FunNature & AnimalWhat is the memory of the oceans and why...

What is the memory of the oceans and why are they losing it?

Let’s start by knowing what the memory of the oceans is. “Ocean memory, the persistence of oceanic conditions , is an important source of predictability in the climate system beyond meteorological time scales,” the researchers explain in a new paper published in Science Advances .

“We show that ocean memory, as measured by the year-to-year persistence of sea surface temperature anomalies, is projected to steadily decline over the next few decades over much of the planet.”

In the research, the team studied sea surface temperatures (SSTs) in the upper shallow layer of the ocean, called the upper ocean mixed layer (MLD).

Despite the relative shallowness of the MLD, which extends to a depth of about 50 meters from the ocean surface, this upper layer of water shows great persistence over time in terms of thermal inertia, especially when compared to observed variations in the atmosphere above.

Going forward, however, models suggest that this “memory” effect of thermal inertia in the upper ocean is going to diminish globally over the rest of the century, with dramatically larger variations in temperature predicted in coming decades.

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

According to the researchers, the effects of sandbars in the MLD will introduce higher levels of water mixing into the upper ocean, thinning the top layer.

This is expected to reduce the thermal inertial capacity of the ocean, making the upper ocean more susceptible to random temperature anomalies.

It’s unclear what this means for marine wildlife , but the researchers note that “population impacts are likely,” although some species are expected to be better adapted than others.

On the other hand, the decline in ocean memory is expected to make forecasting ocean dynamics considerably more difficult , reducing lead times for all types of SST-related forecasts. This will hinder our ability to project monsoons, marine heat waves (OMC), and periods of extreme weather, among other things.

With extreme weather events expected to become more frequent in the future, our need to accurately predict measurements of things like ocean temperatures, precipitation levels, and atmospheric anomalies becomes increasingly important. However, if the ocean loses its memory, we risk going in the opposite direction, say the researchers. “The predicted decline in ocean memory will likely hamper ocean forecasting efforts by reducing lead times in which SST forecasts, including those for MHWs (marine heat waves), are skilful,” the authors write.

“Future warming-induced displacement of the MLD may also alter statistics for extreme temperatures, which, combined with the reduced lead time for predictions based on persistent ocean surface conditions, will pose challenges .” for ecosystem management and preparedness for marine hazards .”

 

Reference:

Shi H et al. 2022. Global decline in ocean memory over the 21st century. Science advances. DOI: 10.1126/sciadv.abm3468

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