Marine Warmth Waves Get Extra Intense, Extra Frequent – Watts Up With That?
Due to the thinning of the sea layer, the water is more susceptible to extreme warming events
COLORADO UNIVERSITY AT BOULDER
PICTURE: BLEACHED CORALS FROM WARM OCEAN WATER TEMPERATURES. Show more CREDIT: NOAA
When the surface layer of the ocean is thick, it acts as a buffer for extreme ocean warming. However, a new study from the University of Colorado Boulder shows that this “mixed layer” is getting shallower from year to year. The thinner it gets, the easier it is to heat. The new work could explain the recent extreme ocean heat waves, pointing to a future with more frequent and more destructive ocean warming events as global temperatures continue to rise.
“Marine heat waves will be more intense and more common in the future,” said Dillon Amaya, CIRES Visiting Fellow and lead author of the study published this week in the American Meteorological Society’s bulletin of Explaining Extreme Events. “And we now understand the mechanics of why. If the mixed layer is thin, it takes less heat to warm the ocean more. “
The mixed layer – the water in which the temperature remains constant – covers the top 20 to 200 meters of the ocean. Its thickness is responsible for heat events: the thicker it is, the more the layer can serve as a buffer to protect the water underneath from hot air flowing in. However, as this armor thins, the mixed layer becomes more susceptible to rapid temperature changes.
“Think of the mixed layer as boiling a pot of water,” said Amaya. “It won’t take any time for an inch of water to boil, but it will take a lot longer for a pot filled to the brim to warm through.”
Amaya and his team used a combination of ocean observations and models to estimate the depth of the mixed layer up to 1980 and to project it into the future. They found that over the past 40 years, the layer has thinned nearly 3 meters in some regions of the North Pacific. And by 2100, the mixed layer will be 4 meters thinner – 30 percent less than it is today. This thin mixed layer combined with warmer global temperatures will create the conditions for drastic fluctuations in ocean temperatures, leading to much more frequent and extreme warming events, the researchers say.
And it’s already happening. Take the 2019 heat wave in the Northeast Pacific. Weaker winds and higher air temperatures warmed the water of the Pacific Ocean by about 3 degrees Celsius. A thinning mixed layer most likely contributed to this rise in warm water, the authors found. And it will get worse.
“If you apply the same wind and ocean conditions as 2019 and apply those to the estimated mixed layer in 2100, you get a marine heat wave 6.5 ° C warmer than what we say in 2019,” he told Amaya. “Such an event would absolutely destroy fragile marine ecosystems along the US west coast.”
Amaya also points out that as the climate continues to warm and the mixed layer continues to thin, scientists may lose the ability to predict sea surface temperatures from year to year. Without the ability to accurately predict sea temperatures, fishing and other coastal operations could be at risk.
Other studies also suggest that ocean heat waves will be more common in the future, but not many have investigated the root cause: the dynamics and physics of the oceans. “In order to model these events and predict them, we need to understand the physics of why this is happening,” said Amaya.