El Niño, La Niña and the Indian Monsoon: A Climate Story from the Pacific Ocean to India
Introduction
Every few years, changes in ocean temperatures thousands of kilometers away in the tropical Pacific Ocean capture global attention. Terms such as El Niño, La Niña, and even Super El Niño begin appearing in weather and climate forecasts and news headlines. These phenomena are part of the El Niño–Southern Oscillation (ENSO), one of the most important drivers of year-to-year climate variability on Earth. But what do these terms mean, and how can changes in the distant Pacific Ocean influence the Indian monsoon? The answers lie in a fascinating scientific story that connects the Pacific Ocean, the Indian monsoon, and the pioneering work of Sir Gilbert Walker, who sought to unlock the secrets of monsoon prediction while serving in India more than a century ago.
What are El Niño and La Niña?
El Niño and La Niña are opposite phases of the El Niño–Southern Oscillation (ENSO), a naturally occurring ocean-atmosphere phenomenon in the tropical Pacific Ocean.
El Niño
El Niño occurs when sea surface temperatures in the central and eastern equatorial Pacific Ocean become warmer than normal for an extended period. The term El Niño originated from fishermen along the coasts of Peru and Ecuador, who observed unusually warm ocean waters appearing around Christmas. They named the phenomenon El Niño, meaning “the Christ Child” in Spanish.
La Niña
La Niña is the opposite phase of ENSO and is characterized by cooler-than-normal sea surface temperatures in the central and eastern equatorial Pacific Ocean. The term La Niña, meaning “the little girl” in Spanish, was adopted as the counterpart to El Niño.
Together, El Niño and La Niña represent the warm and cool phases of ENSO, a recurring climate cycle that influences rainfall, temperature, droughts, floods, tropical cyclones, and many other aspects of weather and climate around the world. Figure 1 summarizes the key characteristics of El Niño and La Niña and their typical association with Indian monsoon rainfall.
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| Figure 1. Schematic illustration of El Niño (top) and La Niña (bottom), showing typical sea surface temperature anomalies in the tropical Pacific Ocean and their associated influence on Indian monsoon rainfall. |
Although El Niño was initially identified as an oceanic phenomenon, scientists later discovered that it is closely linked to large-scale fluctuations in atmospheric pressure across the tropical Pacific Ocean. These atmospheric fluctuations are known as the Southern Oscillation. Together, the oceanic and atmospheric components form the El Niño–Southern Oscillation (ENSO), a coupled ocean–atmosphere phenomenon that influences weather and climate around the world.
In simple terms:
- El Niño = Warm phase of ENSO
- La Niña = Cool phase of ENSO
- El Niño and La Niña = Oceanic manifestations of ENSO
- Southern Oscillation = Atmospheric component of ENSO
- ENSO = Coupled ocean–atmosphere system
Today, ENSO is recognized as the dominant mode of year-to-year climate variability on Earth, influencing weather and climate patterns across many regions of the world.
ENSO is not merely a phenomenon of scientific interest. For India, it is one of the most important sources of year-to-year climate variability. Since agriculture, water resources, hydropower generation, and food security are closely linked to monsoon rainfall, understanding ENSO is essential for improving seasonal forecasting and enhancing preparedness for droughts, floods, and other climate-related risks.
ENSO and the Indian Connection
The story of ENSO has a remarkable connection with India. In the early twentieth century, following a series of devastating droughts and famines during the nineteenth century, the British administration sought a scientific method to predict the Indian monsoon. To address this challenge, Sir Gilbert Walker, who served as the Director-General of Observatories in India from 1904 to 1924—a position that later evolved into the present-day Director-General of Meteorology and head of the India Meteorological Department (IMD)—undertook an ambitious effort to identify the factors responsible for year-to-year variations in the monsoon.
