El Nino, La Nina , ENSO - Geography for UPSC CSE PDF Download

Table of Contents
1. El Nino
2. El Nino-Southern Oscillation (ENSO)
3. Indian Ocean Dipole (IOD)
4. El Nino Modoki
5. La Nina
6. Practical Use of Indices and Forecasts
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El Nino and La Nina are opposite phases known as the El Nino-Southern Oscillation (ENSO) cycle. The ENSO is a recurring climatic pattern involving temperature changes in the eastern and central tropical Pacific Ocean's surface waters. It alters the patterns of upper- and lower-level winds, sea-level pressure, and tropical rainfall across the Pacific Basin.

El Nino

The name El Nino (Spanish for "the Christ Child") was coined by fishermen along the coasts of Ecuador and Peru to describe the warming of central and eastern tropical Pacific waters that often occurs around Christmas. El Nino events are irregular but typically recur every 2-7 years, with an average interval of about 3-4 years. Their duration ranges from a few months to over a year in strong events.

1. El Nino refers to the development of unusually warm ocean surface waters in the central and eastern equatorial Pacific, particularly affecting the coastlines of Ecuador and Peru.
2. Warming suppresses the normal upwelling of cold, nutrient-rich deep water along the South American coast, reducing marine productivity.
3. El Nino episodes often begin during the northern hemisphere winter, around Christmas time and can persist from weeks to many months.
4. Notable prolonged warm events occurred in the early 1990s (beginning 1991 and persisting into 1995) and the very strong event from autumn 1997 to spring 1998.

Normal Pacific Conditions (Non-ENSO)

• In a normal year, a surface low pressure develops over northern Australia and Indonesia and a high-pressure system exists over the eastern Pacific near the coast of Peru.
• Strong east-to-west trade winds push warm surface waters westward, piling warm water in the western Pacific and producing convective storms (thunderstorms) over Indonesia and coastal Australia.
• To replace the westward-moving warm surface water, cold, nutrient-rich deep water upwells along the coast of Peru, supporting rich fisheries.

Walker Circulation (During Normal Years)

• The Walker circulation is a zonal (east-west) atmospheric circulation cell along the equatorial Pacific driven by the pressure contrast between a high over the eastern Pacific and a low over the western Pacific.
• The Walker cell produces rising motion and convection over the warm western Pacific and subsidence over the cooler eastern Pacific.
• The thermocline-the depth separating warmer surface water from colder deep water-is typically shallow in the eastern Pacific and deeper in the west, supporting upwelling in the east.
• Upwelling off Peru and Ecuador is indirectly maintained by the Walker circulation and is critical to marine ecosystems and fisheries.

During an El Nino Year

• Air pressure falls over the central and eastern equatorial Pacific and rises over the western Pacific; this large-scale change in the pressure pattern is the Southern Oscillation.
• The Walker cell weakens or can become displaced or even reversed; trade winds are reduced, and easterly flow weakens.
• Reduced trade winds allow the equatorial counter-current to carry and concentrate warm surface water toward the eastern Pacific and the coasts of Peru and Ecuador.
• This accumulation of warm water causes the thermocline to deepen in the eastern Pacific, cutting off upwelling of cold, nutrient-rich water along South America.
• Climatically, El Nino often produces drought in the western Pacific (Australia, Indonesia) and heavy rainfall and flood risk in the equatorial coast of South America and the central Pacific; it can also modify tropical cyclone activity in the Pacific and Atlantic basins.

Major Effects of El Nino

• Severe negative impact on marine life and fisheries off the coast of Peru and Ecuador because of suppressed upwelling and reduced nutrient supply.
• Lower fish catches in South American coastal waters compared with normal years.
• Severe droughts commonly occur in Australia, Indonesia, India, and parts of southern Africa.
• Unusually heavy rains and floods may affect California, Ecuador, and parts of the Gulf of Mexico.

