Thinning Surface Layer of Ocean Leaves Waters More Susceptible to Extreme Warming Events

Marine Heatwaves Becoming More Intense, More Frequent

When thick, the surface layer of the ocean acts as a buffer to extreme marine heating—but a new study from shows this “mixed layer” is becoming shallower each year. The thinner it becomes, the easier it is to warm. The new work could explain recent extreme marine heatwaves, and points 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 are now understanding the mechanics of why. When the mixed layer is thin, it takes less heat to warm the ocean more.”

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.”

Bleached corals from warm ocean water temperatures. Credit: NOAA

Amaya and his team from NOAA, NCAR and CU Boulder used a combination of ocean observations and models to estimate the depth of the mixed layer back to 1980, and also project out into the future. They determined that over the last 40 years, the layer has thinned by nearly 3 meters (9 feet) in some regions of the North Pacific. And by 2100, the mixed layer will be 4 meters (12 feet) thinner—30 percent less than what it is 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 say in 2019,” said Amaya. “An event like that would absolutely devastate sensitive marine ecosystems along the U.S. west coast.”

Amaya also points out that, as 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.

Other studies also suggest marine heatwaves will become more common in the future, but not many have explored the root cause: ocean dynamics and physics. “In order to simulate these events in models and help predict them, we must understand the physics of why that’s happening,” said Amaya.

Reference: “Are Long-Term Changes in Mixed Layer Depth Influencing North Pacific Marine Heatwaves?” by Dillon J. Amaya, Michael A. Alexander, Antonietta Capotondi, Clara Deser, Kristopher B. Karnauskas, Arthur J. Miller and Nathan J. Mantua, 27 January 2021, Bulletin of the American Meteorological Society.
DOI: 10.1175/BAMS-D-20-0144.1

Climate ChangeClimate ScienceGlobal WarmingOceanographyUniversity of Colorado at Boulder
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  • Clyde Spencer

    Another poorly written article! The original published research has a title that poses a question. This article reads as though the question has been answered and proven.

    The linked research article starts with a phrase that is not defined, and I have not seen before: “anthropogenic mixed layer shoaling.” I doubt that humans have much control over shoaling.

    It appears that something was lost in the translation when the claim is made that “When the mixed layer is thin, it takes less heat to warm the ocean more.” It should have said, “…, it takes less heat to warm the mixed layer in the ocean more.”

    Equally bad is the statement, “… the thicker it is, the more the layer can act as a buffer to shield the waters below from incoming hot air.” Conduction of heat through air proceeds slowly because the rate of thermal conduction through air is so slow that most clothing uses air as an insulator, as in a down-filled jacket. If an air mass is stagnant, a surface layer of air that is saturated with water vapor will suppress evaporation. Even so, most of the heating will arise from solar energy being absorbed and turned to heat. If the air mass is moving and unsaturated (windy conditions), then surface water will be evaporated and the process will cool the surface layer. Making generalizations are risky because there are so many variables to consider.

    Lastly, that absorbed heat isn’t going anywhere because heat that is conducted goes from a warmer body to a cooler body. Thus, the surface of the mixed layer can warm coastal land in the Winter, but most of the heat will eventually work its way into deeper water, unless the surface water evaporates and cools the top of the mixed layer.

    I would expect that the thickness of the mixed layer will be a function of surface opacity, which limits the depth to which sunlight will reach, and the surface windiness, which creates the necessary turbulence for mixing that allows deeper water to cool the warming surface water. I don’t see a case being made for human activities, in particular CO2 emissions, causing local decreases in windiness.

    The Discussion section of the research article contains two sentences that should be bolded: “The anthropogenic contribution to these observed MLD trends is less clear. … Regardless, it is clear that large internal climate variability complicates the detectability of the forced MLD signal in observations.”

    I think that the authors need to be reminded that correlation does not prove causation.