Indian Monsoon and Ocean–Atmosphere Interactions: ENSO, IOD, MJO and Climate Change
भारतीय मानसून और महासागर–वायुमंडल अंतःक्रिया: ENSO, IOD, MJO और जलवायु परिवर्तन
1. Introduction / परिचय
India’s monsoon is not an isolated atmospheric phenomenon. It forms part of a vast and interconnected ocean–atmosphere system extending from the tropical Pacific Ocean to the Indian Ocean. Changes in sea-surface temperature, atmospheric pressure, wind circulation, convection and ocean currents occurring thousands of kilometres away can significantly influence the timing, intensity and spatial distribution of rainfall over the Indian subcontinent.
भारत का मानसून कोई अलग-थलग वायुमंडलीय घटना नहीं है। यह उष्णकटिबंधीय प्रशांत महासागर से हिंद महासागर तक फैली एक विशाल और परस्पर जुड़ी हुई महासागर–वायुमंडल प्रणाली (Ocean–Atmosphere System) का हिस्सा है। हजारों किलोमीटर दूर समुद्री सतह के तापमान, वायुमंडलीय दबाव, पवन परिसंचरण, संवहन तथा महासागरीय धाराओं में होने वाले परिवर्तन भारत में मानसूनी वर्षा के समय, तीव्रता और क्षेत्रीय वितरण को प्रभावित कर सकते हैं।
Among the most important climate drivers influencing the Indian monsoon are:
El Niño–Southern Oscillation (ENSO)
Indian Ocean Dipole (IOD)
Madden–Julian Oscillation (MJO)
Equatorial Indian Ocean variability
Arabian Sea and Bay of Bengal conditions
Long-term warming of the Indian Ocean
Anthropogenic climate change
The interaction among these factors explains why the Indian monsoon can behave differently even when apparently similar global climatic conditions prevail.
भारतीय मानसून को प्रभावित करने वाले प्रमुख जलवायु कारकों में ENSO, Indian Ocean Dipole (IOD), Madden–Julian Oscillation (MJO), भूमध्यरेखीय हिंद महासागर की परिवर्तनशीलता, अरब सागर एवं बंगाल की खाड़ी की स्थितियाँ तथा हिंद महासागर का दीर्घकालिक ऊष्मीकरण शामिल हैं।
इन कारकों की पारस्परिक अंतःक्रिया यह समझने में सहायता करती है कि समान दिखाई देने वाली वैश्विक जलवायु परिस्थितियों में भी भारतीय मानसून का व्यवहार अलग-अलग क्यों हो सकता है।
2. Why Ocean–Atmosphere Interaction Matters / महासागर–वायुमंडल अंतःक्रिया क्यों महत्वपूर्ण है?
Oceans cover most of the Earth’s surface and possess enormous heat capacity. They absorb, store and redistribute vast quantities of solar energy. Consequently, changes in ocean temperatures can alter atmospheric circulation, while atmospheric winds can simultaneously modify ocean currents, vertical mixing and upwelling.
महासागर पृथ्वी की अधिकांश सतह को ढकते हैं और उनकी ऊष्मा धारण क्षमता (Heat Capacity) अत्यधिक होती है। वे विशाल मात्रा में सौर ऊर्जा को अवशोषित, संग्रहित और पुनर्वितरित करते हैं।
महासागर और वायुमंडल के बीच यह संबंध एकतरफा नहीं बल्कि द्विदिशीय (two-way interaction) है।
For example:
Changes in sea-surface temperature (SST) influence atmospheric convection.
Atmospheric winds influence ocean currents.
Winds can strengthen or weaken upwelling.
Ocean temperatures affect moisture availability.
Atmospheric pressure gradients modify wind circulation.
These wind changes can further alter ocean temperatures.
इस प्रकार महासागर वायुमंडल को प्रभावित करता है और वायुमंडल पुनः महासागर की स्थिति को बदल सकता है।
This coupled interaction produces major climate phenomena such as ENSO and IOD.
UPSC Concept
ENSO and IOD should not be understood merely as changes in ocean temperature. They are coupled ocean–atmosphere phenomena, involving simultaneous changes in ocean temperatures, winds, pressure and convection.
3. What is ENSO? / ENSO क्या है?
ENSO stands for El Niño–Southern Oscillation.
It is a coupled ocean–atmosphere phenomenon centred primarily over the tropical Pacific Ocean.
