Tropical Cyclones, Temperate Cyclones, and Anticyclones

1. TROPICAL CYCLONES

Origin

1. Tropical disturbance - Clusters of thunderstorms form over warm oceans. In the Atlantic, most originate from African Easterly Waves that drift westward off the African coast.
2. Tropical depression - Multiple thunderstorm clusters merge, forming a closed low-pressure circulation. Sustained winds: up to 62 km/h.
3. Tropical storm - The system organizes and intensifies. Winds: 63-118 km/h. Gets an official name.
4. Tropical cyclone - Fully mature storm with a defined eye. Winds exceed 119 km/h.

Structure

         OUTFLOW (anticyclonic, >12 km)
              ↑↑  ↑↑  ↑↑
    [Spiral rainbands] → [EYE WALL] → [EYE]
              ↓↓  ↓↓  ↓↓
         INFLOW (cyclonic, 0-3 km)

• Eye: The calm center, typically 30-65 km in diameter. Air sinks here, suppressing cloud formation. The sea surface beneath the eye is violent despite the relatively calm air above.
• Eye Wall: A ring of tall, intense thunderstorms (cumulonimbus) surrounding the eye. This is where the strongest winds and heaviest rainfall occur. Wind speeds can exceed 300 km/h in major cyclones.
• Spiral Rainbands: Curved bands of heavy rain and strong winds extending outward from the eye wall, sometimes hundreds of kilometres.
• Warm Core: The upper troposphere inside the cyclone is significantly warmer than the surrounding environment - this is what distinguishes tropical cyclones from temperate ones.

Regional Names

Examples and Impacts

• Hurricane Katrina (2005): Struck the US Gulf Coast (Louisiana, Mississippi). Category 5 at peak. Over 1,800 deaths, ~$125 billion in damages. The storm surge (up to 8.5 m) was the primary killer, inundating 80% of New Orleans.
• Typhoon Haiyan/Yolanda (2013): One of the strongest landfalling tropical cyclones ever recorded. Struck the Philippines with winds exceeding 315 km/h. Over 6,300 deaths, 4 million displaced.
• Cyclone Bhola (1970): Bay of Bengal. Estimated 300,000-500,000 deaths - the deadliest tropical cyclone in recorded history.
• Cyclone Amphan (2020): Super cyclone in the Bay of Bengal, made landfall in West Bengal/Bangladesh. Over $13 billion in damages.

• Storm surge (most deadly): Sea water pushed ashore by winds, raising sea levels by 2-10 m
• Extreme winds: Structural damage, uprooted trees, flying debris
• Torrential rainfall and flooding: Inland flooding far from the coast
• Landslides in hilly terrain
• Disruption of ecosystems: Coral reef damage, mangrove destruction
• Economic losses: Agriculture, infrastructure, fisheries
• Positive: Replenish freshwater reservoirs in drought-prone regions; redistribute ocean heat

2. TEMPERATE (EXTRATROPICAL) CYCLONES

Origin

1. Frontal boundary: A polar front forms where cold polar air meets warm subtropical air. The boundary is initially stationary.
2. Wave formation: Upper-level jet stream divergence over the front causes surface air to rise, reducing pressure. A wave (kink) develops on the front.
3. Warm and cold fronts develop: The warm front advances poleward; the cold front pushes equatorward and eastward (cold fronts move faster).
4. Deepening: The system deepens as the pressure at the center drops. The warm sector between the two fronts narrows.
5. Occlusion: The cold front eventually overtakes the warm front, lifting the warm air entirely off the surface - forming an occluded front. The cyclone begins to weaken.
6. Dissipation: Once occluded, the energy source (temperature contrast) is cut off.

