TRADE WINDS
WESTERLIES
POLAR EASTERLIES
General Circulation
Hadley, Ferrel & Polar Cells · ITCZ & Trade Winds · Walker Circulation (ENSO) · Seasonal Migration of Belts (28 Micro-Realms)
The Planetary Engine of Heat Transfer (NCERT Chapter 10)
The pattern of planetary winds is known as the General Circulation of the Atmosphere. It is driven by latitudinal insolation gradients and organized by Earth's rotation into the Tri-Cellular Meridional Model: the thermally direct Hadley Cell ($0^\circ\text{–}30^\circ$) which drives the Trade Winds towards the ITCZ; the dynamically driven Ferrel Cell ($30^\circ\text{–}60^\circ$) generating the Westerlies; and the thermally direct Polar Cell ($60^\circ\text{–}90^\circ$) powering Polar Easterlies. Coupled with the latitudinal shifting of these belts (which dictates Monsoonal and Mediterranean climates) and the zonal Pacific Walker Circulation (ENSO), this vast engine transports tropical heat surplus to polar deficits.
I. The Tri-Cellular Meridional Circulation Model (NCERT Core)
General Circulation Def.
- The pattern of planetary winds is known as
- General circulation of the atmosphere
- It also drives the Earth's ocean water circulation!
Driving Forces
- Driven by latitudinal variation in heating
- Emergence of pressure belts
- And modified by Earth's rotation (Coriolis)
Tri-Cellular Model
- Meridional (North-South) cellular circulation
- Hadley Cell ($0^\circ\text{–}30^\circ$)
- Ferrel Cell ($30^\circ\text{–}60^\circ$); Polar Cell ($60^\circ\text{–}90^\circ$)
1. Hadley Cell (0°–30°)
- Thermally direct (driven by heat differences)
- Warm air rises at equator, flows poleward aloft
- Cools and sinks at $30^\circ$ N/S latitude
ITCZ (Thermal Equator)
- Inter-Tropical Convergence Zone
- Low-pressure zone where Trade Winds converge
- Air is forced to rise $\to$ Heavy convectional rain
Rising Limb (Equatorial Low)
- Intense insolation heats surface air
- Air expands, becomes lighter, and ascends
- Creates towering Cumulonimbus clouds (Doldrums)
Subsiding Limb (Subtropical High)
- Equatorial air aloft cools, piles up & sinks at 30°
- Adiabatic warming dissipates clouds
- Creates global hot deserts (Sahara, Kalahari)
II. Ferrel & Polar Cells, Trade Winds & Westerlies (NCERT)
Trade Winds (Tropical Easterlies)
- Surface limb of the Hadley Cell
- Air flowing from 30° High back to Equator Low
- Deflected Westward by Coriolis $\to$ Easterlies
2. Ferrel Cell (30°–60°)
- Dynamically driven "gear" between Hadley/Polar
- Air sinks at 30° and flows poleward at surface
- Forced to rise at 60° by cold polar air
The Westerlies
- Surface limb of the Ferrel Cell
- Flow from 30° Subtropical High to 60° Low
- Deflected Eastward by Coriolis $\to$ Westerlies
Subpolar Low Convergence
- At 60° latitude, warm Westerlies meet
- Cold Polar Easterlies (The Polar Front)
- Air converges & is mechanically forced to rise
3. Polar Cell (60°–90°)
- Thermally direct cold engine
- Air rises at 60°, flows to pole aloft, and sinks
- Creates intense Polar High at 90°
Polar Easterlies
- Surface limb of the Polar Cell
- Cold, dense air blowing from Polar High to 60° Low
- Deflected Westward $\to$ Polar Easterlies
Meridional Heat Transport
- Primary purpose of general circulation
- Transfers thermal energy from equatorial surplus
- To polar deficit zones, balancing Earth's temp
III. Zonal Walker Circulation & ENSO (El Niño / La Niña Mechanics)
Walker Circulation (Zonal)
- Unlike Meridional (N-S) cells, this is ZONAL (E-W)
- Equatorial atmospheric circulation over the Pacific
- Driven by temperature differences between E & W Pacific
Southern Oscillation (SO)
- See-saw flip-flop of atmospheric pressure
- Between Tahiti (East Pacific) and Darwin (Indian Ocean)
- Measured by Southern Oscillation Index (SOI)
ENSO (Coupled System)
- El Niño represents the Oceanic component (Temp)
- Southern Oscillation is the Atmospheric component
- Together they form the coupled ENSO phenomenon
El Niño Phase (Warm East)
DRY
WET
- Trade winds weaken or reverse
- Warm water sloshes back to East Pacific (Peru)
- Drought in Australia/India; Floods in Peru (NCERT)
La Niña Phase (Cold East)
WET
DRY
- "Normal" Walker circulation on steroids!
