Tropical Cyclones Analysis In Depth | UPSC
Fierce and frequent: What causes rapid intensification of cyclones?
WHY IN NEWS:
Lack of understanding of transition from a seedling, like a low-pressure system to a tropical storm or cyclone, limits extending forecast lead times.
SYLLABUS COVERED: GS 3:Earth : Geography : Cyclones : Winds
For PRELIMS rapid intensification is important , GPI definition , factor affecting the cyclone etc.
For MAINS look out for forecasting methods , effects and consequences . Let us dive in !
TROPICAL CYCLONES ANALYSIS IN DEPTH
- 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.
- RETARDATION PROCESS : 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.
- 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.
- Empirical methods have been used for decades to issue seasonal outlooks of expected number of cyclones.
CYCLONE GENESIS POTENTIAL (GPI)
- 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.
FACTORS AFFECTING ROTATION OF THE CYCLONE
- The rotation or vorticity of a low-pressure system at the surface
- Sea surface temperatures or the volume of warm water available
- Vertical motion of air in this low-pressure system
- Amount of humidity available in the middle atmosphere
- 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 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.
SEEDS OF A CYCLONE
- 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.
- 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.
WHY TROPICS ARE PRONE TO CYCLOGENESIS?
- Warm ocean.
- Atmospheric convection.
- Vertical motion.
- Mid-level humidity.
OTHER FACTORS INFLUENCING CYCLONES
- MADDEN-JULIAN OSCILLATIONS : Madden-Julian Oscillations(MJO’s) 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.
- MJOs show a strong association with cyclogenesis, especially for the post-monsoon season.
- MONSOON INTRASEASONAL OSCILLATIONS : There are also northward propagating waves over the Indian Ocean during the monsoon season which we call the Monsoon Intraseasonal Oscillations (MISO).
- EL-NINO AND EL-NINA : 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.
- 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.
THEORIES EVOLVED OVER A PERIOD OF TIME
- 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.
- 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.
- EYE OF THE CYCLONE : The low-pressure centre of the cyclone is called the eye.
- The lower the pressure in the eye, the more intense is the cyclone.
- TROPOPAUSE : 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.
- TROPOPAUSE HEIGHT : The tropopause in the tropics is typically at 15 km altitude.
- HOT TOWERS : 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.
- 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.
SOURCES:DOWNTOEARTH | Tropical Cyclones Analysis In Depth | UPSC