Structure

• Warm sector: The area between the warm front (ahead) and the cold front (behind), with relatively mild temperatures and steady rainfall
• Cold sector: Cold, showery weather behind the cold front
• Warm front zone: Gentle, widespread precipitation (stratus clouds) as warm air rides up over cold air at a shallow angle
• Cold front zone: Steep, narrow band of intense precipitation (cumulonimbus), sometimes with thunderstorms, as cold air undercuts warm air sharply
• Occluded front: Complex structure where warm air has been lifted completely off the surface
• Center (low): The pressure minimum, surrounded by isobars forming roughly concentric rings

Examples and Impacts

• Cyclone Kyrill (2007): Swept across northern Europe, causing 43 deaths and $6.7 billion in insured damages. Wind gusts exceeded 200 km/h.
• The Great Storm of 1987 (UK): Exceptionally deep depression that struck southern England with hurricane-force winds. Felled 15 million trees, killed 18 people, caused £1 billion in damages.
• Northeast US "Nor'easters": Classic temperate cyclones that track up the East Coast. The Blizzard of 1978 and the "Storm of the Century" (1993) are famous examples, bringing heavy snow, coastal flooding, and widespread disruption.
• The Perfect Storm (1991): A powerful nor'easter that absorbed a tropical system off the US East Coast.

• Heavy rain and snow over wide regions
• Strong winds (strongest near the tropopause, but significant surface winds along fronts)
• Coastal storm surges and flooding
• Blizzards in cold air masses
• Aviation disruption due to turbulence and icing
• Positive: Essential for transporting heat poleward from the tropics, maintaining global energy balance

3. ANTICYCLONES

Origin

• Found at roughly 20°-35° latitude in both hemispheres
• Form due to the descending limb of the Hadley Cell - air that rose at the ITCZ (equator) cools, becomes dense, and sinks in the subtropics
• Examples: Azores High (North Atlantic), Pacific High, Bermuda High, Mascarene High (Indian Ocean)
• These are semi-permanent features of global circulation

• Form over cold continental surfaces in winter due to intense surface cooling and air mass densification
• Examples: Siberian High (winters over Asia), Canadian High (North America)
• These are seasonal and can be very intense

• Slow-moving or stationary high-pressure systems that "block" the normal west-to-east progression of weather systems
• Can cause prolonged heat waves, droughts, or cold spells

Structure

• Central high pressure: Air descends and diverges outward at the surface
• Winds spiral outward: Clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere
• Subsidence inversion: As air sinks and compresses, it warms adiabatically. This creates a temperature inversion (a layer where temperature increases with height), which traps pollution and moisture near the surface
• Clear skies dominate: Subsiding air inhibits cloud formation and precipitation
• Gentle winds: The pressure gradient is usually weaker than in cyclones
• Large and diffuse: Anticyclones can span thousands of kilometres and move slowly

• Surface: High pressure, outward-diverging winds
• Aloft: Convergence and sinking of air
• No fronts; no significant vertical convection

Examples and Impacts

• Azores High (North Atlantic): Steers the track of Atlantic hurricanes and brings dry, sunny summers to Mediterranean Europe
• Siberian High: Dominates Asian winter weather. Sends cold Arctic air masses deep into South and East Asia, influencing monsoon dynamics
• European Heat Wave (2003): A blocking anticyclone stalled over Europe for weeks, causing the deadliest weather event in European recorded history - over 70,000 deaths from heat-related causes
• Australian Drought (2017-2019): A persistent blocking high contributed to the catastrophic drought and bushfire season
• Bermuda-Azores High: Its strength and position control summer rainfall patterns across the eastern US

• Positive: Fair, dry, sunny weather; ideal for agriculture and tourism
• Heat waves when blocking anticyclones persist in summer (heat stress, wildfires, crop failure)
• Cold waves when polar anticyclones surge equatorward in winter
• Fog and smog: Subsidence inversions trap cold air and pollutants near the surface
• Drought: Extended anticyclones suppress rainfall
• Air quality crises in cities (e.g., London smog, Los Angeles smog)

Comparative Summary Table

Global Significance

• Tropical cyclones transfer enormous amounts of heat and energy from the tropics poleward, and help regulate sea surface temperatures
• Temperate cyclones are the primary mechanism for poleward energy and moisture transport in mid-latitudes, driving most of the rainfall that feeds agriculture in Europe, North America, and temperate Asia
• Anticyclones provide the stable, descending air that balances the rising motion in cyclones, and their semi-permanent subtropical forms are anchors of the global wind belts (trade winds, westerlies)

