Climates: distribution, characteristics, reasons for distribution

Climates of the Tropics – Distribution, Characteristics & Underlying Reasons

Learning Objectives (AO1‑AO3)

• AO1 – Knowledge: Identify the geographic extent of tropical climates, describe their physical characteristics and the atmospheric/oceanic processes that control them.
• AO2 – Application & Analysis: Analyse how climate interacts with weathering, soils, vegetation, water resources and human activity.
• AO3 – Evaluation: Critically evaluate management options for tropical environments using criteria of effectiveness, sustainability, equity and resilience.

1. Syllabus Scope – What Must Be Covered

AS‑Level Required Topics (Cambridge 9696)	Key Content to Include in This Unit
Hydrology	River regimes, rainfall‑runoff relationships, flood & drought cycles in tropical basins.
Atmospheric Processes	Hadley cell, ITCZ, monsoons, ENSO, climate‑change trends (e.g., +1.2 °C global mean since 1850, CO₂ 415 ppm in 2023).
Earth Processes	Chemical weathering, tropical karst, inselberg formation, soil development (Oxisols, Ultisols, Entisols).
Population & Migration	Population growth rates (≈ 2 % yr⁻¹ in many tropical countries), rural‑urban migration, settlement patterns, cultural‑economic place‑making.
Water Resources	Surface‑water availability, groundwater use, irrigation schemes, dam impacts, water‑security indicators.
Urban Areas	Rapid urbanisation (e.g., Lagos +5 % yr⁻¹), informal settlements, heat‑island effect, infrastructure challenges.

Optional A‑Level Global‑Environment Topics (choose any two for Paper 3/4)

• Coastal environments – sea‑level rise, mangrove dynamics, coastal erosion.
• Arid environments – desertification, water scarcity, oasis agriculture.
• Hazardous environments – tropical cyclones, volcanic activity, landslides.
• Climate‑change impacts – mitigation, adaptation, carbon budgeting.
• Trade, aid & tourism – global commodity chains, eco‑tourism, development assistance.
• Disease & health – malaria, dengue, climate‑linked health risks.

2. Geographic Distribution of Tropical Climates

Tropical climates occupy the band between 23.5° N (Tropic of Cancer) and 23.5° S (Tropic of Capricorn). The Köppen–Geiger classification recognises three main humid tropical types:

Köppen Code	Typical Rainfall Pattern	Key Regions (examples)	Mean Annual Precipitation
Af – Equatorial	High, evenly distributed rainfall year‑round	Amazon Basin (Brazil), Congo Basin (DRC), Borneo (Indonesia/Malaysia), West‑Central Africa	> 2 000 mm
Am – Monsoon	Very wet summer, short dry spell (1–3 months)	India & Bangladesh, West Africa (Nigeria, Ghana), northern Australia	1 500–2 500 mm
Aw/As – Savanna	Distinct wet season (4–6 months) & long dry season (4–8 months)	East Africa (Kenya, Tanzania), Brazil’s Cerrado, northern Australia, Southern Africa (Zambia, Botswana)	800–1 500 mm (wet season)

3. Key Physical Characteristics

1. Equatorial (Af)
   • Mean monthly temperature > 18 °C (average 24–27 °C); annual range < 2 °C.
   • Monthly rainfall rarely < 100 mm; annual total > 2 000 mm.
   • Relative humidity 70–90 %.
   • Vegetation: four‑layered evergreen rainforest; > 300 species ha⁻¹ in some sites.
2. Monsoon (Am)
   • Temperature similar to Af (24–28 °C) with < 3 °C annual range.
   • Rainfall concentrated in high‑sun months; dry period 1–3 months (< 50 mm month⁻¹).
   • Annual precipitation 1 500–2 500 mm.
   • Vegetation: seasonal evergreen or semi‑deciduous forest; brief leaf‑drop.
3. Savanna (Aw/As)
   • Mean monthly temperature > 18 °C; annual range 8–12 °C (22–30 °C).
   • Wet season > 1 000 mm in 4–6 months; dry season < 30 mm month⁻¹.
   • Vegetation: grasses with fire‑adapted trees (Acacia, Baobab); frequent surface fires.

