Elaborate Notes
Tropical Cyclones: An Overview
A tropical cyclone is an intense, rotating, low-pressure weather system with a warm core, originating over tropical or subtropical waters. Characterized by high-velocity winds, a spiral arrangement of thunderstorms, and heavy rainfall, these systems are among the most destructive natural phenomena on Earth. The primary energy source for a tropical cyclone is the latent heat of condensation, which is released when moist air drawn in from the ocean surface rises, cools, and condenses into clouds and rain.
These powerful storms are known by different names in various parts of the world, reflecting their global impact:
- Cyclone: In the Indian Ocean (Bay of Bengal and Arabian Sea) and the South Pacific Ocean.
- Hurricane: In the Atlantic Ocean and the eastern North Pacific Ocean. The term derives from ‘Huracan’, a god of wind and storms for the indigenous Taino people of the Caribbean.
- Typhoon: In the western North Pacific Ocean, particularly the South China Sea.
- Taifu: A localized term used in Japan.
- Bagui: The term used in the Philippines.
- Willy-Willy: An archaic term sometimes used for cyclones in Australia.
Conditions for the Formation of Tropical Cyclones
The genesis of a tropical cyclone, known as cyclogenesis, is a complex process that requires a specific set of atmospheric and oceanic conditions to converge. These conditions were extensively studied and outlined by meteorologists like William M. Gray in the 1960s. The necessary conditions are:
- High Sea Surface Temperature (SST): The ocean surface temperature must be at least 26.5°C to 27°C. This warm water acts as the fuel for the cyclone, providing the necessary heat and moisture to the overlying atmosphere.
- Sufficient Depth of Warm Water: The warm temperature of 27°C must extend to a significant depth, typically 60-70 meters. This ensures a high Ocean Heat Content (OHC). A deep layer of warm water prevents the cyclone from churning up colder water from the depths (upwelling), which would cut off its energy supply and cause it to weaken.
- High Atmospheric Humidity: A high level of moisture must be present in the lower-to-mid troposphere, typically with a relative humidity of 50-60% near the surface. This humid air, when lifted, becomes unstable and fuels the development of deep convective cumulonimbus clouds.
- Low Vertical Wind Shear: Vertical wind shear is the difference in wind speed and/or direction between different altitudes. A minimal wind shear (less than 10 m/s) is crucial. Low shear allows the vertical structure of the storm to remain intact and organized, facilitating the efficient transfer of heat from the ocean to the upper atmosphere. High shear would disrupt this structure, tilting the storm and tearing it apart.
- Sufficient Coriolis Force: This force, named after the French scientist Gaspard-Gustave de Coriolis (1835), imparts the necessary spin or vorticity to the storm. The Coriolis effect is weakest at the Equator and strengthens towards the poles. Consequently, tropical cyclones do not form within about 5 degrees of latitude of the Equator (in the region known as the doldrums). They typically form between 5° and 25° latitude in both hemispheres.
- Pre-existing Low-Pressure Area: Cyclones do not form spontaneously. They require a pre-existing weather disturbance, such as a weak low-pressure trough or an easterly wave migrating from Africa into the Atlantic, which provides the initial focus for convergence and rotation.
- Upper-Air Divergence: For a surface low-pressure system to intensify, there must be a mechanism to remove the rising air at the top of the storm. This is achieved by divergence, or the spreading out of air, in the upper levels of the troposphere (often facilitated by an upper-level anticyclone). This upper-level outflow acts like a chimney, pulling more air up from the surface and causing the surface pressure to drop further, thus intensifying the cyclone.
Formation and Life Cycle
- Incipient Stage: A pre-existing disturbance over warm tropical waters with high humidity leads to strong atmospheric convection. Air converges at the surface, rises, and forms large cumulonimbus clouds.
- Intensification: As the moist air rises and condenses, it releases vast amounts of latent heat. This heat warms the air column, making it lighter and causing it to rise further, which in turn leads to a further drop in surface pressure. This creates a positive feedback loop, drawing in more moist surface air.