Polycyclic Landforms

1. Conceptual Foundation: The Cycle of Erosion

2. What are Polycyclic Landforms?

"Landscapes that show evidence of more than one cycle of erosion are termed polycyclical." - Wikipedia, Cycle of Erosion

The Trigger: Rejuvenation

1. Tectonic uplift - The land surface is raised relative to base level, steepening river gradients
2. Eustatic sea level fall - Global sea level drops (e.g., during glacial periods), effectively lowering the base level
3. Isostatic rebound - Land rises after removal of ice sheet weight (post-glacial uplift)
4. Climate change - Changes in precipitation and runoff alter erosional energy
5. River capture - A more energetic river pirates a neighboring catchment, increasing discharge and erosive power

3. Key Polycyclic Landforms and Processes

A. River Terraces

• During an earlier cycle, a river erodes laterally and creates a broad, flat valley floor (floodplain or strath)
• Rejuvenation causes the river to incise vertically, cutting downward into the old valley floor
• The old valley floor is left stranded as a flat-topped bench above the new river level - a terrace
• If rejuvenation occurs again, a second (lower) terrace forms, leaving the first terrace even higher
• Repeated uplift events produce a staircase of terraces at successively lower elevations

• Paired terraces: Matching terraces at the same elevation on both sides of the valley - indicate rapid, symmetric incision. These are polycyclic terraces.
• Unpaired terraces: Terraces at different elevations on opposite banks - indicate slower, lateral migration during incision (non-cyclic terraces).
• Strath terraces: Terraces cut into bedrock, then covered by a thin alluvial veneer. Each strath represents one cycle of lateral erosion followed by renewed incision.

• Thames Valley terraces, England: The River Thames exhibits a well-documented staircase of seven terraces (e.g., Boyn Hill Terrace, Lynch Hill Terrace, Taplow Terrace, Floodplain Terrace) formed during successive Pleistocene glacial-interglacial cycles. Each terrace records a phase of lateral widening followed by incision triggered by base level fall during glaciation.
• Rhine Valley terraces, Germany: Multiple terrace levels record repeated Quaternary uplift of the Rhenish Massif combined with Pleistocene sea level changes.
• Yellow River (Huang He) terraces, China: Multiple terrace levels record tectonic uplift of the Tibetan Plateau margin and Quaternary climate-driven incision episodes.

B. Peneplain Remnants and Erosion Surfaces

• Appalachian Mountains, USA: W.M. Davis himself used the Appalachians as the classic example of polycyclicity. The broad, accordant summit surfaces at about 1,000 m are interpreted as remnants of an ancient Cretaceous peneplain (the "Schooley Peneplain"). Rivers subsequently re-incised this surface as the region was gently uplifted, creating the current ridge-and-valley topography. A lower erosion surface (the "Harrisburg Surface" at ~150-300 m) represents a second, incomplete cycle.
• Scottish Highlands, UK: Geomorphologists have identified up to three or four planation surfaces at different altitudes (e.g., the "High Plateau," "Main Plateau," and lower valley surfaces), each representing an ancient erosion surface that was uplifted and dissected.
• Western Ghats, India: The broad, flat-topped summit plateaux of the Deccan plateau surface, standing high above deeply incised river gorges cutting seaward, represent classic polycyclic relief - the plateau is a remnant of an older cycle; the gorges are the product of the new cycle triggered by rifting and uplift of the Western Ghats escarpment.

C. Incised (Entrenched) Meanders

• Goosenecks of the San Juan River, Utah, USA: One of the finest examples. The San Juan River has incised over 300 m into the Colorado Plateau, yet retains its mature meander loops almost perfectly. This occurred because the Colorado Plateau was uplifted gently while the river maintained its meandering course.
• Moselle (Mosel) River meanders, Germany/Luxembourg: The deeply incised meanders of the Moselle are polycyclic - formed during the Tertiary on a near-flat surface, then deeply entrenched following tectonic uplift of the Rhenish Massif in the Quaternary.
• Dee and Wye valleys, Wales/England: Classic incised meanders carved during post-glacial rejuvenation.

