Monsoon: Definition, Characteristics, Origin and Relation with El-Nino and IOD

Definition of Monsoon

A monsoon is a seasonal reversing wind system accompanied by corresponding changes in precipitation patterns. The term derives from the Arabic word "Mausim," meaning "season." Technically, monsoons represent a cyclic reversal of pressure and wind systems occurring annually, characterized by a shift in atmospheric circulation patterns. Modern meteorological understanding defines monsoons as large-scale seasonal wind systems blowing persistently in one direction over vast areas of the globe, which reverse direction with the change of season. At their fundamental level, monsoons operate as land and sea breezes on a planetary scale, driven by the differential heating of land and oceanic surfaces in response to incoming solar radiation.^[1]^[2]

Characteristics of Indian Monsoon

The Indian monsoon exhibits distinctive features that make it one of the world's most significant atmospheric phenomena, profoundly influencing the lives of over a billion people in South Asia.

Temporal and Spatial Distribution

The Indian monsoon operates in two distinct phases: the Southwest Monsoon (June to September) and the Northeast Monsoon (October to December). The southwest monsoon dominates the Indian subcontinent's rainfall pattern, accounting for approximately three-fourths of India's total annual rainfall, with the plains receiving an average of about 87% of their rainfall during this period. The monsoon exhibits a characteristic progression, arriving on the Kerala coast around June 1st and sweeping rapidly northward, covering most of India by mid-July.^[3]^[4]

Rainfall Variability and Distribution

Rainfall from the southwest monsoon is highly variable across India. The coefficient of variation (CV) in rainfall demonstrates clear regional patterns: areas with less than 25% variability receive over 100 cm of annual rainfall, while regions with more than 50% variability experience less than 50 cm of annual rainfall. The Western Ghats, as the first highland encountered by southwest monsoon winds, receive copious amounts of rainfall. Notably, Mawsynram in Meghalaya's Khasi Hills records the world's highest average annual rainfall due to intense orographic precipitation.^[5]^[3]

Wind Patterns and Atmospheric Circulation

During the southwest monsoon, winds blow from southwest to northeast due to the pressure gradient between the cooler Indian Ocean and the heated Indian landmass. The monsoon manifests through two primary branches: one travels westward along the Gangetic Plains toward Punjab, while the other moves up the Brahmaputra valley in northeastern India. This branching pattern, along with sub-branches affecting regions like the Garo and Khasi Hills, creates complex spatial rainfall distributions.^[3]^[5]

Temperature and Pressure Changes

The arrival of southwest monsoons brings a significant temperature drop of 3°C to 6°C, occasionally reaching 5°C to 10°C in some regions. This sudden temperature change accompanies the arrival of warm, moisture-laden winds that replace the hot, dry pre-monsoon conditions. The mechanism involves the creation of a low-pressure system over the heated Tibetan Plateau and northern Indian plains, which contrasts sharply with the high-pressure conditions over the cooler Indian Ocean.^[6]^[7]^[3]

Onset and Withdrawal Patterns

The monsoon exhibits characteristic burst behavior rather than a gradual onset. The normal arrival date is June 1st in Kerala and extends to the Indo-Gangetic Plain by mid-July. Delays in monsoon onset directly reduce its duration and rainfall volume. The monsoon's retreat follows a similar pattern, beginning in September and completing by December, when the northeast monsoon takes over.^[8]^[3]

Theories of Indian Monsoon

Multiple complementary theories explain the complex mechanisms underlying Indian monsoon formation and variability.