- Trade winds intensify; intense cold upwelling off Peru
- Heavy rains in Australia & boosts Indian Monsoon!
IV. Seasonal Migration of Belts, Climatic Impact & Jet Streams (NCERT)
Seasonal Migration of Belts
- Pressure belts follow the sun's apparent zenith
- Shift 5°–10° North in July (Summer solstice)
- Shift 5°–10° South in Jan (Winter solstice)
ITCZ Summer Pull (Monsoon)
- In July, ITCZ shifts north over Indian landmass
- Pulls Southern Hemisphere SE Trades across equator
- Coriolis deflects them right $\to$ SW Indian Monsoon!
ITCZ Winter Shift
- In January, ITCZ shifts south over Indian Ocean
- Normal Northern Hemisphere NE Trades dominate India
- Creates dry winter (Northeast Retreating Monsoon)
Mediterranean Climate Control
WESTERLIES
SUBTROP. HIGH
- Caused entirely by seasonal shifting of belts!
- Winter: Westerlies shift equatorward (bring rain)
- Summer: Subtropical High shifts poleward (dry sinking air)
Rossby Waves (Upper Air)
- Giant meanders in high-altitude westerly winds
- Triggered by Rocky/Himalayan topography & heating
- Transport cold polar air south & warm air north
Jet Streams (NCERT)
- Fast-flowing, narrow geostrophic air currents
- Located near the Tropopause altitude breaks
- Guide the paths of temperate frontal cyclones
Polar vs. Subtropical Jets
- Polar Jet (60°): Forms at Ferrel/Polar cell boundary
- Subtropical Jet (30°): Forms at Hadley/Ferrel boundary
- STJ shifts south of Himalayas in winter $\to$ Western Disturbances!
Air Masses (NCERT Source Regions)
- Large body of air with uniform temp & moisture
- c = Continental (Dry); m = Maritime (Wet)
- P = Polar (Cold); T = Tropical (Warm)
Master Tri-Cellular Matrix
- Completes General Circulation (NCERT Ch 10)
- Locks planetary meridional & zonal engines
- Transitions to Realm 71 (Water in the Atmosphere)
Core Distinctions
• Hadley Cell ($0^\circ\text{–}30^\circ$): Thermally driven. Drives the Trade Winds (Easterlies) at the surface.
• Ferrel Cell ($30^\circ\text{–}60^\circ$): Dynamically driven "gear" cell. Drives the Westerlies at the surface.
• Polar Cell ($60^\circ\text{–}90^\circ$): Thermally driven. Drives the Polar Easterlies at the surface.
• El Niño (Warm Phase): Trade winds weaken or reverse. Warm equatorial water flows eastward to Peru. Upsets normal circulation $\to$ Floods in Peru, Drought in Australia & India.
• La Niña (Cool Phase): Trade winds become stronger than normal. Pushes more warm water to Australia/Indonesia, increasing upwelling of cold water off Peru. $\to$ Floods in Australia, Boosts Indian SW Monsoon.
Pressure belts shift 5°–10° following the Sun. Mediterranean Climate ($30^\circ\text{–}40^\circ$ W. Coasts): Summer $\to$ Belts shift poleward, bringing the dry Subtropical High (no rain). Winter $\to$ Belts shift equatorward, bringing the wet Westerlies (winter rain). Indian Monsoon: Summer $\to$ ITCZ pulls north of Equator, dragging SE Trades across equator where Coriolis bends them into the SW Monsoon.
General Circulation Matrix
NCERT Chapter 10 Reference Guide