As one of the largest heat reservoirs in the climate system, the global ocean absorbs more than 90% of the excess energy from ongoing anthropogenic warming. In the last century, the greatest warming in the ocean has occurred in the upper 500 m, with relatively weak warming in the deep ocean, corresponding to a small ocean heat storage efficiency of ~0.1.

Paleoceanographic observations, however, suggest that on long time scales, the deep ocean warming can be comparable to or larger than surface ocean warming, with ocean heat storage efficiency during the last deglaciation about ten times of its modern value. This raises the following question: Which mechanisms are responsible for ocean heat uptake/storage, and how great can their efficiency be?

Recently published in Science Advances, a joint study by an international team of scientists from China and the U.S. has shed light on this issue. By combining state-of-the-art deglacial simulations and proxy-based reconstructions, they resolved the three-dimensional deglacial ocean temperature change and found that the deglacial ocean heat storage efficiency is substantially enhanced to ≥1 by strong warming in intermediate-depth waters in response to deglacial forcing.

"Our simulations and proxy reconstructions demonstrate that the three-dimensional ocean warming during the last deglaciation was strongly nonuniform, with the strongest warming occurring at intermediate depths, in striking contrast to contemporary observations," said Dr. Chenyu Zhu from the Institute of Atmospheric Sciences at the Chinese Academy of Sciences, the study's co-first author.

Utilizing sensitivity experiments, the study revealed that the large warming of intermediate waters can be related to surface warming at mid-to-subpolar latitudes through ventilation in response to greenhouse gases and ice sheet forcing, and substantially enhanced by oceanic circulation change associated with meltwater forcing.

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