Why has May to September been so wet for much of Australia?
For many people in Australia, the past few months has seemed wetter than usual. In fact, according to the Bureau of Meteorology (BoM), the levels of rainfall across much of Australia have been breaking records or getting close. As examples, May to September was the wettest five consecutive months on record for Australia and that same period was the wettest on record for Queensland, New South Wales, South Australia, Tasmania and the Murray-Darling Basin. This is even more surprising considering that before May 2016, large parts of eastern Australia were experiencing significant drought.
How can the change in rainfall levels be so dramatic?
A number of drivers impacts Australia’s climate and two of them, El Niño−Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), have changed dramatically throughout 2015-16. In 2015 and early 2016, a very strong El Niño event occurred (main impacts are less rainfall for northern and eastern Australia and higher average temperatures across most of southern Australia) which ended in May. The ending of the El Niño event coincided with negative IOD values. The negative IOD phase brings higher levels of rainfall to large parts of Australia. The IOD is expected to have peaked in its negative phase in September and is now transitioning toward a more neutral phase meaning the associated higher than average rainfall in eastern Australia will ease. The weakening of the El Niño event coupled with a strong negative IOD created a large area of warm sea surface temperatures, driving evaporation and rainfall off the north-eastern coast of Australia and eventually moving inland.
Is this really unusual?
While it is not unusual that heavy rainfall occurs because of the ending of a strong El Niño; according to the BoM, many of the wettest Australian May to September years have had either the ending of a strong El Niño or a strong IOD negative phase. Thus the alignment of these two systems is reflected in the record breaking rainfall levels over a sustained period of time.
Why does this matter?
As a result of this period of rainfall, it is too simplistic to say “so much for man-made climate change when it’s raining more than ever”. As mentioned above, until May 2016 parts of Australia were in drought reflecting the cyclical nature of ENSO and the IOD systems. The manifestations of the cycles will be dependent on a range of contributing factors that can be independent or related to climate change. The challenge for scientists, policy makers and regulators is to distinguish what the data is saying, a task which is no small feat for an area of study as technical and politically fraught as climate science.
For more information see here for a useful resource published by the CSIRO and Bureau of Meteorology.
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