Tropical cyclones are one of the most destructive types of weather system on the planet. The obvious human interest in tropical cyclones is in their sheer power. Historically tropical cyclones have had devastating impacts on life, agriculture, water supplies and the economic well-being of tropical countries.
This cyclone season (November to April), cyclone activity in the Australian region (5°S-40°S, 90°E-160°E) is likely to be above average. Typically, 12 cyclones develop or move into the region during the tropical cyclone season. The outlook issued by the Bureau’s National Climate Centre suggests an 80% chance of having more than the long-term average number of cyclones in the Australian region during the 2011-12 season.
The forecast is due to the presence of a La Niña event in the Pacific Ocean. Cyclones like warm water, and La Niña events are associated with warmer than usual ocean waters in the Australian region. Historical records indicate that La Niña periods are usually, but not always, associated with an increase in tropical cyclone risk for northern Australia during the cyclone season.
Almost everything about a tropical cyclone is extreme. The winds, rainfall, storm surges and flooding associated with severe storms are at the very extremes of recorded weather. Last cyclone season, the whole country watched with morbid fascination as the biggest system since 1918, Tropical Cyclone Yasi, tracked in from the Coral Sea and crossed the Queensland coast near Mission Beach on February 3. While the damage and cost of the storm was huge, the human impact was thankfully small.
What are cyclones and where do they come from?
A tropical cyclone is a rotating storm system characterised by a low-pressure center and numerous thunderstorms, with associated strong winds and torrential rain. The characteristic that separates tropical cyclones from most other cyclonic systems is that at any height in the atmosphere, the center of a tropical cyclone will be warmer than its surroundings. This is a phenomenon called a “warm core” storm system.
The term “tropical” refers both to the geographical origin of these systems, which usually form in tropical regions, and to their formation in maritime tropical air masses. The term “cyclone” refers to the storms’ cyclonic nature, with counterclockwise low level wind flow (near the surface of the Earth) in the Northern Hemisphere and clockwise low level wind flow in the Southern Hemisphere.
A tropical cyclone is sometimes confused with a tornado, which it is not. When compared to a tropical cyclone, a tornado is a micro-sized rotating system. Around the world, there are lots of different common names for a tropical cyclone. These include simply cyclone, cyclonic storm, tropical storm, hurricane or typhoon.
In the northwest Pacific, typhoon is the regional name for a severe tropical cyclone. Hurricane is the regional term for the northeast Pacific and northern Atlantic, bordering North America. For Australian weather and climate folk, a tropical cyclone is known mostly by its initialisation, “TC”.
TCs form and develop over tropical ocean waters. You can think of warm, tropical waters as the engine that spins a cyclonic weather system into a true TC. If they move over land, cyclones lose their strength due to loss of the warm ocean as an energy source, and to increased surface friction. This is why coastal regions can receive significant wind damage from a TC, while inland regions are relatively safe. However, torrential rains can produce significant flooding inland, as happened with both cyclones Yasi and Anthony last summer.
Devastation caused by a cyclone can be significant enough to reach national disaster proportions, as was the case for severe TC Tracy. Cyclone Tracy moved over Darwin from the Arafura Sea on Christmas Eve 1974. It is one of the most significant tropical cyclones in Australia’s history. It led to the loss of 65 lives and the destruction of most of Darwin, and profoundly affected the Australian view of the tropical cyclone threat. The difference between our readiness for Tracy contrasts strongly with our readiness for Yasi, nearly 40 years later.
More or fewer, stronger or weaker, here or there…
Tropical cyclones rely on warm tropical waters to form and develop. So what should we expect from cyclones as climate change ramps up in the next 100 years? Will we get more severe storms in future? As the oceans warm up outside of the tropics, can we expect storms in places we have never seen them before?
Examining trends in cyclone data is problematic for several reasons. Since TCs are few in number each year and for each region, the overall sample size is small. This makes it hard to find statistically meaningful trends in the number of TCs.
In terms of any changing strength of TCs, the science is hampered by the nature of the data. Historically, and without the benefit of satellites, the strength of each TC was manually analysed by forecast meteorologists. This means that there is no objective consistency in historical data over time and over different regions. For example, differences might be found in the manual analysis of the same storm, as it crossed from one tropical cyclone warning centre’s area of responsibility to another.
Considering all of the problems with the data, examination of recent tropical cyclone activity in the Southern Hemisphere shows no significant trends in the total numbers of TCs, nor in numbers of severe tropical cyclones in the South Indian and the South Pacific Oceans.
In the Australian region, no categorical changes in the total numbers of tropical cyclones, or in the proportion of the most intense tropical cyclones, have been found; though there is considerable year-to-year and decade-to-decade variability.
One study by the Bureau of Meteorology has suggested a decline in the most severe storms crossing the Queensland coast over the last hundred years, but the number of storms available to study is small. In the most studied region on Earth, the North Atlantic’s Gulf of Mexico region, the data makes some suggestion of a local increase in the most severe storms, subject to the many caveats mentioned above.
Modelling future cyclones will test our scientific limits
Whether the characteristics of tropical cyclones have changed, or will change in a warming climate — and if so, how — has been the subject of considerable investigation, with less than clear results. This is because we are posing the existing science of climate modeling a fundamentally very difficult question.
Predicting how temperatures will change due to increasing greenhouse gases is relatively straightforward physics. By way of contrast, predicting how regional rainfall will change, such as the future rainfall over South Australia, is a much harder task.
Predicting future changes in TCs is harder again. In fact, even if scientists had another Earth to play with, they would likely find concrete answers difficult to pin down. This is particularly true for more modest increases in greenhouse gases, such as will occur in the next few decades, as opposed to a doubling of atmospheric carbon dioxide concentrations.
For large increases in greenhouse gases, extensive climate modeling has pointed to some consistent future changes. Future projections based on physics and using high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms. The average intensity will increase with global warming.
Existing modeling studies also consistently project decreases in the globally averaged frequency of tropical cyclones. In other words, there will be fewer overall storms.
However that is balanced against the projected increases in the frequency of the most intense cyclones as global warming intensifies. This suggests a future world where tropical cyclones are less frequent, but those storms which do occur are more dangerous.