How El Nino Impacts the Indian Monsoon

• El Nino and the Indian summer monsoon are generally inversely related: strong El Nino events often reduce monsoon rainfall over India.
• Historical observations indicate that six of the most prominent droughts in India since 1871 have coincided with El Nino events (examples include drought years like 2002 and 2009).
• However, not every El Nino causes a drought in India; for example, the strong 1997-98 El Nino did not produce a drought in India because the effect was modulated by other factors such as the Indian Ocean Dipole (IOD).
• Even a moderate El Nino (e.g., 2002) can coincide with a severe drought in India depending on other climate interactions.
• El Nino adversely affects India's agrarian economy by reducing production of summer crops such as rice, sugarcane, cotton, and oilseeds, with knock-on effects on inflation and GDP growth; agriculture contributes around 14 per cent of India's GDP.

El Nino-Southern Oscillation (ENSO)

• The formation of El Nino (oceanic warming) is closely linked with a Pacific atmospheric pressure change called the Southern Oscillation (a see-saw of pressure between eastern and western Pacific).
• In oceanography and climatology, the Southern Oscillation is a coherent inter-annual fluctuation of atmospheric pressure across the tropical Indo-Pacific region.
• Because El Nino (ocean) and the Southern Oscillation (atmosphere) usually occur together, the combined coupled phenomenon is called ENSO.

Only El Nino = [Warm water in the eastern Pacific + relatively cooler water in the western Pacific].

Only SO = [Low pressure over the eastern Pacific + high pressure over the western Pacific].

ENSO = [Warm water in the eastern Pacific + low pressure in the eastern Pacific] + [Cooler water in the western Pacific + high pressure in the western Pacific].

Southern Oscillation Index (SOI) and the Indian Monsoon

• The Southern Oscillation is a see-saw of pressure changes between the eastern and western tropical Pacific.
• When pressure is high over the equatorial eastern Pacific, it tends to be low over the equatorial western Pacific, and vice versa.
• The pressure pattern produces the Walker circulation, with rising motion over the low-pressure zone and sinking motion over the high-pressure zone.
• When the rising limb of the Walker circulation is located over the western Pacific it is favourable for strong monsoon rainfall in India; when it shifts eastward (as in many El Nino years) it tends to reduce Indian monsoon rainfall.
• ENSO and Southern Oscillation are closely linked, so they are considered jointly as an ENSO event.
• The periodicity of the Southern Oscillation varies and is typically between 2 and 5 years.
• The Southern Oscillation Index (SOI) measures the intensity of the Southern Oscillation as the normalized difference in sea-level pressure between Tahiti (French Polynesia) and Port Darwin (northern Australia).
• Positive and negative SOI values (Tahiti pressure minus Darwin pressure) correlate with different ENSO states and can be used in statistical forecasting of monsoon performance.

Indian Ocean Dipole (IOD)

The Indian Ocean Dipole (IOD) is an inter-annual climate mode of the Indian Ocean analogous to ENSO in the Pacific. It was identified in research during the late 1990s (formally described around 1999). The IOD represents a zonal gradient in sea surface temperature between the western equatorial Indian Ocean (near the Arabian Sea) and the eastern equatorial Indian Ocean (south of Indonesia).

• The IOD develops typically from April-May, and its anomalies usually peak around September-October.
• A positive IOD means the western Indian Ocean (Arabian Sea region) is warmer than the eastern Indian Ocean (near Indonesia), with winds tending to blow from east to west across the equatorial Indian Ocean.
• A negative IOD has the opposite pattern, making the eastern Indian Ocean warmer and rainier.
• Positive IOD events can counteract the negative effect of El Nino on the Indian monsoon; positive IODs were important in years such as 1983, 1994, and 1997, when monsoon outcomes differed from expected ENSO-only signals.
• The two poles of the IOD-the western (Arabian Sea) and the eastern (south of Indonesia)-can influence Indian monsoon rainfall both independently and jointly.

Impact of IOD on Cyclogenesis in the Northern Indian Ocean

• Positive IOD (Arabian Sea warmer relative to the Bay of Bengal) is associated with increased cyclone formation over the Arabian Sea.
• Negative IOD tends to enhance cyclogenesis over the Bay of Bengal while suppressing activity over the Arabian Sea.