ENSO का पूर्ण रूप El Niño–Southern Oscillation है। यह मुख्यतः उष्णकटिबंधीय प्रशांत महासागर में विकसित होने वाली एक संयुक्त महासागर–वायुमंडल जलवायु घटना है।
ENSO has three broad phases:
El Niño
La Niña
ENSO-neutral conditions
The oceanic component of ENSO involves abnormal sea-surface temperature conditions in the tropical Pacific.
The atmospheric component, known as the Southern Oscillation, involves changes in:
Atmospheric pressure
Trade winds
Tropical convection
Walker Circulation
Therefore, ENSO cannot be understood solely by observing Pacific Ocean temperatures.
Prelims Key Point
ENSO = Oceanic variability + Atmospheric variability
El Niño represents primarily the warm phase, while La Niña represents the cool phase of ENSO.
4. Normal Equatorial Pacific Conditions / सामान्य विषुवतीय प्रशांत महासागरीय स्थिति
Understanding normal Pacific conditions is essential before studying El Niño and La Niña.
Under normal conditions, easterly trade winds blow broadly from east to west across the equatorial Pacific.
These winds push warm surface water toward the western Pacific, particularly toward Indonesia and neighbouring regions.
सामान्य परिस्थितियों में भूमध्यरेखीय प्रशांत महासागर पर पूर्वी व्यापारिक पवनें (Easterly Trade Winds) पूर्व से पश्चिम की ओर चलती हैं।
ये पवनें गर्म सतही जल को पश्चिमी प्रशांत महासागर की ओर धकेलती हैं।
Consequently:
Western Pacific waters become relatively warmer.
Sea level becomes relatively higher in the western Pacific.
Warm water encourages strong atmospheric convection.
Heavy rainfall occurs around Indonesia and the western tropical Pacific.
The eastern Pacific remains comparatively cooler.
Cold, nutrient-rich water rises near the western coast of South America through upwelling.
This east–west contrast in ocean temperature is crucial for maintaining normal tropical Pacific atmospheric circulation.
Normal Pacific Pattern
Eastern Pacific → Cooler water → Higher atmospheric pressure
Western Pacific → Warmer water → Lower atmospheric pressure + Strong convection
The resulting pressure difference helps maintain the trade winds and the Walker Circulation.
5. Walker Circulation / वॉकर परिसंचरण
The Walker Circulation is a large-scale east–west atmospheric circulation found over the tropical Pacific.
Under normal conditions:
Warm waters in the western Pacific heat the atmosphere.
Warm and moist air rises over the western Pacific.
Rising air produces clouds, convection and heavy rainfall.
Air moves eastward in the upper atmosphere.
It descends over the relatively cooler eastern Pacific.
Near the surface, air flows westward again as easterly trade winds.
वॉकर परिसंचरण उष्णकटिबंधीय प्रशांत महासागर के ऊपर पाया जाने वाला एक प्रमुख पूर्व–पश्चिम वायुमंडलीय परिसंचरण है।
पश्चिमी प्रशांत महासागर के गर्म जल के ऊपर वायु गर्म होकर ऊपर उठती है। ऊपरी वायुमंडल में यह वायु पूर्व की ओर बढ़ती है तथा अपेक्षाकृत ठंडे पूर्वी प्रशांत महासागर के ऊपर नीचे उतरती है।
इसके बाद सतह के निकट वायु पुनः पूर्व से पश्चिम की ओर व्यापारिक पवनों के रूप में प्रवाहित होती है।
UPSC Importance
Walker Circulation is closely connected with ENSO.
El Niño → Walker Circulation generally weakens or shifts.
La Niña → Walker Circulation generally strengthens.
6. What is El Niño? / एल नीनो क्या है?
El Niño refers to abnormal warming of the central and/or eastern equatorial Pacific Ocean accompanied by significant changes in tropical atmospheric circulation.
एल नीनो भूमध्यरेखीय प्रशांत महासागर के मध्य और/या पूर्वी भाग में समुद्री सतह के तापमान में असामान्य वृद्धि से संबंधित घटना है।
However, El Niño is not merely ocean warming. It is associated with broader atmospheric changes.
During an El Niño event:
Easterly trade winds often weaken.
Warm surface water shifts eastward.
The normal east–west Pacific temperature gradient weakens.
