Oceans’ true boundaries explain source of ocean water — and “garbage patches”

Print Friendly
Oceans’ true boundaries explain the source of ocean water — and “garbage patches”

A deep look at ocean circulation returns a surprise: Currents transport water — and non-degradable, floating plastic — between the ocean basins. Thus, some of the plastic in the South Atlantic “garbage gyre” was actually thrown away in nations bordering the Indian Ocean.

Oceanographers have long known that currents converge on mid-ocean circulating structures called gyres. By email, Erik van Sebille, an oceanographer at the University of South Wales in Australia explained, “We knew about the convergence of water in the middle of the gyres since the early 20th century, when Vagn Walfrid Ekman developed a theory for how winds move water in the ocean around. That theory predicts that there are regions where water sinks, and that anything that is too buoyant to sink with the water accumulates at the surface there. That is exactly why we have garbage patches.”

Map of global surface ocean shows flow speed (red) and direction (arrows) averaged over 48 weeks.

Map of global surface ocean shows flow speed (red) and direction (arrows) averaged over 48 weeks. When currents converge, they form gyres that are infamous for the accumulations of plastic garbage.

Although the discovery of garbage patches has focused concern on the gyres, a new study in the journal Chaos looks instead at how the same flow of ocean currents that makes garbage patches also delineates the functional boundaries of the oceans.

Small task: Map the oceans!

The goal, explained the study’s first author, Gary Froyland, a mathematician at New South Wales, was to “draw the ocean boundaries in a way that minimized the amount of surface water leaking from one ocean to its neighbors, as a scientific way of compartmentalizing oceans, rather than drawing boundaries based on geopolitics.”

The mathematical model described in the paper started with estimates of the speed and direction of ocean currents at the surface. “The current information we are using comes from a highly regarded ocean model, produced by international ocean modeling experts, which has undergone many internationally peer-reviewed tests,” Froyland wrote us.

The catchment areas of the world's oceans don't follow the boundaries on maps. Driven by the wind, currents sometimes move water between oceans.

The catchment areas of the world’s oceans don’t follow the boundaries on maps. Driven by the wind, currents sometimes move water between oceans. Adapted by Why Files from image courtesy Gary Froyland, University of New South Wales.

As required, the model showed gyres at the position of the garbage patches.

The gyres — mid-oceanic congregations of bits of polymer — stay largely in position, Froyland wrote to us. “There may be some slight meander, but because the ocean currents do not substantially change, these convergence areas do not change and garbage will continue to accumulate in these areas.”

However, because “each ocean ‘leaks’ to some extent to its neighbors,” Froyland said, floating junk can, on occasion, play trash-patch tourist and visit a different gyre.

Garbage: It’s a sharing thing!

Just as rivers get water from their watersheds, oceans get their water (and floating debris) from “catchment areas,” and the study’s paramount goal was to mark out catchment areas for the oceans. The result was unexpected: water and garbage from Mozambique, for example, reaches the South Atlantic garbage gyre within 10 years.

“The littering of the ocean is a global problem,” Froyland wrote, “and plastics are not confined to one’s ‘local’ ocean. We hope this realization will promote global action on mitigating further contamination of the world’s oceans by plast

Images credit: NOAA Okeanos Explorer Program

Because seawater cannot be compressed, when wind drives water toward the gyres, some of it is forced downward. From fundamental physics, Froyland said, “oceanographers already had a reasonable idea of how some ocean currents circulate and expected down-welling and regions of convergence on the surface in some oceans, perhaps all.”

Before you’re tempted to give too much scientific credit to the garbage patches for identifying the gyres, Froyland points out that while the notion of garbage patches “has raised the profile of these areas in the public’s mind — and puts another face on ocean pollution — garbage did not play a major role in finding these areas of convergence.”

– David J. Tenenbaum
1 2 3

Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer

Bibliography

  1. How well connected is the surface of the global ocean? Gary Froyland, Robyn M. Stuart, and Erik van Sebille, Chaos: An Interdisciplinary Journal of Nonlinear Science, September 2, 2014.
  2. Mapping ocean garbage
  3. “Garbage iceberg” image credit to Middle of the Pacific.