Teleconnections: El Nino, the Southern Oscillation, the Indian Ocean Dipole and the MJO

Detail

1. What a teleconnection is

• A teleconnection is a link between the weather in two places that are far apart. A change in one ocean can change the rain thousands of kilometres away.
• The Indian monsoon has four main teleconnections:
• ENSO in the Pacific Ocean (El Nino plus the Southern Oscillation).
• IOD in the Indian Ocean.
• MJO, a pulse of cloud and rain that travels around the tropics.

• Weaker links with the Atlantic Ocean and with snow over Eurasia.

• ENSO and IOD act over months and seasons. They decide whether the whole monsoon is good or bad. The MJO acts over weeks. It decides when spells of heavy rain and breaks come within one season.

2. El Nino: the warm Pacific

Name and timing

• El Nino means "the Christ Child" in Spanish. Warm water appears off the coast of Peru (eastern Pacific, South America) around Christmas, which gives it the name.
• It comes every 3-7 years. It brings droughts, floods and other weather extremes in many parts of the world.

How it works (cause → process → result)

• Normal year:
• Strong trade winds (steady winds that blow from east to west near the equator) push warm surface water west towards Indonesia and Australia.
• Off Peru, cold deep water rises to replace it. This rising of cold water is called upwelling.

• The cold, nutrient-rich water feeds plankton (tiny floating plants and animals) and so supports rich fisheries. The Humboldt (Peru) current is the cold current along this coast.

• El Nino year:

• The trade winds weaken.
• Warm water from the western Pacific spreads east towards South America.
• This warm layer stops the cold upwelling.

• Correction to NCERT: El Nino does not "replace" the Humboldt current. It covers the coast with warm water and shuts off the cold water rising from below. (NCERT: El Nino "replaces the cold Peruvian current temporarily".)

• Results:

• The Peru coast warms by up to 10°C.
• Equatorial circulation is distorted: the belts of rising and sinking air along the equator shift east.
• Evaporation becomes irregular.
• Plankton and fish decline, so fish catches off Peru fall.

Measuring El Nino

• Today, the official sea-temperature index for ENSO is Niño 3.4. It is the average sea surface temperature (SST) anomaly (the difference from normal) over 5°N–5°S, 170°W–120°W in the central-eastern Pacific [4].

Link to India

• El Nino shifts the zone of rising air (and rain) east, away from the Indian Ocean. More air sinks over India, so rain is suppressed.
• India uses El Nino in long-range forecasting. The 1990-91 El Nino delayed monsoon onset by 5-12 days over parts of India.
• El Nino years were often drought years, for example 2002, 2009 and 2015.
• The link does not always hold. 1997 had a very strong El Nino, but India's monsoon rain was normal, because a positive IOD offset it (see section 4).
• La Nina is the opposite phase: a cooler-than-normal central and eastern Pacific with stronger trade winds. It usually favours a good monsoon.

Live example: 2025-26

• La Nina conditions lasted from September 2025 to February 2026. The Pacific was ENSO-neutral in March 2026 [3].
• Niño 3.4 anomalies then rose fast. They crossed the El Nino threshold in May 2026 and reached about +2.08°C by August 2026, which counts as strong El Nino [3].
• In August 2026, SST anomalies in the eastern equatorial Pacific were above +2.0°C. IMD's model forecasts strong to very strong El Nino for the coming season [3].
• On 31 July 2026, IMD forecast below-normal rainfall (less than 94% of the Long Period Average) for August–September 2026 [2].
• LPA (Long Period Average): the average rainfall over a long base period, used as the "normal".
• The LPA for August–September is 422.8 mm, and for August alone it is 254.9 mm (base period 1971–2020) [2].

Feature	El Nino	Neutral	La Nina
Central and eastern Pacific SST	Warmer than normal	Near normal	Cooler than normal
Trade winds	Weak	Normal	Strong
Upwelling off Peru	Suppressed	Normal	Stronger
Rain over Indonesia and Australia	Less	Normal	More
Usual effect on Indian monsoon	Weaker, often drought (2002, 2009, 2015)	Mainly depends on other factors	Stronger, often good

3. The Southern Oscillation and ENSO

• The Southern Oscillation is a see-saw of surface air pressure across the tropical Pacific and Indian Oceans. When pressure rises at one end, it falls at the other.
• The two stations used are:
• Tahiti in French Polynesia, South Pacific (NCERT: about 20°S, 140°W).

• Darwin in northern Australia (12°30'S, 131°E).

