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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.
What is not discussed often is how we do not yet know how a cyclone is born. 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. Forecasts worldwide have matured in terms of the intensity and the track of a cyclone once it is born.
India has also made tremendous progress in cyclone forecasting and reduced cyclone-related deaths significantly by evolving appropriate evacuation plans and other hazard mitigation actions in response to forecasts.
The lack of understanding of transition from a seedling of a cyclone, like a low-pressure system to a tropical storm, limits extending the forecast lead times. Nonetheless, empirical methods have been used for decades to issue seasonal outlooks of expected number of cyclones.
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 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.
Cyclone is like a turbine driven by the energy supplied from the ocean in the form of water vapour. Vertical motion and mid-level humidity are 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.
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.
There are also northward propagating waves over the Indian Ocean during the monsoon season which we call the Monsoon Intraseasonal Oscillations (MISO). While the strong vertical shear suppresses cyclones during the monsoon season, MISOs influence cyclone genesis during the pre-monsoon season.
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.
Despite all we know about cyclone genesis potential being controlled by environmental factors, we don’t quite know the magical transition of a few rotational seedlings into full-blown cyclones. Atlantic hurricanes have been a focus for several insights.
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.
It is unclear if this typology extends to the Indian and Pacific Oceans. But the cyclone genesis potential is determined by the same environmental factors over all cyclone regions. Once a cyclone is born, its innate structure itself has the processes to amplify the energy release and wind acceleration to achieve rapid intensification or RI. One of the main ingredients needed to support this amplification is the energy provided by the ocean.
The low-pressure centre of the cyclone is called the eye. The lower the pressure in the eye, the more intense is the cyclone. The eye-wall surrounds the eye with the strongest winds and heaviest rain and is the most destructive part of the cyclone.
The strong convection in the eye-wall produces rain-bearing clouds that rise up to the ceiling of the lower-atmosphere known as the tropopause. The tropopause in the tropics is typically at 15 km altitude.
These clouds are called hot towers. The fast rising air in the hot towers causes its own circulation and drives an equally fast sinking of dry air into the eye from the near the tropopause into the middle atmosphere.
This sinking air compresses and warms up as opposed to the rising air that expands and cools down. This compression causes a warming of up to 10 degrees Celsius and it is this warming that causes a rapid drop in the eye and the intensification leading to acceleration of the winds by 55 km / h or more within 24 hours.
A warming ocean thus tends to favour this cascade of events and prime the climate system for more and more cyclones undergoing RI.
It is this rapid transition from a seedling set down by the fairly benign atmospheric convection and waves to a destructive cyclone that poses truly exciting science questions as well as a great challenge for prediction.
RI energised by the warming ocean and the internal feedback between the rising and expanding air releasing energy, and the sinking warming air demanding more water from the ocean, add to the challenge.
As we focus on what the monsoon will come up during the rest of this season, the post-monsoon cyclone season beckons with challenges of accurate predictions of tracks, intensity and RI. The Atlantic hurricane season, in the meantime, is likely to offer some more displays of RI.