Southern Oscillation

Also called: SO, Southern Oscillation Index, SOI · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

The Southern Oscillation is a see-saw of surface air pressure across the tropical Pacific: when pressure rises over the eastern Pacific, it falls over the Indian Ocean–Australia region, and the other way round. It is the air part of ENSO (El Nino–Southern Oscillation), and El Nino is the ocean part [3]. It matters because the size and sign of this pressure swing are closely tied to the strength of the Indian monsoon. That makes it a key tool in IMD's long-range monsoon forecasts.

Explanation

How the see-saw works

• Normal (neutral) year
• The trade winds (steady easterly winds blowing from the subtropical high-pressure belts towards the equator) push warm surface water west, towards Indonesia and northern Australia.
• Warm water there heats the air. The air rises and brings heavy rain, which creates low pressure at Darwin's end.
• Over the cool east Pacific off Peru, air sinks. This creates high pressure at Tahiti's end.

• This east–west loop of air is the Walker circulation (air rises in the west, flows east high up, sinks in the east and returns west near the surface as the trade winds).

• When the see-saw tips (El Nino)

• The trade winds weaken, and warm water slides east towards South America.
• The zone of rising air and rain moves from Indonesia towards the central and eastern Pacific.

• Pressure falls at Tahiti and rises at Darwin.

• When it tips the other way (La Nina)

• The trade winds get stronger, and even more warm water piles up in the west.
• The Walker circulation becomes stronger. Pressure is high at Tahiti and low at Darwin.

Measuring it: the Southern Oscillation Index (SOI)

• SOI = Tahiti pressure minus Darwin pressure.
• Tahiti (French Polynesia, central-south Pacific) stands for the east.

• Darwin (northern Australia) stands for the west.

• What the readings mean

• Strongly negative SOI means El Nino: the Walker circulation is weak or has moved east.
• Near zero means a neutral year.

• Positive SOI means La Nina: the Walker circulation is strong.

• The SOI measures the air. NOAA declares events from the ocean: the Oceanic Nino Index (ONI), which uses sea-surface temperature anomalies (how far the temperature is from normal) in the Nino 3.4 region (5°N–5°S, 120°–170°W). An anomaly of ≥ +0.5 °C means El Nino conditions and ≤ –0.5 °C means La Nina conditions [2].

Discovery and the ENSO link

• Sir Gilbert Walker, Director-General of the India Meteorological Department (IMD), found the Southern Oscillation in the 1920s. He was looking for ways to forecast the Indian monsoon.
• In 1969, Jacob Bjerknes showed that El Nino (ocean warming) and the Southern Oscillation (pressure swing) are two sides of one event, together called ENSO. He also named the Walker circulation after Walker.

Feature	El Nino	Neutral	La Nina
Tahiti pressure	Falls	Normal	High
Darwin pressure	Rises	Normal	Low
SOI (Tahiti – Darwin)	Strongly negative	Near zero	Positive
Trade winds	Weak	Normal	Strong
Walker circulation	Weak or moved east	Normal	Strong
Usual Indian monsoon	Weaker; drought risk	Normal	Good

Where it occurs

• Equatorial Pacific Ocean: the widest stretch of ocean along the equator. It runs from Indonesia and Australia in the west to Peru and Ecuador in the east.
• Measuring stations: Tahiti (French Polynesia) in the east and Darwin (northern Australia) in the west.
• Western end (Indonesia, northern Australia): in a normal year the Western Pacific Warm Pool brings rising air and heavy rain here. A negative SOI brings sinking air, drought and bushfires.
• Eastern end (Peru, Ecuador): in a normal year this is a dry desert coast with the cold Humboldt (Peru) current. A negative SOI brings heavy rain and floods and the collapse of the anchovy fishery.
• India: a negative SOI often weakens the south-west monsoon. In 2026 the El Nino was "firmly established" by mid-August [5]. IMD cut its monsoon forecast from 92% of LPA in April [6] to 90% in May [7]. By 30 September it reported strong El Nino conditions with a neutral IOD [8].
• China: floods during strong El Nino years.

Don't confuse with

• El Nino: the ocean part of ENSO (warming of sea-surface temperature in the east Pacific). The Southern Oscillation is the air-pressure part [3].
• Walker circulation: the actual east–west loop of air along the equatorial Pacific. The Southern Oscillation is the swing in pressure at the two ends of that loop. The SOI measures how strong the loop is.
• Oceanic Nino Index (ONI): NOAA's index based on sea-surface temperature in Nino 3.4 [2]. The SOI is based on surface air pressure at Tahiti and Darwin.
• Indian Ocean Dipole (IOD): a temperature difference between the western and eastern Indian Ocean. It is separate from the Pacific pressure see-saw. A positive IOD can offset El Nino, as it did in 1997.

Prelims Hooks

• SOI = Tahiti pressure – Darwin pressure. A common trap is to reverse the two stations.
• SOI strongly negative means El Nino; SOI positive means La Nina.
• The Southern Oscillation was found by Sir Gilbert Walker of IMD in the 1920s, while he was looking for monsoon predictors.
• Bjerknes (1969) joined El Nino and the Southern Oscillation into ENSO and named the Walker circulation.
• Station–location pairs: Tahiti is in French Polynesia (east); Darwin is in northern Australia (west).
• A negative SOI does not always mean drought in India: in 1997–98 the monsoon was normal despite a strong El Nino, because of a positive IOD.

Mains Points

• How a Pacific pressure swing reaches India (GS-I)
• A falling SOI means weak trade winds and a Walker circulation that weakens or moves east.
• Rising air moves away from the Indonesia–Indian Ocean side, and sinking air gets stronger over South Asia.

• Less moisture reaches the south-west monsoon, so the risk of drought goes up. But the IOD and El Nino Modoki (warmest water in the central Pacific) can soften or worsen the result.

• Forecasting and disaster preparedness (GS-III)

• Watching the SOI together with Nino 3.4 and the ONI gives warnings months in advance. This feeds IMD's long-range forecasts (MME and MMCFS models) [8].

• These warnings support contingency crop plans, drought declaration rules and heat action plans. In 2026 WMO warned of heat stress and faster droughts and urged preparedness [5].

• Changing ENSO behaviour

• ENSO switched quickly from a weak La Nina in early 2026 [4] to an El Nino expected to become very strong by late 2026 [5]. This came after a triple-dip La Nina (2020–23).
• This makes it risky to forecast the monsoon from the SOI alone. The Pacific, the Indian Ocean (IOD) and central-Pacific patterns need to be watched together.

Related concepts

• Walker circulation
• El Nino
• ENSO
• Nino 3.4 region
• El Nino Modoki

Read more

• The circulation and the oceans: Walker circulation, El Nino and ENSO