• Southern Oscillation Index (SOI): the difference between the pressure at Tahiti and the pressure at Darwin (both standardised). It is the oldest ENSO indicator. Its station records go back to the late 1800s [4].

• Reading the sign:
• El Nino: pressure is below average at Tahiti and above average at Darwin, so the SOI is negative [4][5].
  • High pressure over Darwin and the eastern Indian Ocean means sinking air, so the monsoon tends to be weak.

• La Nina: the pattern reverses, so the SOI is positive [4][5], which tends to favour the monsoon.

• ENSO (El Nino–Southern Oscillation): El Nino is the ocean part (sea temperature). The Southern Oscillation is the atmosphere part (air pressure) [5]. They move together, so they are treated as one system.

• Forecasting: NCERT is dated here.
• NCERT says IMD forecasts the monsoon with 16 indicators (a statistical method).
• Since 2021, IMD has used a Multi-Model Ensemble (MME) system. It combines several coupled global climate models, including IMD's own Monsoon Mission Coupled Forecast System (MMCFS) [2].
  • Coupled model: a computer model in which the ocean and the atmosphere affect each other.
• IMD issues a first-stage forecast in April, an update at the end of May, and monthly outlooks after that. In 2026 these came on 13 April, 29 May, 30 June and 31 July [2].
• IMD Pune, a WMO Regional Climate Centre, publishes a monthly ENSO and IOD Bulletin [3].

4. The Indian Ocean Dipole (IOD)

• Definition: the IOD is an east–west difference in sea surface temperature across the tropical Indian Ocean.
• West pole: the western Arabian Sea and the western equatorial Indian Ocean, near East Africa.

• East pole: the south-eastern Indian Ocean near Sumatra (Indonesia).

• Discovery: it was identified by Saji, Goswami, Vinayachandran and Yamagata in Nature in 1999 [7].

• In that paper the dipole showed cooler-than-normal SST off Sumatra and warmer-than-normal SST in the western Indian Ocean, together with changes in wind and rain [7].

• It arises from ocean–air interaction inside the Indian Ocean and is independent of ENSO [7]. It explains about 12% of Indian Ocean SST variability [7].

• Dipole Mode Index (DMI): the measure of IOD strength. It is the western tropical Indian Ocean SST anomaly minus the south-eastern tropical Indian Ocean SST anomaly. Saji et al. (1999) defined it [6].

Positive IOD (warmer west, cooler east)

• The western Indian Ocean and the Arabian Sea are warm, so more moisture and rising air form over the west.
• Monsoon winds into India get stronger.
• The usual result is more rain over India. A positive IOD can offset El Nino, as in 1997 and 2019.
• Abroad, it brings heavy rain in East Africa and drought in Indonesia [7]. Very high DMI values have gone with extreme September–November rain in tropical East Africa [6].

Negative IOD (cooler west, warmer east)

• Rising air and rain shift towards Sumatra and Australia, so the Indian monsoon is hurt.

Combination	Typical effect on India	Example
El Nino + neutral IOD	Monsoon suppressed; neutral IOD does not reduce El Nino's effect [2]	2026 forecast (Aug–Sep below normal) [2]
El Nino + positive IOD	Monsoon suppression partly or fully offset [2]	1997 (normal rain)
La Nina + positive IOD	Strongly favourable	—
Any ENSO phase + negative IOD	Unfavourable	—

IOD in 2025-26

• Negative IOD from September to December 2025.
• Near-neutral in January 2026, then positive in February–April 2026.
• Leaning negative in May–June 2026, then neutral in July–August 2026 [3].
• Some international centres expected a positive IOD around September 2026 [2][3].

5. The Madden-Julian Oscillation (MJO)

• Definition: the MJO is a moving pulse of cloud, rain, wind and pressure. It travels eastward around the tropics and returns to its starting point every 30–60 days [8].
• It was described by Madden and Julian in 1971-72.

• It is intraseasonal, which means it changes within a single season, unlike ENSO and IOD, which last for months.

• Structure: a wet, rising-air area (enhanced rain) is followed by a dry, sinking-air area (suppressed rain). Both drift east, mainly over the Indian and Pacific Oceans [8].

• Speed: it moves at about 5 m/s [9].
• Maritime Continent "barrier": as the MJO passes over Indonesia and the Philippines (the Maritime Continent: the islands and seas between the Indian and Pacific Oceans), it often weakens or dies out [9].

Effect on India

• Active phase over the Indian Ocean:
• More rising air and convection (warm, moist air rising to form clouds).

