Pressure Belts & Winds
Pressure Belts & Winds
Atmospheric pressure is the force exerted by the weight of the atmosphere on the Earth's surface. The unequal heating of the Earth creates zones of high and low pressure arranged in belts around the globe. Differences in pressure generate winds that flow from high-pressure to low-pressure areas, modified by the Coriolis effect and friction. These pressure belts and wind systems drive global circulation, weather patterns, and the Indian monsoon.
Key Dates
Evangelista Torricelli invented the mercury barometer and measured atmospheric pressure
George Hadley proposed the single-cell model of atmospheric circulation (Hadley Cell)
Gaspard-Gustave de Coriolis mathematically described the Coriolis effect — deflection of moving objects on a rotating Earth
William Ferrel described the mid-latitude circulation cell (Ferrel Cell)
Christoph Buys Ballot formulated Buys Ballot's Law — relating wind direction to pressure distribution
The three-cell model (Hadley, Ferrel, Polar) of global atmospheric circulation was fully developed
India Meteorological Department (IMD) established — one of the oldest weather services in the world; HQ Delhi
Sir Gilbert Walker appointed DG of Indian Observatories; discovered the Southern Oscillation and Walker Circulation
Jet streams discovered by military pilots during World War II at high altitudes over Japan
Concept of Rossby waves (planetary waves in the jet stream) formalized by Carl-Gustaf Rossby
P.K. Das and Indian meteorologists refined the understanding of the Indian monsoon mechanism involving jet streams
2004 Indian Ocean Tsunami prompted establishment of ITEWC at INCOIS, Hyderabad for real-time weather/ocean monitoring
Inter-Tropical Convergence Zone shifts seasonally — over India in summer (~25°N), driving the SW monsoon
Narrow high-altitude wind bands (150-300+ km/h) at 9-12 km altitude; STJ withdrawal triggers Indian monsoon
Atmospheric Pressure — Fundamentals
Atmospheric pressure is the weight of the air column above a unit area on the Earth's surface. At sea level, the standard atmospheric pressure is 1013.25 millibars (mb) or hectopascals (hPa), equivalent to 760 mm of mercury (Hg) or 1 atmosphere (atm). Atmospheric pressure decreases with altitude because the air column above becomes shorter — pressure drops approximately 1 mb for every 10 m increase in altitude near the surface (this rate decreases with altitude). Pressure also varies horizontally due to differential heating: warm air expands, becomes less dense, and rises, creating low pressure at the surface; cold air contracts, becomes denser, and sinks, creating high pressure at the surface. Pressure is measured using barometers — mercury barometers (invented by Torricelli in 1643) and aneroid barometers (no liquid). Lines connecting places of equal pressure on a map are called isobars. Closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate gentle winds. Pressure also varies with temperature (warm air = low pressure, cold air = high pressure), humidity (moist air is lighter than dry air at the same temperature — water vapour has lower molecular weight than N2 and O2), and rotation of the Earth. India's sea-level pressure varies significantly between seasons: a deep low-pressure area develops over the Thar Desert in summer (June), drawing the moisture-laden southwest monsoon; in winter (January), a high-pressure area over central Asia pushes cold, dry air southward over India.
Global Pressure Belts
The Earth's surface has seven major pressure belts arranged latitudinally: (1) Equatorial Low-Pressure Belt (Doldrums) — 0° to 5°N/S; intense heating causes air to rise (convection), creating a zone of low pressure, light winds, calms, heavy rainfall, and frequent thunderstorms; called the doldrums because sailing ships were becalmed here; this is the thermal equator. (2) Subtropical High-Pressure Belts (Horse Latitudes) — centered around 30°N and 30°S; descending air from the upper branches of the Hadley Cell creates high pressure, clear skies, light winds, and arid conditions; most of the world's hot deserts (Sahara, Arabian, Thar, Kalahari, Great Australian) lie in these belts; called "horse latitudes" because Spanish sailors reportedly threw horses overboard to lighten ships when becalmed here. (3) Subpolar Low-Pressure Belts — centered around 60°N and 60°S; convergence of warm subtropical air and cold polar air creates fronts, cyclonic activity (mid-latitude cyclones), and precipitation; these are dynamic lows caused by Earth's rotation and convergence, not primarily by surface heating. (4) Polar High-Pressure Belts — at 90°N and 90°S; extremely cold air is dense and sinks, creating high pressure; stable, dry conditions with very low precipitation (Antarctica is technically a desert). These pressure belts shift north and south with the apparent movement of the Sun (about 5-10° seasonal shift). This shift is critical for India: in summer, the ITCZ shifts northward to about 25°N over the Indo-Gangetic Plain, drawing moisture-laden winds from the Indian Ocean — this is the fundamental driver of the Indian Southwest Monsoon.
