Scientists are working to understand the drivers of Australia’s changing climate. One such driver is the Madden Julian Oscillation and its impacts over northern Australia.
Northwest Australia’s tropical climate means it experiences 2 main seasons: wet and dry. The wet season, which starts in November and ends in April, brings around 90% of the yearly rainfall. Since the 1950s, Australia’s northwest has seen a significant increase in rainfall, with some areas receiving an extra 200 mm of rain per wet season. This trend is in stark contrast to the rest of Australia, which has been drying.
In a recent hub research collaboration, published in Geophysical Research Letters, we found that one potential cause for the increased rainfall is the Madden-Julian Oscillation (MJO). The MJO is an eastward travelling disturbance in rainfall wind and pressure within the monsoon. The MJO starts in the Indian Ocean and takes around 30 to 90 days to reach the cool waters of the Pacific Ocean, where it decays. As part of its journey, it affects the climate and weather of northern Australia.
A source of energy for the MJO is the Indo-Pacific warm pool – the largest body of warm water on the planet. The warm pool has doubled in size since the 1950s (that’s an average increase of one Victoria per year – 2.3 x 105 Km2). The change in warm pool size has modified the MJO’s cycle, with the MJO now spending more time over northern Australia (5-6 days per cycle) and less time over the Pacific Ocean.
Our research found that this increased residence time of the MJO, may be related to the increasing rainfall trend. We found that the daily rainfall increase over the northwest was driven by the increased occurrence of the MJO in its enhanced cloudiness and rainfall phase. This resulted in more rain days per monsoon, with little change in the intensity of rainfall events. There was also very little change in rainfall when the MJO was not present (inactive) and also when the MJO was in its suppressed cloudiness and rainfall phase.
Climate simulations, using a subset of models that have been shown to capture the MJO, have found that the MJO may continue to drive increasing rainfall in the region, with a high emissions scenario predicting a 12% increase in rainfall. However, under low-emission scenarios, the pattern stabilizes, and the rainfall increase almost disappears.
Understanding the cause of these trends is crucial for the health of ecosystems and agriculture in northwest Australia. Northwest Australia is home to many unique plant and animal species only found in these regions.
Mx. Alexander Borowiak
University of Melbourne and the Centre for Climate Excellence
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