia, this study finds it is more important to think about why significant widespread drought-breaking rainfall failed to appear over extended periods as opposed to understanding what caused a drought.
  • The Australian instrumental record (1900-to date) shows significant periods of drought-breaking rainfall in south-eastern Australia generally correspond with and are made more likely by negative Indian Ocean Dipole (IOD) events and La Niñas.
  • The study finds that the longer the period of time since a negative-IOD or La Niña event, the greater the likelihood that SE Australia and the Murray Darling Basin (MDB) in particular will be gripped by drought.
  • The authors conclude that Australia’s vulnerability to drought may depend very much on future changes caused by global warming to the frequency and intensity of La Niña and negative-IODs.

Drivers of droughts in Australia

Recently, considerable scientific and public discussion has focussed on the role of human-caused climate change and its influence on droughts. The many influences that initiate and break droughts make it difficult to disentangle a precise climate change signal. Moreover, the instrumental record and proxy records suggest that current droughts are not unique and that in the past 1000 years in a pre-industrial climate, Australia witnessed droughts considerably longer than those found on the instrumental record.

However, climate researchers can distinguish key climate drivers that affect Australia. These drivers can prompt periods of sustained dryness that lead to drought or encourage widespread deluges that bring droughts to an end.

There are many climate drivers but two of the most important influences, particularly on rainfall in south-east and eastern Australia, are:

The role of negative-IODs and La Niñas in MDB drought

In this study of drought and rainfall over the MDB led by Dr Andrew King, CLEX authors find that the length of time between La Niñas and negative-IODs is an important indicator of the likelihood of drought for this region. This is because on all but two occasions in the instrumental record the neutral phases of the El Niño Southern Oscillation and IOD led to rainfall deficits over the Murray Darling Basin. So, even without an El Niño or positive-IOD, the MDB was still drying.

Major Australian droughts, such as the WW2 Drought (years 1935-1945) and the Millennium Drought (years 1997-2009) , have included unusually long periods without a negative-IOD or La Niña event. The instrumental record also shows rainfall deficits in the MDB mounted up quickly after a negative-IOD or La Niña event had passed.

From the paper: “In the instrumental record, when 6 months have passed since a La Niña or negative-IOD event, 67% of the time (32/48 cases) there was an accumulated rainfall deficit in the MDB. Similarly, for events that were 12, 18 and 24 months after a La Niña or negative-IOD event, the MDB experienced an accumulated rainfall deficit 76% (25/33), 81% (22/27) and 90% (19/21) of the time, respectively”. In short, this means that the longer it is since Australia has experienced a La Niña or negative-IOD, the more likely it is that south-eastern and eastern Australia will find itself with mounting rainfall deficits and on its way to being in the grip of drought.

The role of climate change in drought

Annual rainfall across Australia is highly variable, so it is extremely difficult to detect an overall rainfall trend that suggests an annual climate change signal across the entire continent. However, this is not true at a regional level. Long-term trends can be detected in rainfall over particular regions of Australia– such as the drying of southwest Western Australia (since the 1970s). Long term trends can also be seen at seasonal timescales such as the drop in winter rainfall across much of southern Australia (over the past 20-30 years).

The MDB is a useful Australian case study because it has high year-to-year rainfall variability and there is no statistically significant change in annual rainfall trends over the 20th century and over most of the basin. This means if there is a climate change signal, it is currently hidden by natural variability. Yet, the MDB experiences severe drought, as we have seen over the past few years.   

The team found the drought-breaking influence of a negative-IOD and/or La Niña event were key factors and their absence mattered.

Knowing an extended absence of these drivers coincided with prolonged droughts, the next question had to be would their frequency and intensity change with global warming?

There have already been some tentative suggestions in this area. There are early indications that negative-IOD events may become less common with climate change while La Niñas may increase in intensity. However, definitive conclusions are premature about what this means for droughts because of the degree of variability with these events, the relatively short observational record and climate model limitations that mean a high degree of confidence does not exist for modelled findings. This means we can’t be sure how climate change will impact Australian droughts but we can’t wait to find out either. Based on research to date, there is evidence that some parts of Australia will dry in the future, and be seriously impacted by changes in the IOD and ENSO. That risk is serious and real, and needs to be taken into account in future planning.

Conclusions

The research has revealed that:

  • Without a negative-IOD or La Niña event, there is a strong tendency for rainfall deficits to accumulate in key agricultural areas.
  • To better understand drought in Australia, a research focus on drought-breaking rains may be very useful.
  • We need to understand how the negative-IOD and La Niñas  will change in a warming worlds. Urgent research effort and investment is needed to extend our ability to forecast these events and to understand how they may change in a warming world.