Thermal Concept (Halley's Theory)

The Thermal Concept, proposed by astronomer Halley, represents the foundational explanation for monsoon circulation. According to this theory, the primary cause of Indian monsoon circulation is the differential heating of land and sea. During winter, the vast Asian landmass cools rapidly compared to surrounding oceans, creating a strong high-pressure center over the continent. This pressure difference drives air outflow from land toward the sea, bringing cold, dry conditions. Conversely, in summer, rapid heating of the landmass creates a low-pressure zone that attracts moisture-laden oceanic air inland.^[9]^[5]

However, the thermal concept faces significant criticisms. It fails to explain critical monsoon phenomena including: (1) the sudden burst of monsoons rather than gradual onset, (2) breaks in monsoon rainfall, (3) the spatial and temporal distribution of monsoon, and (4) the complex nature of monsoon rainfall combining orographic, cyclonic, and convectional components. Additionally, low-pressure areas are not stationary, and pressure reversals in ocean currents cannot be fully explained by thermal principles alone.^[9]^[5]

Dynamic Concept (Flohn's Theory - Equatorial Westerlies Theory)

The Dynamic Concept, proposed by German meteorologist Flohn in 1951 and enriched by Krishna Rao's research, represents a more comprehensive explanation of monsoon mechanisms. According to this theory, monsoons result from the seasonal migration of planetary winds and pressure belts in response to the seasonal swing of temperature and pressure zones, associated with the changing overhead position of the sun.^[10]^[11]^[12]

The dynamic theory emphasizes that during the Inter-Tropical Convergence (ITC) zone formation near the equator (around March-September), the northeast trade winds of the northern hemisphere and southeast trade winds of the southern hemisphere converge in a belt of low pressure. Under the influence of the Coriolis force, the southeast trades of the southern hemisphere cross the equator and deflect to blow as southwest to northeast winds, which become the southwest monsoon of India. These winds are moisture-laden after traveling over the warm Indian Ocean. The theory also explains why the equatorial westerlies shift northward to become the southwest monsoon winds as the sun's position shifts toward the Tropic of Cancer.^[12]^[10]

This dynamic framework successfully explains monsoon onset through the mechanism of the northward shift of the Subtropical Jet Stream (STJ) and the positioning of the Tropical Easterly Jet (TEJ). It also accounts for monsoon breaks through changes in the position of the Intertropical Convergence Zone (ITCZ), providing a more realistic representation of monsoon variability than the thermal concept alone.^[11]

Jet Stream Theory

The Jet Stream Theory provides crucial insights into monsoon onset, strength, and withdrawal. The Subtropical Westerly Jet Stream (STJ) flows between 25° to 35°N latitude in the upper troposphere at heights of 12-14 km, with wind speeds typically ranging from 150-300 km/h. The southern branch of the STJ, with an average speed of about 240 km/h, creates a strong latitudinal thermal gradient responsible for subsiding air that produces dry conditions over northwestern India.^[13]

The burst of the monsoon depends critically on the northward shift of the STJ by the end of May. When this shift occurs, the high-pressure system over northwestern India weakens, allowing the Equatorial Trough (ITCZ) to push northward. Simultaneously, an Easterly Tropical Jet emerges over peninsular India at 15°N latitude, creating strong convection and heavy rainfall. This jet positioning explains why central India experiences the most intense rainfall during the peak monsoon months.^[14]^[13]

The strength of the southwest monsoon is directly determined by the strength of the easterly tropical jet over central India: a strong jet produces vigorous monsoon circulation and abundant rainfall, while a weak jet results in subdued monsoon activity. Variations in jet stream positioning explain monsoon timing and intensity variability from year to year.^[14]

Role of Tibetan Plateau as an Elevated Heat Source

The Tibetan Plateau functions as a critical elevated heat source that profoundly influences monsoon dynamics. The plateau heats rapidly during summer, becoming 2°C to 3°C warmer than surrounding regions. This intensive heating creates a strong low-pressure system over Tibet, which acts as a major attractor for moisture-laden Arabian Sea and Bay of Bengal winds.^[15]^[16]

