Hidden Heatwaves Far Below the Ocean Surface Come in Seven Shapes

Marine heatwaves hidden below the sea surface fall into seven recurring vertical shapes and can hold those shapes for months to years, according to a study published Nov. 14, 2026, in npj Climate and Atmospheric Science. Shujing Zhang of the Institute for Marine and Antarctic Studies at the University of Tasmania and colleagues cataloged more than 500 of the events in the top 1,000 meters of the ocean from 1993 to 2020.

None of those events was observed directly at depth. The paper identifies them in an ocean temperature reanalysis, a model reconstruction of past conditions, which the authors turned to because long records of measurements below the surface are sparse. They describe the catalog as a basis for better detection of deep heatwaves and for testing whether such events can be forecast.

The authors report that the events were most frequent near the equator and least frequent in the subtropics, and that grouping them by shape produced the seven recurring types. Five of those types were significantly associated with El Niño and La Niña phases. The events also rarely spread between the ocean's warm surface layer and the cooler water below it.

The same method traced the depth of two named events. Among them, the northeast Pacific Blob reached hundreds of meters below the surface, and the Ningaloo Niño off Western Australia extended past the surface footprint it is usually defined by.

The analysis covers most of the world's ocean but leaves out the polar seas. Funding came from the Australian Research Council Centre of Excellence for Climate Extremes, and the paper is open access.