Planetary Wind Systems
Planetary (permanent) winds blow consistently from high-pressure belts to low-pressure belts throughout the year, modified by the Coriolis effect: (1) Trade Winds — blow from the subtropical highs (~30°) toward the equatorial low; in the Northern Hemisphere, they are Northeast Trades (deflected right by Coriolis); in the Southern Hemisphere, Southeast Trades (deflected left); the most consistent winds on Earth; historically crucial for sailing trade routes (hence the name); they converge at the ITCZ, where they rise and produce heavy equatorial rainfall. (2) Westerlies — blow from the subtropical highs (~30°) toward the subpolar lows (~60°); in the Northern Hemisphere, they are Southwest Winds; in the Southern Hemisphere, Northwest Winds (but commonly called "Brave West Winds" or Roaring Forties (40°S), Furious Fifties (50°S), and Screaming Sixties (60°S) due to the absence of landmasses to slow them); westerlies bring variable weather and precipitation to mid-latitude regions; Western Disturbances that bring winter rainfall to India are carried by the westerly wind belt. (3) Polar Easterlies — blow from the polar highs toward the subpolar lows; cold, dry winds; in the Northern Hemisphere, they blow from the northeast; weaker and less consistent than trades or westerlies. The convergence of trade winds at the ITCZ and the convergence of westerlies and polar easterlies at the Polar Front are major zones of weather activity. The Coriolis effect, named after French mathematician Gaspard-Gustave de Coriolis (1835), causes deflection of moving air (and ocean currents) — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator and maximum at the poles.
Three-Cell Model of Global Circulation
The global atmospheric circulation is best explained by the three-cell model: (1) Hadley Cell (0°-30°) — proposed by George Hadley in 1735; at the equator, intense heating causes air to rise (ITCZ); this air moves poleward at upper levels, gradually cooling; by about 30°N/S, it has cooled and become dense enough to sink — creating the subtropical high-pressure belt; the sinking air diverges at the surface — part returns to the equator as trade winds (completing the cell) and part moves poleward as westerlies. The Hadley Cell is the largest, most consistent, and most powerful circulation cell, driven primarily by thermal energy. (2) Ferrel Cell (30°-60°) — an indirect (thermally indirect) cell between the Hadley and Polar cells; surface winds (westerlies) blow from subtropical highs toward subpolar lows; air rises at about 60° (polar front) and descends at about 30°; this cell is weaker and maintained by the momentum transfer from the other two cells rather than direct thermal driving. (3) Polar Cell (60°-90°) — cold dense air sinks at the poles and flows toward the subpolar low as polar easterlies; at about 60°, it meets the warmer westerlies and is forced to rise (polar front); this rising air moves toward the pole at upper levels, completing the cell. The boundaries between cells are critical weather zones: the ITCZ (Hadley-Hadley convergence), the Horse Latitudes (Hadley-Ferrel subsidence), and the Polar Front (Ferrel-Polar convergence, where mid-latitude cyclones form). In the Indian context, the Hadley Cell's seasonal migration is the engine of the monsoon system — when the ITCZ shifts north over India in summer, it reverses the trade winds, creating the moisture-laden southwest monsoon.
ITCZ, Monsoon Trough, and the Indian Monsoon Mechanism
The Inter-Tropical Convergence Zone (ITCZ) is the zone where the northeast and southeast trade winds converge near the equator. It is characterized by low pressure, rising air, clouds, and heavy rainfall. The ITCZ is not fixed — it migrates seasonally, following the Sun's apparent movement. This migration is the fundamental driver of the Indian monsoon. In summer (June-September), the ITCZ shifts northward to about 25°N, positioning itself over the Indo-Gangetic Plain. This northward-positioned ITCZ is called the "monsoon trough" in Indian meteorology. The monsoon trough is a low-pressure zone that draws moisture-laden air from the Indian Ocean. When the trough is positioned over the plains, active monsoon conditions prevail with widespread rainfall. When it shifts northward to the Himalayan foothills, the plains experience dry spells called "monsoon breaks." The monsoon trough's position oscillates during the season, causing alternating active and break phases. Multiple factors drive the Indian monsoon: (1) Differential heating of land (Indian subcontinent heats up rapidly) and sea (Indian Ocean remains cooler), creating a strong pressure gradient; (2) Northward shift of the ITCZ over the Ganga Plain; (3) The Tibetan Plateau acts as an elevated heat source in summer, strengthening the upper-level anticyclone and sustaining the Tropical Easterly Jet; (4) The Somali Jet carries moisture from the Indian Ocean; (5) The withdrawal of the Subtropical Jet Stream from south of the Himalayas triggers the sudden onset; (6) El Nino/La Nina and IOD modulate monsoon intensity. In winter (October-February), the ITCZ shifts south of the equator, the monsoon trough disappears, and the northeast monsoon prevails.
