Researchers find North Atlantic Ocean has memory of nearly two decades

A research team, led by the University of Liverpool, has found that the North Atlantic Ocean has a memory of approximately one to two decades, significantly surpassing previous estimates of a few years.

Ocean memory refers to the persistence of ocean conditions and is a major source of predictability in the climate system beyond weather time scales. However, the fundamental question of “how long does the ocean remember?” has been unclear. The difficulty in identifying ocean memory is due to continual changes in atmospheric forcing masking the longer-term effects of ocean circulation.

Reported in a paper published in the journal Geophysical Research Letters, the research team were able to quantify ocean memory by tracking the evolution of ocean temperatures in response to a single extreme atmospheric event.  The authors find that there are fast and slow responses involving an immediate atmospheric effect on surface temperature and a delayed ocean redistribution of heat.

The authors attribute the extended memory of the North Atlantic Ocean to the slow pace of changes in the ocean circulation and heat transport.

Their findings suggest that fluctuations in ocean temperatures exert a significant influence on regional climates, such as air temperatures, for over a decade.

It also offers support that careful analyses of observations may enhance future climate projections over a decade.

Dr Hemant Khatri, from the School of Environmental Sciences at the University of Liverpool and lead author of the paper, said: “The study addresses a fundamental question of what ocean memory truly is. The new ocean memory framework reveals physical mechanisms responsible for multi-year ocean memory and paves the way for new methods for evaluating climate models.”

Professor Ric Williams, also from the University of Liverpool and co-author of the paper, said: “This is the first time we’ve been able to measure ocean memory. We find that, in contrast to observation-based estimates, state-of-the-art climate models significantly underestimate ocean memory, estimating it at only eight to ten years. This significant discrepancy in ocean memory could have substantial implications for the accuracy of decadal climate predictions from these models.”

“Our work shows how the ocean affects our climate and how understanding the ocean temperature changes leads to enhanced skill over the next decade in climate projections”.

The study involves the University of Liverpool, the University of Oxford and  the UK Met Office with support from UKRI Natural Environment Research Council (NERC).

The paper “An ocean memory perspective: Disentangling atmospheric control of decadal variability in the North Atlantic Ocean” (doi: 10.1029/2024GL110333) is published in Geophysical Research Letters.