At that time, the mechanisms driving climate variability were largely unknown. Walker analyzed vast amounts of meteorological data from stations around the world and searched for statistical relationships between India's monsoon rainfall and weather conditions elsewhere. Through this pioneering work, he identified a large-scale atmospheric seesaw in surface pressure across the tropical Pacific Ocean. One of its most prominent signatures was the pressure difference between Darwin, Australia, in the western Pacific and Tahiti in the eastern Pacific. Interestingly, both stations are located south of the Equator in the Southern Hemisphere. He termed this phenomenon the Southern Oscillation and demonstrated that variations in this pressure pattern were linked to fluctuations in Indian monsoon rainfall. When the Southern Oscillation was strong, the monsoon generally tended to be stronger, whereas a weaker oscillation was often associated with drought conditions over India.
Although Walker was unaware of the ocean temperature variations that we now associate with El Niño and La Niña, his discovery laid the foundation for modern ENSO science and seasonal climate prediction. More than a century later, ENSO remains one of the most important predictors used in operational seasonal forecasting of the Indian summer monsoon. Today, the difference in sea-level pressure between Tahiti and Darwin forms the basis of the Southern Oscillation Index (SOI), one of the key indicators used to monitor ENSO conditions.
This remarkable scientific journey highlights how India's quest to understand and predict the monsoon contributed to one of the most important discoveries in modern climate science.
How are El Niño and La Niña Measured and Monitored?
Scientists monitor El Niño and La Niña using sea surface temperature anomalies in several predefined regions of the equatorial Pacific Ocean, known as the Niño regions. Among these, the Niño-3.4 region is the most widely used for operational monitoring and classification of ENSO events. The geographical locations of these Niño regions are shown in Figure 2. An anomaly represents the departure from the long-term average temperature. When sea surface temperatures remain sufficiently warmer than average for several consecutive months, El Niño conditions develop. When temperatures remain sufficiently cooler than average, La Niña conditions develop. The Oceanic Niño Index (ONI) is the most widely used indicator for monitoring ENSO conditions.
Operationally, NOAA and many meteorological agencies declare El Niño or La Niña conditions when the three-month running mean sea surface temperature anomaly in the Niño-3.4 region exceeds ±0.5°C for several consecutive overlapping seasons.
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| Figure 2: The geographical extent of the Niño regions in the equatorial Pacific Ocean. The Niño-3.4 region (5°N–5°S, 170°W–120°W) is most widely used for operational monitoring and classification of El Niño and La Niña events through the Oceanic Niño Index (ONI). [Source: NOAA] |
El Niño and La Niña events can vary considerably in their intensity. To quantify their strength, scientists use the Oceanic Niño Index (ONI), which is based on sea surface temperature anomalies in the Niño-3.4 region of the equatorial Pacific Ocean. Based on the magnitude of these anomalies, ENSO events are classified into weak, moderate, strong, and very strong categories. Table 1 summarizes the commonly used classification of El Niño and La Niña events.
| Category | El Niño (ONI) | La Niña (ONI) |
|---|---|---|
| Weak | +0.5 to +0.9°C | -0.5 to -0.9°C |
| Moderate | +1.0 to +1.4°C | -1.0 to -1.4°C |
| Strong | +1.5 to +1.9°C | -1.5 to -1.9°C |
| Very Strong | ≥ +2.0°C | ≤ -2.0°C |
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| Figure 3. Historical Niño-3.4 sea surface temperature anomalies. Positive anomalies (red) represent El Niño conditions and negative anomalies (blue) represent La Niña conditions. The exceptionally strong El Niño events of 1982–83, 1997–98, and 2015–16, often referred to as Super El Niño events, are highlighted. |
What is a "Super El Niño"?
