Researchers have looked at the climate patterns that occur alongside coral bleaching in the Great Barrier Reef.

Bleaching in the Great Barrier Reef (GBR) happens when ocean temperatures exceed a certain threshold for long periods. This issue is closely linked to climate patterns like the El Niño Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). 

A recent study reveals how these phenomena together impact coral bleaching in the GBR.

The GBR is known for its biodiversity, cultural importance, and economic value, contributing around $6.4 billion annually to the Australian economy. However, rising ocean temperatures due to climate change threaten its survival.

The new study highlights how the MJO and ENSO influence GBR weather, contributing to coral bleaching risks.

It was conducted by researchers from the Institute for Marine and Antarctic Studies and the ARC Centre of Excellence for Climate Extremes, led by UNSW Sydney. 

Corals are highly sensitive to weather changes. 

Sunny, calm weather increases ocean temperatures, raising the risk of bleaching, while stormy, rainy weather can cool the ocean, protecting corals. 

Calm, sunny days heat the ocean's surface, while storms mix cooler, deeper waters with warmer surface waters.

The study found that ENSO, involving temperature changes in the central and eastern tropical Pacific Ocean, affects GBR weather seasonally. 

Meanwhile, the MJO, a tropical weather fluctuation on weekly to monthly timescales, can change these patterns in the short term, leading to unexpected impacts on ocean temperatures and corals.

“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 Catherine Gregory, the study's lead author and a PhD candidate at the Institute for Marine and Antarctic Studies, said. 

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

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

ENSO phases like El Niño and La Niña impact weather and ocean temperatures, with El Niño often causing higher ocean temperatures and bleaching risk, while La Niña brings cooler conditions. 

However, the MJO can disrupt these patterns, causing unexpected weather variations affecting ocean temperatures and coral health.

“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.”

Gregory said there is a strong need to consider these climate interactions.

“While ENSO provides insight into the expected synoptic states, it lacks details of anticipated sub-seasonal weather variability at local scales,” she said. 

The findings suggest forecasting models should include both ENSO and MJO impacts to predict and manage coral bleaching events better. 

The full study is accessible here.