• Result: active monsoon spells. It can trigger monsoon onset over Kerala and help form low-pressure systems and cyclones in the Bay of Bengal and Arabian Sea.

• MJO absent or in its dry phase over India:

• Sinking air and few clouds.

• Result: break-in-monsoon spells (dry gaps of days to weeks in the middle of the season).

• Forecast use: the MJO is the main basis for IMD's extended range forecasts. These are 7-day averaged forecasts for the next four weeks, updated every Thursday [2].

Feature	ENSO	IOD	MJO
Ocean or region	Equatorial Pacific	Tropical Indian Ocean	Whole tropics, moving east
Time scale	Every 3-7 years; lasts months	Seasonal; peaks around Sep–Nov	30–60 days
Index	Niño 3.4, SOI	DMI	Phase (location) and amplitude (strength)
Main effect on India	Total season's rain	Total season's rain; can offset ENSO	Timing of onset, active spells and breaks

6. Other links and climate change

• Eurasian and Himalayan snow:
• Heavy winter and spring snow means the land uses more of the sun's heat to melt snow, so it heats up slowly.
• The land–sea temperature contrast is weaker, so the low pressure over north-west India forms late or weak.

• Result: a weaker monsoon. This was one of the earliest predictors used.

• Atlantic link: sea temperatures and pressure patterns in the North Atlantic can change the jet stream (a fast, narrow band of wind high in the atmosphere) over Eurasia. This in turn affects how strong the monsoon is.

• Climate change: studies suggest a warming climate may weaken or change the ENSO–monsoon link. Warming of the Indian Ocean and the land can partly cut the old "El Nino means drought" rule. This makes forecasts based on only one indicator less reliable, which is one reason for the move to coupled models (see the climate-change section).
• The ocean mechanics of currents and upwelling are covered in the ocean-movements section.

Prelims Hooks

• El Nino = "the Christ Child". Warm water off Peru around Christmas. Comes every 3-7 years. Peru coast warms by up to 10°C.
• Trap: El Nino does not reverse or replace the Humboldt current. Weak trade winds suppress cold upwelling off Peru.
• Southern Oscillation stations: Tahiti (South Pacific) and Darwin (12°30'S, 131°E, northern Australia). Negative SOI = El Nino; positive SOI = La Nina [4].
• Niño 3.4 region (5°N–5°S, 170°W–120°W) is the official SST index for ENSO [4].
• IOD was identified in 1999 by Saji et al. Its index is the Dipole Mode Index (western tropical Indian Ocean minus south-eastern tropical Indian Ocean near Sumatra) [6][7].
• Positive IOD = warmer west, cooler east near Sumatra. It aids the Indian monsoon, brings East African rain and Indonesian drought, and offset El Nino in 1997 [7].
• MJO moves eastward, cycle 30–60 days, about 5 m/s. It controls active spells and breaks, not the total for the season [8][9].
• El Nino drought years: 2002, 2009, 2015. 1997 was a strong El Nino with normal rain.
• IMD has used a Multi-Model Ensemble (MME) for long-range forecasts since 2021, replacing the older 16-indicator statistical model [2].
• 2026: El Nino threshold crossed in May 2026; Niño 3.4 about +2.08°C in August 2026 [3].

Mains Points

• Why El Nino does not always mean drought (GS-I):
• ENSO shifts the Walker circulation (the east–west loop of air along the equator), so air sinks over India.
• A positive IOD (1997, 2019), a well-timed MJO, or local warming of the Indian Ocean can offset it.

• So monsoon forecasting needs many indicators. IMD has moved from 16 statistical indicators to coupled multi-model ensembles (MME, since 2021) [2].

• Effects of ENSO-driven monsoon failure (GS-III):

• Less kharif sowing, lower reservoir storage, more groundwater pumping and food inflation.

• Responses include the extended range forecast (four weeks ahead, issued weekly) [2], contingency crop plans, drought declaration under NDMA guidelines, and crop insurance.

• Changing teleconnections under climate change (GS-I/III):

• The ENSO–monsoon link may be weakening or changing, and the Indian Ocean is warming.
• Positive-IOD-type events and extreme rain spells are getting more attention.

• This argues for more ocean observation (INCOIS) and better coupled modelling (Monsoon Mission).

• Global reach:

• The same El Nino or positive IOD that hurts India can bring floods to Peru and East Africa and drought to Indonesia and Australia [7].
• This makes the case for regional early-warning cooperation, such as IMD's role as a WMO Regional Climate Centre [3].