- Rotation: The inflowing surface winds are deflected by the Coriolis force, initiating a cyclonic rotation (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). As the storm intensifies, wind speeds increase.
- Mature Stage: When wind speeds reach a sustained level (e.g., 119 km/hr or 74 mph), the storm is classified as a mature hurricane, typhoon, or cyclone. At this stage, a distinct eye forms. Strong rotational forces (conservation of angular momentum) prevent air from reaching the absolute center; instead, some of the rising air in the eyewall is forced to descend in the center, creating a calm, clear, and warm eye due to compressional (adiabatic) warming.
- Movement and Dissipation: The cyclone moves under the influence of large-scale steering winds, primarily the trade winds, which typically push it from east to west. The storm begins to weaken and dissipate when it loses its primary energy source. This occurs when it:
- Makes Landfall: The supply of warm, moist air from the ocean is cut off. Increased friction over the land surface also helps to disrupt the organized circulation.
- Moves over Colder Waters: Encountering sea surface temperatures below 26.5°C starves the storm of its fuel.
- Encounters High Vertical Wind Shear: Moving into an area with strong upper-level winds can disrupt its vertical structure.
Structure of a Mature Tropical Cyclone
- The Eye: The calm, often cloud-free center of the storm, typically 5-50 km in diameter. It is characterized by light winds, the lowest atmospheric pressure of the storm, and the highest temperatures (due to sinking air).
- The Eyewall: A ring of towering cumulonimbus clouds immediately surrounding the eye, typically 10-20 km wide. This is the most destructive part of the cyclone, featuring the strongest winds, heaviest rainfall, and most intense convection.
- Spiral Bands (or Rainbands): Long, curved bands of thunderstorms that spiral outwards from the eyewall, giving the cyclone a galaxy-like appearance when viewed from space. These bands can extend for hundreds of kilometers and are associated with heavy rain and gusty winds.
- Annular Zone: A less common feature observed in some very intense cyclones. It is a zone of suppressed cloudiness and relatively dry air that can separate the inner core (eye and eyewall) from the outer rainbands. It signifies a very stable and powerful storm.
- Outer Convective Bands: These are located at the periphery of the storm system, marking the extent of the cyclone’s influence. They are characterized by intermittent convection and instability.
Naming System of Cyclones in the North Indian Ocean
To facilitate clear communication and avoid confusion, the World Meteorological Organization (WMO) and its Regional Specialized Meteorological Centres (RSMCs) oversee the naming of tropical cyclones.
- For the North Indian Ocean region, a formal naming system was initiated in 2004. Initially, a panel of eight member countries—Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka, and Thailand—contributed a list of names.
- The RSMC in New Delhi, operated by the India Meteorological Department (IMD), is responsible for assigning these names sequentially once a cyclonic storm forms and its wind speed crosses the threshold of 34 knots (approximately 62 km/hr).
- The naming conventions stipulate that names must be neutral with respect to politics, religion, culture, and gender. They should be short, easy to pronounce, and not be offensive to any group.
- The original list of 64 names was exhausted with Cyclone Amphan in 2020. A new list, effective from 2020, was created with contributions from the original eight countries plus five new members: Iran, Qatar, Saudi Arabia, the United Arab Emirates, and Yemen, making a total of 13 countries.
Recurvature of Cyclones
Recurvature is the phenomenon where a tropical cyclone, after moving in a generally westward direction, changes its path and curves poleward and then eastward.
- Initially, cyclones are steered by the low-level easterly trade winds.
- As they drift poleward (northward in the Northern Hemisphere), they may come under the influence of the mid-latitude westerlies, which are prevailing winds that blow from west to east.
- This interaction with the westerlies can cause the cyclone to “re-curve.”
- The prediction of recurvature is one of the most significant challenges in cyclone forecasting. An incorrect prediction of the turning point can lead to disastrous consequences for coastal populations. Cyclone Ockhi in 2017 (not 2018) was a notable example of a storm with a highly erratic and difficult-to-predict track involving recurvature.