4. Why Tropical Climates Are Distributed the Way They Are

4.1 Solar Insolation & Energy Budget

• Equatorial zones receive > 400 W m⁻² of average short‑wave radiation year‑round.
• High, stable insolation keeps surface temperatures above the 18 °C Köppen threshold.

4.2 Large‑Scale Atmospheric Circulation

• Hadley Cell: Warm, moist air rises in the Intertropical Convergence Zone (ITCZ), creating deep convection and heavy rain (Af).
• Air diverges aloft and descends at ≈ 30° N/S, forming subtropical anticyclones that define the poleward limits of tropical climates.
• Seasonal north‑south migration of the ITCZ (≈ ± 15° latitude) produces a wet season where the ITCZ passes overhead and a dry season when it is displaced – the fundamental driver of Am and Aw/As patterns.

4.3 Monsoon Mechanism (Land‑Sea Thermal Contrast)

• Summer: Continental interiors heat faster than adjacent oceans → low pressure over land → on‑shore moist flow → intense summer rains (Am).
• Winter: Land cools more rapidly → high pressure over land → reversal of winds → short dry spell.

4.4 ENSO (El Niño–Southern Oscillation)

• El Niño: Warm central/eastern Pacific weakens Walker circulation, suppresses convection over western Pacific → ↓ rainfall in equatorial Africa, Amazon, SE Asia.
• La Niña: Enhanced west‑Pacific convection → ↑ rainfall over Indian subcontinent and parts of African savanna.
• ENSO contributes 10–30 % of inter‑annual rainfall variability in many tropical regions.

4.5 Climate‑Change Trend (AO2)

• Global mean surface temperature +1.2 °C since pre‑industrial (1850–1900).
• Projected tropical temperature increase 1.5–2.5 °C by 2050 (RCP 4.5).
• Rainfall projections: ↑ intensity of extreme events, ↑ seasonality in monsoon regions, ↑ drought risk in savannas.

5. Hydrology of the Tropics (AO2)

• River Regimes: Perennial rivers dominate Af zones; highly seasonal discharge in Am and Aw/As (e.g., Niger, Amazon flood pulse).
• Runoff Coefficients: Af ≈ 0.7–0.9 (most rainfall becomes runoff); Savanna ≈ 0.3–0.5 due to high evapotranspiration in dry season.
• Flood & Drought: Monsoon floods (e.g., 2010 Pakistan) and savanna droughts (e.g., 2015–2016 Horn of Africa) illustrate climate‑hydrology linkages.
• Water‑Security Indicators: Annual renewable freshwater per capita < 1 000 m³ in many tropical countries – classified as water‑stress.

6. Earth Processes – Weathering, Landforms & Soils (AO2)

6.1 Chemical Weathering

• High temperature & humidity accelerate hydrolysis, oxidation and solution.
• Typical rates: silicate hydrolysis ≈ 10⁻⁶ mol m⁻² yr⁻¹ in tropical rainforests (10× higher than temperate zones).

6.2 Characteristic Landforms

• Granite Terrains: Bornhardts (massive domes), koppies (rounded inselbergs), exfoliation sheets.
• Limestone Terrains: Tropical karst – sinkholes, dolines, tower karst (e.g., Phang Nga Bay), poljes.

