Tropical Cyclone Monty (14S) over Western Australia. Credit: NASA

Fewer cyclones but they'll hit harder

In the future fewer tropical cyclones may form off Western Australia but they may become more intense, shows new research from a Western Australian climate research collaboration.

  • 21 December 2011 | Updated 9 July 2013

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Glen Paul: G'day, and welcome to CSIROpod, I'm Glen Paul. The destructive force of tropical cyclones has been experienced many times by people living in northern Australia, but now new research is showing that in the future fewer tropical cyclones may form off Western Australia, with up to a 50 per cent reduction in the number of storms in the second half of this Century. But before you let out a cheer, the bad news is that these fewer cyclones will be more severe and destructive.

To discuss these findings I'm joined by Dr Debbie Abbs from CSIRO Marine and Atmospheric Research. Debbie, this is obviously related to the impacts of climate change, but what exactly will bring about this change in cyclone behaviour?

Dr Abbs: The change in cyclone behaviour is due to a number of things. Firstly there’s an increase in sea surface temperatures, and in the early days of climate research we thought that increase in sea surface temperatures would result in more tropical cyclones. However in the world that we live in today cyclones are due to more than just high sea surface temperatures.

They are also affected by other aspects of the atmosphere, so the wind shear, which is the change in wind speed and direction with height, and then other characteristics that are related to the temperature and humidity of the atmosphere. And this is important because it’s the temperature and humidity of the atmosphere that allows the large thunderstorm type complexes to form, which are the precursors to tropical cyclones.

So if we get changes in either the wind shear or in the temperature and humidity characteristics of the atmosphere, then that will affect the ability of the atmosphere to form tropical cyclones, and we find that is happening, but the relative contributions are still under investigation. Some of our climate models say that it’s the shear is the most important, and other climate models say it’s the temperature and humidity characteristics that are the most important.

Glen Paul: Right, though either way there’ll be impact. Is this only going to affect the west coast of Australia, or will there be a change in the number and strength of cyclones right across the top end?

Dr Abbs: We’re finding that these results are for Australia wide. Our modelling shows a very consistent picture of decreases in cyclone occurrence across the Australian continent as a whole.

Glen Paul: So basically while we’re going to get less cyclones overall, the ones that we do get are going to be bigger. With the tropical cyclone intensity scale going up to category five, does that mean we’re going to see cyclones along the lines of Cyclone Yasi, which was a category five?

Dr Abbs: There’s a greater likelihood, or greater risk, that the cyclones that do form will form in the category three, four and five level. To say that they’ll be Cyclone Yasi is pushing it a little bit too far, but there’s a greater risk that they will be at that level.

Glen Paul: OK. Now, cyclones can be up to 2,000 kilometres across, is that likely to change as well, as in a larger diameter encompassing more surface area?

Dr Abbs: Our research is showing that they are changing in size. We’re finding larger cyclones, and that’s cause for concern because it’s the size of cyclones that affects the destruction from waves and storm surge.

Glen Paul: Hmm, and with the larger towns and cities further south that could be a real problem. Is there a chance that the cyclones could move further south?

Dr Abbs: On average our results are showing approximately a one degree, or a hundred kilometre further southward movement in cyclones, so that means that when they eventually peter out or decay it’s going to be a little bit further south than we expect today.

Glen Paul: And how did you model all of these changes?

Dr Abbs: We used the outputs from global climate models and regional climate models, to look for weather systems that have the characteristics of the tropical cyclone. The global climate models are very course, and they have a grid spacing of around 300 kilometres. So with those sorts of assistance we can see weather systems that look like tropical cyclones, but they’re a bit washed out, they don’t have the intensities that we see in reality.

Regional climate models are a little bit finer. The ones that we investigated had a grid spacing of around 60 kilometres, so we start to see some of the higher intensities that we experience, but still not the very high intensities that cause all the damage.

So we go through all of these outputs and we look for, as I said for weather systems that look like tropical cyclones that have the physical characteristics such as high wind speeds, and warmer temperatures in the centre of the cyclone than outside.

Glen Paul: So, when are we likely to see the impact of these changes begin to kick in?

Dr Abbs: The work that I’ve been talking about is for the end of the 21st Century. We haven’t done the work ourselves, but there’s been some work completed in the U.S. which has looked at the emergence time to start to see the changes in the characteristics of cyclones in our climate record, and their suggestions or calculations are for an 80 year emergence time. So I wouldn’t expect that we would know for certain that climate change was affecting our cyclone record until late in the 21st Century.

Glen Paul: Hmm. And in preparation for that is the report being looked at now and acted upon?

Dr Abbs: The Institute of Engineers is using information related to our work to upgrade their technical specifications for wind loadings for buildings. Another part of the Institute of Engineers are revising the Australian rainfall and runoff, and that’s used to design major dams, and subdivisions, gutters, and so on, drainage. They’re interested in upgrading their technical specifications related to change in extreme rainfall, and for many parts of northern Australia extreme rainfall is due to tropical cyclones.

Other groups, such as the State Government and Local Government Authorities, are very interested in what might happen to tropical cyclones from a planning point of view. For instance we don’t want to see subdivisions and so on being put in areas that might be flood prone or storm surge prone in the future.

Glen Paul: Absolutely. Well it’s an interesting twist in the tail of cyclones, and certainly something to think about over summer. Thank you very much for discussing it with us today, Debbie.

Dr Abbs: OK. Thank you.

Glen Paul: Dr Debbie Abbs. For more information find us online at www.csiro.au. You can like us on Facebook, or follow us on Twitter at CSIROnews.