New modelling suggests most of the world’s ocean is losing its year-to-year memory as a result of global warming. 

Compared with the fast weather fluctuations of the atmosphere, the slowly varying ocean exhibits strong persistence, or “memory”, meaning the ocean temperature tomorrow is likely to look a lot like it does today, with only slight changes. As a result, ocean memory is often used for predicting ocean conditions. 

Ocean memory decline is found as a collective response across the climate models to human-induced warming. 

As greenhouse-gas concentrations continue to rise, such memory decline will become increasingly evident.

“We discovered this phenomenon by examining the similarity in ocean surface temperature from one year to the next as a simple metric for ocean memory,” says Hui Shi, lead author of a new paper using future projections from the latest generation of Earth System Models. 

“It's almost as if the ocean is developing amnesia.” 

Ocean memory is found to be related to the thickness of the uppermost layer of the ocean, known as the mixed layer. 

Deeper mixed layers have greater heat content, which confers more thermal inertia that translates into memory. However, the mixed layer over most oceans will become shallower in response to continued anthropogenic warming, resulting in a decline in ocean memory. 

“Other processes, such as changes in ocean currents and changes in the energy exchange between the atmosphere and ocean, also contribute to changes in ocean memory, but the shoaling of the mixed layer depth and resulting memory decline happens in all regions of the globe, and this makes it an important factor to consider for future climate predictions,” says Dr Robert Jnglin Wills, co-author of the research.

Along with ocean memory decline, the thinning mixed layer is also found to increase the random fluctuations of the sea surface temperature. As a result, although the ocean will not become much more variable from one year to the next in the future, the fraction of helpful signals for prediction largely reduces.

Ocean memory loss does not just impact the prediction of physical variables, but could also influence the way we manage sensitive marine ecosystems. 

“Reduced memory means less time in advance for a forecast to be made. This could hinder our ability to predict and prepare for ocean change including marine heatwaves, which are known to have caused sudden and pronounced changes in ocean ecosystems around the world,” says Dr Michael Jacox, another co-author of the research. 

The full study is accessible here.