The El Niño Southern Oscillation (ENSO) and Australian Climate Variability

The El Niño Southern Oscillation (ENSO) Overview

• Presenter: Associate Professor Jatin Kala (J.Kala@murdoch.edu.au) from Murdoch University.

• Lecture Focus: Climate variability in Australia, specifically the El Niño Southern Oscillation (ENSO).

Impact of Extreme Weather in Australia

Extreme weather events, often influenced by climate variability, have significant impacts, particularly on the agricultural sector in Australia:

• Cattle Welfare and Heatwaves:

  • Severe hot weather forces farmers to find ways to keep cattle cool.

  • Example (Jan 2024): A heatwave over a long weekend in Queensland resulted in the deaths of $320$ cattle in feedlots, as reported by ABC Rural and Queensland Country Life.

• Cattle Welfare and Floods:

  • Paradoxically, after years of drought, floods can also lead to catastrophic losses.

  • Example 1 (Feb 2019): Up to $500,000$ drought-stressed cattle were killed in Queensland floods, creating a humanitarian crisis, as reported by The Guardian.

  • Example 2 (Jan/Feb 2019): Catastrophic Queensland floods killed approximately $600,000$ cattle and devastated native species.

    • The floods were so vast they could only be accurately captured from space, forming a wall of water up to $70$ km wide.

    • Record depths were reached along $500$ km of the Flinders River, submerging $25,000$ square kilometers of country.

    • Cattle were stranded and many drowned.

    • Beyond livestock, wildlife such as kangaroos, marsupial mice, and birds were also severely impacted.

Atmospheric Circulation and Climate Variability Drivers

• Australian climate variability is driven by several atmospheric features:

  • Hadley cell

  • Trade winds (Northeast and Southeast)

  • Doldrums

  • Horse latitudes

  • Westerlies

  • Polar front

  • Polar easterlies

  • Polar high

The Walker Circulation and ENSO Phases

The Walker Circulation is a major atmospheric circulation system over the equatorial Pacific Ocean that significantly influences global weather patterns, particularly in Australia.

Neutral Phase (Normal Conditions)

• Mechanism: The Walker circulation drives trade winds from the east to the west Pacific.

  • Warm, moist air rises and creates rain in the western Pacific (near Australia, $140°E$).

  • Dry air returns eastward, cools, and sinks in the eastern Pacific (near South America, $100°W$).

  • Strong trade winds push warm surface water westward, leading to a deeper thermocline in the west and upwelling of cool, nutrient-rich water in the east.

• Australian Impact: Generally balanced conditions with no particular influence noted in the slide, implying typical seasonal rainfall for Australia.

El Niño Phase

• Mechanism: The Walker Circulation is significantly weakened or even reversed.

  • Trade Winds: Weaker than normal trade winds across the Pacific.

  • Ocean Temperatures: Warm water pools shift eastward in the equatorial Pacific. Sea surface temperatures (SSTs) are warmer than normal in the central and eastern Pacific and cooler than usual in the west Pacific (near Australia).

  • Convection: Reduced convection (rising moist air) in the western Pacific and increased convection in the central/eastern Pacific.

  • Upwelling: Suppressed upwelling of cold water off the coast of South America.

• Australian Impact: Results in dry conditions and a reduced chance of rain for Australia. The $97/98$ event was cited as a strong El Niño.

La Niña Phase

• Mechanism: The Walker Circulation intensifies.

  • Trade Winds: Stronger than normal trade winds, pushing more warm water towards the western Pacific.

  • Ocean Temperatures: SSTs become warmer than normal in the western Pacific (near Australia) and cooler than normal in the central and eastern Pacific.

  • Convection: Increased convection (rising moist air, leading to rain) in the western Pacific.

  • Upwelling: Stronger upwelling of cold water off the coast of South America.

• Australian Impact: Brings warm oceans and increased chance of rain and wet conditions to eastern Australia.

Southern Oscillation Index (SOI)

• Definition: The SOI is a standardized index based on the surface pressure difference between Tahiti (in the central South Pacific) and Darwin (Northeast Australia).

• La Niña and SOI:

  • Stronger Trade Winds.

  • Increased convection (lower pressure) near Darwin.

  • Results in a positive SOI (> +7).

