11.1 Tropical cyclone formation and development

Tropical cyclones form in specific ocean regions with warm waters and low wind shear. They start as disturbances, grow into storms, and can become powerful hurricanes. Understanding their formation is key to predicting their paths and potential impacts.

As cyclones develop, they intensify through stages, from tropical depressions to full-blown hurricanes. Warm oceans fuel their growth, while atmospheric conditions like low wind shear allow them to strengthen. The Coriolis effect gives them their distinctive spin.

Tropical Cyclone Formation

Ocean and Atmospheric Conditions

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• Warm ocean waters exceeding 26.5°C (80°F) to a depth of at least 50 meters fuel tropical cyclone formation
• Low vertical wind shear less than 10 m/s (20 knots) between surface and upper troposphere enables cyclone development
• Moist mid-troposphere with relative humidity values of at least 50-60% supports convection and cyclone growth
• Pre-existing atmospheric disturbance or weak low-pressure system provides starting point for cyclone formation
• Distance of at least 5° latitude from equator supplies sufficient Coriolis force for cyclone rotation
• Upper-level divergence and lower-level convergence in atmosphere create favorable environment for cyclone intensification
• Sea surface temperatures warmer than overlying air promote instability and convection

Examples of Favorable Formation Regions

• Western Pacific Ocean (South China Sea, Philippine Sea)
• Eastern Pacific Ocean (off the coast of Mexico and Central America)
• North Atlantic Ocean (Caribbean Sea, Gulf of Mexico)
• Bay of Bengal and Arabian Sea in the Indian Ocean
• Southwest Indian Ocean (east of Madagascar)

Stages of Tropical Cyclone Development

Early Stages

• Tropical disturbance forms as cluster of thunderstorms with little or no circulation and winds below 20 knots (23 mph)
• Tropical depression develops with organized system of clouds and thunderstorms, defined circulation, and maximum sustained winds up to 33 knots (38 mph)
• Tropical storm emerges as organized system with defined circular rotation and maximum sustained winds between 34-63 knots (39-73 mph)
  • Tropical storms receive names from predetermined lists for each basin

Hurricane Stage

• Hurricane (or typhoon/cyclone) intensifies into intense tropical weather system with well-defined circulation and maximum sustained winds of 64 knots (74 mph) or higher
• Hurricanes categorized on Saffir-Simpson Hurricane Wind Scale
  • Category 1: 74-95 mph
  • Category 2: 96-110 mph
  • Category 3: 111-129 mph
  • Category 4: 130-156 mph
  • Category 5: 157 mph or higher
• Eye and eyewall structure typically form during hurricane stage
  • Most intense winds and heaviest precipitation occur in eyewall
• Central pressure drops as system intensifies
• Wind field expands, increasing overall size and potential impact of hurricane

Examples of Notable Hurricanes

• Hurricane Katrina (2005): Category 5, devastated New Orleans and Gulf Coast
• Hurricane Patricia (2015): Strongest recorded hurricane in Western Hemisphere with 215 mph winds
• Typhoon Tip (1979): Largest tropical cyclone on record with a diameter of 1,380 miles

Conditions for Intensification

Oceanic Factors

• Warm ocean waters act as primary energy source for tropical cyclones
• Sea surface temperatures above 26.5°C (80°F) allow sufficient evaporation and latent heat release
• Oceanic heat content determines available energy supply for sustained intensification
  • Deeper warm water layer supports longer-lasting and more intense cyclones
• Examples of high oceanic heat content regions
  • Loop Current in Gulf of Mexico
  • Western Pacific Warm Pool

Atmospheric Factors

• Low vertical wind shear allows vertical alignment of cyclone's structure
• Wind shear below 10 m/s (20 knots) prevents disruption of cyclone's warm core
• Low shear enables development of deep convection around center
• Combination of warm waters and low shear creates positive feedback loop
  • Increased convection leads to lower surface pressure
  • Enhanced surface winds and evaporation further fuel storm
• Rapid intensification occurs with increase in maximum sustained winds of at least 30 knots (35 mph) in 24 hours
  • More likely when both warm waters and low shear present

Examples of Rapid Intensification

• Hurricane Wilma (2005): Pressure dropped 97 mb in 24 hours, fastest intensification on record
• Hurricane Patricia (2015): Intensified from Category 1 to Category 5 in 24 hours

Coriolis Effect and Rotation

Fundamentals of Coriolis Effect

• Coriolis effect causes apparent deflection of moving objects relative to Earth's surface
• Results from planet's rotation
• Northern Hemisphere cyclones rotate counterclockwise
• Southern Hemisphere cyclones rotate clockwise
• Coriolis force negligible near equator (within about 5° latitude)
• Strength increases towards poles

Impact on Tropical Cyclones

• Coriolis effect initiates and maintains rotation of tropical cyclones
• Deflects air parcels as they flow towards low-pressure center
• Balance between pressure gradient force and Coriolis force leads to gradient wind balance
  • Crucial for cyclone intensification
• Coriolis effect becomes more pronounced as tropical cyclone intensifies
  • Contributes to tightening of pressure gradient and increased wind speeds
• Explains why tropical cyclones rarely form near equator
• Influences cyclone track, causing poleward curve as they move away from tropics

Examples of Coriolis Effect on Cyclone Tracks

• Hurricane Irma (2017): Curved northward from Caribbean towards Florida
• Typhoon Hagibis (2019): Recurved from tropical Pacific towards Japan