Although there is no universally accepted scientific definition of a Super El Niño, the term is widely used in the media and scientific literature to describe exceptionally strong El Niño events in which Niño-3.4 sea surface temperature anomalies exceed about +2.0°C. Historically, the most notable Super El Niño events occurred during:
- 1982–83
- 1997–98
- 2015–16
These events produced major climate impacts across many parts of the world. However, it is important to remember that a Super El Niño does not automatically imply catastrophic impacts everywhere. The actual consequences depend on how ENSO interacts with other climate drivers such as the Indian Ocean Dipole (IOD), Madden–Julian Oscillation (MJO), regional ocean conditions, and local weather systems. For India, a Super El Niño increases the risk of below-normal monsoon rainfall, but it does not guarantee drought conditions.
While exceptionally strong El Niño events are often referred to as "Super El Niño" events, an equivalent term is not commonly used for La Niña. Instead, very intense cool events are generally described as strong or very strong La Niña episodes.
It is also worth noting that not all El Niño events are identical. While conventional El Niño events are characterized by maximum warming in the eastern equatorial Pacific Ocean, some events exhibit stronger warming in the central Pacific and are referred to as Central Pacific El Niño or El Niño Modoki. The location of the warming influences atmospheric circulation patterns and can result in different impacts on the Indian monsoon.
El Niño, La Niña, and the Indian Monsoon
Every year, millions of people across India eagerly await the arrival of the southwest monsoon. The monsoon provides nearly 75% of the country's annual rainfall and remains the lifeline for agriculture, water resources, energy production, and ecosystems. Yet, one question often arises whenever seasonal forecasts are discussed: How do El Niño and La Niña influence the Indian monsoon?
News headlines frequently associate El Niño with drought and La Niña with floods. While there is some truth in these statements, the reality is more nuanced. El Niño and La Niña do not directly control India's rainfall; rather, they alter the large-scale atmospheric circulation that influences the strength of the monsoon system.
The key connection between ENSO and the Indian monsoon lies in the Walker Circulation, an east-west circulation along the equatorial Pacific.
Under normal conditions:
- Warm waters in the western Pacific and the Maritime Continent support vigorous convection and rainfall.
- Air rises over these warm regions and descends over the cooler eastern Pacific.
- Surface winds complete the circulation by blowing from east to west.
Changes in Pacific Ocean temperatures during El Niño and La Niña alter this circulation, which in turn affects the Indian monsoon.
The influence of ENSO on the Indian monsoon is mediated through a series of interconnected atmospheric and oceanic processes involving tropical convection, the Walker circulation, and moisture transport. These mechanisms are summarized schematically in Figure 4.
How El Niño Suppresses the Indian Monsoon
During an El Niño event, sea surface temperatures become warmer than normal over the central and eastern Pacific Ocean. The warmer waters enhance convection and rainfall in the central Pacific. As a result, the rising branch of the Walker Circulation shifts eastward away from the western Pacific and the Indian Ocean region (Figure 4, Top panel). This shift has several consequences for India:
i. Suppressed Convection over India
As convection intensifies over the central Pacific, compensating downward motion develops over parts of the Indian region. Descending air inhibits cloud formation and suppresses rainfall-producing systems.
ii. Weaker Monsoon Circulation
The large-scale circulation changes weaken the monsoon flow toward the Indian subcontinent. The cross-equatorial winds that transport moisture from the southern Indian Ocean become less vigorous.
The strength of the Somali Jet and the southwesterly monsoon winds often decreases during El Niño years. Consequently, less moisture reaches India from the Arabian Sea and the Bay of Bengal.
Many El Niño years experience a reduction in the frequency and intensity of low-pressure systems and monsoon depressions originating over the Bay of Bengal. Since these systems contribute significantly to seasonal rainfall, their reduction can adversely affect monsoon performance.
The combined effect of these processes often results in below-normal rainfall over India.