Upwelling near the South American coast may weaken.
Convection shifts eastward.
Walker Circulation becomes weaker or reorganised.
Atmospheric pressure patterns change.
These changes can influence weather conditions far beyond the Pacific Ocean through teleconnections.
Simplified Mechanism
Trade winds weaken → Warm water moves eastward → Eastern Pacific warms → Upwelling weakens → Convection shifts → Walker Circulation changes → Global weather patterns are affected
7. What is La Niña? / ला नीना क्या है?
La Niña is broadly characterised by cooler-than-normal sea-surface temperatures in the central and eastern equatorial Pacific Ocean.
ला नीना के दौरान मध्य और पूर्वी भूमध्यरेखीय प्रशांत महासागर का समुद्री सतही तापमान सामान्य से कम हो जाता है।
It generally strengthens the normal east–west temperature contrast of the tropical Pacific.
During many La Niña events:
Easterly trade winds strengthen.
Warm surface water becomes more concentrated in the western Pacific.
Eastern Pacific waters become relatively cooler.
Upwelling strengthens.
The east–west temperature gradient increases.
Walker Circulation tends to intensify.
La Niña often produces climatic effects broadly opposite to those associated with El Niño.
However, this should not be interpreted as a universal rule because actual impacts vary according to region, season and interactions with other climate systems.
UPSC Trap
El Niño and La Niña are not simply exact opposites in terms of their impacts.
Their effects depend on the location, season, intensity and interaction with other climate drivers.
8. Southern Oscillation / दक्षिणी दोलन
The Southern Oscillation represents large-scale fluctuations in atmospheric pressure across the tropical Pacific.
It demonstrates that El Niño and La Niña are not merely ocean-temperature phenomena.
दक्षिणी दोलन उष्णकटिबंधीय प्रशांत महासागर के विभिन्न भागों के बीच वायुमंडलीय दबाव में होने वाले बड़े पैमाने के उतार-चढ़ाव को दर्शाता है।
Ocean and atmosphere behave as a coupled system.
This combination of:
El Niño/La Niña oceanic conditions + Southern Oscillation atmospheric conditions
is collectively referred to as ENSO.
9. Southern Oscillation Index (SOI) / दक्षिणी दोलन सूचकांक
The Southern Oscillation Index (SOI) is an atmospheric indicator used to monitor ENSO-related conditions.
It is broadly calculated using atmospheric pressure differences between locations in the tropical Pacific, traditionally Tahiti and Darwin.
सामान्य रूप से:
Persistent negative SOI values are associated with El Niño-type atmospheric conditions.
Persistent positive SOI values are associated with La Niña-type atmospheric conditions.
UPSC Trap
A single day's SOI value should not be used to declare an El Niño or La Niña event.
ENSO assessment requires:
Persistence of anomalies
Sea-surface temperature observations
Atmospheric pressure patterns
Trade-wind behaviour
Convection patterns
Therefore, ENSO is assessed using multiple oceanic and atmospheric indicators rather than one isolated measurement.
10. Thermocline / थर्मोक्लाइन
The thermocline is a layer within the ocean where temperature decreases rapidly with increasing depth.
थर्मोक्लाइन महासागर की वह परत है जिसमें गहराई बढ़ने के साथ तापमान तेजी से घटता है।
Under normal equatorial Pacific conditions:
Thermocline is generally deeper in the western Pacific.
Thermocline is relatively shallower in the eastern Pacific.
This produces an east–west tilt.
During El Niño
The east–west thermocline tilt generally weakens.
Warm water spreads eastward, and the thermocline in the eastern Pacific becomes deeper than under normal conditions.
During La Niña
The normal thermocline gradient can become stronger.
Thermocline depth is important because it influences:
Sea-surface temperature
Upwelling
Nutrient availability
Marine productivity
Fisheries
11. Upwelling / अपवेलिंग
Upwelling is the upward movement of deeper, colder and generally nutrient-rich ocean water toward the surface.
अपवेलिंग वह प्रक्रिया है जिसमें महासागर की गहराई से ठंडा और पोषक तत्वों से भरपूर जल सतह की ओर ऊपर आता है।
The eastern tropical Pacific, particularly near the western coast of South America, is an important upwelling region.
Under normal conditions, trade winds help maintain this process.
