PRE-EXISTING LOW
VIOLENT EYEWALL (↑)
ARABIAN SEA RISING
Tropical Cyclones
Formation Conditions · Structure of Eye & Eyewall · Path & Landfall · The Shifting Arabian Sea Dynamics (28 Micro-Realms)
The Latent Heat Engine: Genesis, Structure, and the Indian Ocean Shift
Tropical Cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing large scale destruction caused by violent winds, very heavy rainfall and storm surges. Their formation (cyclogenesis) requires specific conditions: Sea Surface Temperature (SST) > $26.5^\circ\text{C}$, presence of the Coriolis force (hence none at the equator), small variations in vertical wind speed (low shear), and upper-level divergence. Structurally, they are defined by a paradoxical core: a calm, clear, subsiding Eye surrounded by a towering, violent Eyewall of rising cumulonimbus clouds fueled by the latent heat of condensation. Historically dominant in the Bay of Bengal, climate change is currently driving a dangerous increase in the frequency and intensity of cyclones in the traditionally colder Arabian Sea (e.g., Biparjoy, Tauktae).
I. Cyclogenesis & The Five Essential Prerequisites (NCERT Core)
Tropical Cyclone Definition
- Intense low-pressure systems originating over warm tropical oceans
- Characterized by destructive winds and torrential rain
- One of the most devastating natural calamities on Earth
The 5 Prerequisites (NCERT)
- Favorable conditions for formation and intensification
- Must exist simultaneously over tropical oceans
- Listed explicitly in NCERT Chapter 10
1. SST > $26.5^\circ\text{C}$ (Warm Water)
- Provides the continuous supply of moisture
- Must be at least 60 meters deep to prevent cold upwelling
- Fuel for the latent heat engine
2. Presence of Coriolis Force
- Required to impart rotational spin to converging air
- Coriolis is zero at the Equator
- Therefore, cyclones do NOT form between $0^\circ\text{–}5^\circ$ N/S!
3. Low Vertical Wind Shear
- Small variations in vertical wind speed
- Prevents the storm's latent heat core from being sheared apart
- Allows towering thunderstorms to organize vertically
4 & 5. Trigger & Exhaust
- 4. A pre-existing weak low-pressure area (trigger)
- 5. Upper divergence above the sea level system
- Acts as a chimney exhausting rising air to sustain the vacuum
II. Storm Structure, The Latent Heat Engine & Landfall Dissipation
The Eye (Lowest Pressure)
- The calm, circular center of the storm (10–50 km wide)
- Air is gently descending (subsiding) and warming
- Clear skies, no rain, lowest barometric pressure!
The Eyewall (Max Intensity)
- Ring of towering Cumulonimbus clouds surrounding the eye
- Air is violently ascending
- Most destructive winds and torrential rainfall (NCERT)
Spiral Rainbands
- Bands of clouds spiraling outward from the eyewall
- Can extend for hundreds of kilometers
- Bring intermittent squalls and heavy rain
The Latent Heat Engine
- Energy that intensifies the storm comes from condensation
- Water vapor in towering clouds condenses, releasing heat
- Drives further uplift and pressure drops
Path & Movement (E to W)
- Generally move East to West in the Tropics
- Steered by the prevailing Easterly Trade Winds
- Speed averages 15–30 km/h over open ocean
Recurvature (Poleward Bend)
- Upon reaching western edges of oceans (~20°–30° Lat)
- They often recurve poleward and then eastward
- Caught in the Mid-Latitude Westerlies (NCERT)
Landfall & Dissipation
- Place where tropical cyclone crosses the coast
- Storm dissipates rapidly over land because:
- Its moisture source (latent heat) is cut off & friction increases
Storm Surge (Deadliest Hazard)
- A dome of water pushed ashore by intense winds & low pressure
- Causes catastrophic coastal inundation
- Responsible for 90% of tropical cyclone fatalities
III. Indian Ocean Dynamics: Bay of Bengal vs. Arabian Sea Shift
Global Basin Nomenclature
- Different regional names for the same phenomenon
- Indian Ocean specific term is "Cyclone"
Bay of Bengal Dominance
- Historically sees 4x more cyclones than Arabian Sea
- Post-monsoon (Oct-Nov) is the most active season
- Devastates Odisha, Andhra Pradesh, and Bengal coasts
Why BoB is More Active
- Higher SST and sluggish winds (funnel shape)
- Lower salinity from river influx reduces density/mixing
- Typhoon remnants from South China Sea enter BoB
Traditional Arabian Sea
- Historically colder due to Somali jet upwelling
- High vertical wind shear disrupts formation
- Typically saw fewer and less intense cyclones
The Arabian Sea Shift
- Recent decades show rapid warming of Arabian Sea
- Resulting in a 52% increase in cyclonic storms
- Poses new threats to Gujarat & Maharashtra coasts
Recent Arabian Cyclones
- Tauktae: Extremely severe, hit Gujarat coast
- Nisarga: Rare landfall near Mumbai
- Biparjoy: Exceptionally long lifespan (13 days)
Cyclone Biparjoy (2023)
- Fed by abnormally high Arabian Sea SST
- Remained over open water for 13+ days
- Erratic movement made forecasting landfall difficult
IV. Tropical vs. Temperate, Nomenclature & Early Warning Systems
IMD Naming Convention
- Panel of 13 countries in the North Indian Ocean region
- Each contributes 13 names (Total 169 names)
- Used sequentially to identify storms and aid warnings
Origin & Fronts Contrast
NO FRONTS
FRONTAL ZONES
- Tropical: Form only over warm oceans, no fronts exist
- Temperate: Form over land/sea along polar fronts where warm & cold air clash
Energy Source Contrast
TEMPERATE: HORIZONTAL TEMP GRADIENT
- Tropical: Driven by latent heat; die over land
- Temperate: Driven by density/temp differences of air masses
Path & Velocity Contrast
- Tropical: Move East to West (Trade winds), violent speed, small area
- Temperate: Move West to East (Westerlies), slower, massive area
Early Warning Systems
- IMD utilizes INSAT geostationary satellites
- Doppler Weather Radar (DWR) tracks coastal approach
- Ocean buoys monitor SST and pressure drops
Dvorak Technique
- Developed by Vernon Dvorak
- Estimates tropical cyclone intensity purely from
- Visible and infrared satellite imagery cloud patterns
Master Cyclone Matrix
- Completes Tropical Cyclones framework
- Highlights the Bay of Bengal vs Arabian Sea shift
- Final atmospheric storm synthesis
Core Distinctions
• The Eye: The exact center of the storm. Despite having the lowest barometric pressure, it is characterized by calm winds, clear skies, and gently subsiding (sinking) air.
• The Eyewall: The ring of towering Cumulonimbus clouds immediately surrounding the Eye. It is characterized by violent ascending (rising) air, the most destructive winds, and the heaviest torrential rainfall.
• Tropical Cyclones are driven entirely by the latent heat of condensation extracted from warm oceans (SST > 26.5°C). They have no fronts and dissipate rapidly over land because their moisture source is cut off.
• Temperate Cyclones are driven by the horizontal temperature contrast between cold polar air and warm tropical air (Fronts). They can form over land or sea and cover vastly larger areas.
Historically, the Bay of Bengal saw far more cyclones due to higher SST and lower salinity. However, rapid ocean warming due to climate change has altered the Arabian Sea's thermal profile, leading to a recent, dangerous increase in the frequency, intensity, and duration (e.g., Biparjoy) of cyclones striking India's west coast.
Tropical Cyclone Matrix
NCERT Chapter 10 & Regional Mechanics