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| Figure 4. Schematic representation of the atmospheric and oceanic mechanisms linking ENSO and the Indian summer monsoon. El Niño (Top panel) generally weakens monsoon circulation and favors below-normal rainfall over India, whereas La Niña (Bottom panel) strengthens monsoon circulation and favors above-normal rainfall. |
How La Niña Enhances the Indian Monsoon
La Niña represents the opposite phase of ENSO. During La Niña, the central and eastern Pacific Ocean becomes cooler than normal. Convection remains concentrated over the western Pacific and Maritime Continent, strengthening the Walker Circulation (Figure 4, Bottom panel). This generally favors the Indian monsoon through several mechanisms:
The strongest tropical convection remains closer to the Indian Ocean region. This promotes large-scale rising motion over the broader Indo-Pacific sector.
The strengthened Walker Circulation often enhances the monsoon circulation over South Asia. Cross-equatorial flow and southwesterly winds become stronger.
iii. Increased Moisture Supply
Stronger monsoon winds transport greater amounts of moisture from the Arabian Sea, Bay of Bengal, and equatorial Indian Ocean toward the Indian subcontinent.
La Niña years frequently witness more low-pressure systems and monsoon depressions, leading to enhanced rainfall and more active monsoon spells.
As a result, India often experiences above-normal seasonal rainfall during La Niña years.
Another important mechanism involves the temperature difference between the Asian landmass and surrounding oceans. The summer monsoon is fundamentally driven by the thermal contrast between the warm land and relatively cooler oceans. El Niño tends to warm the tropical atmosphere globally, reducing this thermal contrast and weakening the monsoon circulation. La Niña generally supports a stronger thermal gradient, helping sustain a more vigorous monsoon circulation.
Why El Niño Does Not Always Mean Drought
Although ENSO is one of the strongest drivers of monsoon variability, it is not the only factor. Several other climate drivers influence the Indian monsoon, including:
- Indian Ocean Dipole (IOD)
- Madden–Julian Oscillation (MJO)
- Eurasian snow cover
- North Atlantic variability
- Regional ocean-atmosphere interactions
- Climate change-induced warming
These factors can either amplify or offset the effects of ENSO. For example, India received near-normal seasonal rainfall during the strong El Niño of 1997, largely because of a strong positive Indian Ocean Dipole. Similarly, despite the strong El Niño conditions in 2023, the seasonal rainfall over India was close to normal due to the influence of other favorable climate drivers.
One of the most important messages for climate communication is that El Niño and La Niña influence the probability of wet or dry monsoon conditions; they do not determine the outcome with certainty. During El Niño years, the odds of below-normal rainfall increase. During La Niña years, the odds of above-normal rainfall increase. However, the final outcome depends on the interaction of multiple climate drivers operating across different spatial and temporal scales.
Conclusion
Advances in climate science and seasonal prediction systems have significantly improved our ability to monitor ENSO and assess its potential impacts months in advance. Today, meteorological agencies combine information from ENSO, conditions over the Indian Ocean, atmospheric circulation patterns, and state-of-the-art climate models to provide seasonal outlooks and early warnings. Understanding how El Niño and La Niña influence the monsoon helps policymakers, farmers, water managers, disaster management agencies, and the public make better-informed decisions. As climate services continue to evolve, translating complex climate phenomena into actionable information remains essential for building resilience to weather and climate variability.
More than a century after Sir Gilbert Walker's pioneering efforts to understand and predict the Indian monsoon, ENSO remains one of the most important sources of year-to-year climate variability affecting India. In simple terms, El Niño tends to tilt the odds toward a weaker monsoon, while La Niña tends to favor a stronger monsoon. However, the Indian monsoon is shaped by a complex interplay of multiple climate drivers, and no single factor alone determines the final seasonal outcome. Understanding ENSO therefore provides a valuable window into the year-to-year variability of the Indian monsoon and highlights how conditions in the distant Pacific Ocean can influence the lives and livelihoods of millions across India.
Disclaimer: The views and interpretations presented in this blog are solely those of the author and are intended for educational and science communication purposes. They do not necessarily represent the official views, policies, forecasts, or positions of the India Meteorological Department (IMD) or any other organization with which the author is affiliated.
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