El Nino Modoki

• El Nino Modoki (Japanese for "similar but different") is a variant of El Nino characterised by anomalous warm SSTs concentrated in the central equatorial Pacific with cooler anomalies in both the eastern and western Pacific.
• Conventional El Nino features strong warming in the eastern equatorial Pacific, whereas El Nino Modoki shows a central Pacific (near the dateline) maximum of warming.
• El Nino Modoki produces different atmospheric responses-particularly in the Walker circulation-often leading to a two-cell anomalous Walker circulation with a wetter central Pacific region and distinct teleconnection patterns compared with conventional El Nino.

Impacts of El Nino Modoki

• Because the warm anomalies are concentrated in the central Pacific, the resulting atmospheric heating and convection shift accordingly, producing a distinct pattern of remote climatic impacts (teleconnections).
• El Nino Modoki can influence rainfall patterns in regions differently from conventional El Nino-for example, certain parts of India, Australia, and the Americas may respond oppositely or with different intensity compared with a canonical El Nino.

La Nina

• La Nina is the cool phase of ENSO, occurring when trade winds strengthen and colder-than-normal sea surface temperatures develop in the central and eastern equatorial Pacific.
• La Nina is often preceded by or follows an El Nino, but it can also occur independently.
• A strong La Nina occurred in 1988, which is associated with drought conditions in parts of North America and, in some subsequent years, very active Atlantic hurricane seasons (e.g., the late 1990s).
• Powerful La Nina events have been associated with intense tropical storms and exceptional weather extremes in various basins (for example, Hurricane Mitch in the late 1990s was among the strongest October hurricanes observed in one century of records).

Effects of La Nina

• La Nina is characterised by lower-than-normal air pressure over the western Pacific, which favours enhanced convection and increased rainfall in that region.
• Regions that typically experience above-normal rainfall during La Nina include India (especially northwest India), Bangladesh, Southeast Asia, eastern Australia, southeastern Africa, and northern Brazil.
• La Nina tends to produce cooler winters in western Canada and the northwestern United States and winter drought conditions in the southern United States.
• Strong La Nina events are often associated with catastrophic floods in northern Australia.
• La Nina enhances upwelling along the west coast of tropical South America, benefiting fisheries by bringing nutrient-rich cold waters to the surface.
• Regions that can become drier than normal during La Nina include the west coast of tropical South America, the Gulf Coast of the United States, and parts of southern South America (the pampas).

Practical Use of Indices and Forecasts

• Operational agencies use indices such as the SOI, Pacific and Indian Ocean SST anomalies, and the IOD index to develop seasonal outlooks for rainfall, drought, and cyclone risk.
• Ensemble climate forecasts from coupled ocean-atmosphere models (e.g., seasonal forecast centres) provide probabilistic guidance; combining model output with statistical relationships improves region-specific predictions.
• For policy and planning, ENSO signals are best used as part of integrated risk-management frameworks that include socio-economic vulnerability, adaptive measures, and contingency planning.

Key Terms (for quick reference)

• El Nino - warm phase of ENSO; anomalous warming of central/eastern equatorial Pacific SSTs.
• La Nina - cold phase of ENSO; anomalous cooling of central/eastern equatorial Pacific SSTs.
• ENSO - coupled ocean-atmosphere phenomenon linking El Nino and the Southern Oscillation.
• Southern Oscillation Index (SOI) - pressure-based index (Tahiti minus Port Darwin) used to diagnose Southern Oscillation intensity.
• Walker circulation - east-west atmospheric circulation cell across the tropical Pacific.
• Thermocline - layer separating warm surface water and cold deep water; its depth controls upwelling.
• Indian Ocean Dipole (IOD) - zonal SST gradient in the equatorial Indian Ocean influencing monsoon and regional climate.
• El Nino Modoki - central-Pacific warming variant of El Nino with different teleconnections.