The strong El Niño currently underway in the Pacific Ocean raises the spectre of marine heatwaves and coral bleaching once again threatening Australia’s iconic Great Barrier Reef.
And now new research has revealed the complex interplay of atmospheric patterns and weather systems thousands of kilometres apart that can combine in a recipe potentially fatal for coral reefs.
Sea-surface temperatures in the Coral Sea off northern Queensland often increase during periods known as the ‘doldrums’, characterised by weak winds, reduced cloud cover and dry, descending air. These conditions are unusual for the humid tropics, where we would typically expect plenty of cloud cover, thunderstorms and rainfall.
Dr Lara Richards conducted the research as a PhD student with the ARC Centre of Excellence for 21st Century Weather at Monash University, before joining the University of Reading as a Research Fellow.
“We wanted to understand why these doldrums form and what makes some last longer,” Dr Richards said.
“When doldrums hang around, the ocean remains in a heating phase for longer, which can produce rapid increases in temperature and contribute to coral bleaching.”
The doldrums can happen at any time, and are not caused by the El Niño-Southern Oscillation (ENSO). Nevertheless, an active El Niño raises the level of background threat, and a prolonged spell of doldrums during a strong El Niño phase could contribute to the development of a marine heatwave and increase the risk of harmful mass coral bleaching.
Normally, the easterly trade winds naturally cool the waters around the Reef, stirring the upper ocean and helping heat escape its surface. But when doldrum conditions persist, this natural cooling process is suppressed.
The relationship between weak winds, reduced cloud cover, ocean heating, and coral bleaching is well understood, but scientists have struggled to fully explain why doldrums themselves occur, including why some pass quickly and others persist.
New work led by 21st Century Weather researchers based at Monash University and the University of Melbourne has tackled that challenge to produce our most comprehensive understanding to date of the weather conditions that can produce sustained doldrums and increase the risk of a mass bleaching event.
Crucially, contributing author Dr Yi Huang of 21st Century Weather and the University of Melbourne said the team focused on interactions between weather systems over the tropics and the mid-latitudes.
“A lot of previous research has focused on what happens locally over the Great Barrier Reef. This study looks beyond the local weather to understand how much larger weather systems interact to create those conditions,” Dr Huang said.
Typically, strong trade winds blow from east to west in the Southern Hemisphere, coming from the Pacific Ocean and moving across the Coral Sea and the Great Barrier Reef.
However, these trade winds can be disrupted by a sequence of large-scale waves of atmospheric pressure, known by scientists as ‘Rossby waves’. Low-pressure systems associated with the Rossby waves produce unusual westerly winds over the Reef, counteracting the normal prevailing easterly winds.
What happens next can determine how long the doldrums last.
If the low-pressure system moves quickly eastward into the Pacific, the trade winds can return and the doldrums tend to end. If the system stalls over eastern Australia, the unusual westerly winds can remain in place and the doldrums can persist.
The stalling is associated with repeated Rossby-wave breaking over Australia: a large-scale distortion and overturning of the upper-level jet stream. This breaking weakens the normal west-to-east atmospheric flow and strengthens a blocking high near New Zealand.
Together, the low-pressure system over eastern Australia and the blocking high near New Zealand can help to hold the wider weather pattern in place, prolonging the interruption to cooling trade winds.
According to Dr Richards, it’s not just the Great Barrier Reef that suffers when these atmospheric conditions and weather patterns persist. In early 2022, a series of stalled low-pressure systems led to persistent heavy rain and flooding over northern New South Wales and southern Queensland, while a mass bleaching event struck the Great Barrier Reef at the same time.
“The Lismore floods are a striking example of how the same weather system can produce very different conditions across Australia. While the low-pressure system draws moist air south towards the east coast, its circulation can also steer dry air north across the continent and towards the Great Barrier Reef,” Dr Richards said.
Rather than the active El Niño underway in 2026, the Lismore floods and mass bleaching event in 2022 occurred during a La Niña phase, demonstrating the major role of mid-latitude weather systems and atmospheric patterns in threatening the Great Barrier Reef, independently of ENSO.
In addition to the weather and atmosphere over the mid-latitudes, conditions in the tropics, including equatorial waves, can also align for long-lasting doldrums and bleaching conditions to develop.
The Madden-Julian Oscillation, a large pulse of cloud, rainfall and winds that travels eastward through the tropics, and westward-moving Equatorial Rossby waves, can both influence winds, cloud cover and rainfall over the Reef.
During longer doldrum events, the suppressed (dry) phases of these two patterns were more likely to align over the Great Barrier Reef. This reinforced the unusual westerly winds, further counteracting the normal easterly trade winds and helping the calm conditions to persist.
The formation of the dry descending air over the Reef, which helps inhibit cloud cover, was traced to originate in the mid-latitudes, where it is guided over the continent's dry interior by the stalling low-pressure system before descending over the Great Barrier Reef.
By combining expertise in tropical weather, mid-latitude dynamics, Rossby waves and air-parcel trajectories, the researchers have produced the most comprehensive explanation yet of the atmospheric and weather processes that cause doldrums to form over the Great Barrier Reef and determine how long they persist.
Looking ahead to future research directions, Dr Huang said: “The next question is whether we can predict these events using the computer models and tools we already have.
"Ultimately, understanding coral-bleaching events will require high-resolution coupled climate models that allow the atmosphere and ocean to interact directly.”
The research has been published in Weather and Climate Dynamics and is available here.
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