World’s ocean is steadily losing its year-to-year memory

Earth suffers ‘ocean amnesia’ symptoms of global warming.


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Ocean memory, the persistence of ocean conditions, is a significant source of predictability in the climate system beyond weather time scales. It is often used for predicting ocean conditions.

A study by a University of Hawaiʻi at Mānoa atmospheric scientist suggests that the world’s ocean is steadily losing its year-to-year memory due to global warming. Scientists discovered this by assessing future projections from the latest generation of Earth System Models.

Ocean memory has an excellent correlation with the thickness of the uppermost layer of the ocean, known as the mixed layer. The greater heat content within the deeper mixed layers confers more thermal inertia that translates into memory. When exposed to anthropogenic warming, the mixed layer becomes shallower, prompting a decline in ocean memory.

Scientists in this study discovered this phenomenon by analyzing the similarity in ocean surface temperature from one year to the next as a simple metric for ocean memory.

Hui Shi, lead author and researcher at the Farallon Institute in Petaluma, California, said, “It’s almost as if the ocean is developing amnesia.”

What’s more, scientists also found that the thinning mixed layer increases the random fluctuations of the sea surface temperature. This won’t lead to much more variability in the ocean from one year to the next in the future, the fraction of helpful signals for prediction largely reduces.

Fei-Fei Jin, an atmospheric sciences professor at the UH Mānoa School of Ocean and Earth Science and Technology, said, “Reduced ocean memory and increased random fluctuations suggest intrinsic changes in the system and new challenges in prediction under warming.”

However, ocean memory does not significantly influence the prediction of physical variables, but it affects how we manage sensitive marine ecosystems.

Scientists noted“Reduced ocean memory might render such estimation inaccurate and calls for new approaches in ecosystem-based fisheries management to include real-time ocean monitoring and other efforts alike. Ocean memory decline also likely exerts impacts on populations of biological resources. Depending on whether the species are adapted to constant or more variable environmental conditions, future changes in their population can be better estimated and predicted by considering ocean memory loss.”

“Besides ocean prediction, forecasting land-based impacts on temperature, precipitation, and extreme events might also be affected by ocean memory decline due to their dependence on the persistence of sea surface temperature as a predictability source. As ocean memory continues to decline, researchers will likely be challenged to search for alternative predictors for skillful predictions.”

Journal Reference:

  1. Hui Shi et al. Global decline in ocean memory over the 21st century. DOI: 10.1126/sciadv.abm3468


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