How do seeds from vines in Mexico end up in Norway and Japanese fishing floats find their way to Alaska? The same way 29,000 bathtub toys that escaped their containers during a storm in the North Pacific in 1992 were washed up almost 2500 miles away on the shores of southwestern Alaska ten months later. The short answer is they were transported by currents, then held by giant eddies called gyres, and finally released into other currents.
Ocean currents flow in complex patterns affected by wind, water density, salinity, solar heating near the equator; topography of the ocean floor and the rotation of the earth. These currents are relatively constant and flow in one direction only, in contrast to the tidal currents closer to shore. Surface currents are generally wind-driven and deep ocean currents are driven by the other factors.
Wind causes large surface currents that flow year round. At the equator the direct rays of the sun cause intense heating. As the hot air rises, it leaves areas of low pressure in its place. Warm air from the equator begins to cool and sink about 30 degrees north and south of the equator. The rest of the air flows toward the poles. The air movements toward the equator are called trade winds: warm, steady breezes that blow almost continuously. The rotation of the earth also affects the currents through the Coriolis force. This force causes water to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It exists because ocean water at the equator is moving at the same speed as the Earth. As you move north, there is less friction from the earth beneath it. A wind blowing for ten hours across the ocean will cause the surface waters to flow at about 2% of the wind speed (University of Southern California, earth sciences).
The second effect of the sun is to alter the density of the ocean surface water by changing its temperature and its salinity. If water is cooled or becomes saltier through evaporation, it becomes denser. This can result in the water column becoming unstable, setting up density-dependent currents, also known as the thermohaline (thermo – temperature, haline – salinity) circulation. Ocean currents, flowing under the surface of the ocean, are hidden from immediate detection. These submarine rivers are often called ‘global conveyor belts’. For example, deep water forms in the North Atlantic, sinks, moves south and circulates around Antarctica and then moves northward to the Indian, Pacific, and Atlantic basins. It can take a thousand years for water from the North Atlantic to find its way into the North Pacific.
Currents help transfer oxygen from the atmosphere into the deep ocean. The sinking water is very cold and contains high concentrations of dissolved oxygen acquired at the surface (cold water can hold more oxygen than warm water). During their flow, they mix with ‘older’ water that has been away from the surface for a longer time, thus ensuring that the bottom waters of the ocean are supplied with oxygen. Currents also transport nutrients and trash.
The American National Aeronautics and Space Administration (NASA) have developed a visual tool to see and predict currents. The program is called OSCURS (Ocean Surface Current Simulator) and can be found at: http://oceanmotion.org/html/resources/oscar.htm.
Now that we have had a brief overview of ocean currents we can understand the development of large ocean eddies called gyres. A gyre is a large system of rotating ocean currents formed by currents generated by surface winds, the movement of the earth and land masses. The major gyres of the earth’s oceans are named for their locations: North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean gyres. The map shows a simplistic drawing of the gyres. There are several smaller gyres; the large gyres contain sub-gyres. For example, the Atlantic and the Pacific Ocean have four gyres each. The gyres rotate in either a clockwise or counter-clockwise direction depending on the hemisphere. The gyres are huge – the size of continents – and it can take years for a piece of flotsam to exit the gyre. For example, the size of the North Atlantic gyre is 1,200 x 3,000 nautical miles, with an 8,000 nautical mile circumference. The orbital period is 3.3 years, but only half of the stuff in the giant gyre will escape in each full revolution; thus there is a one percent chance that a given object would remain adrift after seven orbits – 23 years. These figures are taken from the book Flotsametrics and the Floating World (Curt Ebbsemeyer and Eric Scigliano, 2009. HarperCollins Press).
Let’s get back to the tub toys and their story. On 10 January 1992 a ship, sailing from Hong Kong, lost a container overboard and released 29,000 bath toys into the ocean. Caught in the North Pacific Gyre (counter-clockwise ocean current in the Bering Sea, between Alaska and Siberia), the ducks took ten months to begin landing on the shores of Alaska. By 2005 some of the toys washed up on beaches in eastern North America between Maine and Massachusetts. In 2007 the first tub toys were found on beaches in England.
It is not just rubber ducks that get caught in gyres. Trash enters coastal currents and is fed into the gyres creating huge floating islands of garbage. The North Pacific gyre is often called the garbage patch. By all accounts this gyre is a mass of plastic and other debris that resists decomposition. Dead birds can be found with bellies full of plastic debris.
Devi Sharp is a retired wildlife biologist and is exploring the Caribbean with her husband, Hunter and Bert, their rubber duck on their sailboat Arctic Tern.
To read the full gyrating history of the tub toys try the following websites: http://beachcombersalert.org/RubberDuckies.html or http://www.rubaduck.com/news/rubber_duck_news-200302-duckies_around_the_world.htm
For more information about trash and gyre, visit:
http://www.greatgarbagepatch.org
http://www.5gyres.org
http://www.greatgarbagepatch.org
http://earthsky.org/water/pacific-ocean-gyre-filled-with-plastic-trash