Cyclones in the Indian Ocean: Bay of Bengal vs. Arabian Sea
Historically, the Bay of Bengal has been a more active basin for tropical cyclones than the Arabian Sea, with a formation ratio of approximately 4:1. The key reasons include:
- Higher Sea Surface Temperatures: The Bay of Bengal is semi-enclosed, receives massive freshwater and sediment influx from large rivers like the Ganga-Brahmaputra and Mahanadi, which leads to stratification and traps heat in the upper layers. It generally maintains higher SSTs for longer durations compared to the Arabian Sea.
- Weaker Vertical Wind Shear: The Arabian Sea, particularly during the southwest monsoon, experiences stronger vertical wind shear, which is hostile to cyclone formation. The Bay of Bengal generally has more favourable, low-shear conditions.
- Influx of Cyclone Remnants: The Bay of Bengal frequently receives the remnants of typhoons from the South China Sea, which cross over Southeast Asia and emerge as weakened low-pressure systems. These remnants can readily re-intensify into cyclones over the warm bay waters.
IMD’s Colour-Coded Weather Warning System
The India Meteorological Department (IMD) uses a four-stage, colour-coded warning system to alert the public and disaster management agencies about the severity of impending hazardous weather. This universal system is designed for easy comprehension and to prompt appropriate action.
- Green (No Warning): No severe weather is expected. All is well.
- Yellow (Be Aware): Indicates the potential for moderately bad weather. It suggests that people should keep themselves updated.
- Orange (Be Prepared): A warning for extremely bad weather that could disrupt daily life and pose a threat. Authorities are advised to be prepared for necessary action.
- Red (Take Action): The highest level of warning, issued for exceptionally severe weather conditions. It signals that authorities must take immediate action to protect lives and property, which may include ordering evacuations.
Difference between Tropical and Temperate Cyclones
| Feature | Tropical Cyclone | Temperate Cyclone (Extra-Tropical) |
|---|---|---|
| Origin | Thermal origin; forms only over warm ocean waters. | Dynamic origin; forms due to the interaction of cold and warm air masses (frontogenesis). Can form over land and sea. |
| Latitude | Confined to 5° - 25° North and South. | Forms in mid to high latitudes, typically 35° - 65° North and South. |
| Energy Source | Latent heat of condensation from warm, moist air. | Baroclinic instability; the potential energy from the horizontal temperature contrast between air masses. |
| Structure | Symmetrical, with a warm core. A distinct, calm ‘eye’ is present in mature storms. Isobars are circular. | Asymmetrical, with a cold core. Associated with fronts (cold, warm, occluded). No distinct eye. Isobars are V-shaped. |
| Wind Intensity | Wind speeds are much higher and more destructive, but concentrated over a smaller area. | Winds are less intense but are spread over a much larger geographical area. |
| Movement | Move from East to West under the influence of trade winds. | Move from West to East under the influence of westerlies. |
| Season | Primarily late summer and autumn when ocean waters are warmest. | Occur throughout the year, but are most frequent and intense during winter. |
| Dissipation | Dissipate quickly upon making landfall. | Can intensify over land if temperature contrast persists. |
Prelims Pointers
- A Tropical Cyclone is a low-pressure, high-velocity wind system.
- Regional Names:
- Indian Ocean: Cyclone
- Atlantic Ocean: Hurricane
- South China Sea: Typhoon
- Japan: Taifu
- Philippines: Baguio
- Australia: Willy-willy
- Conditions for Cyclone Formation:
- Sea Surface Temperature > 27°C.
- Depth of warm water: 60-70 meters.
- High humidity: 50-60% near the surface.
- Low vertical wind shear.
- Presence of Coriolis force (forms between 5°-25° N/S latitude).
- Pre-existing low-pressure disturbance.
- Upper air divergence.
- The primary energy source for a tropical cyclone is the latent heat of condensation.
- Structure of a Cyclone:
- Eye: Calm center with lowest pressure and highest temperature.
- Eyewall: Surrounds the eye; location of the strongest winds and heaviest rain.
- Spiral Bands: Outer bands of thunderstorms.
- Cyclones rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- Cyclones dissipate on landfall or when moving over cold ocean waters.
- The North Indian Ocean cyclone naming system began in 2004.
