How Ocean Currents Move Pollution Around the World

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The image of our home Blue Planet is increasingly stained and dotted with human contaminants. It is not news that we are in the midst of a pollution crisis spanning both over land and the seas. 

Unfortunately, however, oceans end up taking up much of our land pollution, making them especially vulnerable to the numerous mishandlings.

We are all appalled by the amount of plastic pollution that floats around our seas. Especially striking are the images of what were supposed to be pristine beaches on remote islands, nevertheless littered with trash (More: This Remote Island Has the Highest Density of Trash in the World). It seems that no place can exclude itself from the infestation of plastics.

Plastic trash bag on the ocean floor.
Plastic trash bag on the ocean floor. Image: NOAA.

But the rage of hydrocarbons doesn’t stop with plastic litter. Oil spills are even more dramatic catastrophes, bound to happen periodically as long as the oil is extracted from the seafloor and shipped around the ocean. Then, like in the case of plastics, waters and coasts away from the original point of accident can fall victim to crude oil pollution.

How do these endless swathes of plastic and oil stains end up in such faraway places in the first place? How does the pollution move around Earth’s oceans?

The short would be – ocean circulations.

As ocean current systems and patterns are sometimes hard to grasp, we will focus on two particular examples: the Great Pacific Garbage Patch and the 1989 Exxon Valdez oil spill.

These two case studies will give you an insight into the power and the complexity of the oceanic currents and their interaction with the dropped by-products of our civilization.

The Great Pacific Garbage Patch 

The Great Pacific Garbage Patch (GPGP), also known as the Pacific trash vortex, is an accumulation of marine debris located in the North Pacific Ocean, spanning from the West Coast of North America to Japan.

Concentrations of marine debris known as the Ocean Garbage Patch in the North Pacific Ocean. Map: NOAA.
Concentrations of marine debris known as the Ocean Garbage Patch in the North Pacific Ocean. Map: NOAA.

A significant fraction of the debris is plastic – mostly microplastics and fishing gear. The suspended particles can never fully degrade and move around in the water column in a soup like-mixture. Unfortunately, they are commonly consumed by marine life. Except for the apparent death cases, the consequences of this plastic diet are not yet fully known.

The estimated surface of the GPGP is 1.6 million square kilometers, making it twice bigger than Texas and three times larger than France. In the manner of all large geographic entities, the patch has its “regions” – the Western Garbage Patch near Japan, and the Eastern Garbage Patch, located between Hawaii and California. 

How did the plastic trash from our coasts end up in the middle of the ocean? 

Gyres guided them.

What are Ocean Gyres?

gyre is a large, swirling system of circulating ocean currents. The world’s oceans host five of them:

  • The North Atlantic gyre,
  • The South Atlantic gyre, 
  • The North Pacific gyre, 
  • The South Pacific gyre,
  • The Indian Ocean gyre. 
Map showing the world's ocean gyres.  Source: NOAA.
Map showing the world’s ocean gyres. Source: NOAA.

All of the gyres have a significant role in ocean circulation, driving the so-called oceanic conveyor belt that circulates ocean waters around the globe. The circulation also includes and draws in the coastal ocean waters – and the pollution suspended in them.

The “patron gyre” of the GPGP is The North Pacific Subtropical Gyre. Four currents form this gyre:

  • The California current
  • The North Equatorial current
  • The Kuroshio current
  • The North Pacific current

The currents rotate clockwise around an area of 7.7 million square miles (20 million square kilometers). 

The circular motion at the gyre’s edges draws debris into the calm and stable gyre center. It is precisely within this slowdown that all the debris – and junk – accumulates and becomes trapped.

To add to the dynamics, the warm water from the South Pacific meets the cool water from the Arctic, creating the North Pacific Subtropical Convergence Zone. Located a few hundred kilometers north of Hawaii, the convergence zone acts like a freeway that links the debris in the Western Garbage Patch and the Eastern Garbage Patch.

How Garbage and Plastic Pollution Enter Ocean Gyres

National Geographic has envisioned an excellent thought experiment. Let’s imagine that someone has thrown a plastic bottle off the coast of California. The bottle will first take the California Current south toward Mexico. Then, as if swapping trains, it can catch the North Equatorial Current, which helps it cross the immense Pacific. Once near the coast of Japan, our bottle can travel north on the powerful Kuroshiro Current, catching the North Pacific Current that carries it. At the end of its journey, the gentle vortexes of the Eastern and Western Garbage Patches gradually draw in the bottle.

NASA carried out a visualization of how plastic debris can collect in oceans with this simulation:

To avoid fueling a common misconception, we have to point out that, unlike our imaginary bottle, the real-life plastic litter rarely reaches the garbage patch intact. Instead, it gradually breaks down into smaller pieces, eventually becoming microplastic which composes most of the GPGP “soup.”

While the Great Pacific Garbage patch gets the most attention, unfortunately, it is not the sole garbage patch in the ocean. Researchers have discovered one more in the South Pacific Ocean and another one in the North Atlantic. In the same manner, only powered by different currents, the plastic soup also circulates in these marine areas, entering the food chain and posing a possible health risk to animals. Lastly, it may also reach our plates via seafood, completing the circle, in a way.

