Tropical cyclones remain the deadliest natural climate hazard that cause 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.

Rapid intensification (RI) is making cyclone forecasts harder and intense cyclones with RI are expected to grow in number.

  • RI is defined as an increase in maximum sustained winds by at least 55 kilometre / hour in a 24-hour period. Such acceleration can only come with a rapid drop in the pressure in the eye of the cyclone.
  • It is only after its birth that a cyclone is easily identifiable with an eye, spiral bands of clouds and the associated strong wind speeds.
  • What is known, though, is the environmental landscape in which cyclones are born. Rotation of the earth forces the cyclone to rotate in the same direction.

The Cyclone Genesis Potential (GPI)to estimate the number of cyclones that may be born in a season is defined based on the variables that occur during the birth of cyclones. GPI also help project how cyclones will respond to global warming.

The most important environmental factors for cyclone genesis are:

  1. The rotation or vorticity of a low-pressure system at the surface.
  2. Sea surface temperatures or the volume of warm water available.
  3. The vertical motion of air in this low-pressure system.
  4. The amount of humidity available in the middle atmosphere.
  5. The vertical shear or the change in winds from the surface to the upper atmosphere.

Cyclone is like a turbine driven by the energy supplied from the ocean in the form of water vapour. Vertical motion and mid-level humidityare the speed and the amount of energy being pumped by the ocean into the turbine.

  • Vertical shear is like a force that can twist this pipeline supplying the energy from the ocean to the turbine, making it difficult for the turbine to rotate.
  • The seeds of a cyclone, namely the rotating low-pressure system, are typically spun off by atmospheric convection, which is the generic term for moisture converging near the surface when warm light air rises and takes the evaporated water with it. Rising air expands, cools and condenses to release energy in the middle atmosphere.
  • The heat release in a convective system also tends to set off waves that travel eastward and westward from the convection center. These waves affect the vertical motion, mid-level humidity and the vertical shear along their path. The waves, thus, affect the cyclone genesis potential.
  • Tropics have all the critical environmental ingredients for cyclogenesis: Warm ocean, atmospheric convection, vertical motion and mid-level humidity.
  • Seasons and regions of low vertical shear then become the ocean gardens where seeds of cyclones can grow rapidly into towering turbines. Any weather and climate phenomenon that affects these parameters will also affect the cyclone genesis potential.

For example, at sub-seasonal timescales — periods shorter than a season — there are eastward propagating waves that are generated in the western tropical Indian Ocean off the coast of east Africa. 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.