When the wind blows – tropical cyclones

When the wind blows

.. the cradle will rock. So the rhyme goes, reminding us that the elements are indiscriminate in their destructive actions. At the time of writing Tropical Cyclone Iggy lingers off our shore, still a little undecided on if and where to make landfall.

TC Iggy

The Australian Bureau of Meteorology website provides a useful introduction to cyclone climatology. The northwest WA coast is the most cyclone-affected coastline in Australia. The annual averages are:

  • 5 tropical cyclones form off the northwest coast
  • 2 cyclones come ashore
  • 1 of these will be severe, most often in March or April

Making tracks

Cyclone tracks vary considerably, some making U-turns or even loops. However, BOM notes that there is a preferred route starting in a south westerly direction, then turning more southerly and eventually south easterly when they travel as far south as the Pilbara.

The link with melioidosis

The path cyclones follow has been a matter of interest to the Micrognome for some time because of the link between melioidosis and severe weather in northern Australia. His team noticed there was a correlation between the direction cyclones took in a given year and the number of cases of severe disease.

Cyclone dynamics

Cyclone physics is complex and reflected in the variability of their destructive effect, wind speed, rainfall and duration. In general, BOM notes four main stages in the life-cycle of a tropical cyclone: formative, immature, mature and decay. We noted that the years when more cases occurred were years in which cyclones made landfall over the Northern Territory then followed a south westerly track. Years with cyclones approaching exclusively from the West and making landfall over the Kimberley or Pilbara coast were not bad years for severe melioidosis.

The fluid dynamics of interaction between a cyclone and the land it passes over  are most definitely complex. An added level of complexity is the rainfall that occurs in the cyclone’s decay stage. This looks like it may explain some of the excess cases observed in the NT.

At least we can predict cyclones like TC Iggy are unlikely to cause extra cases of melioidosis, while the likes of TC Carlos (see moving image) are a different kettle of fish.

Melioidosis info sources

 

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