Frequency
The northwest Australian coastline between Broome and Exmouth is the most cyclone-prone region of the entire Australian coastline, having the highest frequency of coastal crossings as shown below. On average about five tropical cyclones occur during each tropical cyclone season over the warm ocean waters off the northwest coast between 105 and 125°E. On average about two cyclones cross the coast, one of which is severe. Indeed about 75 per cent of severe cyclone crossings in Australia between 1970-71 and 2007-08 were in WA. A map of the average annual number of tropical cyclones shows the area off the northwest coast having the highest incidence of cyclones in the southern hemisphere.
Figure 1. Coastal crossing points between 1970-71 tropical cyclone season and 2007-08 season. Red dots represent severe tropical cyclones (category three or higher) and black dots represent non-severe cyclones (sub-category three).
The Australian cyclone season officially runs from November to April, although very few have occurred in November. The earliest cyclone to impact the northwest coast in a season was on 19 November 1910 when the eye passed over Broome. The latest cyclone was Herbie that formed near Cocos Islands and passed over Shark Bay on 21 May 1988. At the start of the cyclone season, the most likely area to be affected by tropical cyclones is the Kimberley and Pilbara coastline. Later in the season, the area threatened extends further south including the west coast. The chance of experiencing an intense category 4 or 5 cyclone is highest in March and April.
The monthly distribution of tropical cyclones affecting specific locations can be found on Tropical Cyclones affecting WA towns.
Monthly frequency of cyclones off northwestern Australia 1988/89 to 2003/04. Intense cyclones (category 4 and 5) are shown in blue.
Tracks
The map of cyclone tracks in the Australian region (see below) shows that cyclones can move in many different directions. However, cyclones off the northwest coast do have a preferred movement. Typically they are initially steered to the west-southwest at speeds of 5-20 km/h and then take a more southerly track as they move further to the south. If they do move south of about 22°S or cross the Pilbara coast they tend to curve to the south-southeast and accelerate. Those that affect the lower part of the west coast may reach speeds of over 60 km/h.
However, the path of each storm varies considerably in response to the weather patterns occurring at the time. Tropical cyclones can be thought of as being steered by the surrounding environmental flow particularly in the middle parts of the atmosphere (from 2 to 6 km above the surface). The larger and more intense systems do influence the surrounding environment and in so doing affect their movement. Short-term fluctuations in the track are common for intense cyclones, for example Bobby (1995) made several loops and changed speed as it approached the Pilbara coast (see track of Bobby based on radar observations). Others can make dramatic shifts in their track, such as cyclone Lena (1993) which was moving to the west but made a U-turn and returned close to its original path. By contrast Vance (1999) followed a much steadier course (see comparison tracks of Lena and Vance).
Overall cyclones off the northwest coast follow a much more predictable path than off the Queensland coast. Indeed the southwest Pacific basin has the highest percentage of cyclones having 'erratic' tracks of anywhere in the world. Those systems that markedly change their course or intensity close to the coast present the greatest challenge to forecasters and decision-makers in the community.
See the Interactive Tropical Cyclone Plotting web page to access tracks of historical tropical cyclones.
Tracks of cyclones in the Australian region from 1989/90 to 2002/03.
Life-cycle
Tropical cyclones have a distinct life cycle of about 4-7 days although some weak ones only briefly reach gale force while others can be sustained for weeks. For cyclones that reach at least severe (category 3 or higher having wind gusts of at least 170 km/h) the life-cycle may be divided into four stages. For non-severe cyclones, their development is constrained by one or more of a number of factors such as being located in an unfavourable atmospheric environment, movement over cooler water or making landfall.
- 1. The formative stage
- On satellite images the disturbance appears as an unusually active, but poorly organised, area of convection (thunderstorms) typically between 5 and 15°S. The circulation centre is usually ill-defined but sometimes curved cumulus cloud bands spiralling towards an active area of thunderstorms indicate the location of the centre. Initially the amount of convection near the centre is dependent upon the normal diurnal cycle of tropical convection, increasing overnight and subsiding during the day. As development occurs the convection persists throughout the day. The strongest surface winds may be well removed from the centre, tend to occur in disorganised squalls and are often confined to one quadrant, for example the northwesterly monsoon winds to the north of the centre. Apart from local squalls the maximum wind is usually less than gale force. When formative stage tropical cyclones move inland they produce little or no damage on landfall but are often associated with heavy rain and sometimes flooding over northern Australia.
