The Earth suffers from “ocean amnesia”, a symptom of global warming

Decline of ocean memory (blue) by the end of the 21st century. (Photo credit: Shi et al. 2022)

According to a study published in Scientists progress co-authored by a university of Hawaii at atmospheric scientist Mānoa. The research team discovered this while evaluating future projections from the latest generation of Earth system models.

Compared to the rapid weather fluctuations of the atmosphere, the slowly varying ocean exhibits strong persistence, or “memory,” meaning that tomorrow’s ocean temperature is likely to be much like today’s, with only slight changes. As a result, ocean memory is often used to predict ocean conditions.

In all climate models, declining ocean memory has been found to be a collective response to human-induced warming. As greenhouse gas concentrations continue to rise, this decline in memory will become increasingly evident.

Global average ocean memory under low to high emission scenarios. (Photo credit: Shi et al. 2022)

“We discovered this phenomenon by examining the similarity of ocean surface temperature from year to year as a simple measure of ocean memory,” said lead author and researcher Hui Shi. at the Farallon Institute in Petaluma, California. “It’s almost like the ocean is developing amnesia.”

Ocean memory is related to the thickness of the upper layer of the ocean, known as the mixed layer. Deeper mixed layers have greater heat content, which imparts more thermal inertia which translates into memory. However, the mixed layer over most oceans will become shallower in response to continued anthropogenic warming, causing ocean memory to decline.

New challenges for ocean forecasting

Along with declining ocean memory, thinning of the mixed layer also increases random fluctuations in sea surface temperature. Else in the future, the fraction of useful signals for prediction decreases greatly.

“The reduced ocean memory together with the increased random fluctuations suggest intrinsic changes in the system and new challenges in prediction under warming,” said Fei Fei Jinprofessor of atmospheric science at uh Mānoa School of Ocean and Earth Science and Technology, and research co-author.

Impacts on ocean management and more

Ocean memory loss not only impacts the prediction of physical variables, but could also influence how we manage sensitive marine ecosystems.

In fisheries management, the biological parameters used for stock assessment are estimated assuming a stable environment represented by the recent past. Reduced ocean memory could make this estimate inaccurate and calls for new approaches in ecosystem-based fisheries management to include real-time ocean monitoring and other similar efforts. The decline of ocean memory is also likely to have impacts on populations of biological resources. Depending on whether species are adapted to constant or more variable environmental conditions, the future evolutions of their population can be better estimated and predicted by taking into account the loss of oceanic memory.

In addition to ocean forecasting, the forecasting of land-based impacts on temperature, precipitation as well as extreme events could also be affected by the decline of ocean memory due to their dependence on the persistence of sea surface temperature as a what a source of predictability. As ocean memory continues to decline, researchers will likely be challenged to seek alternative predictors for skillful predictions.

This work is an example of uh Mānoa’s goal of research excellence: to advance the enterprise of research and creative work (PDF) and Building a Sustainable and Resilient Campus Environment: Part of the Global Movement for Sustainability and Climate Resilience (PDF), two of the four objectives defined in the Strategic Plan 2015-2025 (PDF), updated December 2020.

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