Climate patterns influence coral bleaching on the Great Barrier Reef

A recent study has identified significant impacts of the Madden-Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO) on coral bleaching events in the Great Barrier Reef (GBR). 

The Great Barrier Reef, spanning nearly 350,000 square kilometers along Australia’s northeast coast, is the largest coral ecosystem globally. 

The GBR contributes approximately $6.4 billion annually to the Australian economy, but faces severe threats from rising ocean temperatures due to climate change.

Focus of the study

“Local meteorology over the Great Barrier Reef (GBR) can significantly influence ocean temperatures, which in turn impacts coral ecosystems. While El Niño–Southern Oscillation (ENSO) provides insight into the expected synoptic states, it lacks details of anticipated sub-seasonal weather variability at local scales,” wrote the researchers.

“This study explores the influence of the Madden-Julian oscillation (MJO) on Australian tropical climate, both independently and in combination with ENSO, focusing on GBR impacts.”

Bleaching on the Great Barrier Reef 

Corals in the Great Barrier Reef are particularly sensitive to weather conditions. Sunny, calm weather usually raises ocean temperatures, heightening the risk of coral bleaching, while stormy, rainy weather can cool the ocean, offering some protection to corals. 

Calm, sunny days allow more sunlight to penetrate the water, heating the surface, whereas storms and rain increase cloud cover and wind, promoting the mixing of cooler, deeper waters with warmer surface waters.

“Bleaching is likely when tropical corals are exposed to ocean temperatures above a threshold for a prolonged period,” noted the researchers.

Recurring climate patterns on the Great Barrier Reef

The study found that ENSO, a recurring climate pattern involving temperature changes in the central and eastern tropical Pacific Ocean, influences weather patterns over the Great Barrier Reef on a seasonal scale. 

Meanwhile, the MJO, a major fluctuation in tropical weather on weekly to monthly timescales, can alter these patterns on shorter, sub-seasonal timescales. This results in unexpected impacts on ocean temperatures and corals.

Compounding impacts of multiple drivers 

“We find that the MJO can significantly influence the weather variability over the GBR, altering the expected states of El Niño and La Niña periods,” said lead author Catherine Gregory, a doctoral candidate at the Institute for Marine and Antarctic Studies.

“While these findings do not tell us all the possible causes of extreme warming and coral bleaching in the GBR, they emphasize the need to consider drivers beyond ENSO, including the compounding impacts of multiple drivers.”

Anticipating coral bleaching events 

Understanding how ENSO and MJO influence weather patterns can help anticipate coral bleaching events. 

ENSO phases, such as El Niño and La Niña, impact weather and ocean temperatures, with El Niño often leading to higher ocean temperatures and increased coral bleaching risk, while La Niña can bring cooler conditions. 

However, the MJO can disrupt these patterns, leading to unexpected weather variations that also affect ocean temperatures and coral health.

Bleaching on the Great Barrier Reef 

“I’d often heard in the media that during El Niño periods the GBR is more likely to experience bleaching. Then during 2022, the reef experienced mass bleaching during a La Niña period, and it was reported that La Niña should mean cooling in the GBR,” Gregory said.

“However, in my research, I had examined this relationship and not found a strong connection between the ENSO index and ocean temperatures in this region. This motivated me to understand other drivers that could be influential.”

“The MJO, as the leading driver of sub-seasonal weather variability, seemed like an important one to consider. While ENSO provides insight into the expected synoptic states, it lacks details of anticipated sub-seasonal weather variability at local scales.”

These findings underscore the need for comprehensive forecasting models that include both ENSO and MJO impacts to better predict and manage coral bleaching events.

As climate change continues to pose the greatest threat to corals, understanding these climatic drivers and their interactions is crucial. This knowledge could be instrumental in developing adaptive strategies to safeguard the future of the Great Barrier Reef.

The study is published in the journal Geophysical Research Letters.

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