Syllabus: General Studies Paper 1(Indian Geography)
What are tropical cyclones ? Give reasons for its increasing intensity. Examine how Titli cyclone is different.
Tropical cyclones are one of the nature’s most violent manifestations and potentially the deadliest of all meteorological phenomena. Tropical cyclones are of thermal origin. That is, it owes its origin to the warmness of ocean water. It occurs between Tropic of Cancer and Tropic of Capricorn. It is an an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain. Recent cyclones like ockhi cyclone and Gaja, and titli cyclone best describes the increased intensity of 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 due to violent winds (squalls), very heavy rainfall (torrential rainfall) and storm surge.
There are several conditions which are favorable for the formation of a tropical cyclone.
- Tropical cyclones develop in the vicinity of inter tropical convergence zone (ITCZ) or near equatorial trough where relative cyclonic vorticity is already present as a quasi-steady feature. This zone is found nearly 5° north or south of the equator.
- Tropical cyclones develop in maritime air mass over sea areas where sea surface temperature > 26.5°C and overlying tropical atmosphere is convectively unstable.
- Formation of a well-marked low pressure area or depression close to ITCZ /near equatorial trough which later on develops into a full-fledged tropical cyclone is favoured by the arrival of the low pressure wave from the east.
- It is found that over the regions which are climatologically favourable for the development of tropical cyclones, the vertical shear in the zonal horizontal flow is weak. The cyclones generally do not form when the shear of the zonal flow between 950 and 200 hPa exceeds 10 m /s.
- There should be enough moisture up to the mid atmospheric level.
- There should be pre-existing vortex in the low level of the atmosphere which will support in the formation of tropical cyclones.
Along the ITCZ, when SST exceeds 26°c the convergence of the trade winds takes place. The convection of trade winds takes along the pre-existing vortex. Convection leads to release of latent of condensation which acts as a source of energy and helps in maintenance of low pressure. With the greater convection, cyclone is fully developed with cumulonimbus clouds. A fully developed cyclone consists of Eye region of low pressure and eye wall (clouds).
Reasons for increasing intensity of cyclones.
Climate change warms sea surfaces, the heat available to power hurricanes has increased, raising the limit for potential hurricane wind speed and with that an exponential increase in potential wind damage. There is strong evidence that climate change may be responsible for the recent observed increase in the intensity, as measured by wind speeds and central pressure, of tropical cyclones.
Three main factors responsible for intensifying tropical cyclones are warm ocean temperatures (hurricanes occur when temperatures exceed 79 degrees Fahrenheit/26 degrees Celsius), low vertical wind shear, and high humidity.
- Warm ocean temperature:
- Factors like ocean heat content and water vapour have both increased over the past several decades, primarily due to the burning of fossil fuels and continual deforestation.
- This has in turn increased carbon dioxide emissions in the atmosphere. According to the study, “CO2 and other heat-trapping gases act like an insulating blanket that warms the land and ocean and increases evaporation.”
- This has directly lead to a significant increase in storm intensity and tropical storm occurrences.
- “Analyses of model simulations suggest that for each 1°C increase in tropical sea surface temperatures, hurricane surface wind speeds will increase by 1 to 8 percent and core rainfall rates by 6 to 18 percent.”
- Low vertical wind shear:
- One important factor is the wind shear, or the change in direction and speed of the winds from bottom to the top of the atmosphere.
- For a tropical depression to intensify into a tropical cyclone, the warm core (powered by the latent heat of evaporation, as the water vapour from the ocean condenses) must be positioned over the centre of lowest air pressure at sea level. Without this mechanism of intensification via latent heat, the depression will simply dissipate. Horizontal wind shear greater than 10 ms-1 inhibits intensification.
- Generally, this wind shear is relatively strong in the Arabian Sea compared to the Bay of Bengal.
- Opposing winds prevent cyclones from developing vertically, this wind shear is weakening in the Arabian Sea with increased carbon dioxide in the atmosphere.
- The air pollution over the Indian sub-continent changes wind circulation patterns reducing wind shear in the Northern Indian Ocean providing more potential for intense cyclones to form.
- As per some geo-physicists the active eruption of volcanoes of Barren island, Karkatoa etc. along Indonesian arc (Andaman trench, Sunda trench regions etc), is warming the waters of North Indian Ocean. This is leading to increase in SST of Bay of Bengal. This is considered to be one of the important factors for increasing frequency and intensity of tropical cyclones of Bay of Bengal.
- Aerosols, greenhouse gases, volcanic activity, solar variability, and internal climate variability also add degrees to sea surface temperature, making the conducive weather conditions for tropical cyclones.
- A La Niña (a weather pattern in the Pacific Ocean that is less damaging than El Niño) event is unfolding over the Pacific. Studies have shown that La Niña conditions change the atmospheric circulation over the north Indian Ocean and make them favourable for cyclogenesis.
- Indian Meteorological Department declared Titli cyclone a rarest of the rare occurrence and categorised it as ‘very severe cyclone’.
- Wind speed of normal tropical cyclone ranges around 118-120 km per hour. But Wind speed of Titli Cyclone was around 150 km per hour, which is rare occurrence.
- Reoccurrence or re-emergence: Usually cyclones die after landfall. But Titli Cyclone re-emerged again after landfall on Odisha coast.
- Turned in Northeastern direction: Usually cyclones die after landfall, but Titli took a Northeast turn and continued.
- These are some of the reasons because of which Titli Cyclone has been declared as rare occurrence.
- The most important factors responsible for increase in frequency of tropical cyclones are global warming and climate change.
- According to recent IPCC report, the world is already 1°C hotter than what it was 150 years ago. It could witness greater frequency of droughts and floods, tropical cyclones, etc. if the planet heats by a further 0.5°C. That could happen anytime between 2030 and 2050.
- The need of the hour is to implement the conventions like Paris Accord and INDCs in letter and spirit to control the rising global temperatures.
- Conserving the existing ecosystem is the best way to counter climate change.
Frequency and severity of storms are increasing. It has been argued that Climate Change can be one of the reason behind it. The shocking truth about global warming emissions is that they remain in the atmosphere for decades. But one of the best ways we can make sure that we are doing our part in taking care of the environment is by investing and promoting alternative forms of energy and environmental sustainability efforts the technology to increase energy efficiency and reduce dangerous carbon emissions is readily available. Let’s just hope we can implement a significant change for the well being of future generations.
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