Definition
Tropical cyclones
Tropical cyclones are rapidly rotating storm systems that develop over warm ocean waters in the tropics.
- Tropical cyclones have a low-pressure centre (the eye) surrounded by spiralling bands of thunderstorms.
- These systems are characterized by:
- Very strong winds (often exceeding 119 km/h)
- Heavy rainfall and flooding
- Thunderstorms and storm surges along coasts
- Once sustained wind speeds reach 119 km/h or more, they are classified as:
- Hurricanes in the Atlantic Ocean and eastern Pacific
- Typhoons in the western Pacific Ocean
- Cyclones in the Indian Ocean and South Pacific
Note
Tropical cyclones are among the most dangerous natural hazards to humans, causing widespread destruction and loss of life.
The Structure of a Tropical Cyclone
- Eye: Central calm region of low pressure.
- Eyewall: Zone of maximum wind and rain surrounding the eye.
- Rainbands: Spiraling arms of heavy rain and thunderstorms extending outward.
- Outflow: Air diverging aloft, which helps maintain low pressure below.
Conditions Required for Formation
- Warm ocean surface: Sea-surface temperatures must exceed 26.5°C to supply sufficient latent heat.
- Moist atmosphere: Abundant water vapour fuels condensation and releases energy.
- Low wind shear: Calm upper winds allow vertical storm development.
- Coriolis effect: Earth’s rotation causes the system to spin, giving it its spiral structure.
- Pre-existing disturbance: A small low-pressure area acts as the seed for cyclone development.
Tip
Cyclones do not form directly at the equator because the Coriolis effect is zero there, preventing the necessary rotational movement.
Formation Process
- Warm, moist air rises over the ocean surface.
- Rising air cools and condenses, forming clouds and releasing latent heat energy.
- The release of latent heat warms surrounding air, causing further uplift.
- This process reduces surface pressure, drawing in more warm air.
- Continuous evaporation and condensation maintain and intensify the storm.
- The Coriolis effect causes the storm to rotate cyclonically (anticlockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere).
Note
The eye is calm, warm, and clear, while the eyewall, a ring of towering thunderstorms contains the strongest winds and heaviest rain.
Impacts of Tropical Cyclones
Environmental Impacts
- Massive rainfall and flooding destroy vegetation and erode coastlines.
- Saltwater intrusion damages freshwater ecosystems.
- Coral reefs and mangroves are damaged by wave action and storm surges.
Human and Economic Impacts
- Loss of life and widespread displacement of communities.
- Damage to infrastructure, housing, and transport networks.
- Economic loss from destroyed crops, ports, and industries.
- Health risks such as waterborne diseases and mental trauma.
Common Mistake
- Do not confuse tropical cyclones with tornadoes.
- Tornadoes are much smaller (hundreds of metres wide) and last minutes, while cyclones can be hundreds of kilometres wide and last for days.
Rising Ocean Temperatures and Cyclone Intensity
- Global warming caused by greenhouse gas emissions is increasing ocean temperatures.
- Warmer water provides more latent heat energy, the main fuel for tropical cyclones.
- This leads to:
- Stronger storms with more powerful winds
- Heavier rainfall due to greater atmospheric moisture
- Higher storm surges because of thermal expansion and sea-level rise
- Warmer air can retain more moisture, amplifying rainfall and flooding during storms.
Note
- Latent heat is the energy released when water vapour condenses into liquid.
- This energy powers the cyclone’s vertical convection and wind acceleration.
Link Between Climate Change and Cyclone Behaviour
- Warmer seas increase the potential energy available for storm formation.
- Higher sea levels mean coastal flooding is more extensive.
- Longer cyclone seasons may occur as tropical waters remain warm for extended periods.
- The geographic spread of storms may expand toward higher latitudes, as previously cooler waters become warm enough for cyclone development.
Example
Studies show that Category 4 and 5 hurricanes in the Atlantic Ocean have become more common over the past few decades as ocean heat content rises.
Theory of Knowledge
The uncertainty of climate models illustrates the challenges of predicting complex Earth systems. While trends are observable, precise forecasting of cyclone intensity remains difficult.
Scientific Evidence and Observed Trends
1. Sea Surface Temperature (SST) Rise
- The average global ocean temperature has increased by about 0.8°C since the mid-20th century.
- This rise has provided additional latent heat energy to strengthen tropical storms.
2. Observed Cyclone Trends
- Data from the National Oceanic and Atmospheric Administration (NOAA) show a significant rise in major hurricanes (Category 4–5) in the Atlantic since the 1980s.
- In the Western Pacific, the frequency of intense typhoons has also increased.
3. Rainfall and Storm Surge
- Warmer oceans lead to higher evaporation rates, causing more intense rainfall.
- Rising sea levels due to thermal expansion and melting ice enhance storm surge impacts during landfall.
Example
Regional Examples
- Hurricane Harvey (2017):
- Caused catastrophic flooding in Texas due to record rainfall.
- Warmer Gulf of Mexico waters contributed to the storm’s intensity and persistence.
- Typhoon Haiyan (2013):
- One of the strongest cyclones ever recorded, with winds exceeding 315 km/h.
- The unusually high sea-surface temperatures in the western Pacific intensified its strength.
Self review
- Explain the process that drives the energy of a tropical cyclone.
- Distinguish between hurricanes, typhoons, and cyclones.
- Describe how global warming affects the intensity and frequency of tropical cyclones.
- Explain why tropical cyclones cannot form near the equator.
- Discuss the relationship between sea-level rise and storm surge impacts during cyclones.
- Analyse how global climate models are used to predict future trends in tropical cyclone behaviour.