During El Niño
When trade winds weaken:
Upwelling weakens → Nutrient supply decreases → Marine productivity may decline → Fisheries may suffer
Thus, ENSO is not merely a meteorological phenomenon.
It can have significant:
Ecological impacts
Fisheries impacts
Economic impacts
Food-security implications
12. Teleconnection / टेलीकनेक्शन
A teleconnection is a climate relationship through which atmospheric or oceanic anomalies in one geographical region influence weather or climate in distant regions.
टेलीकनेक्शन का अर्थ ऐसी जलवायवीय अंतःसंबद्धता से है जिसमें एक क्षेत्र में उत्पन्न जलवायु विसंगति वायुमंडलीय या महासागरीय परिसंचरण के माध्यम से दूर स्थित क्षेत्रों के मौसम अथवा जलवायु को प्रभावित करती है।
ENSO's influence on the Indian monsoon is an important example.
Changes occurring in the tropical Pacific can alter atmospheric circulation patterns and subsequently influence rainfall thousands of kilometres away over India.
UPSC Trap
Teleconnection does not mean that two distant regions are physically connected by a single ocean current.
It refers to dynamically and statistically linked climate anomalies operating through the larger climate system.
13. El Niño and the Indian Summer Monsoon / एल नीनो और भारतीय ग्रीष्मकालीन मानसून
Historically, many El Niño years have been associated with relatively weaker Indian summer monsoon rainfall and, in some cases, drought-like conditions.
The broad mechanism involves changes in:
Walker Circulation
Tropical convection
Atmospheric pressure
Moisture transport
Large-scale monsoon circulation
During El Niño, eastward displacement of Pacific convection can alter circulation patterns that would otherwise favour strong monsoon rainfall over the Indian subcontinent.
इस कारण एल नीनो की परिस्थितियाँ कई बार भारतीय मानसून के लिए प्रतिकूल मानी जाती हैं।
However, an extremely important qualification is necessary:
El Niño does not automatically cause drought in India.
ENSO modifies the probability of particular monsoon outcomes; it does not mechanically determine India's rainfall.
This distinction is highly important for both UPSC Prelims and Mains.
14. Why El Niño Does Not Always Cause Drought in India / एल नीनो हमेशा भारत में सूखा क्यों नहीं लाता?
The Indian monsoon is a complex climate system influenced by numerous interacting factors.
These include:
Indian Ocean Dipole (IOD)
Madden–Julian Oscillation (MJO)
Eurasian snow conditions
Land–sea thermal contrast
Monsoon depressions
Arabian Sea conditions
Bay of Bengal convection
Monsoon trough position
Intra-seasonal oscillations
Local and regional circulation
Indian Ocean thermal conditions
Therefore, an El Niño event may create an unfavourable background condition, but other factors can strengthen or weaken its eventual impact on India.
For example, a favourable positive IOD may sometimes partly compensate for the adverse influence associated with El Niño.
Core UPSC Principle
ENSO is a probabilistic climate driver, not a deterministic controller of the Indian monsoon.
This means:
El Niño ≠ Guaranteed drought
and
La Niña ≠ Guaranteed excess rainfall
15. La Niña and the Indian Monsoon / ला नीना और भारतीय मानसून
La Niña is often associated with atmospheric conditions that are relatively favourable for a stronger Indian summer monsoon.
ला नीना की स्थिति में प्रशांत महासागर का सामान्य पूर्व–पश्चिम तापमान अंतर अधिक मजबूत हो सकता है तथा Walker Circulation भी तीव्र हो सकती है। इससे कई परिस्थितियों में भारतीय मानसून के लिए अनुकूल पृष्ठभूमि बन सकती है।
However:
La Niña does not guarantee above-normal rainfall throughout India.
Even during a nationally good monsoon year:
Some districts may experience drought.
Some regions may receive excessive rainfall.
Long dry spells may occur.
Rainfall may arrive at unsuitable stages of crop growth.
Extreme rainfall may be concentrated within only a few days.
Therefore, the quality of the monsoon cannot be judged merely by the all-India seasonal rainfall total.
UPSC Analytical Point
For agriculture and water security:
Rainfall distribution + timing + intensity + dry-spell duration
can be as important as:
Total seasonal rainfall.
This explains why a statistically “normal” monsoon can still produce agricultural distress in some regions and devastating floods in others.
Attempt : UPSC 2027 GS1 Quiz: ENSO, IOD, MJO and Indian Monsoon