• El Niño and SOI:

  • Weaker Trade Winds.

  • Increased convection (lower pressure) over South America (associated with higher pressure at Darwin).

  • Results in a negative SOI (< -7).

• Historical Data: The Southern Oscillation Index has been recorded monthly since $1876$.

  • Example: The Queensland floods of $2010/2011$ occurred during a strong La Niña event, reflected in a highly positive SOI during that period.

ENSO and Australian Rainfall Patterns

• El Niño and Rainfall (Winter/Spring): Historically associated with significantly lower than average (drier) rainfall across large parts of eastern, central, and northern Australia.

• La Niña and Rainfall (Winter/Spring): Historically associated with significantly higher than average (wetter) rainfall across large parts of eastern, central, and northern Australia.

Current ENSO Status (July/August 2025)

Based on observations for July/August $2025$:

• Sea Surface Temperature (SST) Anomalies:

  • As of July $2, 2025$, average SST anomalies (°C) show a mix, but the overall Pacific equatorial region appears to be in a neutral-like state, with some warmer anomalies in the far western Pacific and cooler in parts of the eastern Pacific, but not definitively El Niño or La Niña.

  • For the week ending July $27, 2025$, waters across much of the Australian region were warmer than average (up to $2$ °C warmer, with patches up to $3$ °C warmer in the southwest and Coral Sea), except for waters to Australia's north, which were near average.

  • SSTs around Australia for June $2025$ were $+0.96$ °C above the $1991-2020$ average, making it the second-warmest June on record since $1900$.

  • From July $2024$ to June $2025$, SSTs have been the warmest or second warmest on record for each respective month globally.

  • Forecasts for the next $3$ months (July-Sept $2025$) indicate SSTs around Australia are likely to be warmer than average to the north, south, and east, with waters near to slightly above average for Western Australia.

• Southern Oscillation Index (SOI):

  • As of August $3, 2025$, the latest $30$-day running average SOI value was $+5.7$.

  • This value falls within the neutral range ($-7$ to $+7$).

• Overall Summary (Issued July 29, 2025): The El Niño-Southern Oscillation and Indian Ocean Dipole remain neutral; however, a negative IOD is possible in late winter to early spring.

Key Takeaways and Implications

• ENSO as a Primary Driver: The El Niño Southern Oscillation (ENSO) is the most important driver of natural climate variability over Australia.

• Three Phases of ENSO:

  • Neutral/Inactive: Southern Oscillation Index (SOI) is between $-7$ and $+7$.

  • El Niño: Sustained negative SOI below $-7$.

  • La Niña: Sustained positive SOI above $+7$.

• Climate Change and ENSO: The influence of climate change on ENSO is an active area of research. While a clear answer is not yet available, some studies suggest that climate change could lead to more extreme or stronger ENSO events.

• Specific Concerns: Very hot conditions, especially in eastern Australia, can be particularly concerning when they occur during a La Niña event, as La Niña typically brings wetter conditions, making combined extremes more impactful.

• Forecasting Importance: Accurate seasonal forecasts of ENSO phases are critical for better planning in areas like cattle management and welfare.

• Limitations of ENSO: ENSO does not explain all climate variability; other factors also play a role.

• Probabilistic Nature of Forecasts: Climate forecasts are probabilistic, meaning they indicate the likelihood of certain conditions rather than deterministic outcomes.

Challenges and Controversies in ENSO Forecasting

• Recent Criticism (Jan 2024): The Bureau of Meteorology (BOM) faced widespread criticism regarding its El Niño forecast for the summer of $2023-2024$.

  • BOM had predicted a hot and dry summer, indicative of an El Niño, but many regions experienced widespread rain and flooding instead.

  • Farmers expressed frustration, having destocked their herds based on BOM's forecast, only to then face wet conditions.

• BOM's Defense: BOM defended its long-range forecasting, explaining that predicting chaotic systems is about probability, not certainty. Even with an El Niño declaration, local weather can deviate, and specific outcomes are not guaranteed. Climate models provide probabilities, not definite outcomes for localized weather events, and this nuance is crucial for understanding forecasts. (As highlighted by Christian Jakob, Director, ARC Centre of Excellence for the Weather of the $21$st Century, Monash University).