People who live in North Queensland learn to keep an eye on cyclones as long as they exist, whether they look like being a direct threat or not. I followed the progress of cyclone Nathan, via the BoM’s Cyclone Tracker, day by day and it turned out to be a good example of their unpredicability.
The thumbnails here are screen shots from the BoM site at around noon each day from Monday 16 March until Nathan was declared ‘no longer a cyclone’ on Wednesday 25 March; click on them for larger images, as usual, or a slide show. A satellite image showing Nathan’s remnants off the Kimberley coast on Saturday 28 March completes the sequence.
By the time I started recording Nathan, the cyclone had formed in the Coral Sea, drifted slowly West towards Cooktown, stalled, back-tracked slightly further North and drifted South and East to be almost exactly East of its position five days earlier. The prediction at that stage (March 16) was for intensification from category 2 to 3 and a track to the South and West which might have struck the coast anywhere between Cairns and Cardwell a few days later.
In the event, Nathan tracked further to the North, crossing the coast just North of Cooktown as a category 4. When a cyclone moves over the land, it loses its source of energy and moisture. Wind speeds fall quickly and the storm can be downgraded from (e.g.) category 4 to 2 in a matter of hours and, if it stays over land, to a tropical low or rain depression.
A rain depression can still be a very significant weather event, with hundreds of millimetres of rain falling in 12 or 24 hours, but without the destructive winds.
Townsville’s “Night of Noah” in 1998 was one such event, the result of the remnants of cyclone Sid; it brought us a metre of rain in a day or so, more than enough to stick in the memory. Wikipedia has quite a big page on it, and the BoM produced a comprehensive report (pdf).
If the weather system moves back to sea, however, it can re-form into a cyclone. Nathan did just that after crossing Cape York, and struck the Eastern tip of the Northern Territory during the night of 21-22 March as a category 2 system.
The people of Nhulunbuy must have felt life was totally unfair, since they had suffered through cyclone Lam (bigger but not striking them so directly) only a month earlier.
After passing very briefly over land, Nathan tracked West along the Arnhem Land coast, just far enough out to sea to pick up more energy and remain at category 2, before dipping South-west over Maningrida on March 24. That was its last day as a cyclone. It continued over land to the South-west, passing South of Darwin and then out to sea as a low. It dumped 100 – 200mm of rain on the way, flooding remote communities and cutting roads.
Four general points about cyclones are worth making:
- The Bureau of Meteorology does a terrific job of tracking and predicting cyclones. Predictions for 24 hours ahead consistently come true, and the cyclone’s track further into the future stays within the widening grey cone of possible tracks.
- How much damage a cyclone does to human life and property depends as much on its track as on its strength. When Nathan crossed the coast North of Cooktown as a category 4, its footprint covered Cooktown (pop. 2500) and a few smaller communities – a total of perhaps 5000 people. When cyclone Larry crossed the coast near Innisfail (pop. 8500 and in a much more closely settled region) in 2006 as a category 4 it did $1.5 billion worth of damage.
- Cyclones (typhoons) striking areas with greater population density, e.g. the Philippines (and especially Haiyan), or less-robust infrastructure, e.g. Cyclone Pam in Vanuatu often have far more dire effects.
- Climate change fuels stronger cyclones. According to Kerry Emmanuel on RealClimate there is a “strengthening consensus that the frequency of high category tropical cyclones should increase as the planet warms.”
Basic theory and a variety of numerical simulations support this, as well as the projection that tropical cyclones should produce substantially more rain, owing to the increased moisture content of the tropical atmosphere. This is important because most destruction and loss of life are caused by high category storms and their attendant storm surges, and by freshwater flooding from torrential rains. Most of the disagreement in the literature on tropical cyclone projections concerns the incidence of weak storms, but these are usually far less consequential in spite of being more numerous.
… While Pam and Haiyan, as well as other recent tropical cyclone disasters, cannot be uniquely pinned on global warming, they have no doubt been influenced by natural and anthropogenic climate change and they do remind us of our continuing vulnerability to such storms.