The surface layers of the ocean that help slow down the extreme aquatic temperature spike. Unfortunately, a new study from the University of Colorado Boulder showed that the said layer is becoming shallower every year.
The thinner this "mixed layer" becomes, the easier the ocean gets warmer. The new work could explain recent extreme marine heatwaves and point at a future of more frequent and destructive ocean warming events as global temperatures continue to climb.
"Marine heatwaves will be more intense and happen more often in the future," said Dillon Amaya, a CIRES Visiting Fellow and lead author on the study out this week in the Bulletin of the American Meteorological Society's Explaining Extreme Events. "And we now understand the mechanics of why. When the mixed layer is thin, it takes less heat to warm the ocean more."
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"Mixed Layer"
The mixed layer-the water in which temperature remains consistent-blankets the top 20-200 meters of the ocean. Its thickness is responsible for heat events: the thicker it is, the more the layer can act as a buffer to shield the waters below from incoming hot air. But as this armor thins, the mixed layer becomes more susceptible to rapid swings in temperature.
"Think of the mixed layer as boiling a pot of water," said Amaya. "It will take no time at all for an inch of water to come to a boil, but much longer for a pot filled to the brim to heat through."
Amaya and his team combined ocean observations and other models to estimate the depth of the layer during the 1980s. They also used the methods to predict potential possibilities.
They determined that over the last 40 years, the layer has thinned by nearly 3 meters (9 feet) in some North Pacific regions. And by 2100, the mixed layer will be 4 meters (12 feet) thinner-30 percent less than today. This thin mixed layer combined with warmer global temperatures will set the stage for drastic swings in ocean temperatures, leading to much more frequent and extreme heating events, the researchers say.
And, it's already happening.
Take the 2019 heatwave in the Northeast Pacific. Weakened winds and higher air temperatures came together to warm Pacific Ocean waters by about 3 degrees C (5.5 F). A thinning mixed layer most likely contributed to this surge of warm waters, the authors found. And it will get worse.
"If you take the same wind and ocean conditions that occurred in 2019 and you apply them to the estimated mixed layer in 2100, you get a marine heatwave that is 6.5 degrees C (12 F) warmer than what we said in 2019," said Amaya. "An event like that would absolutely devastate sensitive marine ecosystems along the U.S. west coast."
Aquatic Studies
Amaya also points out that, as the climate continues to warm and the mixed layer continues to thin, scientists might start to lose the ability to predict year-to-year ocean surface temperatures. Without the ability to accurately forecast ocean temperatures, fisheries and other coastal operations could be in danger.
According to Amiya, "to simulate these events in models and help predict them, we must understand the physics of why that's happening." They suggested that more similar studies be conducted about ocean dynamics and physics.
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