During El Niño, a perfect storm of weather systems could spell disaster for the Great Barrier Reef

During an ongoing El Niño in the Pacific Ocean, there is a heightened danger of marine heat waves and coral bleaching that could threaten Australia's Great Barrier Reef. A recent study has uncovered the intricate web of atmospheric patterns and weather systems spanning thousands of kilometers that can come together to potentially cause catastrophic harm to coral reefs.

Sea-surface temperatures in the Coral Sea, close to northern Queensland, often rise during the "doldrums", a period marked by weak winds, less cloud cover, and dry air. These conditions are uncommon in the typically humid tropics, where one would typically expect abundant cloud cover, thunderstorms, and rainfall. Dr. Lara Richards, who conducted the research as a Ph.D. student at Monash University and later joined the University of Reading as a research fellow, explained that understanding why these doldrums form and why some last longer is crucial.

When doldrums persist, the ocean stays in a heating phase for an extended period, which can lead to rapid temperature increases and contribute to coral bleaching. While doldrums can occur at any time and are not solely caused by the El Niño-Southern Oscillation (ENSO), a strong El Niño raises the overall risk, and a prolonged period of doldrums during a strong El Niño phase could lead to the formation of a marine heat wave and increase the likelihood of harmful mass coral bleaching.

The research team, led by 21st Century Weather researchers from Monash University and the University of Melbourne, focused on the interactions between weather systems in the tropics and the mid-latitudes to explain why sustained doldrums occur and how they heighten the risk of a mass bleaching event. They discovered that the disruption of strong trade winds by Rossby waves, low-pressure systems associated with these waves, can lead to unusual westerly winds over the reef, which counteract the normal easterly winds.

If the low-pressure system moves quickly eastward into the Pacific, the trade winds return, and the doldrums usually end. However, if the system stalls over eastern Australia, the unusual westerly winds can persist, and the doldrums can last longer. This situation is associated with repeated Rossby-wave breaking over Australia, which weakens the normal west-to-east atmospheric flow and strengthens a blocking high near New Zealand.

The researchers found that these atmospheric conditions and weather patterns can not only damage the Great Barrier Reef but also cause heavy rain and flooding in northern New South Wales and southern Queensland. In 2022, a series of stalled low-pressure systems led to both persistent heavy rain and flooding in the region, while a mass bleaching event simultaneously struck the Great Barrier Reef.