D. Knickpoints and Waterfalls

• Afon Cynfal, Wales: Described as a good example of "polycyclic relief" (Howe and Thomas, 1963). The river responds to at least three distinct base levels, with two main platform levels at 400-500 m and 200 m, producing a series of gorges and waterfalls.
• Victoria Falls (Zambezi River, Zambia/Zimbabwe): The Zambezi's profile shows multiple knickpoints and gorge systems, with successive gorges (1st through 7th gorge downstream) each representing an earlier position of the waterfall, recording episodic downcutting related to tectonic and climatic rejuvenation.
• Niagara Falls, USA/Canada: The falls are a knickpoint migrating upstream (at ~1 m/year historically), with a gorge downstream representing the upstream migration of the knickpoint since postglacial times.

E. Raised Beaches and Coastal Terraces

• Isostatic uplift (land rising after ice unloading)
• Eustatic sea level fall (past interglacial high sea levels, now stranded)

• Scottish coastline, UK: Raised beaches at 8 m, 15 m, and 30 m above present sea level record successive postglacial isostatic uplift stages as Scotland "bounced back" after the retreat of the Pleistocene ice sheet. The "Main Postglacial Shoreline" (about 7-8 m) and the "Main Lateglacial Shoreline" (~15 m) are the best documented.
• Pacific coast of Chile and Peru: Multiple marine terraces step upward from the coast, each recording a past interglacial sea level high combined with ongoing tectonic uplift. Some sequences span the entire Quaternary.
• Mediterranean coastlines: Raised beaches record both Pleistocene interglacial sea level highs and ongoing tectonic activity.

F. Gorges and Canyon-within-Canyon

• Grand Canyon, Colorado Plateau, USA: Arguably the world's most famous polycyclic landscape. The broad upper canyon walls represent older cycles of erosion across the Plateau surface; the Inner Gorge (Vishnu schist) represents much more recent, rapid incision by the Colorado River following renewed Quaternary uplift. The Colorado Plateau's flat surface is a remnant of the older cycle; the canyon is the new cycle.
• Indus, Brahmaputra, and Sutlej gorges, Himalayas: These rivers were graded across a gentler landscape before the Himalayan uplift began. As the mountains rose, the rivers maintained their courses (antecedent drainage) and cut some of the world's deepest gorges - the Indus gorge at Nanga Parbat reaches over 5,000 m depth. The high, broad valley shoulders represent the pre-uplift surface; the gorges represent the new cycle.

4. Monadnocks - Polycyclic Survivors

• Inselbergs of Africa (e.g., Uluru/Ayers Rock, Australia): Some inselbergs are considered polycyclic - they survived multiple erosion cycles because their rock is exceptionally resistant (quartzite, granite) while surrounding softer rock was planed away.
• Sugarloaf Mountain (Pão de Açúcar), Rio de Janeiro, Brazil: A classic inselberg/monadnock rising abruptly from a near-flat coastal plain, interpreted as a survivor of ancient erosion cycles acting on the Brazilian Shield.

5. Schematic Illustration of a Polycyclic Valley

Elevation
   |
   |   [OLD PENEPLAIN REMNANT]        [OLD PENEPLAIN REMNANT]
   |___________↓___________________________________↓__________
   |        Knickpoint                         Wide valley
   |            ↓ waterfall                   (1st cycle)
   |           [River Terrace]  [River Terrace]
   |            _____________  ______________
   |                  ↓ new incision ↓
   |              [Deep gorge - 2nd cycle]
   |                     ~~~river~~~
   |_______________________________________________ Base Level

6. King's Pediplanation Cycle: An Alternative View

7. Critical Evaluation

• Provides a logical framework for interpreting complex landscapes with multiple generations of landforms
• Widely used in denudation chronology (dating the history of landscape development)
• Supported by evidence from terraces, knickpoints, and erosion surfaces worldwide

• Davis assumed rapid uplift followed by complete stillstand - unrealistic, as uplift and erosion overlap continuously
• The idea of discrete, complete cycles is idealized; real landscapes show continuous, overlapping processes
• Climate change and lithological variation create similar effects to rejuvenation without needing tectonic uplift
• Modern process geomorphology (dynamic equilibrium theory, stream power models) challenges the cyclic framework but does not invalidate the observation that landscapes preserve multiple generations of landforms

Summary Table: Polycyclic Landforms at a Glance