Elaborate Notes
Walker Circulation
The Walker Circulation is a large-scale atmospheric circulation cell in the tropical Pacific, conceptualized as the zonal (east-west) counterpart to the meridional (north-south) Hadley Cells. It is the fundamental driver of weather patterns in the tropical Pacific under normal, or non-El Niño, conditions.
- Discovery and Naming: The circulation is named after Sir Gilbert Walker, who, as the Director-General of Observatories in India in the early 20th century, sought to understand the variability of the Indian monsoon. Through his research published in the 1920s and 1930s, he identified a seesaw-like pattern of atmospheric pressure between the eastern and western Pacific, which he termed the “Southern Oscillation.” The atmospheric circulation driven by this pressure difference was later named the Walker Circulation.
- Mechanism under Normal Conditions:
- Upwelling in the Eastern Pacific: Along the western coast of South America (specifically Peru and Ecuador), persistent southeasterly trade winds blow parallel to the coast. These winds, influenced by the Coriolis effect, drive surface water offshore (Ekman Transport). This allows cold, nutrient-rich water from the deep ocean to rise to the surface in a process called upwelling. This is further amplified by the cold, northward-flowing Humboldt (or Peru) Current.
- High Pressure in the East: The cold surface water cools the air above it. This cool, dense air sinks (subsidence), creating a stable atmospheric column with suppressed convection and minimal cloud formation. This results in a persistent zone of high atmospheric pressure. The coastal areas, like the Atacama Desert in Chile and Peru, are consequently arid.
- Westward Flow and Warming: The surface water, pushed westward by the trade winds, forms the South Equatorial Current. As this water traverses the vast expanse of the tropical Pacific, it is intensely heated by the sun, creating a deep layer of warm water in the Western Pacific, known as the “Western Pacific Warm Pool” (near Indonesia and Australia).
- Low Pressure in the West: Sea surface temperatures (SSTs) in the Western Pacific Warm Pool can exceed 28-29°C. The warm, moist air above this pool becomes unstable, rises vigorously through convection, and forms deep cumulonimbus clouds. This process releases vast amounts of latent heat, leading to heavy precipitation and creating a permanent region of low atmospheric pressure.
- Upper-Air Return Flow: The rising air reaches the tropopause, diverges, and flows eastward in the upper troposphere. Upon reaching the Eastern Pacific, this now cool and dry air sinks, completing the circulation loop.
- Summary of Normal Conditions: The Walker Circulation results in a distinct pattern: high pressure, cool, dry conditions, and upwelling in the Eastern Pacific (South American coast), and low pressure, warm, wet conditions with heavy rainfall in the Western Pacific (Indonesia, Australia).
El Niño
El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO) phenomenon. It represents a significant departure from the normal Walker Circulation, characterized by the anomalous warming of the central and eastern tropical Pacific Ocean.
- Historical Context and Etymology: The term “El Niño,” meaning ‘The Little Boy’ or ‘Christ Child’ in Spanish, was coined by Peruvian fishermen in the 19th century. They observed that a warm ocean current would occasionally appear off their coast around Christmas, leading to a drastic decline in their fish catch.
- Scientific Understanding: The link between the oceanic warming (El Niño) and the atmospheric pressure changes (Southern Oscillation) was definitively established by Norwegian-American meteorologist Jacob Bjerknes in a landmark paper in 1969. He proposed that they were two facets of the same coupled ocean-atmosphere phenomenon, which he termed ENSO.
- Mechanism:
- Weakening of Trade Winds: The cycle, occurring every 3-7 years, is initiated by a significant weakening of the southeasterly trade winds. The precise trigger for this weakening is still a subject of active research, but it disrupts the normal atmospheric and oceanic flow.
- Suppression of Upwelling: As the trade winds weaken, their ability to push surface water westward diminishes. This drastically reduces or even stops the upwelling of cold, deep water along the South American coast.
- Eastward Propagation of Warm Water: The large volume of warm surface water accumulated in the Western Pacific Warm Pool begins to flow eastward, in what is known as an oceanic Kelvin wave. This warm water spreads across the central and eastern Pacific.
- Reversal of the Walker Circulation: The warming of the Eastern Pacific heats the air above it, causing it to become unstable and rise. This shifts the primary zone of convection and rainfall from the Western Pacific to the central and eastern Pacific. Consequently, the atmospheric pressure drops over the east and rises in the west, reversing the normal pressure gradient. This atmospheric see-saw is the Southern Oscillation.
