Tropical cyclones remain the deadliest natural climate hazard that causes an unacceptably high loss of life, property and infrastructure.  Global warming has already resulted in a detectable increase in the number of higher intensity cyclones as well as their intensification.

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[Burning Issue] Tropical Cyclones and India

Try this question:

Q. The Marsupial Theory often seen in news is related to which of the climatic phenomena?

a) Heatwaves b) Monsoon Variability c) Formation of Cyclones d) Thunderstorms

What is Rapid Intensification of Cyclones?

• RI is defined as an increase in maximum sustained winds by at least 55 km/hour in a 24-hour period.
• Such acceleration can only come with a rapid drop in the pressure in the eye of the cyclone.
• Rapid intensification (RI) is making cyclone forecasts harder and intense cyclones with RI are expected to grow in number.
• The lack of understanding of the transition from a seedling of a cyclone, like a low-pressure system to a tropical storm, limits extending the forecast lead times.

Factors causing RI

The most important environmental factors for cyclone genesis are-

• the rotation or vorticity of a low-pressure system at the surface;
• sea surface temperatures or the volume of warm water available;
• the vertical motion of air in this low-pressure system;
• the amount of humidity available in the middle atmosphere and
• the vertical shear or the change in winds from the surface to the upper atmosphere.

MJO and Cyclones

• Madden-Julian Oscillations as they are known, dominate the tropics during October-April by propagating from the western Indian Ocean into the eastern Indian Ocean, across the Indonesian seas into the Pacific Ocean.
• Referred to as MJOs, these Madden-Julian Oscillations throw seeds of rotational low-pressure systems over the Indian and the Pacific Oceans.
• And thus, MJOs show a strong association with cyclogenesis, especially for the post-monsoon season.

Impacts of MISO

• Monsoon Intraseasonal Oscillations (MISO) are alternating periods of heavy and minimal rainfall, each lasting for about a month or so and tending to follow a cyclical, northward shifting pattern from the equator to southern Asia.
• While the strong vertical shear suppresses cyclones during the monsoon season, MISOs influence cyclone genesis during the pre-monsoon season.

Other factors

• At longer timescales, phenomena like the El Niño and La Niña influence not only the number of cyclone seeds but also the location and the expanse of warm water.
• For example, during the pre-monsoon season of La Niña year, the region of warm water over the Bay of Bengal increases. This leads cyclones to travel longer and grow stronger than during an El Niño year.
• Over the Pacific Ocean, on the other hand, it is the El Niño that provides a larger swath of warm water and more intense cyclones.
• West Africa produces waves called easterly waves that propagate west from land onto the tropical Atlantic Ocean and sow the seeds for most hurricanes.
• Extensive analysis has produced theories that are evocatively called the Marsupial Theory — a wave pouch that allows cyclones to grow, or waves interacting to produce a Kelvin cat’s eye, which is a ‘sweet-spot’ for the birth of a cyclone.

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