A team of researchers has found that with the ongoing global warming, the ocean is gradually losing its “memory”. These results, published on May 6th in the journal Science Advances, have strong implications for seasonal predictability and the socio-economic sectors that rely on it.
The ocean has a significant thermal inertia. A familiar example is the seaside where the ground quickly heats up under the sun in summer, while the water barely changes its temperature. Conversely, at night, the ground cools relative to the sea. A similar contrast can be observed on a seasonal scale between ocean basins and continental masses.
The link between oceanic memory and the mixing layer
Water has a higher heat capacity than air or land, meaning more energy needs to be added or removed to change its temperature. Additionally, the upper part of the ocean is well mixed. As a result, temperature variation is not limited to the water’s surface but spreads over a layer of a few tens of meters thick, which further dampens changes. Due to this high thermal inertia, it is said that the ocean has a significant “memory”.
By analyzing observations and climate model responses to a continuous increase in atmospheric greenhouse gas concentration, a group of researchers has recently discovered that the ocean is losing a significant portion of this memory. The main cause is the thinning of the oceanic mixing layer, in other words, the thickness over which the water experiences turbulent stirring. Regionally, dynamic changes in ocean currents also contribute to this thinning.
A significant deterioration of the predictability horizon
As the water layer to be heated or cooled becomes less significant, temperature anomalies occur and disappear more rapidly. It is easier to change the temperature of the first ten meters of the ocean than the first twenty, for example. “We discovered this phenomenon by examining the similarity of ocean surface temperatures from one year to another as a simple measure of ocean memory,” said Hui Shi, the study’s lead author. “It’s almost as if the ocean is developing amnesia.”
Used in seasonal forecasting due to their persistence, surface sea temperature anomalies are expected to become more volatile from one year to the next. A direct implication of this evolution is a reduced ability to anticipate major seasonal trends globally (warmer or cooler seasons, probabilities of droughts, anomalies in monsoons, etc.). Major sectors such as fishing, agriculture, energy, or health depend on these forecasts.
“A reduced memory means less time in advance to make a forecast,” said Michael Jacox, one of the paper’s co-authors. “This could hinder our ability to predict and prepare for oceanic fluctuations, including marine heatwaves known to cause sudden and pronounced changes in ecosystems worldwide.”
