The ocean is beginning to lose memory, scientists warn

The oceans around us are changing. As our climate changes, the world’s waters are also shifting, and abnormalities are evident not only in the temperature of the ocean, but also in its structure, currents, and even its color.

As these changes manifest, the ocean’s typically stable environment is becoming increasingly unpredictable and irregular, and in some ways this phenomenon is similar to the ocean losing memory, scientists suggest.

“Ocean memory, the persistence of ocean conditions, is a major source of predictability in climate systems outside of time scales,” explain researchers in a new paper led by first author and climate researcher Hui Shi of the Farallon Institute in Petaluma, California.

“We show that ocean memory, measured by the persistence of sea surface temperature anomalies from year to year, is predicted to decline in much of the world in the coming decades.”

In the study, the team studied sea surface temperatures (SST) in the shallow upper ocean layer, called the mixed upper ocean layer (MLD).

Despite the relative shallowness of MLD – which extends only to a depth of about 50 meters below the ocean surface – this upper water layer shows a lot of stability over time in terms of thermal inertia, especially compared to variations seen in the atmosphere above.

In the future, however, modeling suggests that this ‘memory’ effect of thermal inertia in the upper ocean should decrease globally for the rest of the century, with dramatically greater variations in temperature predicted over the coming decades.

“We discovered this phenomenon by examining the similarity of ocean surface temperature from year to year as a simple metric for ocean memory,” explains Hui.

According to researchers, the effects of shallows in MLD will lead to higher levels of water mixing in the upper ocean, effectively diluting the upper layer.

This is expected to reduce the ocean’s ability for thermal inertia, making the upper ocean more susceptible to accidental temperature anomalies.

It is unclear what this means for wildlife in the sea, but researchers note that they are “likely consequential impacts on populations,” although some species are expected to fare better than others in terms of adaptation.

On the other hand, declining ocean memory is expected to make it significantly more difficult for scientists to predict upcoming ocean dynamics, reducing reliable time for all types of SST-related predictions. This will interfere with our ability to project monsoons, sea heat waves (MHW) and periods of extreme weather, among others.

As extreme weather is predicted to become more common in the future, our need to accurately predict measurements for things like ocean temperature, precipitation levels and atmospheric anomalies is only becoming more important – but if the ocean loses memory, we risk heading in the opposite direction, researchers say.

“The projected decline in ocean memory is likely to hamper ocean prediction efforts by reducing the time at which SST forecasts, including those for MHW, are skilled,” the authors write.

“Future shallower MLDs caused by warming may also change temperature extreme statistics … which, combined with shortened time to predict ocean surface status based on resilience, will pose challenges for ecosystem management and preparedness for marine hazards.”

Findings are reported in Advances in science.

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