Causes & Characteristics of Tropical Cyclones (Edexcel GCSE Geography A)

Formation of Tropical Cyclones

Global atmospheric circulation and tropical cyclones

• The global atmospheric circulation affects the Earth's climate

• It creates differences in air pressure, which is measured in millibars (mb)

• This causes some areas to develop certain types of weather more frequently than others

• The link between tropical cyclones and atmospheric circulation is related to:

  • The Hadley cell

  • The Coriolis effect

  • Equatorial trade winds

• Equatorial regions are the warmest because the Sun is directly overhead

• This intense solar heating raises ocean temperatures 

• The warm, moist air rises and leads to an intense low-pressure zone between the two Hadley cells, also known as the Intertropical Convergence Zone or ITCZ 

• This generates thunderstorms, strong winds and intense rainfall at the surface

  • These are typical weather conditions at the rising arm of the Hadley cell

  • Further along, dry air descends, creating a high-pressure zone at the surface

  • This generates pressure gradients and air rushes to the low-pressure zone, forming the winds of a tropical cyclone

• The trade winds move in a westerly direction from the equator 

• Only from 5° north and south of the equator is the Coriolis effect strong enough to begin spinning the air 

• The spin in these low-pressure zones is:

  • Anti-clockwise (anticyclone) in the northern hemisphere

  • Clockwise (cyclone) in the southern hemisphere

  • The greater the low-pressure, the greater the winds, the greater the spin and the larger the tropical storm becomes

Effects of global circulation on tropical cyclones

Sequence of events

• Under the right conditions, a tropical cyclone can form rapidly

• They follow a number of stages:

  • Sea temperatures must be 27°C and above

    • Allowing warm, moist air to rapidly rise

    • This forms an area of very low pressure

  • Meanwhile, air from high-pressure areas rush to take the place of the rising air

    • The rising air draws further moist, warm air up from the ocean's surface, generating stronger winds

  • The air spirals upwards, cools, condenses and forms large cumulonimbus clouds 

    • These clouds form the eye wall of the storm and produce heavy rainfall

  • Cold air sinks at the centre, creating a calm, dry area known as the eye of the storm

• Tropical cyclones will die out if heat energy and moisture from the ocean are no longer available to drive the storm

Characteristics & Distribution of Tropical Cyclones

• Characteristics include:

  • Lasting 7–14 days

  • Heavy rainfall

  • High wind speeds (over 119 kmh)

  • High waves and storm surges

• Tropical storms can vary in diameter (100–1000 km) 

• Winds spiral rapidly around a calm central area known as the eye

  • The eye is a column of descending high pressure, cold air with light winds and no clouds or rain

• The winds of the storm are not constant across its diameter

  • The outer edges of the storm have lighter wind speeds, smaller and more scattered clouds, and less intense rain,  temperatures begin to increase

  • The strongest and most destructive winds are found within the eyewall, with spiralling storm clouds, torrential rainfall and low temperatures 

Tropical cyclone formation

Distribution of tropical cyclones

• They develop over the warm tropical oceans, usually between 5° and 15° north and south of the equator, although they can extend to as high as 30°

  • This area has the warmest waters

  • The Coriolis effect takes effect 500 km from the equator

  • Ocean temperatures must be in excess of 27°C and to a depth of 50–60 metres

• Tropical storms are rotating, intense low-pressure systems (below 950mb)

• They are known as:

  • Typhoons in the South China Sea and west Pacific Ocean

  • Hurricanes in the Gulf of Mexico, Caribbean Sea and west coast of Mexico

  • Cyclones in the Bay of Bengal, Indian Ocean and northern Australia

The global distribution of tropical cyclones

A map showing distribution and direction of tropical cyclones

• The distribution of tropical cyclones could change if global ocean temperatures continue to rise

• This could affect:

  • Areas further from the Equator

  • Parts of the sub-tropics, the South Atlantic and the Northeast USA

  • Larger parts of the world through a wider distribution

Tropical cyclone movement

• Movement is determined by the prevailing winds and ocean currents

• Tropical cyclones can travel up to 600 km at 40 km/h a day

• A cyclone's track is determined by the distance it travels across the ocean

  • The warmer the ocean, the more moisture it will collect and the stronger it will become, the further it will track

• It is possible to forecast a tropical cyclones movement using satellite images

Frequency

• It depends on where and which hemisphere, but on average, over 80 tropical storms are generated by tropical oceans every year

• Tropical storms begin when tropical oceans are at their warmest, which is usually late summer (November–April in the southern hemisphere and June–November in the northern hemisphere)

• The Pacific Ocean sees the largest number of tropical storms, followed by the Indian Ocean, and lastly, the Atlantic

• Tropical storms in the western Pacific are the strongest 

Average Frequency of Tropical Storms

Changes in frequency

• Although the number of Atlantic storms has increased, the overall global frequency remains steady

• However, tropical storm intensity has increased by 70% over the last 30 years

• El Niño cycles increase wind strength high over the Atlantic, which has the effect of decreasing storm activity overall (winds are too strong and will tear through developing storms above the ocean surface)

• Global warming has the potential to increase the frequency, distribution and intensity of tropical storms in the future