Calm Seas and Doldrum Days: The Surprising Cause of Coral Bleaching

New research by Monash University and the ARC Center of Excellence for the Weather of the 21st Century analyzed close to three decades of weather data during the coral bleaching season and identified the prevalence of "doldrum days," and the absence of the trade winds, as a key factor in the mass bleaching events threatening the Great Barrier Reef. Doldrums is an old nautical expression. Hundreds of years ago, the prolonged absence of wind in the tropical ocean could spell disaster for sailing ships and their crews, who rapidly ran out of drinking water as they prayed for a life-saving change in the weather. Now, it is a world heritage area that is suffering in the doldrums. Building on work published in 2024, the study, published in Weather and Climate Dynamics, found that when large planetary atmospheric waves are present, this can lower atmospheric pressure and disrupt the trade winds, increasing the chances of doldrums forming. In particular, losing these classic trade winds during December and April, respectively, means less ocean cooling and could contribute to a build-up of early summer heat stress and the persistence of heat later into the season. Lead researcher and Ph.D. candidate in the ARC Center of Excellence for the Weather of the 21st Century, Lara Richards, said trade winds act like nature's air conditioner for the reef. "When they fail, the ocean heats up quickly, and corals suffer. Understanding these patterns gives us a better chance to predict and potentially mitigate the impacts of future bleaching events," said Ms. Richards. Years with mass bleaching had more "calm and clear" days during the hottest months and fewer strong trade wind days at the start and end of the season. This combination allows heat to build early and linger later, creating ideal conditions for marine heat waves. Professor Steven Siems from the School of Earth, Atmosphere and Environment at Monash University said when the reef suffers mass coral bleaching, doldrums happen more often and last longer. "These calm spells, along with other breaks in the usual cooling trade winds, allow ocean temperatures to spike, putting corals under stress. The problem is even worse when the trade winds fail to show up at the start and end of the summer period. Without these cooling winds, heat builds early and lingers later, making bleaching much more likely," said Professor Siems. "While we already knew that calm conditions cause warming, the research highlights how missing cooling events at the edges of the season can turn an ordinary year into a bleaching year." The findings highlight the importance of monitoring atmospheric patterns to improve early-warning systems for coral bleaching. As climate change continues to disrupt weather systems, the role of trade winds in protecting the Great Barrier Reef becomes even more critical. "As local weather conditions act as drivers of Great Barrier Reef ocean temperatures, furthering our understanding of the trade wind structure and how they break down is crucial for the continued monitoring and forecasting of marine heat waves and thermal bleaching events," Ms. Richards said.