- 2. The immature stage
- In this stage the area of convection persists and becomes more organised. Intensification occurs simultaneously. The minimum surface pressure rapidly drops below 1000 hPa and convection becomes organised into long bands spiralling inwards. Gale-force winds develop with the strengthening pressure gradient, and the maximum winds (which now may be storm-force or more) are concentrated in a tight band close to the centre. The circulation centre is well defined and subsequently an eye may begin to form. In satellite images several well organised curved bands of active convection may be seen spiralling in towards a central dense mass of clouds covering the focal point of the banding, or surrounding the centre. The eye (if it exists) may be masked by a canopy of cirrus cloud, which itself may contain curved striations associated with the outflow at the top of the tropical cyclone. The immature tropical cyclone can cause devastating wind and storm surge effects upon landfall, although damage is usually confined to a relatively small area. In this stage of development very rapid intensification can occur and the associated structural changes observed when the cyclone is under radar surveillance can sometimes be confusing.
- 3. The mature stage
- During this stage the tropical cyclone acquires a quasi-steady state with only random fluctuations in central pressure and maximum wind speed. However, the cyclonic circulation and extent of the gales increase markedly. Variations in the wind field may also become more pronounced. In satellite images the cloud field is highly organised and becomes more symmetrical. The more intense cyclones are characterised by circular area of widespread thunderstorms containing a distinct round eye. The surrounding convective bands are tightly coiled. Typically a cyclone spends just a day or so at maximum intensity before it begins to weaken, unless the cyclone remains in a highly favourable environment. Most cyclones off northwestern Australia reach maximum intensity between 10 and 20°S.
- 4. The decay stage
- The warm core is destroyed during this stage, the central pressure rises, and the belt of maximum wind expands away from near the centre. Decay may occur very rapidly if the system moves into an unfavourable atmospheric or geographic environment, but sometimes only the tropical characteristics are modified while the cyclonic circulation moves on to higher latitudes.
- In satellite images the decaying stage is characterised by the weakening of organised convection near the centre and disappearance of major curved convective bands. The low-level circulation centre may still be very well defined by narrow bands of low clouds. Those cyclones that cross the coast and weaken over land may continue to produce heavy rain a considerable distance inland.
- In the Western Australian area cyclones weaken over land or, for those remaining over water, weaken as they move southwards encountering an unfavourable environment (strengthening wind shear) and move over cooler water. They usually increase in speed as the steering winds in the mid-levels of the atmosphere increase and move to the south or southeast.
- Occasionally a cyclone moves south and interacts with a mid-latitude trough and undergoes extra-tropical transition changing from a warm-cored tropical low to a cold-cored mid-latitude low. The structure, distribution of winds and rainfall changes significantly. For more information see Tropical Cyclones affecting Perth.
An abbreviated life cycle of tropical cyclone Paul, April 2000.
- Formative stage: 11 April.
- Immature stage: 14 April.
- Mature stage (920 hPa): 15 April.
- Decay stage: 20 April.
Long-term frequency
Trends in tropical cyclone activity in the Australian region (south of equator; 105-160°E) show that the total number of cyclones has decreased in recent decades (see graph). This decrease may partly be due to an improved discrimination between tropical cyclones and sub-cyclone intensity tropical lows. If weak cyclones are excluded from the analysis, the trend is more gradual and follows the downward trend in the Southern Oscillation Index suggesting that the decrease in cyclone numbers may be related to the greater number of El Niño events since the mid-1970s. However, the number of severe cyclones has increased and this does not appear to be due to improved discrimination between cyclones or trends in the Southern Oscillation Index. The actual cause of this is unknown.
According to the International Panel on Climate Change (IPCC), tropical cyclone activity has risen in the northwest Pacific and north Atlantic since 1950. However, there has been little change in the number of very strong typhoon-force or hurricane-force systems. There has been little change in the number of tropical cyclones in the north Indian Ocean, southwest Indian Ocean and southwest Pacific Ocean east of 160°E.
Despite advances in computer modelling of the global climate using various scenarios of greenhouse gas emissions, making projections of how tropical cyclones may change in frequency and intensity remains a significant challenge. Since tropical cyclone activity is modulated by the El Niño Southern Oscillation (ENSO), projections of cyclone frequency will partly depend on the projections of future ENSOs. It is uncertain how ENSO will change in a warmer world. Some studies have suggested that peak winds in cyclones may increase by 5-10 % and peak rainfall rates may rise by 20-30 %. For more details see Climate Change in Australia (Chapter 5.9.1).
