Context: Meteorologists say Tropical Cyclone Ilsa is forecast to be the most powerful storm to hit Australia in eight years, bringing wind gusts of up to 196 miles per hour as it crosses the northwest coast.
About Cyclones
- Cyclones are the centres of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure.
- The winds move anti clockwise in northern hemisphere and clockwise in southern hemisphere.
- Based on location, cyclones are classified in two major types:
- Tropical cyclones
- Extra Tropical cyclones
Tropical Cyclones
- Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges.
- This is one of the most devastating natural calamities. They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia.
- Tropical cyclones originate and intensify over warm tropical oceans.
The conditions favourable for the formation and intensification of tropical storms are
(i) Large sea surface with temperature higher than 27° C;
(ii) Presence of the Coriolis force;
(iii) Small variations in the vertical wind speed;
(iv) A pre-existing weaklow-pressure area or low-level-cyclonic circulation;
(v) Upper divergence above the sea level system.
Mechanism
- The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm.
- With continuous supply of moisture from the sea, the storm is further strengthened.
- On reaching the land the moisture supply is cut off and the storm dissipates.
- The place where a tropical cyclone crosses the coast is called the landfall of the cyclone. The cyclones, which cross 20 ° N latitude generally, recurve and they are more destructive.
- A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air.
- Around the eye is the eye wall, where there is a strong spiralling ascent of air to greater height reaching the tropopause. Torrential rain occurs here.
- From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region. The cyclone creates storm surges and they inundate the coastal low lands. The storm peters out on the land.
Extra-Tropical Cyclones
- The systems developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones.
- The passage of front causes abrupt changes in the weather conditions over the area in the middle and high latitudes
- Extra tropical cyclones form along the polar front. Initially, the front is stationary.
- In the northern hemisphere, warm air blows from the south and cold air from the north of the front. When the pressure drops along the front, the warm air moves northwards and the cold air move towards, south setting in motion an anticlockwise cyclonic circulation.
- The cyclonic circulation leads to a well-developed extra tropical cyclone, with a warm front and a cold front.
- There are pockets of warm air or warm sector wedged between the forward and the rear cold air or cold sector.
- The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation.
- The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front.
- The cold front moves faster than the warm front ultimately overtaking the warm front.
- The warm air is completely lifted up and the front is occluded and the cyclone dissipates.
- The processes of wind circulation both at the surface and aloft are closely interlinked.
Similarities between tropical cyclones and extra tropical cyclones:
- Tropical cyclones and extratropical cyclones are both symmetrical.
- They also have surface areas of low pressure with winds that rotate counter-clockwise.
- Both produce very heavy precipitation and often results in flooding.
- Both tropical cyclones and mid-latitude cyclones can last for several days, and sometimes if a week or more.
- Often, a tropical cyclone will transform into an extra-tropical cyclone as it recurves poleward.
Differences between tropical cyclones and extratropical cyclones:
Read also: ITCZ – Fluctuations and Impact on Monsoon
Climate Change increasing cyclonic intensity and frequency
According to Council on Energy, Environment and Water, in last 50 years India has recorded a 12-fold surge in number of associated cyclonic events such as extreme rainfall, floods, sea-level rise, and thunderstorms.
- Increased sea surface temperature: Over the past 50 years, the global ocean has absorbed 90% of the excess heat generated due to man-made climate change leading to higher convection and rapid intensification of cyclones.
- Rising sea level: on account of Antarctic melting has increased the moisture availability for cyclones.
- Micro-climatic changes on land: local heating of coastal land is pulled by the adjacent ocean further heating it up.
- Changing weather events: El-Nino and rising marine heat waves lead to prolonged warm periods over oceans by reducing the ocean upwelling.
- Higher Atmospheric moisture: due to anthropogenic global warming increase cyclonic precipitation rates thereby increasing the frequency.
- Changes in wind systems: Occasionally intense winds drive the low-pressure regions to other areas rising the frequency in those areas. E.g.- Gulab cyclone shifted to Arabian sea from B.O.B
- Rapid Intensification: According to NOAA, when the speed of a storm increases by 55km/hr within the span of 24 hours.
