IB Environmental systems and societies 2.4F Tropical Cyclones Notes

Tropical Cyclones: Warm Oceans Power the Most Destructive Storms

1. A tropical cyclone is an intense low-pressure system with a calm centre, the eye, surrounded by spiralling bands of thunderstorms.
2. Every tropical cyclone brings three main hazards:
   1. Very strong winds, often exceeding 119 km/h.
   2. Heavy rainfall and flooding.
   3. Thunderstorms and coastal storm surges.
3. Once sustained winds reach 119 km/h, the same type of storm takes a different name by region:
   1. Hurricanes in the Atlantic and eastern Pacific.
   2. Typhoons in the western Pacific.
   3. Cyclones in the Indian Ocean and South Pacific.

The Structure of a Tropical Cyclone

1. Eye: the central, calm region of lowest pressure.
2. Eyewall: the ring of towering thunderstorms with the strongest winds and heaviest rain.
3. Rainbands: spiralling arms of heavy rain and storms extending outward.
4. Outflow: air diverging aloft, which helps maintain the low pressure below.

Conditions Required for Formation

1. Warm ocean surface: sea-surface temperatures must exceed 26.5°C to supply enough latent heat.
2. Moist atmosphere: abundant water vapour fuels condensation and releases energy.
3. Low wind shear: calm upper winds let the storm build vertically.
4. Coriolis effect: the Earth's rotation makes the system spin, giving it its spiral shape.
5. Pre-existing disturbance: a small low-pressure area acts as the seed for the cyclone.

Formation Process

1. Warm, moist air rises over the ocean surface.
2. The rising air cools and condenses, forming clouds and releasing latent heat.
3. This latent heat warms the surrounding air, causing further uplift.
4. Falling surface pressure draws in more warm, moist air from the sides.
5. Continuous evaporation and condensation feed and intensify the storm.
6. The Coriolis effect makes it rotate: anticlockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

Impacts of Tropical Cyclones

Environmental Impacts

1. Massive rainfall and flooding destroy vegetation and erode coastlines.
2. Saltwater intrusion damages freshwater ecosystems.
3. Coral reefs and mangroves are damaged by wave action and storm surges.

Human and Economic Impacts

1. Loss of life and widespread displacement of communities.
2. Damage to infrastructure, housing, and transport networks.
3. Economic loss from destroyed crops, ports, and industries.
4. Health risks such as waterborne diseases and mental trauma.

Warmer Oceans Make Cyclones Stronger

1. Global warming from greenhouse gas emissions is raising ocean temperatures.
2. Warmer water provides more latent heat, the main fuel for tropical cyclones.
3. This makes storms more intense in three ways:
   1. Stronger winds.
   2. Heavier rainfall, because warmer air holds more moisture.
   3. Higher storm surges, because of thermal expansion and sea-level rise.

How Climate Change Alters Cyclone Behaviour

1. Warmer seas increase the energy available for storm formation.
2. Higher sea levels make coastal flooding more extensive.
3. Longer cyclone seasons occur as tropical waters stay warm for longer.
4. The geographic range of storms may spread toward higher latitudes as cooler waters warm.

Scientific Evidence and Observed Trends

Sea-Surface Temperature Rise

1. Average global ocean temperature has risen by about 0.8°C since the mid-20th century.
2. This has supplied additional latent heat to strengthen tropical storms.

Observed Cyclone Trends

1. NOAA data show a significant rise in major hurricanes (Category 4–5) in the Atlantic since the 1980s.
2. The frequency of intense typhoons in the western Pacific has also increased.

Rainfall and Storm Surge

1. Warmer oceans raise evaporation rates, causing more intense rainfall.
2. Rising sea levels, from thermal expansion and melting ice, worsen storm surges at landfall.

Case Studies: Hurricane Harvey and Typhoon Haiyan

1. Hurricane Harvey (2017): caused catastrophic flooding in Texas from record rainfall.
   1. Warmer Gulf of Mexico waters added to the storm's intensity and persistence.
2. Typhoon Haiyan (2013): one of the strongest cyclones ever recorded, with winds over 315 km/h.
   1. Unusually high sea-surface temperatures in the western Pacific intensified it.