The Indian Monsoon — Two Branches and Rainfall Distribution
The Southwest Monsoon (June-September) brings about 75% of India's annual rainfall and arrives in two branches: (1) Arabian Sea Branch — hits the Western Ghats around June 1 (Kerala); the Western Ghats act as an orographic barrier, causing heavy rainfall on their windward (western) side (3,000-5,000 mm annually in some places) while the leeward (eastern) side gets much less (rain shadow effect — e.g., Bangalore gets only ~900 mm despite being south of the Ghats); this branch then moves northwestward along the coast, reaching Gujarat and Rajasthan by early July; the Thar Desert receives very little monsoon rain because the branch has lost most moisture by then, and the desert's high albedo creates a thermal low that draws air but provides insufficient lifting. (2) Bay of Bengal Branch — enters through Myanmar, moves into northeast India (making the Khasi and Jaintia Hills receive extremely heavy rainfall — Mawsynram 11,872 mm/year is the world's wettest place), then turns westward along the Ganga Plain guided by the monsoon trough; this branch is the primary rain-giver for the Indo-Gangetic Plain and central India. The two branches merge in the Punjab region by early July, completing the monsoon advance by July 15 (approximately). Rainfall distribution: India's annual rainfall average is about 1,150 mm; but distribution is highly uneven — Mawsynram (~11,872 mm) vs Jaisalmer (~170 mm). Monsoon retreat begins from northwest India in September and reaches Tamil Nadu by November. The Northeast Monsoon (October-December) brings crucial rainfall to Tamil Nadu — winds blow from the northeast, pick up moisture from the Bay of Bengal, and hit the Coromandel Coast; this gives Tamil Nadu its rainfall maximum in October-November when the rest of India is dry.
Local and Periodic Winds
Local winds are caused by local temperature and pressure differences, while periodic winds reverse direction seasonally. Monsoon Winds — the most important periodic wind system; derived from the Arabic "mausim" (season); the Indian monsoon is driven by differential heating of land and sea, the northward shift of the ITCZ, and the role of the Tibetan Plateau and Somali Jet; Southwest Monsoon (June-September) brings about 75% of India's annual rainfall; Northeast Monsoon (October-December) brings rainfall mainly to Tamil Nadu and parts of Andhra Pradesh. Land and Sea Breezes — diurnal cycle; during the day, land heats faster than the sea, creating a sea breeze (from sea to land); at night, land cools faster, creating a land breeze (from land to sea); significant in coastal cities like Mumbai, Chennai, Kolkata. Mountain and Valley Breezes — during the day, valley slopes heat up and warm air rises upslope (valley breeze or anabatic wind); at night, slopes cool rapidly and cold air drains downslope (mountain breeze or katabatic wind); common in Himalayan valleys. Important local winds in India and the world: Loo — hot, dry wind in the Indo-Gangetic Plain (May-June, temperatures up to 45-50°C); Chinook — warm, dry wind on the eastern slopes of the Rockies ("snow-eater"); Foehn — warm, dry wind in the Alps; Sirocco — hot, dry wind from the Sahara blowing across the Mediterranean; Mistral — cold, dry northwesterly in southern France; Harmattan — dry, dusty wind from the Sahara across West Africa (called "the doctor" for its cooling effect); Bora — cold northeasterly in the Adriatic region; Santa Ana — hot, dry wind in southern California; Berg — hot, dry wind in South Africa.
