From Western Disturbance to El Niño: The climate terms you’re hearing this summer, explained

As summer intensifies across India, several climate terms — Western Disturbance, El Niño, La Niña, Indian Ocean Dipole, heatwave — have become part of everyd...

What Happened

As summer intensifies across India, several climate terms — Western Disturbance, El Niño, La Niña, Indian Ocean Dipole, heatwave — have become part of everyday weather reporting, yet their mechanisms and UPSC relevance remain underexplored in mainstream coverage.
The Indian Meteorological Department (IMD) has flagged above-normal temperatures and early heat-stress conditions in parts of northern and central India in the 2026 summer season.
Climate events such as ENSO phases and the Indian Ocean Dipole are actively shaping monsoon forecasts, making this an important conceptual cluster for aspirants.

Static Topic Bridges

Western Disturbance

A Western Disturbance (WD) is an extratropical cyclone that originates over the Mediterranean Sea, Caspian Sea, and Black Sea region and travels eastward embedded in the mid-latitude subtropical westerly jet stream. It is called "western" because it approaches India from the west.

WDs are non-tropical low-pressure systems that carry moisture from the Mediterranean, Caspian, and Black Seas.
They travel at speeds of up to 12 m/s along the subtropical westerly jet stream (~200 hPa level).
On reaching the Himalayas, the orographic barrier weakens the system, causing precipitation: rain in the plains of Punjab, Haryana, and Delhi, and heavy snowfall over Jammu & Kashmir, Himachal Pradesh, and Uttarakhand.
They are most frequent and intense between December and March (winter) but can occur year-round.
Recent studies (Copernicus WCD, 2024) show that WDs are increasing in frequency and seasonal length, linked to a strengthening subtropical jet stream.

Connection to this news: Western Disturbances are the primary source of winter and pre-monsoon precipitation in North India. Reduced or untimely WDs contribute directly to spring heatwave conditions, making them a bridge concept between winter climate and summer heat stress.

El Niño and La Niña (ENSO)

The El Niño-Southern Oscillation (ENSO) is a periodic climate pattern driven by anomalous sea-surface temperatures (SSTs) in the equatorial central and eastern Pacific Ocean. It alternates between two phases: El Niño (warm phase) and La Niña (cool phase).

El Niño: Anomalously warm SSTs in the central/eastern Pacific weaken the Walker Circulation, suppressing convection over the Indian subcontinent. This reduces the moisture-laden southwest monsoon winds, causing deficit or drought conditions in India (~60% of El Niño years coincide with below-normal monsoon).
La Niña: Opposite conditions — cooler eastern Pacific, stronger Walker Circulation, enhanced convection over India, typically producing above-normal monsoon rainfall.
The Walker Circulation refers to the large-scale east-west atmospheric circulation over the equatorial Pacific driven by SST gradients.
ENSO cycles recur every 2–7 years; each phase typically lasts 9–12 months.
India's Long Period Average (LPA) for monsoon rainfall is 87 cm (for June–September).

Connection to this news: ENSO is the single most important global driver of Indian monsoon variability. A developing El Niño during April–June can be a reliable predictor of a below-normal monsoon, making it the most tracked seasonal climate signal by IMD.

Indian Ocean Dipole (IOD)

The Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the Indian Ocean, where SST anomalies develop between the western Indian Ocean (Arabian Sea region) and the eastern Indian Ocean (near Sumatra/Indonesia).

Measured by the Dipole Mode Index (DMI): difference between SST anomalies in the western pole (50–70°E, 10°S–10°N) and eastern pole (90–110°E, 10°S–0°N).
Positive IOD: Western Indian Ocean warmer, eastern cooler → enhanced moisture flux toward India → 10–20% above-normal monsoon rainfall.
Negative IOD: Western cooler, eastern warmer → suppressed monsoon.
The IOD can offset ENSO impacts: the landmark 2019 monsoon achieved 110% of LPA despite a moderate El Niño because a strong positive IOD compensated.
IOD events typically develop between June and October.

Connection to this news: The IOD is India's "home field advantage" against El Niño. Understanding how IOD and ENSO interact is essential for interpreting seasonal monsoon forecasts issued by IMD and WMO each April.

IMD Heatwave Definition

A heatwave is a condition of abnormally high temperatures over a region that can be dangerous to human health. IMD has precise criteria distinguishing it from a simple hot day.

IMD declares a heatwave when the maximum temperature of a plains station reaches at least 40°C (30°C for hill stations; 37°C for coastal stations) AND is at least 4.5°C above the normal maximum.
A severe heatwave is declared when the departure from normal is 6.4°C or more.
Alternatively, if the actual maximum temperature is 45°C or above regardless of departure, it qualifies as a heatwave.
Heat dome conditions — blocking high-pressure systems that trap warm air — amplify heatwave duration.
The National Disaster Management Authority (NDMA) classifies heatwaves as a notified natural disaster.

Connection to this news: IMD's technical definition underpins all official heat action plan triggers across states. Understanding the threshold criteria helps decode IMD bulletins and is a direct Prelims fact.

Monsoon Onset Mechanics — ITCZ and Jet Streams

The southwest monsoon's onset is governed by a convergence of atmospheric drivers, with the Intertropical Convergence Zone (ITCZ) playing a central role.

The ITCZ is a belt of low pressure encircling the Earth near the equator where trade winds from both hemispheres converge. It shifts northward in Northern Hemisphere summer following the thermal equator (zone of maximum insolation).
Monsoon onset over Kerala (standard date: June 1) is triggered when the ITCZ crosses approximately 8°N near the Kerala coast, drawing in moisture-laden southwesterly winds from the Arabian Sea.
IMD's three official monsoon onset criteria: (1) rainfall over at least 14 of 60 designated Kerala stations for two consecutive days; (2) outgoing longwave radiation (OLR) below 200 W/m²; (3) lower-tropospheric westerly winds of 15–20 knots at 925 hPa.
The subtropical jet stream (at ~200 hPa) shifts northward to the Tibetan Plateau in early June, removing the upper-level barrier that had previously prevented the monsoon's northward penetration — this is called the "break" of the jet stream.
Somali Jet (Low Level Jet): A powerful low-level wind current off the East African coast that intensifies in June, channelling moisture into the Arabian Sea and amplifying the monsoon.

Connection to this news: The ITCZ-monsoon link is a perennial UPSC Prelims question. Knowing onset criteria, jet stream dynamics, and the role of the Somali Jet provides a complete mechanistic picture of why the monsoon arrives when it does.

Key Facts & Data

Western Disturbances originate over the Mediterranean, Caspian, and Black Seas and travel eastward along the subtropical westerly jet stream.
El Niño suppresses Indian monsoon in approximately 60% of El Niño years; La Niña typically enhances it.
The ENSO cycle recurs every 2–7 years; each phase lasts 9–12 months on average.
India's monsoon Long Period Average (LPA): 87 cm over June–September.
Positive IOD can offset El Niño impacts — demonstrated in 2019 when a strong positive IOD delivered 110% of LPA despite moderate El Niño conditions.
IMD heatwave threshold for plains: maximum temperature ≥ 40°C AND ≥ 4.5°C above normal; severe heatwave at ≥ 6.4°C departure or actual temp ≥ 45°C.
Standard monsoon onset date over Kerala: June 1 (declared officially by IMD based on three criteria).
NIH studies show occupational paraquat exposure increases Parkinson's risk 2.5× (cross-reference with pesticide topic).
IOD is measured by the Dipole Mode Index (DMI) — difference between western (50–70°E) and eastern (90–110°E) Indian Ocean SST anomalies.