Science

Ocean eddies are getting more intense and could affect the climate

Today there is evidence that the oceans are warming and water levels are rising, threatening island and coastal populations around the world. A new study today reveals another anomaly: ocean currents change over time and become more and more energetic. A parameter that could have a major impact on the future climate of the planet.

This study is based on three decades of data, collected via satellite observations. The latter highlight areas where oceanic eddies are particularly numerous and intense. These include the Gulf Stream in the North Atlantic, the Kuroshio Current in the North Pacific, the Southern Ocean surrounding Antarctica and the Eastern Australian Current.

These eddies, 10 to 100 kilometers in diameter, play a major role in ocean circulation. They move masses of hot and cold water from one place to another, mixing together carbon, nutrients and microorganisms in the oceans. In this way, they regulate the climate both regionally and globally. But scientists have observed significant changes in these vortices, never before detected.

5% stronger eddies every decade

Observation satellites orbiting the Earth are able to detect the smallest changes, such as a few centimeters in sea level rise. These elevation changes, together with ocean surface temperature data, can then be translated into ocean flow velocities, allowing scientists to assess how “energetic” a particular ocean vortex is. However, by carefully analyzing the observational data, researchers at the Australian National University have discovered significant changes in the distribution and strength of ocean eddies.

Until the early 1990s, scientists could only observe changes in ocean eddies using sparse ocean measurements or relatively limited satellite data. They now have sufficient satellite recording to draw solid conclusions about likely longer-term trends in vortex behavior. The data available cover the period from 1993 to 2020.

In particular, the team found that over time, regions already rich in eddies have even more. ” This includes the Southern Ocean around Antarctica, as well as some of the major boundary currents [ndlr : dont la dynamique est influencée par la présence d’un littoral] like the Gulf Stream and the current is Australian », Explains Josué Martínez Moreno, researcher at theARC Center of Excellence for Climate Extremes from Australian National University and lead author of the study. And on average, these whirlpools get 2 to 5% more energetic each decade.

(a, b) Trends in sea level height, (c, d) mean kinetic energy of ocean surface eddies, (e, f) trends in kinetic energy of ocean eddies, between 1993 and 2019. © J. Martinez-Moreno et al.

It turns out that the Southern Ocean is particularly affected by the phenomenon; analyzes have shown a massive increase in the activity of its vortices (+ 5% per decade). However, this ocean is known to be a critical area for the absorption of ocean heat and the storage of carbon dioxide.

If the activity of eddies has increased globally in the world, it has on the other hand decreased in the tropical oceans. The study’s authors believe that as such, the observed changes could affect the exchange of heat and carbon dioxide between the ocean and the atmosphere, which in turn would alter weather patterns and currents. ” What we have discovered is a global reorganization of ocean energy over the past three decades. », Summarizes Martínez-Moreno.

Unknown effects on oceanic absorption of CO2

For the Australian team, the discovery is consistent with other large-scale climate research. This coincides in particular with the fact that drylands are becoming more dry, just as wetlands tend to become more humid. Because eddies play a fundamental role in the climate and the balance of ecosystems, the newly obtained results also have profound implications for climate and fisheries.

Problem: in studies of global warming, the behavior of ocean eddies is often not taken into account in projections. Indeed, as they are relatively small, they remain practically “invisible” in the current models used to predict the future climate. Their impact is therefore greatly underestimated, if not completely ignored.

This omission is of particular concern in light of this new study, which shows that the vortices are becoming more and more intense. ” The world’s oceans absorb most of the carbon dioxide that humans release into the atmosphere. The Southern Ocean, in particular, contributes up to 40% and a large part of this absorption is carried out via oceanic eddies. », Explains Janet Sprintall, oceanographer at the Scripps Institution of Oceanography, California, which did not participate in the study.

However, to date, we do not yet know what the consequences of the changes observed in the vortices will be. They could potentially impact the ability of the oceans to absorb carbon that we may continue to emit in the future. ” It could have devastating effects on global society Sprintall emphasizes. The researchers therefore believe that it is crucial to incorporate oceanic eddies into future climate projections. ” Without solving this component of the ocean flow, we might be missing out on something critical for the climate. », Warns Martínez-Moreno.

Nature Climate Change, J. Martinez-Moreno et al.

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