Subtropical gyres are enormous rotating ocean currents that can be found just to the north and south of the equator. These gyres are slow moving whirlpools that circulate within massive basins around the world, gathering up nutrients, organisms, and sometimes trash.
Oceanographers have been perplexed over conflicting observations for a long time. Microbes that feed the rest of the ocean food chain and are responsible for sucking up a significant portion of the atmosphere's carbon dioxide can be found at the surface.
According to what scientists know about the dynamics of gyres, the currents wouldn't be able to keep up with the amount of food they were seeing. The microbes were able to thrive.
MIT researchers found that the delivery vehicle for the nutrients may be eddies that swirl at the edges of a gyre. The eddies pull in high-nutrient equatorial regions and push them into the center of a gyre, where they are taken up by other currents and pumped to the surface.
The team found that ocean eddies are an important source of nutrition. The ocean's ability to sequester carbon from the atmosphere is dependent on the health of the population of phytoplankton. Climate models tend to project a decline in the ocean's ability to sequester carbon over the next decades, but this "nutrient relay" could help sustain carbon storage over the ocean.
As climate continues to change, there is a lot of uncertainty about how the carbon cycle of the ocean will evolve. The carbon distribution is not straightforward and depends on understanding the role of eddies and other fine-scale motions.
The findings of Gupta and his colleagues are reported in the Proceedings of the National Academy of Sciences. The study's co- authors are Jonathan Lauderdale, Oliver Jahn, Christopher Hill, and Michael Follows at MIT.
There is a puzzle in the snow.
A cross-section of an ocean gyre is similar to a stack of bowls with different densities at different depths. The ocean's top sunlit layers are where the organisms need sunlight, warm temperatures, and nutrition to grow.
They sink through the ocean's layers as marine snow. Some of the snow goes back into the current to feed the microbes. The snow leaves the gyre and goes down to the ocean. The deeper the snow is, the harder it is to get it to come up. The snow is trapped or sequestered after being released.
Recirculating marine snow is thought to be the main source of nutrition in the gyres. As a portion of the snow inevitably sinks to the bottom, there must be another source of nutrition. The oceanography community is a little confused by what that source is.
At the edge, there are swirls.
The team wanted to see what other dynamics were doing in the gyre. The North Pacific gyre is one of the five major gyres on the planet and spans more than 20 million square kilometers.
The team started with a general circulation model called the MITg cm. To reproduce the North Pacific gyre's dynamics as realistic as possible, the team used an MITg cm algorithm, which was previously developed at NASA and MIT, to tune the model to match actual observations of the ocean.
Lauderdale says that they use a simulation of the ocean that is as realistic as they can get.
Compared to other models that have a more limited resolution, the realistic model has a better resolution. The simulation of the ocean's physical behavior was combined with the Darwin model to show how the organisms grow and evolve.
Over the course of a decade, the team ran a simulation of the North Pacific gyre and created animations to show the pattern of currents. The small eddies that ran along the edges of the huge gyre appeared to be rich in vitamins and minerals.
Lauderdale says they were picking up on weather systems in the ocean. We wondered if these transfers made a difference.
The nutrients move deeper away from the sun before being returned to where they came from. The team found that the ocean eddies can provide up to 50 percent of the nutrition in the gyres.
Gupta says that's very significant. There is more to the story than the vertical process. The other half is replenish the eddies. The world's oceans are a significant part of the subtropical gyres.
More information: Gupta, Mukund, A nutrient relay sustains subtropical ocean productivity, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2206504119. doi.org/10.1073/pnas.2206504119 Journal information: Proceedings of the National Academy of Sciences
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