A Blue View: The Ocean's Flow

The First Years Inc. always expected its line of plastic bath toys to end up in water, but it never dreamed they would ride ocean currents 17,000 miles around the world.

Published June 23, 2015

In January 1992, a shipment of 29,000 plastic yellow ducks, blue turtles, red beavers and green frogs washed into the Pacific Ocean on the way from Hong Kong, China, to Washington, USA. The container broke open, and the packaging quickly deteriorated, sending the toys on a journey that would teach us what we know today about our oceans’ currents.

Using computer simulation of ocean currents to map the toys’ journey, experts were able to determine the speed of these currents and how long it takes objects to complete a full circuit around the North Pacific Gyre. Today, around 2,000 of the bath toys are still bobbing in the currents of the North Pacific Gyre, where they’re most likely trapped in the Great Pacific Garbage Patch, a dense mass of marine debris.

Understanding the Global Ocean Conveyor Belt

Deep underwater currents are powered by a process called thermohaline circulation, which distributes heat and moisture throughout our oceans. Also known as the global ocean conveyor belt, this system of deep-ocean circulation is generated by temperature and salinity.

Though its force is 16 times stronger than all of the world’s rivers combined, the conveyor belt churns at a much slower speed than wind-driven or tidal currents, taking nearly 1,000 years to complete a single cycle.

To understand how it works, it’s important to know that water density depends on temperature and salinity. The colder and saltier the water, the denser it is. So when water reaches the poles, it freezes, but all of the salt doesn’t freeze with it, so the cold, salty water that’s left behind sinks to the bottom of the ocean. More water rushes in to take its place, creating a current. This new water continues the cycle as it gets cold and sinks.

Powered by this system, the ocean’s water travels south, between Africa and South America, around the edge of Antarctica and then splits into two sections: One travels into the Indian Ocean, while the other moves into the Pacific Ocean. Both move northward toward the equator, and then loop back southward and westward to the South Atlantic, eventually meeting again in the North Atlantic, where the cycle repeats.

The conveyor belt plays an essential role in our oceans’ ecosystems, distributing nutrients throughout our planet’s waters and helping support the world’s food chain.

Global warming has the potential to cause an influx of warm freshwater onto the sea surface due to increased rainfall in the North Atlantic and the melting of glaciers and sea ice. This unusual influx of warm freshwater into the North Atlantic could prevent sea ice from forming, potentially disrupting the thermohaline cycle.

To learn more about the impact plastics and microplastics are having on our blue planet, tune into our Google+ Hangout tomorrow, June 24, at 12:15pm! 

Episode Transcript

Go with the flow. For some people, this is a life philosophy; for oceanographers, it describes the very dynamics of our global ocean.

Early seafaring cultures knew of dependable ocean currents that moved their craft swiftly across the sea, like conveyor belts. The Polynesians were among the first to develop oceanographic maps. Made from shells and sticks of bamboo, these stick charts enabled them to deftly navigate Pacific currents. 

Mapped in 1762 and named by Benjamin Franklin, the Gulf Stream is the Atlantic Ocean’s best-known current. Functioning like a gigantic river, it carries a greater volume of water than all of the world’s freshwater rivers combined. 

Starting with the Gulf of Mexico’s loop current, the stream flows around the southern tip of the Florida peninsula and runs up the Eastern Seaboard. Here in Maryland, at Ocean City, Gulf Stream eddies make for great sport fishing. It then speeds up the coast into New England, finally taking a right at Cape Cod to cross the Atlantic to Europe, influencing the climate of every landmass it passes.

For example, Penzance, on England’s southern tip, is warmer than any other area at that latitude thanks to the Gulf Stream's tropical effect. In spite of its location, its environment even supports palm trees!

But the map of global ocean currents wasn't drawn with precision until fairly recent times. And that was literally a windfall, involving a now-famous shipment of rubber duckies. 

In 1992, a ship on its way from Hong Kong to the United States was carrying, among other things, a container of bath toys. The container was accidentally lost overboard in the middle of the Pacific, where it broke open and 28,000 plastic ducks were let loose to sail (or more accurately, bob) across the high seas. 

Oceanographers call this fleet the "Friendly Floatees," and they became a boon to science, functioning as current-tracking mini-buoys for years.

Among other things, the duckies provided proof of the existence of the North Pacific Gyre, where 2,000 ducks still float, locked in a loop with other plastics from all over the world in the Great Pacific Garbage Patch.

In 2007, a duckie appeared on a beach in France, marking a 15-year, 12,000-mile journey around the globe, from the Pacific to the Atlantic. Another was found trapped in Arctic ice. The message from the duckies is clear: Our one world ocean is truly interconnected.

While the Gulf Stream and the North Pacific Gyre are composed of surface currents driven by wind and the Earth's rotation, there are also great, moving rivers in the deep ocean.

Driven by temperature and salinity gradients, the thermohaline conveyor—called thermo for temperature and haline for salt—begins as the warm Gulf Steam cools in the northern Norwegian Sea. Ice forms, taking with it freshwater and leaving behind dense, salty, cold water that sinks to the bottom of the ocean.

Amazingly, this global ocean conveyor carries a volume of water almost 5,000 times greater than Niagara Falls, transporting 20 million cubic meters of water per second. That's equivalent to half a million Olympic-sized swimming pools every minute. 

This global circulation system slowly mixes and churns all of the planet’s seawater in a process that can take a thousand years.

Scientists believe climate change may already be impacting this important system and, by extension, the nutrient distribution that is the foundation of the world's fisheries, underscoring again the importance of mitigating the effects of global climate change.

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