Recent research reveals that the intensity and duration of Tibetan Plateau heating directly correlates with Indian monsoon rainfall amounts. Critically, if snow over the Tibetan Plateau does not melt sufficiently, the easterly jet fails to develop adequately, hampering monsoon rainfall. Consequently, years with thick and widespread snow coverage over Tibet typically precede weak monsoon years with reduced rainfall. The plateau also influences monsoon circulation through the Hadley Cell mechanism, where ascending air above Tibet gradually spreads southward to a descending limb over the north Indian Ocean near the Mascarene High.^[17]^[16]

ITCZ Shifting and Monsoon Trough Formation

The Inter-Tropical Convergence Zone (ITCZ) plays a fundamental role in monsoon development and rainfall distribution. During the monsoon season, the ITCZ shifts northward to approximately 20°-25°N latitude, positioning itself in the Indo-Gangetic Plain, where it is termed the Monsoon Trough. This monsoon trough represents the area of ascending air, maximum cloud formation, and heaviest rainfall in India.^[18]^[4]^[6]

The ITCZ positioning results from the convergence of southeast trade winds from the southern hemisphere (deflected by Coriolis force to become southwest winds) and air flowing from the subtropical high-pressure zone. The northward shift of the ITCZ to this latitude over the Indo-Gangetic Plain creates an exceptionally active zone of convection during July when monsoon intensity peaks. The monsoon front, marking the boundary between southwest monsoons and northeast trade winds, receives concentrated rainfall due to the convergence of winds with different temperatures and moisture content.^[4]^[18]

Walker Circulation and Hadley Cell Dynamics

The Indian monsoon performance is influenced by the relative intensity of two major atmospheric circulation cells: the Hadley Cell and the Walker Cell. Good monsoons are associated with more intense Hadley Circulations and relatively weak Walker Cells, while poor monsoons occur when the Walker Cell strengthens and the Hadley Cell weakens.^[17]

The Hadley Cell for the Asian Summer Monsoon involves ascending air above the Tibetan Plateau heat source spreading gradually southward to a descending limb near the Mascarene High, with southwesterly return currents at the surface forming the southwest monsoon. An additional east-west Walker Cell influences summer monsoon performance, with its ascending branch located over semi-arid regions of northwest India, Pakistan, and the Middle East.^[17]

Role of El-Niño on Indian Monsoon

El-Niño represents a major oceanic-atmospheric phenomenon in the Pacific Ocean that significantly impacts Indian monsoon rainfall through teleconnections—long-distance atmospheric connections linking climate patterns across ocean basins.

Mechanism of El-Niño Impact

El-Niño conditions involve warming of the eastern Pacific Ocean, which weakens the trade winds responsible for transporting moisture toward the Indian subcontinent. The reduction in moisture transport and altered atmospheric circulation associated with El-Niño result in deficient rainfall across various parts of India. The relationship is inverse: El-Niño years typically coincide with below-average Indian monsoon rainfall and drought conditions.^[19]^[20]

El-Niño disrupts the global Walker Circulation, the east-west atmospheric circulation pattern that normally strengthens trade winds blowing from the Indian Ocean toward India. When El-Niño develops, this circulation weakens significantly, reducing the supply of moisture-laden air to the Indian subcontinent.^[19]

Impacts on Monsoon Characteristics

The specific impacts of El-Niño on Indian monsoons include:

Weakened Monsoon Winds and Delayed Onset: During El-Niño, the trade winds carrying moisture across the Indian Ocean weaken substantially. The normal monsoon onset around June may be delayed, resulting in a late start to the rainy season and reduced overall rainfall duration.^[19]

Deficient Rainfall and Regional Variations: El-Niño events typically lead to below-average rainfall during the monsoon season, though this effect is not uniform spatially. Central and northern India are particularly susceptible to experiencing below-average rainfall during El-Niño years, while some southern Indian regions may receive near-normal or even above-normal rainfall due to local circulation patterns.^[19]

Agricultural and Economic Consequences: El-Niño-induced monsoon deficiency directly impacts India's agrarian economy by reducing summer crop production, particularly affecting rice, sugarcane, cotton, and oilseeds. These agricultural impacts cascade through the economy, contributing to high inflation and reduced gross domestic product growth, as agriculture comprises approximately 14% of India's economy.^[20]