Pre-Monsoon and Regional Winds of India
India experiences several regionally important winds and weather phenomena beyond the monsoon: Norwesters (Kalbaishakhi) — violent thunderstorms in eastern India (West Bengal, Bihar, Assam) during April-May; caused by the convergence of moist air from the Bay of Bengal with hot dry air from the northwest; produce sudden rainfall with gusty winds (60-100 km/h), hail, and lightning; critically important for jute and tea cultivation in West Bengal and Assam; Kolkata experiences 30-40 Norwesters per season. Mango Showers — pre-monsoon showers along the Kerala and Karnataka coast in March-May; caused by local convection and offshore troughs; help in the ripening of mangoes; also called "blossom showers" because they aid flowering of coffee in Karnataka. Cherry Blossoms (Kaal Baisakhi in Assam) — April-May thunderstorms in Assam that bring pre-monsoon rain; beneficial for tea gardens. Bardoli Chherha — strong, destructive winds in Assam and north Bengal during pre-monsoon period. Western Disturbances — extra-tropical cyclones that originate over the Mediterranean Sea and travel eastward carried by the subtropical westerly jet stream; they bring significant winter and early spring rainfall to J&K, Ladakh, Himachal Pradesh, Uttarakhand, Punjab, and Haryana (December-March); this rainfall is critical for the rabi (winter) crop, especially wheat; about 5-6 Western Disturbances affect India per winter season; they can cause heavy snowfall in the Himalayas and occasionally bring rain as far south as Gujarat and Madhya Pradesh. October Heat — the period between the retreat of the SW monsoon and the onset of the NE monsoon (late September-October); characterized by high temperatures and high humidity in northern India; oppressive conditions until the NE monsoon cools the subcontinent.
Jet Streams and Upper-Air Circulation
Jet streams are narrow bands of very fast-moving air currents in the upper troposphere and lower stratosphere (typically at 9-12 km altitude), flowing from west to east. Wind speeds in jet streams range from 150 to 300 km/h, sometimes exceeding 400 km/h. They are caused by the sharp temperature gradients between adjacent air masses and the Coriolis effect. Major jet streams: (1) Subtropical Jet Stream (STJ) — located at about 30°N/S; associated with the boundary of the Hadley Cell; strongest in winter; in the Indian context, the STJ runs south of the Himalayas during winter, bringing Western Disturbances that cause winter rainfall in north India; during summer (June), the STJ shifts north of the Himalayas, and its withdrawal from the Indian subcontinent is associated with the sudden onset of the southwest monsoon — this is a critical trigger mechanism. (2) Polar Front Jet Stream — located at about 60°N/S; weaker and more variable; associated with the boundary of the Ferrel and Polar cells; responsible for guiding mid-latitude cyclones. (3) Tropical Easterly Jet (TEJ) — flows from east to west over peninsular India during summer (June-September) at about 14°N latitude; associated with the upper-level anticyclone over the Tibetan Plateau; its presence is linked to good monsoon performance; if the TEJ weakens, the monsoon is likely to be weak. (4) Somali Jet (East African Low-Level Jet) — a low-level jet at about 1.5 km altitude; crosses the equator from the Southern Hemisphere, turns right due to the Coriolis effect, and enters India as a moisture-laden stream along the Western Coast; it is the primary moisture carrier for the Indian monsoon and is strengthened by the cold Somali Current. Jet streams meander in wave-like patterns called Rossby waves — these influence weather patterns over large areas and can cause prolonged spells of heat, cold, or rain.
Western Disturbances — Impact on Indian Weather
Western Disturbances (WDs) are extra-tropical cyclones that originate over the Mediterranean Sea and Atlantic Ocean and travel eastward across West Asia and Central Asia, carried by the subtropical westerly jet stream. They are the primary source of winter precipitation in northern India and are critical for the country's wheat production and water resources. WDs typically affect India from November to April, with peak activity during December-February. On average, 5-8 WDs affect India each winter. When a WD approaches northwestern India, it creates a low-pressure trough in the upper atmosphere, causing convergence and lifting of moist air. This produces widespread cloudiness, rain in the plains (Punjab, Haryana, UP), and heavy snowfall in the mountains (J&K, Ladakh, HP, Uttarakhand). The WD-driven snowfall in the Himalayas is critical because it feeds rivers (Indus system, Ganga tributaries) during the subsequent summer melt season, providing water for irrigation and hydroelectric power. The interaction between WDs and the monsoon trough can sometimes cause "out of season" rainfall — WDs arriving late (May-June) can interact with the advancing monsoon, sometimes causing extreme rainfall events. Climate change is expected to alter WD frequency and intensity — some studies suggest WDs may bring more intense but less frequent precipitation events, increasing flood and landslide risk in the Himalayas. The IMD tracks WDs using satellite imagery and numerical weather prediction models. WDs also bring cold waves to northern India when they are followed by cold air intrusion from Central Asia.