- Global Impacts of El Niño:
- Oceanic: The warm layer suppresses upwelling, devastating the marine food web. The lack of nutrients inhibits phytoplankton growth, impacting fish stocks (especially anchovy off the coast of Peru), marine mammals, and seabirds. This had catastrophic impacts on the Peruvian fishing and guano (fertilizer) industries, notably during the severe 1972-73 event. The decomposition of dead marine life can release hydrogen sulfide, causing a phenomenon known as “Aguaje” or “red tide,” which makes the water toxic. Mass coral bleaching events are strongly correlated with El Niño years due to thermal stress on the corals.
- Climatic:
- Americas: Causes heavy rainfall and flooding in the normally arid coastal deserts of Peru and Chile. It can also lead to wetter conditions in the southern United States and warmer, drier conditions in the Pacific Northwest and Canada.
- Asia-Australia: Leads to severe drought, suppressed rainfall, and increased risk of forest and bushfires in Indonesia, Australia, and parts of India. The 1997-98 El Niño, one of the strongest on record, was linked to massive, uncontrolled fires in Indonesia that blanketed Southeast Asia in haze. The Indian Summer Monsoon is often weakened or delayed, leading to rainfall deficits.
- Global Weather: The disruption of the Pacific Walker Cell has teleconnections (long-distance impacts) that alter jet streams and storm tracks globally, affecting weather in Africa, Europe, and beyond.
- Tropical Cyclones: It tends to suppress hurricane activity in the Atlantic basin due to increased wind shear, but can increase typhoon activity in the central and eastern Pacific.
- Notable Events: The strongest recorded El Niño events occurred in 1982-83, 1997-98, and 2015-16, each causing billions of dollars in damage and significant ecological disruption worldwide.
La Niña
La Niña (‘The Little Girl’) is the cold phase of ENSO and is considered an intensification or enhancement of the normal Walker Circulation. It is often, but not always, preceded by an El Niño event.
- Mechanism: It is characterized by stronger-than-average trade winds blowing across the Pacific.
- Enhanced Upwelling: The strengthened trade winds push more warm surface water to the west, leading to more intense upwelling of cold water in the Eastern Pacific.
- SST Anomalies: This results in sea surface temperatures in the central and eastern Pacific becoming significantly colder than normal. Concurrently, the Western Pacific Warm Pool becomes even warmer and larger.
- Strengthened Walker Circulation: The temperature and pressure difference between the east and west Pacific is amplified. This leads to extremely strong convection and very heavy rainfall over the Western Pacific (Indonesia, Australia) and more pronounced drought conditions along the South American coast.
- Impacts:
- Australia and Southeast Asia: Often associated with increased rainfall, leading to widespread flooding.
- India: Generally correlates with a stronger-than-average Indian Summer Monsoon, sometimes leading to floods.
- Americas: Tends to bring drought conditions to the southwestern United States and heavy rainfall to the Pacific Northwest. Promotes a more active Atlantic hurricane season due to reduced wind shear.
- Triple-Dip La Niña: A rare phenomenon where La Niña conditions persist for three consecutive years. A recent example occurred from 2020 to early 2023, having prolonged impacts on global weather patterns.
El Niño Modoki
El Niño Modoki is a distinct pattern of Pacific Ocean warming that differs from the canonical El Niño. The term is Japanese for “similar, but different.”
- Discovery: This pattern was identified and described by researchers like Toshio Yamagata in the early 2000s, who noticed that not all warming events in the Pacific followed the classic El Niño pattern.
- Mechanism: Instead of the warmest sea surface temperature anomalies being located in the Eastern Pacific, in El Niño Modoki they are concentrated in the central tropical Pacific. This central warming is flanked by cooler-than-average SSTs in both the eastern and western Pacific.
- Atmospheric Response: This unique SST pattern creates a different atmospheric response. Instead of the entire Walker cell shifting or reversing, it splits into two smaller circulation cells. Air rises over the warm central Pacific and sinks over the cooler eastern and western Pacific.
- Impacts: The teleconnections from El Niño Modoki differ from those of a conventional El Niño.
- Tropical Cyclones: It is associated with an increase in the frequency of hurricanes in the Atlantic basin, contrasting with the suppression seen during a typical El Niño. In the Indian Ocean, it tends to shift cyclogenesis towards the Arabian Sea rather than the Bay of Bengal.
- Precipitation: It causes drought in the western Pacific and eastern Pacific, while bringing heavy rainfall to the central Pacific. Its impact on the Indian Monsoon is more complex and less consistent than that of a canonical El Niño.
Madden-Julian Oscillation (MJO)
The MJO is the largest element of the intraseasonal (30-90 day) variability in the tropical atmosphere. Unlike the stationary ENSO, the MJO is a traversing, eastward-propagating pulse of cloud and rainfall.
