Tropical Cyclones - (Part - 1) | Additional Study Material for UPSC PDF Download

• 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.
• They are irregular wind movements involving closed circulation of air around a low pressure center. This closed air circulation (whirling motion) is a result of rapid upward movement of hot air which is subjected to Coriolis force. The low pressure at the center is responsible for the wind speeds.
  Squall = A sudden violent gust of wind or localized storm, especially one bringing rain, snow, or sleet.
  Torrent = A strong and fast-moving stream of water or other liquid. 
• The cyclonic wind movements are anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere (This is due to Coriolis force). 
• The cyclones are often characterized by existence of an anticyclone between two cyclones.

• Large sea surface with temperature higher than 27° C, 
• Presence of the Coriolis force enough to create a cyclonic vortex, 
• Small variations in the vertical wind speed, 
• A pre-existing weak low-pressure area or low-level-cyclonic circulation, 
• Upper divergence above the sea level system,

• Ocean waters having temperatures of 27° C or more is the source of moisture which feeds the storm. The condensation of moisture releases enough latent heat of condensation to drive the storm.

Why tropical cyclones form mostly on the western margins of the oceans? 

OR 

Why tropical cyclones don’t form in the eastern tropical oceans?

• The depth of warm water (26-27°C) should extend for 60-70 m from surface of the ocean/sea, so that deep convection currents within the water do not churn and mix the cooler water below with the warmer water near the surface. 
• The above condition occurs only in western tropical oceans because of warm ocean currents (easterly trade winds pushes ocean waters towards west) that flow from east towards west forming a thick layer of water with temperatures greater than 27°C. This supplies enough moisture to the storm. 
• The cold currents lower the surface temperatures of the eastern parts of the tropical oceans making them unfit for the breeding of cyclonic storms.

One Exception : During strong El Nino years, strong hurricanes occur in the eastern Pacific. This is due to the accumulation of warm waters in the eastern Pacific due to weak Walker Cell

Why cyclones occur mostly in late summers?

• Whirling motion is enhanced when the doldrums (region within ITCZ) over oceans are farthest from the equator. This happens during the autumnal equinox (August-September). At this time, there are two advantages—the air is overheated and the sun is exactly over the equator. 

Due to high specific heat of water, and mixing, the ocean waters in northern hemisphere attain maximum temperatures in August. (Continents attain maximum temperatures in June-July)

• The Coriolis force is zero at the equator (no cyclones at equator because of zero Coriolis Force) but it increases with latitude. Coriolis force at 5° latitude is significant enough to create a storm [cyclonic vortex]. 
• About 65 per cent of cyclonic activity occurs between 10° and 20° latitude.

• Low-level disturbance (thunderstorms – they are the seeds of cyclones) in the form of easterly wave disturbances in the Inter-Tropical Convergence Zone (ITCZ) should pre-­exist.
  
• Small local differences in the temperature of water and of air produce various low pressure centers of small size. A weak cyclonic circulation develops around these areas. 
• Then, because of the rising warm humid air, a true cyclonic vortex may develop very rapidly. However, only a few of these disturbances develop into cyclones. 

Rising of humid air =>  adiabatic lapse rate => fall in temperature of air => condensation of moisture in air => latent heat of condensation released => air gets more hot and lighter => air is further uplifted => more air comes in to fill the gap => new moisture available for condensation => latent heat of condensation and the cycle repeats

• Trade winds from both the hemispheres meet along inter-tropical front. Temperature contrasts between these air masses must exist when the ITCZ is farthest, from the equator. 
• Thus, the convergence of these air masses of different temperatures and the resulting instability are the prerequisites for the origin and growth of violent tropical storms.

• The remains of an upper tropospheric cyclone from the Westerlies move deep into the tropical latitude regions. As divergence prevails on the eastern side of the troughs, a rising motion occurs; this leads to the development of thunderstorms. 
• Further, these old abandoned troughs (remnants of temperate cyclones) usually have cold cores, suggesting that the environmental lapse rate is steeper and unstable below these troughs. Such instability encourages thunderstorms (child cyclones).
  

• Wind Shear = differences between wind speeds at different heights. 
• Tropical cyclones develop when the wind is uniform. 
• Because of weak vertical wind shear, cyclone formation processes are limited to latitude equator ward of the subtropical jet stream. [Jet streams] 
• In the temperate regions, wind shear is high due to westerlies and this inhibits convective cyclone formation.

• A well – developed divergence in the upper layers of the atmosphere is necessary so that the rising air currents within the cyclone continue to be pumped out and a low pressure maintained at the center.
  

• High humidity (around 50 to 60 per cent) is required in the mid-troposphere, since the presence of moist air leads to the formation of cumulonimbus cloud. 
• Such conditions exist over the equatorial doldrums, especially in western margins of oceans (this is because of east to west movement of ocean currents), which have great moisture, carrying capacity because the trade winds continuously replace the saturated air.

ORIGIN AND DEVELOPMENT OF TROPICAL CYCLONES

• The tropical cyclones have a thermal origin, and they develop over tropical seas during late summers (August to mid-November). 
• At these locations, the strong local convectional currents acquire a whirling motion because of the Coriolis force. 
• After developing, these cyclones advance till they find a weak spot in the trade wind belt.

Origin

• Under favorable conditions, multiple thunderstorms originate over the oceans. These thunderstorms merge and create an intense low pressure system (wind is warm and lighter).

• In the thunderstorm, air is uplifted as it is warm and light. At certain height, due to lapse rate and adiabatic lapse rate, the temperature of air falls and moisture in the air undergoes condensation. 
• Condensation releases latent heat of condensation making the air more warmer. It becomes much lighter and is further uplifted. 
• The space is filled by fresh moisture laden air. Condensation occurs in this air and the cycle is repeated as long as the moisture is supplied. 
• Due to excess moisture over oceans, the thunderstorm intensifies and sucks in air at much faster rate. The air from surroundings rushes in and undergoes deflection due to Coriolis force creating a cyclonic vortex (spiraling air column. Similar to tornado).
  
• Due to centripetal acceleration (centripetal force pulling towards the center is countered by an opposing force called centrifugal force), the air in the vortex is forced to form a region of calmness called an eye at the center of the cyclone. The inner surface of the vortex forms the eye wall, the most violent region of the cyclone.
  

[Eye is created due to tangential force acting on wind that is following a curvy path]

• All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the surface through the cylindrical eye region and at the edges of the cyclone. 
• Continuous supply of moisture from the sea is the major driving force behind every cyclone. On reaching the land the moisture supply is cut off and the storm dissipates. 
• If ocean can supply more moisture, the storm will reach a mature stage.

Mature stage

• At this stage, the spiraling winds create multiple convective cells with successive calm and violent regions. 
• The regions with cumulonimbus cloud (rising limbs of convective cell) formation are called rain bands below which intense rainfall occurs. 
• The ascending air will lose moisture at some point and descends (subsides) back to surface through the calm regions (descending limbs of convection cell – subsiding air) that exist between two rain bands. 
• Cloud formation is dense at the center. The cloud size decreases from center to periphery. 
• Rain bands are mostly made up of cumulonimbus clouds. The ones at the periphery are made up of nimbostratus and cumulus clouds. 
• The dense overcast at the upper levels of troposphere is due to cirrus clouds which are mostly made up of hexagonal ice crystals. 
• The dry air flowing along the central dense overcast descends at the periphery and the eye region.