Cyclones — Tropical and Extra-Tropical
Tropical cyclones are intense rotating weather systems that form over warm tropical oceans (SST >26.5°C) with sustained winds exceeding 62 km/h. They are called hurricanes (Atlantic/NE Pacific), typhoons (NW Pacific), and cyclones (Indian Ocean/South Pacific). Formation requires: warm ocean surface (>26.5°C to a depth of 60 m), sufficient Coriolis force (at least 5° from equator), low wind shear, existing disturbance (usually an easterly wave or ITCZ oscillation). Cyclones rotate counterclockwise in the NH and clockwise in the SH. Structure: eye (calm centre, 20-50 km), eyewall (most intense winds and rain), rain bands (outer spiraling cloud bands). Categories: Depression (<31 knots), Deep Depression (31-33 knots), Cyclonic Storm (34-47 knots), Severe Cyclonic Storm (48-63 knots), Very Severe (64-89 knots), Extremely Severe (90-119 knots), Super Cyclonic Storm (>120 knots). Indian context: Cyclones form in the Bay of Bengal (more frequent — 5-6 per year; warm SST 28-30°C, receives freshwater from rivers) and Arabian Sea (1-2 per year). Peak season: October-December (for Bay of Bengal, post-monsoon). Odisha, Andhra Pradesh, Tamil Nadu, West Bengal, and Bangladesh coastlines are most vulnerable. Major cyclones: 1999 Odisha Super Cyclone (260 km/h winds, ~10,000 killed), Cyclone Hudhud (2014, AP), Cyclone Fani (2019, Odisha — successful evacuation), Cyclone Amphan (2020, WB), Cyclone Biparjoy (2023, Gujarat). Cyclone naming: rotational list maintained by WMO Regional Committee; 13 countries suggest names for the North Indian Ocean (India's recent suggestions: Gati, Tej, Murasu).
Pressure Belts and the Indian Monsoon — Summary
The Indian monsoon system is fundamentally driven by the seasonal shift of pressure belts and the ITCZ. In summer (June-September): intense heating of the Indian landmass creates a deep low-pressure area over the Thar Desert and northwest India, while the Indian Ocean remains relatively cooler with higher pressure. This pressure gradient draws moisture-laden air from the Indian Ocean toward the landmass — the Southwest Monsoon. The ITCZ shifts northward to about 25°N, positioned roughly along the Ganga Plain (called the monsoon trough). The monsoon arrives in two branches: the Arabian Sea branch (hits the Western Ghats first around June 1, then Gujarat and Rajasthan) and the Bay of Bengal branch (moves through Myanmar, northeast India, and then westward along the Ganga Plain). The Western Ghats cause intense orographic rainfall on their windward (western) side — Mawsynram/Cherrapunji on the Khasi Hills receive the world's highest rainfall. In winter (October-February): the landmass cools, pressure rises over central Asia (Siberian High), and the ITCZ shifts south of the equator. Cold, dry winds blow from the northeast (Northeast Monsoon) — generally dry for most of India but bringing crucial rainfall to Tamil Nadu. The monsoon trough position and intensity determine the spatial and temporal distribution of rainfall. When the trough shifts northward to the Himalayan foothills, the plains experience dry spells (monsoon breaks); when it is over the plains, active conditions prevail.
Climate Change and Monsoon Variability
Climate change is significantly impacting India's pressure patterns, wind systems, and monsoon behaviour. Key observed and projected changes: (1) Monsoon Variability — while long-term average monsoon rainfall has remained stable, the variability has increased; there are more extreme rainfall events and more dry spells within the monsoon season; central India has seen a significant increase in the frequency of very heavy rainfall events (>150 mm/day) since the 1950s. (2) Delayed Onset — some studies indicate the monsoon onset date is becoming more variable, though not consistently delayed. (3) Weakening of Monsoon Circulation — the land-sea temperature gradient, which drives the monsoon, may be affected by differential warming; aerosol pollution over the Indo-Gangetic Plain may be weakening the monsoon by reducing surface heating. (4) Increased Cyclone Intensity — while the total number of cyclones in the North Indian Ocean may not change significantly, the proportion of very severe cyclones is increasing; cyclones in the Arabian Sea are becoming more frequent (possibly due to warming SST). (5) Jet Stream Changes — warming is reducing the equator-to-pole temperature gradient, potentially weakening jet streams and causing them to meander more (larger Rossby waves), leading to more persistent weather patterns (prolonged heat waves, cold spells, or rain). (6) ENSO and IOD — El Nino events may become more frequent and intense, increasing monsoon uncertainty; however, positive IOD events can counteract El Nino effects (as seen in 2019 and 2023 when India had normal/above-normal rainfall despite El Nino). The IPCC AR6 projects that Indian summer monsoon rainfall will increase by the end of the 21st century under all emission scenarios, but with greater year-to-year variability.
