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Causes of Tropical Storms

Imagine a giant engine that runs entirely on warm ocean water. When atmospheric and oceanic conditions are exactly right, this engine powers up to create the most devastating storm systems on Earth.

A requires specific prerequisites to form. The must be 27°C or higher to provide sufficient thermal energy. Furthermore, the ocean must be warm to a depth of at least 60–70 metres to ensure a continuous heat source as the storm churns the water.

Storms only form between 5° and 30° north and south of the equator, often within the . Crucially, they do not form within 5° of the equator. This is because the is too weak at the equator to start the rising air spinning. Finally, low wind shear is necessary so the storm's vertical structure is not blown apart.

The Mechanism of Extreme Weather

When these prerequisites are met, extreme weather conditions are generated through a clear chain of events. Warm ocean water causes rapid evaporation, making warm, moist air rise and creating a strong at the surface.

As this air rises, it cools and undergoes to form massive cumulonimbus clouds. This releases , warming the surrounding air and causing it to rise even faster, which lowers the surface pressure further. The intense low pressure violently sucks in air from surrounding areas. The forces this inward-rushing air to spiral, generating sustained winds exceeding 74 mph (118 km/h).

Structurally, a storm has a calm where air descends (creating high pressure and clear skies). Surrounding this is the , a ring of intense thunderstorms containing the heaviest torrential rain and strongest winds. The combination of low atmospheric pressure and high winds also pushes seawater onshore, creating devastating storm surges.

Normal Conditions and the ENSO Cycle

Every few years, changes in Pacific Ocean currents and winds flip global weather patterns upside down. Under normal conditions, permanent easterly blow East to West, pushing warm surface water toward Australasia.

This creates low pressure (rising air and rain) in the West, and high pressure (sinking air) in the East. In the East, brings cold, nutrient-rich water to the surface off the coast of South America. This entire atmospheric loop across the equatorial Pacific is called the .

El Niño and La Niña Drought Mechanisms

is the periodic fluctuation of these Pacific temperatures and pressures, causing extreme weather worldwide.

During an event (every 3 to 7 years), weaken or reverse. Without these winds, warm surface water shifts East toward South America. Because the warm water has moved away, the water near Australia cools. The air above Australia becomes denser and descends. This descending air creates high pressure, preventing moisture from rising and condensing. With no cloud formation, severe hits Australia and Indonesia.

During a event (every 3 to 5 years), become significantly stronger than normal, pushing even more warm water West. Intense drops Eastern Pacific temperatures by 3°C to 5°C below average. This extreme cold water makes the air above it dense, causing it to sink rapidly. This strong high-pressure system prevents , causing severe in Peru, Chile, and the Southern USA.

Case Study: El Niño and The 'Big Dry'

Real-world climate events show how shifts in atmospheric circulation have devastating human impacts. The 'Big Dry' in Australia (2002–2009) was a national-scale disaster triggered by an event that caused high pressure and descending air over the continent.

Environmentally, the Murray-Darling River basin, which is home to 2 million people, experienced extreme and toxic algal blooms. The prolonged also triggered secondary impacts, such as an increase in severe wildfires.

The brought severe economic and social hardship. Water bills rose by 20% in 2008, and 10,000 people in the cotton-growing industry were negatively affected. Socially, the immense pressure led to an increase in suicide rates among rural farmers and forced significant migration from rural areas to cities.

Students often forget that during El Niño, the water near Australia actually cools relative to the new warm pool in the East; this cooling is exactly what creates the sinking air and high-pressure zone causing drought.

For 'Outline' questions on tropical storms, examiners expect you to explicitly link the prerequisite to the weather process, e.g., 'a 27°C sea causes rapid evaporation, leading to low pressure'.

When explaining ENSO mechanisms, memorise the cause-and-effect chain: weakening trade winds → warm water shifts East → high pressure forms over Australia → descending air prevents cloud formation → drought.

Always use specific geographical terminology like 'latent heat' and 'Walker Circulation' to explain extreme weather, rather than vague phrases like 'stormy weather' or 'hot air'.

A low-pressure weather system that develops over warm tropical oceans with sustained winds of at least 74 mph.

The temperature of the top layer of the ocean, which must be 27°C or higher for a tropical storm to form.

A belt of low pressure near the equator where trade winds meet, often acting as a nursery for tropical storm development.

The force caused by the Earth's rotation that deflects moving air, causing tropical storms to spin.

An area where air is rising, cooling, and condensing, leading to cloud formation and precipitation.

The process where water vapour turns into liquid water, forming clouds and releasing latent heat.

The energy released into the atmosphere when water vapour condenses, which powers and intensifies tropical storms.

The calm, central part of a tropical storm where air descends, creating high pressure and clear skies.

The ring of intense thunderstorms surrounding the eye, containing the strongest winds and heaviest rain.

Permanent, constant easterly winds that blow East to West in the Earth's equatorial region.

The process where deep, cold, nutrient-rich water rises to the surface, typically occurring off the coast of Peru.

The atmospheric loop of air moving across the equatorial Pacific, driven by the temperature difference between the Eastern and Western Pacific.

The periodic fluctuation in sea surface temperature and the air pressure of the overlying atmosphere across the equatorial Pacific Ocean.

A climate pattern involving the unusual warming of surface waters in the eastern tropical Pacific Ocean, which weakens trade winds.

The cool phase of the ENSO cycle, characterized by unusually cold ocean temperatures in the Equatorial Pacific and stronger trade winds.

A prolonged period of abnormally low rainfall, leading to a severe water shortage.

A situation where the demand for water exceeds the available amount, or poor quality restricts its use.