Residents along the northwest coast of Western Australia braced for Tropical Cyclone Ilsa expected to become one of the most destructive storms to hit the region in more than a decade.
The Bureau of Meteorology say Cyclone Ilsa has intensified and is now classified as a Category Four system. It is predicted to cross the WA coast between Port Hedland and Bidyadanga on Thursday evening or Friday morning.
Tropical cyclones are massive low-pressure systems that form in tropical waters. They can bring extreme gusts of wind, heavy rains and damaging waves, destroy infrastructure and the environment, and cause injuries and deaths.
Let’s see how Cyclone Ilsa developed and what we can expect from cyclones in this region in the future.
@uptownerd/AAP
Why did Cyclone Ilsa intensify?
Tropical Cyclone Ilsa is the first Category Four or greater system to cross the Australian coast since then Cyclone Trevor crossed the coast of the Gulf of Carpentaria in 2019.
Ilsa formed off the coast of the Northern Territory before moving southwest into the Kimberley region of Western Australia. It quickly developed into a category two system on Tuesday, what means wind gusts between 125 km/h and 164 km/h.
The cyclone intensified into a category four storm on Thursday, with wind gusts between 225 km/h and 279 km/h. This was due to two main factors: high sea surface temperatures and favorable conditions in the upper atmosphere.
Tropical cyclones (TCs) require sea surface temperatures above 27℃. This creates warm, moist air that generates a tremendous amount of energy and fuels the cyclone.
Upper atmospheric conditions affect wind speed. Air is sucked in toward the center of a tropical cyclone. In the Southern Hemisphere, the air spirals clockwise upward and then moves outward into the upper troposphere, away from the storm. This air is known as “outflow”.
Cyclone Ilsa’s path led it to an area where upper-level winds were relatively light, favoring outflow.
As the air moved outward, more wind or “inflow” from the sea surface was drawn into the center of the system, bringing heat and moisture with it. This allowed Cyclone Ilsa to rapidly intensify.
Australia’s cyclone capital
Northwest WA is Australia’s most cyclone-prone region. Showing records since 1970 about 75% of severe cyclones to make landfall in Australia occur in this region.
But why? It comes down to two things: the high sea surface temperatures in this part of the Indian Ocean and the orientation of the coast.
Tropical cyclones tend move pole direction and, in the Southern Hemisphere, often curving southeast. The coast of northwest WA is oriented northeast/southwest, so perfectly aligned to intercept these cyclones.
Several intense tropical cyclones have developed in the warm waters off the coast of northwest WA in recent years. However, the number of people reaching land in this region is lower than average. That’s because mid-to-higher-level atmospheric winds that steer tropical cyclones have led many of them away from the WA coast.
Bomb
What about climate change?
Climate change is expected to alter tropical cyclone patterns. The total number is expected to decrease, but their intensity is likely to increase increasewith stronger wind and heavier rain.
more intense tropical cyclones are expected because higher sea surface temperatures will make the atmosphere warmer and more humid. Cyclones thrive in such conditions.
But the general frequency of tropical cyclones is expected to increase Reduce under climate change in most ocean basins, including the Indian Ocean.
Tropical cyclones usually form when there is a large difference between the temperature at the Earth’s surface and the upper atmosphere. As the climate warms, this temperature difference becomes smaller probably narrow.
Research from last year showed that the annual number of tropical cyclones that form worldwide has decreased by about 13% in the 20th century compared to the 19th.
The activity of tropical cyclones in an ocean basin during a year is measured by what is known as the “accumulated cyclone energy” or ACE Table of contents.
The index is calculated by measuring the wind speed of the cyclone every six hours by squaring it and then adding these values together.
An investigation has shown that index values for cyclonic activity in the southern Indian Ocean have declined significantly since 1990.
I specialize in reconstructing natural records of long-term extreme events. Research tropical cyclone activity along the WA coast has been shown by myself and colleagues to be at its lowest since about 500 CE – about 1500 years ago.
Read more: Cyclone Seroja just devastated parts of WA – and our warming world will bring more of the same
Richard Wainwright/AAP
We’re not off the hook
Tropical cyclones conserve energy over warm water and lose energy as they move over land or cooler oceans.
Cyclone Ilsa is expected to weaken Friday night as it moves eastward into the Northern Territory.
Climate change will generally lead to fewer tropical cyclones. But the ones that do occur will be more intense and damaging. So unfortunately WA can expect regular cyclone effects even as the climate warms.
Read more: Tropical cyclone frequency drops to centuries low in Australia – but will the lull last?