6.3 Dominant Soil Orders (with quantitative properties)

Soil Order	Typical Location	Key Physical‑Chemical Properties	Typical Fertility (N kg ha⁻¹ yr⁻¹)
Oxisols (lateritic)	Old, weathered plateaus & rainforest interiors	Highly leached, pH 4.0–5.5, Fe‑/Al‑oxides dominate, low CEC	≈ 30–50 (requires fertiliser)
Ultisols (red/yellow)	Savanna margins, degraded rainforest	Moderate leaching, clay‑rich, acidic (pH 5.0–6.0)	≈ 70–100 (suitable for millet, sorghum)
Entisols (young alluvial/volcanic)	River valleys, volcanic islands	Thin, high organic matter where moisture adequate, high base saturation	≈ 120–180 (supports intensive crops)

7. Population, Migration & Place‑Making (AO2)

• Population growth in the tropics ≈ 2 % yr⁻¹ (UN 2022); urban share rising from 30 % (1990) to > 45 % (2022).
• Rural‑urban migration driven by agricultural mechanisation, climate‑induced livelihood loss, and search for services.
• Place‑making: Cultural practices (shifting cultivation, agroforestry) and economic activities (cash‑crop plantations, mining) shape land‑use patterns.
• DEI note: Indigenous peoples (e.g., Amazonian tribes, Australian Aboriginal groups) often experience disproportionate impacts from land‑use change and climate policies.

8. Water Resources Management (AO2‑AO3)

• Surface Water: Large river basins (Amazon, Congo, Mekong) provide hydro‑electric potential; dam construction alters flow regimes and sediment transport.
• Groundwater: Aquifer recharge rates high in humid zones (> 200 mm yr⁻¹) but over‑extraction in savanna agriculture leads to declining water tables.
• Irrigation: Irrigated area in tropical Africa grew from 3 % (1990) to 6 % (2020); water‑use efficiency remains low (~ 1.2 m³ m⁻³).
• Management Evaluation Prompt (AO3): Assess the sustainability of large‑scale dam projects in the Congo Basin using criteria of ecological flow maintenance, carbon emissions from reservoir inundation, and socio‑economic benefit distribution.

9. Urban Areas in the Tropics (AO2‑AO3)

• Rapid urban growth: Lagos (population +5 % yr⁻¹), Jakarta (+2 % yr⁻¹); informal settlements house > 60 % of urban residents in many African cities.
• Key challenges: inadequate drainage → flood risk, heat‑island effect (+ 2–4 °C vs. rural), water‑quality degradation.
• Management strategies: green infrastructure (urban wetlands), slum upgrading, participatory planning.
• Evaluation Prompt (AO3): Compare “green‑belt” versus “hard‑engineered” flood mitigation in Jakarta, considering effectiveness, cost, community acceptance and climate‑change resilience.

10. Integrated Case‑Study: Amazon Rainforest (Af)

Use the structure below to develop a full answer for a typical Cambridge exam question (AO1‑AO3).

1. Baseline Climate & Vegetation – Mean annual temperature 26 °C, precipitation 2 200–2 800 mm, evergreen four‑layer rainforest, > 300 species ha⁻¹.
2. Deforestation Trends (2000‑2020) – Approx. 17 % forest loss (≈ 5 M km²); drivers: cattle ranching (≈ 70 % of cleared area), soy cultivation, illegal logging, road expansion.
3. Climate Feedbacks – Reduced evapotranspiration lowers regional rainfall by 5–10 %; increased surface albedo and heat flux contribute to a stronger local ENSO‑like oscillation, amplifying dry spells.
4. Management Responses (evaluate using AO3 criteria)
   • Protected Area Network – 60 % of remaining forest under protection; effectiveness measured by forest‑cover change (± 0.5 % yr⁻¹), biodiversity indices, and enforcement capacity.
   • REDD+ Projects – Carbon‑sequestration payments averaging US $5 t⁻¹CO₂; evaluation should consider carbon outcomes, benefit‑sharing with indigenous communities, permanence of forest protection, and monitoring robustness.
   • Community‑Based Forest Management – Success linked to secure land tenure, clear benefit‑sharing, and participatory monitoring; challenges include market access and policy support.