- Discovery: It was discovered in 1971 by American meteorologists Roland Madden and Paul Julian while analyzing zonal wind patterns in the tropical Pacific.
- Structure and Propagation: The MJO consists of two main components that move eastward around the globe, typically at about 5 meters per second:
- Enhanced Convective Phase: A large region of increased convection, cloudiness, and heavy rainfall.
- Suppressed Convective Phase: A region of sinking air, drier conditions, and suppressed rainfall that follows the wet phase.
- Global Influence: As this dipole of weather propagates, it modulates weather patterns worldwide.
- Monsoon: The MJO strongly influences the Indian and Australian monsoons. The active (heavy rain) and break (dry spells) phases of the Indian monsoon are often linked to the passage of the MJO’s convective and suppressed phases over the Indian Ocean.
- Tropical Cyclones: The passage of the MJO’s enhanced convective phase provides favorable conditions (moisture, low vertical wind shear) for the genesis of tropical cyclones in the Pacific, Indian, and Atlantic oceans.
- Interaction with ENSO: The MJO can interact with the ENSO cycle. Strong MJO activity can sometimes trigger the onset of an El Niño event by generating westerly wind bursts in the Pacific. It can also either amplify or diminish the regional impacts of an existing El Niño or La Niña.
Indian Ocean Dipole (IOD)
The IOD, also known as the Indian Niño, is a coupled ocean-atmosphere phenomenon in the Indian Ocean, analogous to the ENSO in the Pacific. It was formally described and named by a team of researchers including N. H. Saji in a 1999 paper.
- Mechanism: The IOD is defined by the difference in sea surface temperature between two poles—a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
- Positive IOD Phase:
- SST Pattern: The western pole (Arabian Sea) becomes warmer than average, while the eastern pole (off Sumatra) becomes cooler than average.
- Atmospheric Response: This temperature gradient alters the overlying atmospheric circulation. Winds along the equator in the Indian Ocean blow from east to west. This causes upwelling of cold water in the east and piling up of warm water in the west. This results in rising air and greater rainfall over the western Indian Ocean and East Africa, and sinking air (subsidence) and drought over Indonesia and Australia.
- Impact on India: A positive IOD is generally beneficial for the Indian Summer Monsoon, often leading to increased rainfall. It can sometimes counteract the negative rainfall impact of an El Niño event, leading to a near-normal monsoon even in an El Niño year. It also leads to a higher frequency of cyclones in the Arabian Sea.
- Negative IOD Phase:
- SST Pattern: The western pole becomes cooler and the eastern pole becomes warmer than average.
- Atmospheric Response: The pattern reverses. Convection and rainfall are enhanced over the eastern Indian Ocean (Indonesia), leading to floods there, while the western Indian Ocean and parts of India experience suppressed rainfall.
- Impact on India: A negative IOD is detrimental to the Indian monsoon and can exacerbate the drought conditions caused by an El Niño.
Prelims Pointers
- Walker Circulation: An east-west atmospheric circulation cell over the tropical Pacific, driven by a pressure gradient.
- Normal Conditions (Walker Cell): High pressure and cold, upwelling water in the Eastern Pacific (Peru coast); Low pressure and warm, rainy conditions in the Western Pacific (Indonesia/Australia).
- El Niño: Anomalous warming of the central and eastern tropical Pacific Ocean.
- Southern Oscillation: The see-saw pattern of atmospheric pressure between the eastern and western Pacific.
- ENSO: The coupled ocean-atmosphere phenomenon combining El Niño (oceanic) and the Southern Oscillation (atmospheric). Discovered by Jacob Bjerknes (1969).
- El Niño Effects:
- Weakening of trade winds.
- Suppression of upwelling off the Peru coast.
- Drought in Western Pacific (Australia, Indonesia).
- Heavy rain and floods in Eastern Pacific (Peru, Chile).
- Negative impact on Indian Monsoon.
- Suppressed hurricane activity in the Atlantic.
- La Niña: Anomalous cooling of the central and eastern tropical Pacific; an intensification of the normal Walker Circulation.
- La Niña Effects:
- Strengthening of trade winds.
- Enhanced upwelling off the Peru coast.
- Floods in Western Pacific (Australia, Indonesia).
- Drought in Eastern Pacific (Peru, Chile).
- Positive impact on Indian Monsoon.
- Enhanced hurricane activity in the Atlantic.
- Triple-Dip La Niña: The occurrence of La Niña conditions for three consecutive years (e.g., 2020-2023).
- El Niño Modoki: A warming pattern concentrated in the central Pacific, with cooling in the east and west.