Temperature Extremes: The reduced cloud cover and rainfall associated with El-Niño monsoons can lead to increased temperatures during the monsoon season. This creates heatwave conditions in some regions, posing health risks and impacting human well-being.^[19]

Historical Examples of El-Niño Impact

The 1997-1998 El-Niño represents one of the strongest on record, with complex impacts on Indian monsoons. While the 1997 El-Niño was record-breaking, India experienced normal monsoon rainfall—a response that required explanation beyond simple El-Niño forcing. This apparent anomaly led to the discovery of the Indian Ocean Dipole's moderating role.^[21]^[20]

The 1982-1983 El-Niño caused severe droughts across Australia, Indonesia, India, and southern Africa. However, in 1983, a positive Indian Ocean Dipole simultaneously developed, which prevented the expected drought in India despite the strong El-Niño.^[22]^[20]

In contrast, the 2002 El-Niño—a relatively moderate event—resulted in one of the worst droughts in India, demonstrating that El-Niño intensity does not always correlate directly with monsoon impact severity. This variability highlights the importance of understanding modulating factors like the Indian Ocean Dipole.^[20]

Role of Indian Ocean Dipole (IOD) on Indian Monsoon

The Indian Ocean Dipole (IOD) represents a seesaw ocean-atmosphere system in the Indian Ocean analogous to El-Niño in the Pacific, discovered in 1999. The IOD exhibits distinct positive and negative phases with contrasting impacts on Indian monsoon rainfall.^[20]

IOD Mechanism and Definition

The IOD is characterized by continuous changes in sea-surface temperature (SST) between the western Indian Ocean (off the African coast) and the eastern Indian Ocean (around Indonesia). The dipole phenomenon involves independent and cumulative effects of the eastern pole (around Indonesia) and the western pole (off the African coast) on monsoon rainfall in the Indian subcontinent.^[20]^[19]

Positive IOD Phase Impacts

During a positive IOD phase, specific atmospheric and oceanic changes enhance Indian monsoon rainfall:

Enhanced Rainfall and Convergence Patterns: A positive IOD enhances rainfall along the African coastline and over the Indian subcontinent, particularly over central India. This enhancement occurs through anomalous convergence patterns strengthened over the Bay of Bengal, which intensify the monsoon trough over central India.^[23]^[24]^[19]

Modulation of Atmospheric Circulation: Positive IOD events induce anomalous warm conditions in the western Indian Ocean box, creating zones of anomalous convergence. These circulation modifications alter moisture transport patterns, with positive tropospheric moisture anomalies together with anomalous easterly transports over the eastern Indian Ocean, providing additional moisture supply to the Indian subcontinent.^[24]^[25]

Spatial Rainfall Asymmetry: Positive IOD creates a distinct meridional tripolar rainfall anomaly pattern with above-normal rainfall in central India and below-normal rainfall to the north and south of central regions. This pattern differs from the simple El-Niño response, reflecting distinct teleconnection pathways.^[24]

Negative IOD Phase Impacts

During a negative IOD phase, opposite conditions prevail:

Suppressed Rainfall: A negative IOD suppresses rainfall over affected Indian regions, as high temperatures and rainfall patterns reverse from positive IOD conditions.^[26]^[19]

Zonal Dipolar Pattern: Unlike the meridional tripole of positive IOD, negative IOD produces a zonally-oriented dipolar pattern with positive rainfall anomalies in central and western parts and negative anomalies in eastern regions. This reflects different convergence-divergence patterns, with convergence on the western side of India during negative IOD events.^[24]

Moisture Transport Alterations: Negative IOD causes negative tropospheric moisture anomalies together with westerly transport anomalies over the eastern Indian Ocean, reducing moisture availability for Indian monsoon systems.^[24]