11. Optional A‑Level Global‑Environment Topics – Quick Overviews

Topic	Core Focus (AO1‑AO3)
Coastal Environments	Sea‑level rise, mangrove ecology, coastal erosion, integrated coastal zone management.
Arid Environments	Desertification drivers, water‑scarcity solutions, oasis agriculture, land‑degradation‑neutrality.
Hazardous Environments	Tropical cyclones, volcanic hazards, landslide risk mapping, disaster risk reduction.
Climate‑Change Impacts	Mitigation pathways, adaptation strategies, carbon budgeting, climate justice.
Trade, Aid & Tourism	Global commodity chains, eco‑tourism best practice, aid effectiveness, tourism‑induced land‑use change.
Disease & Health	Vector‑borne diseases, climate‑linked health risks, public‑health responses.

12. Summary Comparison Table – Key Contrasts (AO1)

Feature	Equatorial (Af)	Monsoon (Am)	Savanna (Aw/As)
Temperature (°C)	24–27 (± 2)	24–28 (± 3)	22–30 (± 8)
Annual Rainfall	> 2 000 mm, even	1 500–2 500 mm, summer peak	800–1 500 mm, wet season
Dry Season Length	None	1–3 months	4–8 months
Dominant Vegetation	Evergreen rainforest (4 layers)	Seasonal evergreen / semi‑deciduous forest	Grassland with fire‑adapted trees
Typical Soils	Oxisols (highly leached)	Oxisols / Ultisols (moderate leaching)	Ultisols, Entisols (more fertile)
Key Atmospheric Process	ITCZ overhead year‑round	Seasonal ITCZ migration (± 15°)	ITCZ passes twice a year → distinct wet/dry
Major Human Pressures	Logging, mining, road building	Large‑scale agriculture, dam construction	Cattle ranching, fire expansion, climate‑induced drought
Scale of Analysis	Global to local (forest patch)	Regional (monsoon basin)	Landscape (savanna mosaic)

13. Suggested Classroom Diagrams (for visual support)

• Cross‑section of the Hadley Cell showing rising air at the ITCZ, subsidence at 30° N/S, and the seasonal migration of the ITCZ.
• World map with seasonal ITCZ movement (June vs. December) highlighting wet‑season zones for Am and Aw/As climates.
• Diagram of monsoon pressure system (land‑sea thermal contrast) with wind arrows.
• Illustration of chemical weathering in granite (bornhardts, exfoliation) and limestone (karst towers, sinkholes).
• Flow chart of nutrient cycling in a tropical rainforest (litter → decomposition → uptake → canopy).
• Urban heat‑island schematic for a tropical megacity (surface, canopy, built‑up).

14. Quick Revision Questions (with AO3 prompts)

1. Why does the equatorial (Af) climate lack a true dry season? Relate your answer to the position and persistence of the ITCZ.
2. Explain how the seasonal north‑south migration of the ITCZ produces the wet and dry seasons of a savanna (Aw/As) climate.
3. Compare the influence of continental size on the intensity of monsoon (Am) versus savanna (Aw/As) climates.
4. Describe two ways in which ENSO can modify rainfall patterns in tropical rainforest regions, giving specific examples.
5. Identify the main chemical weathering processes that create bornhardts and tropical karst, and state why they are most intense in the tropics.
6. Discuss the role of fire in maintaining savanna ecosystems, and evaluate how altered fire regimes (e.g., increased anthropogenic burning) may affect biodiversity and carbon storage.
7. Using the Amazon case study, evaluate the effectiveness of REDD+ projects in delivering carbon sequestration, socio‑economic benefits and long‑term forest protection.
8. Assess the sustainability of large‑scale dam construction in the Congo Basin, considering ecological flow, greenhouse‑gas emissions from reservoirs, and impacts on local communities.

15. Evaluation Checklist for Teachers (AO3)

• Do students identify at least three quantitative indicators (e.g., precipitation totals, deforestation rates, population growth) when describing a climate zone?
• Are students able to link climate processes to land‑use change and feedback loops (e.g., reduced evapotranspiration → less rainfall)?
• Do practice answers include a structured evaluation using criteria: effectiveness, sustainability, equity, resilience?
• Is DEI incorporated by asking how different social groups experience climate‑related changes?
• Are students encouraged to use diagrams to support analytical arguments?