- El Niño Modoki Effects: Increased hurricane frequency in the Atlantic; increased cyclone frequency in the Arabian Sea compared to the Bay of Bengal.
- Madden-Julian Oscillation (MJO): An eastward-moving pulse of clouds and rainfall in the tropics with a cycle of 30-60 days. It is an intraseasonal phenomenon.
- MJO Effects: Influences active and break phases of monsoons; affects tropical cyclone genesis.
- Indian Ocean Dipole (IOD): Difference in sea-surface temperature between the Western Arabian Sea and the eastern Indian Ocean.
- Positive IOD: Warmer west, cooler east. Good for Indian monsoon. Can counter El Niño’s negative impact.
- Negative IOD: Cooler west, warmer east. Bad for Indian monsoon. Can worsen El Niño’s impact.
- Upwelling: The process where deep, cold, nutrient-rich water rises to the surface. Crucial for marine fisheries off the coast of Peru.
Mains Insights
Interplay of Climatic Phenomena and Impact on Indian Monsoon
The Indian Summer Monsoon is not governed by a single factor but is the result of a complex interplay of multiple global and regional climatic phenomena. Understanding these interactions is crucial for effective forecasting and policy-making.
- ENSO-Monsoon Relationship: The most well-established teleconnection is the inverse relationship between El Niño and the Indian Monsoon. El Niño’s altered Walker Circulation creates subsidence over the Indian subcontinent, weakening the monsoon trough and leading to deficient rainfall. However, this is a correlation, not a certainty; not all El Niño years result in drought in India.
- The IOD as a Modulator: The Indian Ocean Dipole can significantly modulate ENSO’s impact.
- El Niño + Positive IOD: A strong positive IOD can create favorable conditions for the monsoon (warmer Arabian Sea, increased moisture supply) that can partially or fully offset the negative impact of an El Niño. The year 1997 is a classic example where a very strong El Niño was accompanied by a positive IOD, resulting in a near-normal monsoon.
- El Niño + Negative IOD: This combination is often catastrophic, as both phenomena work to suppress monsoon rainfall, leading to severe drought.
- The Role of the MJO: The MJO acts as an intraseasonal control knob.
- Its eastward propagation across the Indian Ocean can trigger the onset of the monsoon.
- The passage of its wet (convective) phase can initiate active monsoon spells, while its dry (suppressed) phase is linked to monsoon breaks. The timing and strength of the MJO can determine the distribution of rainfall within a single monsoon season, even if the seasonal total is influenced by ENSO or IOD.
Socio-Economic and Environmental Consequences
The variability caused by these phenomena has profound implications, particularly for a country like India which is heavily dependent on agriculture and water resources.
- Agricultural Distress: El Niño-induced droughts can lead to widespread crop failure, impacting food security, causing price inflation for essential commodities, and pushing marginal farmers into debt. This links directly to GS Paper III (Indian Economy, Agriculture).
- Water Scarcity: Deficient monsoons lead to reduced reservoir levels, affecting drinking water supply, hydroelectric power generation, and water availability for industries. This has cascading effects on urban planning and energy security.
- Disaster Management: While droughts are a major concern during El Niño, La Niña can bring extreme rainfall and devastating floods, as seen in various parts of India. El Niño Modoki and Positive IOD can alter cyclone tracks, increasing the vulnerability of regions like the west coast of India, which is traditionally less cyclone-prone. This necessitates a dynamic and forecast-based disaster management strategy (GS Paper III).
- Ecological Impact: El Niño events cause global mass coral bleaching, including in India’s reefs in the Andaman & Nicobar Islands and Lakshadweep. Droughts lead to increased forest fire incidents. The disruption of marine ecosystems has a direct impact on the livelihoods of coastal fishing communities (GS Paper I - Society & GS Paper III - Environment).
Climate Change and Future Projections
There is an ongoing scientific debate and research on how anthropogenic climate change is affecting these natural climate variabilities.
- Frequency and Intensity: A key question is whether climate change is making events like El Niño more frequent or more intense. While consensus is yet to be reached, some climate models project an increase in the frequency of extreme El Niño and La Niña events in a warming world.
- Changing Teleconnections: Global warming could alter the established relationships (teleconnections). For instance, the traditional ENSO-monsoon link has shown signs of weakening in recent decades, suggesting that other factors (like IOD or Atlantic warming) may be playing a larger role.
- Policy Imperative: This uncertainty underscores the need for enhanced investment in climate science, improved long-range forecasting models, and building climate resilience. Policies must shift from being merely reactive to proactive, focusing on water conservation, crop diversification, and strengthening early warning systems (GS Paper III/IV - Governance & Ethics in policy making).