Some years ago we did a trip from South Africa up as far as Northern Kenya. Up there we had to cross the Equator and one of the ways the locals make some money, is to demonstrate the difference between the movement of wind and water in the two hemispheres of the earth. Going a short way north of the Equator, a matter of a few metres, water poured down a funnel rotates in an anticlockwise direction. Moving the same distance south of the Equator the water rotates in a clockwise direction.
This brings me to the subject of tropical storms, called Cyclones in South Africa and Australia, Hurricanes in the West Indies and America and Typhoons in Asia. Driven by the Coriolis force, which is what spins water in different directions north and south of the equator, these tropical storms rotate in exactly the same way.
Moving over warm tropical seas, heated air and moisture are drawn inwards and upwards in a low pressure system which rotates around a calm centre called the eye. These huge storms can be hundreds of kilometers across with an eye of 50 kilometers and winds of nearly 300 kilometers an hour. The whole system is driven forward by the prevailing winds.
Cyclones do an important job for the Earth. They help move heat from warm tropical places to the cooler temperate zone. This makes them an important part of the global atmospheric circulation mechanism. To do this, they typically form between 5 to 15 degrees latitude north and south of the equator.
In recent years we have seen some very powerful and destructive cyclones like the one that hit New Orleans. The severity of these storms is driven by water temperature and with global warming and the oceans heating up we are likely to see even worse cyclones.
Tropical cyclones can produce extremely powerful winds and torrential rain. They are also able to produce high waves which result in damaging storm surges. With a lot of human development along coastlines, these mega-storms cause a huge loss of life and damage to infrastructure as well as affecting food supplies.
An anticyclone is a high pressure system in which the wind generally travels downwards and outwards. Depending on where they form on the globe, they can bring clear weather, but can also cause desertification in hot dry areas and are responsible for heavy rains such as the monsoon.
A tornado is another low pressure system that is a violently rotating column of air in contact with both the surface of the earth and a cumulonimbus cloud. They are often referred to as twisters. Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes in the world occur in the so-called “Tornado Alley” region of the United States. They also occasionally occur in south-central and eastern Asia, northern and east-central South America, Southern Africa, north-western and southeast Europe, western and south-eastern Australia, and New Zealand.
In South African terms the cyclone that sticks in everyone’s mind is Demoina. Though it was 28 years ago, it was at the time considered the most devastating tropical cyclone to have struck south-east Africa in the last century.
This changed in 2000 when a level 4 storm, Tropical cyclone Eline, devastated parts of Madagascar, practically crippled Mozambique and impacted heavily on Zimbabwe. In South Africa and Swaziland, Cyclone Eline caused minimum damage to buildings but torrential rains flooded parts of the north eastern coast.
Cyclone Domoina resulted in the loss of 214 human lives, and the lives of countless livestock and wildlife. Eight bridges were washed away by the flood-waters in KwaZulu-Natal, including the bridge connecting the town of St Lucia to the mainland.
Major geo-morphological changes occurred at the mouth of Lake St Lucia, where all man-made structures were obliterated, as the two river channels at the mouth were scoured from 2–3 metres to 10–14 metres in depth and widened by up to 300 metres, while the shoreline between the two channels retreated in places up to 100 metres, with an estimated 16,000,000 cubic metres of sediment being removed from the lower reaches of the system. Some 700mm or 28 inches of rain were recorded over 24 hours. The losses were estimated at one hundred million rand, which 28 years ago was a huge amount of money.
Effects on wildlife
The impact of tropical cyclones on coastal vegetation and wildlife can be devastating. Winds that can attain speeds of well over 200kms per hour, rip through coastal shrub and forest, in some cases removing it completely. Even mangroves, with their enormous root systems, can be stripped away completely. The wildlife at all levels that inhabit these areas is also wiped out.
When cyclonic winds strip vegetation and topple trees, a large pulse of litter fall (fallen leaves, branches, and other natural debris) is generated. This can be as much as 1.4 times the annual litter fall rate. Storm-induced litter fall may contain up to 3 to 5 times more nitrogen, phosphorous, magnesium, and potassium than the average, annual litter fall for an area.
The decomposition of this large amount of organic matter, especially that which has fallen into wetlands and/or reverie habitats, can lead to low levels of oxygen as this matter decays. This drop in available oxygen may cause the aquatic environment to become hypoxic or anoxic (low or no available oxygen, respectively). This then impacts fish and other animals living in these areas.
Turtle nesting sites
Cyclones appear to have a significant impact on sea turtle nest survival. Sea turtle nests can be inundated, washed out, or buried by sand brought in with the high tides and increased wave action associated with a tropical system.
Fixed species such as oysters are also significantly impacted by tropical cyclones. Tropical storms detrimentally impact oyster reefs through physical disturbance (waves pounding into the reefs cause breakage), sedimentation, and extreme salinity changes. These effects are typically due to the storm surge, high winds, and high rainfall associated with each storm.
Damage to coral reefs
The waves generated by cyclones are larger and more powerful than those experienced under normal conditions. These large waves can significantly impact coral reef systems. Damage to coral reefs can vary from almost total destruction to no effects at all over a distance of just a few meters.
The amount of tropical cyclone damage to corals is species-specific because the vulnerability of colonies is a function of their shape, strength of their skeletons and anchor positions, as well as their orientation. Delicate, branching corals are more vulnerable to wave damage than corals with a boulder-like growth form.
Dislodged coral pieces can cause further damage during a cyclone event as they are propelled onto other parts of the reef. Excessive sedimentation reduces available light, inhibiting photosynthesis by the coral’s symbiotic algae. Silt may also settle on the coral surface, blocking feeding and respiration.
The destruction of nesting, roosting and feeding sites can cause the local extinction of a range of organisms from insects and reptiles to birds.
Article by Gwynne Howels, KZN Wildlife