The Exxon Valdez Oil Spill

The Exxon Valdez oil spill happened on March 24, 1989, and to this date, it remains the worst oil spill in terms of damage to the environment. The spill occurred when the Exxon Valdez tanker struck Blight Reef in the Prince William Sound, off the coast of Alaska. 

As a result, nearly 11 million gallons (35,000 metric tons) of crude oil leaked out into the water over several days. The inaccessibility of the location made efforts to contain the spill largely unsuccessful, and the oil was able to spread around the region, affecting 1,300 miles (2,100 km) of coastline. Heavy and moderate oil pollution occurred along 200 miles (320 km) of the coast.

How exactly did the oil manage to cover such a large area?

While it’s not super hard to deduce that ocean currents are the force moving the oil around, the complete picture paints a complex relationship between oceanic and meteorological factors. Right after the accident, in 1990, Exxon itself funded an analysis in which a team of scientists reconstructed the exact pattern of the oil spill dissemination in the region. 

The main conduit for Exxon Valdez oil was the coastal circulation of the northwest Gulf of Alaska and Prince William Sound, with a particular coastal convergence being the reason for such an even spread of the spill. The convergence has two main agents – the influx of coastal freshwater and the favorable downwelling winds. 

Prince William Sound is fed by fresh water from a narrow but abundant coastal drainage region, dominated by small streams powered by plentiful precipitation and glacier melts. The freshwater discharges tend to move surface waters offshore. Since Prince William Sound is located hundreds of miles downstream, it experiences a cumulative effect from the discharge.  

Timeline of recovery from the Exxon Valdez spill.  Image: NOAA.
Timeline of recovery from the Exxon Valdez spill. Image: NOAA.

Then, there are the winds. The same storm systems bringing abundant precipitation are also responsible for winds that create coastal convergences in the ocean and the downwelling of the surface waters. The cyclonic, storm-associated easterly winds are steered by the nearby mountains and fed by latent heat from the coastal precipitation. The main energy source for the entire process is the North Pacific Ocean, which supplies the atmosphere with moisture and heat. Vital for the Exxon Valdez spill, the winds move surface water to the right of the wind direction, causing convergence and accumulation of surface waters near the coast.

The freshwater discharges that move surface waters offshore and the favorable downwelling winds which move surface waters onshore join their forces to create a remarkably consistent coastal convergence. Once the oil was inside the current system, it was carried along the coast for hundreds of miles. The floating hydrocarbons didn’t stay in the proximity of the accident for more than a few days.

If another accident were to happen in the Prince William Sound in the future, it would probably follow the same pattern and trajectory of the Exxon Valdez spill, with the speed of the spread depending upon the time of the year. The team behind the study claimed that the distribution would probably be even speedier than in the case of the Valdez spill. 

The Future of Our Floating Trash

Studying the movement of anthropogenic pollution and all the factors that drive them is not essential just for calculating the scale of the contamination and figuring out how to manage it in the present. It is also vital for the future. 

By studying past accidents and phenomenons and utilizing the ever-more-powerful computer models, scientists can now predict how the drifting pollution will behave and where it will go. That is critical for future mitigation of accidents such as the Exxon Valdez spill. By knowing what to expect, there is a greater chance of preventing an accident from turning into a disaster.

Also, knowing where ocean currents have a propensity to accumulate our litter can help initiatives such as the Ocean Cleanup to find their targets more efficiently. 

Hopefully, one day, when our civilization matures and learns how to handle its waste, we can get back to studying ocean currents and wind patterns for their own sake – and marvel at their power unstained by our hydrocarbons.

References

The Study:

Royer, R.C, et al. (1990). Ocean circulation influencing the Exxon Valdez oil spill. Oceanography, November 1990 https://tos.org/oceanography/assets/docs/3-2_royer.pdf

Articles

Lunn, J. (2017, June 8). Ocean currents push Mainland pollution to remote islands. Eos. https://eos.org/research-spotlights/ocean-currents-push-mainland-pollution-to-remote-islands

National Geographic Society. (2012, October 9). Great Pacific garbage patchhttps://www.nationalgeographic.org/encyclopedia/great-pacific-garbage-patch/

NOAA ocean podcast: Ocean garbage patches. (n.d.). NOAA’s National Ocean Service. https://oceanservice.noaa.gov/podcast/mar18/nop14-ocean-garbage-patches.html

Questions and Answers about the Spill. History of the Spill. (n.d.). Exxon Valdez Oil Spill Trustee Council. https://evostc.state.ak.us/oil-spill-facts/q-and-a/

The Great Pacific Garbage Patch. (2020, February 11). The Ocean Cleanup. https://theoceancleanup.com/great-pacific-garbage-patch/

What is a gyre? (2021, February 26). NOAA’s National Ocean Service. https://oceanservice.noaa.gov/facts/gyre.html

What is the global ocean conveyor belt? (2021, February 26). NOAA’s National Ocean Service. https://oceanservice.noaa.gov/facts/conveyor.html

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