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 the surface ocean warming, with ocean heat storage efficiency during the last deglaciation about ten times of its modern value. This raises the following question: what mechanisms are responsible for ocean heat uptake/storage and how large can its efficiency be?

Recently published in Science Advances, a joint study by an international team of scientists from China and USA 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.

Melting ice in the Southern Ocean. (Image by SHI Jiuxin)

“Our simulations and proxy reconstructions demonstrate that the three-dimensional ocean warming during the last deglaciation was strongly nonuniform with strongest warming occurring at intermediate depths, in striking contrast to contemporary observations,” said Dr. ZHU Chenyu from 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. “The unique ocean warming structure facilitates a large ocean heat storage efficiency. In particular, this resolves the paradox suggested by the conventional view that warming occurred at sites of deep-water formation that remained covered by sea ice”, said Prof. Zhengyu Liu, one of the corresponding authors of the study from The Ohio State University.

“These results have valuable implications. For example, if strong surface warming and strong ventilation are collocated like in our simulations, then the ocean will absorb more heat from the atmosphere, potentially slowing the rate of atmospheric warming” said Prof. Peter U. Clark, another corresponding author of the study from Oregon State University.

The study underscores the important role of surface warming pattern and oceanic circulation change in long-term ocean heat storage change and suggests that “the ocean can serve as a far greater reservoir of energy in the climate system than implied by contemporary observations,” according to the study.