- The Regional Specialized Meteorological Centre (RSMC) in New Delhi is responsible for naming cyclones in this region.
- A storm is named when wind speeds cross 62 km/hr.
- Initially, 8 countries were part of the naming panel; it was expanded to 13 countries in 2020.
- Recurvature: The change in a cyclone’s track from westward to an eastward direction under the influence of westerlies.
- The ratio of cyclones in the Bay of Bengal to the Arabian Sea is approximately 4:1.
- IMD Colour Codes for Weather Warnings:
- Green: No Warning
- Yellow: Be Aware
- Orange: Be Prepared
- Red: Take Action
- Temperate cyclones are also known as extra-tropical cyclones or wave cyclones.
- Temperate cyclones derive energy from the temperature contrast between air masses (fronts).
Mains Insights
1. Tropical Cyclones, Climate Change, and Disaster Management (GS-III)
- Cause-Effect Relationship: Climate change is a significant driver of cyclone characteristics. Rising global temperatures lead to higher Sea Surface Temperatures (SST) and increased Ocean Heat Content (OHC), providing more “fuel” for cyclones.
- Debate and Nuances: While the link between climate change and an increase in the frequency of all cyclones is still debated among scientists, there is a strong consensus, supported by IPCC reports, that climate change is leading to an increase in the intensity (higher wind speeds, more rainfall) of the strongest cyclones. Events like rapid intensification are also becoming more common.
- Changing Patterns: The recent increase in the frequency and intensity of cyclones in the Arabian Sea (e.g., Tauktae, Biparjoy) is a critical development. This challenges the traditional disaster management preparedness which has been historically focused on India’s more vulnerable east coast. This “new normal” necessitates a re-evaluation of infrastructure, early warning systems, and community resilience on the west coast.
- Policy Implications: This trend underscores the need for a robust, technology-driven disaster management framework as outlined in the National Disaster Management Plan (NDMP). This includes improving forecasting models (especially for recurvature), strengthening early warning dissemination (IMD’s color codes), building cyclone-resilient infrastructure (National Cyclone Risk Mitigation Project), and enhancing community-based disaster preparedness.
2. Socio-Economic Impact of Cyclones (GS-I, GS-III)
- Differential Vulnerability: The impact of a cyclone is not uniform. It disproportionately affects marginalized communities, such as small-scale farmers and fisherfolk, who lose their livelihoods and homes. The destruction of infrastructure like roads, power lines, and communication networks cripples the local economy.
- Importance of Naming: The standardized naming system for cyclones is a crucial tool in risk communication. A simple, relatable name helps in media reporting, public awareness, and effective dissemination of warnings, thereby reducing ambiguity and encouraging timely action from citizens. It humanizes the threat, making it more tangible than a technical identification number.
- Post-Disaster Challenges: The aftermath of a cyclone presents significant challenges, including the risk of water-borne diseases, food and water scarcity, and long-term psychological trauma. Effective governance requires not just immediate rescue and relief but also a long-term strategy for rehabilitation, livelihood restoration, and building back better.
3. Science, Technology, and Governance in Cyclone Mitigation (GS-II, GS-III)
- Role of Technology: Advances in remote sensing (satellites like INSAT series), Doppler Weather Radars, and numerical weather prediction (NWP) models have dramatically improved the accuracy and lead time of cyclone forecasts in India. This has led to a significant reduction in casualties, as seen in the successful evacuations during Cyclone Fani (2019) and Amphan (2020).
- Forecasting Challenges: Despite advancements, challenges remain. Accurately predicting the intensity changes (especially rapid intensification) and the exact track (especially during recurvature) remains difficult. This highlights the need for continued investment in research and development and international collaboration.
- Institutional Synergy: Effective cyclone management is a testament to the synergy between various institutions. The IMD provides the forecast, the National Disaster Management Authority (NDMA) and State Disaster Management Authorities (SDMAs) formulate plans, and the National Disaster Response Force (NDRF) carries out ground-level operations. This coordinated effort, from scientific forecasting to last-mile connectivity, is a model of effective governance in disaster risk reduction.