Historical Examples of IOD Impact

The 1997 Monsoon: The most dramatic example of IOD's importance occurred in 1997, when India experienced near-normal monsoon rainfall (2% above normal) despite the record-breaking 1997-1998 El-Niño. This unexpected outcome was explained by the simultaneous positive IOD phase. The positive IOD's anomalous convergence over the Bay of Bengal neutralized the ENSO-induced anomalous subsidence, allowing normal moisture influx to Indian regions despite the powerful El-Niño.^[21]^[24]

The 1983 Monsoon: Similar to 1997, the 1983 El-Niño coincided with a positive IOD that facilitated normal or excess rainfall over India despite unfavorable ENSO conditions.^[22]

The 1994 Monsoon: Positive IOD in 1994 again produced good rainfall despite simultaneous El-Niño conditions, further confirming the IOD's buffering capacity against El-Niño drought impacts.^[22]^[20]

The 1992 Monsoon: In contrast, the combined effects of negative IOD and El-Niño in 1992 cooperatively produced deficient rainfall, demonstrating that when both factors align negatively, monsoon suppression is particularly severe.^[22]

The 2019 Monsoon: A striking recent example demonstrates IOD's growing importance in modern climate. In 2019, June experienced 30% rainfall deficiency due to developing El-Niño effects, but a strong positive IOD that developed during late monsoon was so powerful that it compensated for the deficit rainfall, resulting in normal seasonal totals. Although the developing El-Niño eventually fizzled out later in the year, the IOD's positive phase enabled substantial rainfall recovery.^[23]

The 2015 Monsoon: The 2015 monsoon season experienced complex IOD-monsoon interactions, with positive IOD events contributing to rainfall patterns across the Indian subcontinent.^[24]

Interplay Between El-Niño and IOD

The Indian monsoon's response to climate variability depends critically on the combined effects of both El-Niño (ENSO) and IOD, rather than either phenomenon acting independently.^[20]^[22]

Constructive and Destructive Interference

When positive IOD coincides with El-Niño, the two phenomena produce constructive interference in the atmosphere, with positive IOD's moisture enhancement counteracting El-Niño's moisture suppression. The result is near-normal to above-normal monsoon rainfall despite El-Niño's typical drought-inducing effects.^[22]^[20]^[24]

Conversely, when negative IOD coincides with El-Niño, destructive interference amplifies monsoon suppression, producing particularly severe droughts. The 1992 drought exemplifies this combined negative effect.^[22]

Evolving ENSO-Monsoon Relationship

Recent decades have witnessed a weakening of the traditional ENSO-monsoon relationship. Although ENSO was historically effective in explaining Indian monsoon variability and past droughts, this relationship has attenuated in recent decades. The 1997 El-Niño dramatically highlighted this shift: while strong El-Niños typically precede droughts, the 1997 El-Niño failed to produce the expected severe drought in India. This anomaly prompted meteorological research leading to IOD's discovery and recognition of its role in modulating ENSO's monsoon impacts.^[20]

Predictive Significance

The understanding of IOD as a moderating factor has enhanced monsoon rainfall forecasting accuracy. For instance, in 2023, while El-Niño was firmly established in the Pacific, the India Meteorological Department anticipated approximately 80% probability for positive IOD conditions during June-August, suggesting potential for monsoon rainfall close to normal despite El-Niño's presence. This integrated forecasting approach reflects modern understanding of multiple climate drivers' interplay.^[23]

Conclusion

The Indian monsoon represents a complex atmospheric phenomenon shaped by multiple interacting mechanisms—thermal dynamics, planetary wind shifts, jet stream positioning, Tibetan Plateau heating, and ITCZ migration. Modern monsoon science recognizes that rainfall variability results not from single factors but from the combined influences of global climate oscillations, particularly the interplay between El-Niño and the Indian Ocean Dipole. Understanding these relationships has become essential for agricultural planning, water resource management, and economic forecasting across South Asia.