Measurement, Forecasting, and Key Agencies
Atmospheric pressure and wind measurement are fundamental to weather forecasting. Instruments: Mercury Barometer — a glass tube filled with mercury inverted in a mercury dish; atmospheric pressure supports the mercury column; standard pressure = 760 mm Hg; Aneroid Barometer — a sealed metal box from which air has been partially removed; it expands/contracts with pressure changes, moving a needle on a calibrated dial; Barograph — a recording aneroid barometer that traces pressure on a chart; Anemometer — measures wind speed using rotating cups; Wind Vane — indicates wind direction (named by the direction FROM which the wind blows). Weather maps show pressure patterns using isobars. Key pressure systems: Cyclone (Low) — isobars encircle a low-pressure center; winds spiral inward counterclockwise in the Northern Hemisphere (clockwise in the SH) due to the Coriolis effect; associated with ascending air, clouds, and precipitation. Anticyclone (High) — isobars encircle a high-pressure center; winds spiral outward clockwise in the NH; associated with descending air, clear skies, and stable weather. Buys Ballot's Law (1857): if you stand with your back to the wind in the Northern Hemisphere, low pressure is to your left and high pressure to your right (reversed in the SH). In India, the India Meteorological Department (IMD), established in 1875 and headquartered in Delhi, is the national meteorological agency responsible for weather observation, forecasting, and cyclone warnings. IMD operates a network of 679+ surface observatories, 62 upper-air stations, 37 Doppler weather radars, satellite imagery (INSAT-3D/3DR), and Numerical Weather Prediction (NWP) models. The Indian Institute of Tropical Meteorology (IITM, Pune) leads monsoon and ENSO research. INCOIS (Hyderabad) provides ocean and tsunami services.
Walker Circulation and Its Role in the Monsoon
The Walker Circulation is an east-west atmospheric circulation cell over the tropical Pacific Ocean, discovered by Sir Gilbert Walker during his tenure as Director-General of Indian Observatories (1904-1924). Walker was attempting to predict Indian monsoon failures when he identified the Southern Oscillation — a see-saw pattern of atmospheric pressure between the eastern and western tropical Pacific. In normal conditions, the Walker Circulation has: rising air over the Maritime Continent (Indonesia/Australia) where SSTs are warm, upper-level westward flow across the Pacific, sinking air over the eastern Pacific (Peru/Ecuador) where SSTs are cool due to upwelling, and surface eastward flow (trade winds). This creates high pressure over the eastern Pacific and low pressure over the western Pacific. The Southern Oscillation Index (SOI) measures the pressure difference between Tahiti and Darwin — positive SOI indicates La Nina (normal/enhanced pattern), negative SOI indicates El Nino (weakened/reversed pattern). During El Nino, the Walker Circulation weakens or reverses: trade winds weaken, warm water shifts eastward, the Indonesian low weakens, and the Indian monsoon is often (but not always) suppressed — about 60-65% of El Nino years coincide with below-normal Indian monsoon rainfall. During La Nina, the Walker Circulation strengthens, and the Indian monsoon is generally above-normal. Walker also identified correlations between the Southern Oscillation and Indian monsoon, East African rainfall, and Australian weather — laying the foundation for modern climate teleconnection studies. The concept was later named the "Walker Circulation" in his honour by Jacob Bjerknes, who linked it to El Nino in the 1960s.
Relevant Exams
Pressure belts and winds is a critical topic for UPSC, particularly the monsoon mechanism, jet streams, ITCZ, and the three-cell model. SSC and banking exams test factual recall — names of wind systems, local winds (Loo, Chinook), Coriolis effect, and pressure belts. Questions on Western Disturbances, the role of the Subtropical Jet Stream, the Somali Jet, Walker Circulation, and cyclone formation are UPSC favorites. Understanding monsoon variability and climate change impacts is increasingly important for mains.