What this lesson teaches
I can explain how monsoons, tropical cyclones and ENSO change rainfall in the tropics.
Syllabus 9173, Topic 2.1, 1. Tropical Climates. How does synoptic scale circulation affect rainfall in the tropics?
Make a guess
The sea is warmer than 27 degrees C, yet no tropical cyclone forms. What might be missing?
- Nothing; warm water makes a cyclone.
- The sea must be colder for cyclones to form.
- Cyclones form right on the equator.
- Low wind shear to keep the storm organised.
Show the answer
Low wind shear to keep the storm organised.
Warm water fuels a cyclone, but it also needs moist unstable air, rotation and little change in wind with height. Strong shear tears the circulation apart.
Tropical cyclones need warm ocean water, moisture and organised rotation, and their rain can keep falling inland after the winds weaken.
A tropical cyclone is an organised, rotating low-pressure system powered by heat and moisture from a warm ocean. It works at the regional, or synoptic, scale. Formation usually needs sea temperatures of about 26.5 to 27 degrees C or more, over a deep warm layer. A thin warm layer can cool quickly, because the winds stir up colder water from below.
The syllabus focuses on storms with sustained winds of at least 119 km/h: hurricane or typhoon strength. They are called hurricanes in the Atlantic and eastern North Pacific, typhoons in the western North Pacific, and cyclones in the Indian Ocean and near Australia. Weather agencies also use tropical cyclone as a wider term that includes weaker depressions and storms, and classifications vary by ocean.
Moist, unstable air feeds tall thunderstorms. An existing disturbance and winds meeting at the surface help organise the rising air. Low wind shear, meaning little change in wind speed or direction with height, keeps the circulation and thunderstorms lined up. Enough Coriolis effect is also needed to start the spin, so cyclones rarely form within about 5 degrees of the equator. These are typical conditions, not an absolute rule.
Warm seawater evaporates and supplies water vapour. As moist air rises and condenses, it releases latent heat, which warms the storm's core. High up, air flows outwards, while at the surface air spirals inwards. This keeps pressure low and the storms going. The strongest rising air and heaviest rain are usually in the eyewall and spiral rainbands. In the eye, air usually sinks, so it is calmer.
Slow-moving storms can drop extreme rain over a long time, and mountains add more by forcing moist air upwards. Over land, a storm loses its ocean heat supply and usually weakens, but it can still cause serious inland floods. Storm surge is different: it is seawater flooding the coast, not rivers overflowing after rain.
- Tropical cyclone
- Rotating low-pressure storm fuelled by a warm ocean.
- Conditions
- Sea above about 26.5-27 degrees C, low wind shear, rotation.
- 119 km/h
- Hurricane or typhoon strength in the syllabus.
Step by step
Supply heat and moisture
A deep warm ocean fuels evaporation.
Organise the storm
Meeting winds, rotation and low shear.
Concentrate the rain
Rainbands, slow movement and mountains.
Real examples to use
Typhoon Haiyan, Philippines, 2013
It was one of the strongest storms ever recorded at landfall. Its storm surge and winds killed over 6,000 people.
Worked example: Two equally warm seas
Both seas are 28 degrees C. One has strong wind shear that tears the storms away from the surface circulation. The other has low shear and a spinning disturbance. The second is more likely to form a cyclone, but it is not guaranteed.
Watch out for this
A cyclone that weakens over land can no longer cause floods.
Wind and rain are different hazards. Slow storms and mountains can still bring flooding rain.
Check your understanding
Which change most directly disrupts a forming tropical cyclone?
- A deeper layer of warm water beneath the storm.
- Moist air condensing in rising clouds near its centre.
- Strong wind shear tearing its circulation apart.
Show the answer
Strong wind shear tearing its circulation apart.
Yes. Shear tears the storm apart.