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Scientists warn that as the planet warms up, the oceans will lose oxygen. This process, known as “deoxygenation”, threatens marine eco-systems around the world.
New modeling has revealed a new twist for the Pacific Ocean.
The well-ventilated subtropical zones are expected to become older and less oxygen rich, while the tropical oxygen minimum zone that is poorly ventilated may be “younger”.
Oceanographers are trying to figure out a mystery: Why do the waters that were once “young” age while those waters which had been “old” rejuvenated?
The water-mass age is the time since the last contact of seawater with atmosphere. Freshly ventilated waters, which are rich in oxygen, make up young water. The older the water, the more oxygen it has been consumed by marine life.
Oxygen disappears from Earth’s water
Climate models consistently show that climate change impacts this balance. The waters in the North Pacific subtropical, which usually move down toward the thermocline and remain relatively “young,” have begun to age. In the opposite direction, “old” tropical Pacific waters are getting “younger.”
Researchers used an intermediate-complexity ocean circulation and biogeochemistry model to test the dynamics. The researchers used both global uniform warming and regionally different scenarios.
Two key processes emerged:
As the Pacific Ocean warms, the temperatures and layers of its surface will increase.
The subtropical circulation has thinned (less depth), and become more horizontal. As a result of this thinning, the circulation of the subtropical-gyre has become more horizontal (less vertical motion) and thinner.
Southern Ocean influence: As the Southern Ocean temperature increases, the local upwelling decreases, but the distant upwelling (of oxygen-poor old water) from the Southern Ocean into the tropical Pacific also declines. The Kelvin wave effect is responsible for this change. It results in a younger tropical Pacific with less oxygenation.
The river is rapidly losing oxygen, putting aquatic life at risk.
The Pacific thermocline is a paradoxical response that combines these two processes to create a “young gets older and old gets younger”.
Notes by the Authors The CMIP6 model consistently simulates this ‘young getting older but old getting younger’ pattern across the tropical and subtropical North Pacific under strong radiative forces.
The ocean is not deoxygenated in the same way everywhere. Some parts of subtropical gyre, which have a variety of marine species and are oxygen poor, may remain relatively unaltered. In order to make accurate predictions on the type of adaptations likely to occur within marine species as a result of changes in oxygen levels, we must first understand how they vary geographically.
This study shows that it is necessary to continue monitoring the biogeochemical Argo System and to develop better ways to model how upper oceans are ventilated in both high-and midlatitudes.
The warming of the oceans slows down the vertical currents.
As a result, water that has high levels of oxygen in it will move less into the subtropics. Water with lower levels of oxygen moving upwards into the Tropics is also less likely to do so. Subtropical water will also become oxygenated, and therefore younger.
Tropical waters are more oxygenated.
The ocean will be altered in complex and contradictory ways by climate change. The “paradox” in the Pacific shows that the oceans are not responding uniformly to the global warming.
It is important to understand why the oceans react differently when temperatures rise in order to protect marine life and the environment over the coming decades.
New factor revealed in Antarctic Ice Shelf Melting
Notes by the Authors Our experiments indicate that the decrease in tropical upwelling is primarily due to an adjustment of ocean currents caused by remote forces from the Southern Ocean.
The equatorial region of the Pacific Ocean and the eastern border regions appear to be particularly sensitive to remote warming because they are located along the Kelvin Waveguide, which is responsible for the dynamical thermocline adjustments at basin scale. Oxygenation on the eastern borders in these experiments seems to be primarily related with this downwelling rather than local surface density as emphasized by Gnanadesikan et. al.
We have caused surface densities to be lighter everywhere.
Journal Reference
- Benjamin A. Taylor, Yassir A. Eddebbar, Yohei Takano, Lynne D.
Talley, Shang-Ping Xie. The Contrasting Patterns of Projected Pacific Oxygenation are Explained by Warming-Induced Circulation.
AGU Advances. DOI: 10.1029/2025AV002278
