May 2, 2011 | 2
In Moby-Duck, Donovan Hohn tracks the fate of 28,800 plastic bath toys (“rubber” ducks, frogs, turtles and beavers) across the northwestern coast to their origins in China and even through the Northwest Passage. But how did these bath toys come to be spread on the shores of Alaska, Washington, Hawaii and Russia?
On January 10th 1992, the Ever Laurel, a large container cargo ship, was caught in a storm in the North Pacific with severe waves rolling her from side to side. Under the strain of the pitching rolling seas, several of the 40 foot long containers snapped their retaining links and crashed into the icy Pacific. The spill happened at 44.7N, 178.1E, about 500 miles south of Shemaya Island in the Western Aleutian Islands and 2,000 miles northwest of the Hawaiian Islands.
Within a day or two at most, the boxes and individual packaging for each set of four floating toys (one of each type) broke down enough for the toys to be set free, though likely not all of them were set free at the same time, which we’ll see later might matter. By November some of the toys had washed up on the beaches of Chickagof Island in southern Alaska’s Inside Passage region, where they were discovered by beachcombers.
The following spring the bath toys were discovered all over 500 miles of the south Alaskan coast from Kayak Island in the north to Coronation Island in the south. In the fall they began washing ashore on Shimaya Island at the western end of the Aleutian Island chain. Since 1992 ducks have washed ashore in waves on the beaches of southern Alaska. Peak numbers of toys washed ashore every 2 years at first, but as they have become weathered by sun and waves the cycle has lengthened with peaks occurring every 3-4 years.
In 1995 some of the toys were collected on Washington state’s beaches. (Figure 1) They have been recovered in Russia, on Kure Atoll (1,200 miles due south of the spill site) and Lanai Island, Hawaii. Their travels have also landed them in more than one scientific journal and at least one widely used introduction to oceanography text.
According to computer models, some of the toys could reach Australia, and South America in the Pacific, the Indian Ocean, and could cross the Arctic to be found on New England beaches or in England. Most, however, will probably be broken down by sun and waves to end up as small bits of plastic in the North Pacific
Figure 1) Confirmed landings of the First Years bath toys. Major current patterns in light blue. – Source: Eric Heupel; created in Google Earth, currents hand drawn over Google Earth; bathy, rubber duck icon created in photoshop.
But how did the the toys come to be on those beaches from Shimaya to Lanai?
When each of the bath toys popped free of their containers and packaging and set off on its journey, that journey was controlled by the same four forces that drive the surface currents of the ocean circulation: wind, friction, the Coriolis force and pressure gradient force (essentially gravity).
The process begins as the winds blow across the water. Friction transfers some of the energy from the winds to the surface layer of water setting it in motion, generally at about 2% of the wind’s speed. A sustained wind of 20mph could generate up to a 0.4mph current.
The water, however, does not move directly with the wind. Because of the Earth’s rotation, the surface layer of water is deflected from the direction of the prevailing wind. This is the Coriolis force in action. The amount of deflection depends on how far north or south of the equator one is and the direction depends on which hemisphere the movement is happening. On average, the surface skin of water will be deflected 45° to the right or left of the wind direction in the Northern or Southern hemisphere respectively.
As the energy is transferred to deeper layers of water it becomes weaker and continues to be turned by Coriolis forces. Over the surface 100 to 150m of water, the net transport of water is 90 degrees to the right of the sustained average winds (known as the Eckman transport, Figure 2).
Because of the longterm average pattern of winds – easterly trade winds in the tropics, westerlies in the mid latitudes and easterlies again in the polar and sub-polar region – and the effects of Eckman transport, water is piled up in the center of mid-latitude gyres creating a hill or high pressure zone. Similarly, the wind patterns cause low pressure gyres in the sub-polar region. Because Coriolis forces are very weak in the tropics and 0 at the equator, gyres do not form in those regions.
Figure 2) Schematic of Eckman Transport. Image Public Domain, crated by NOAA. Available at http://oceanservice.noaa.gov/education/kits/currents/media/supp_cur05e.html
In many ways, the high and low pressure gyres in the ocean are just like the high and low pressure systems in the atmosphere. Just like winds circle high pressure cells clockwise and low pressure cells anti-clockwise in the northern hemisphere (and the opposite in the southern), geostrophic currents form around high and low pressure cells in the ocean, flowing clockwise around high pressure cells and anti-clockwise around low pressure cells.
In the North Pacific there are two gyres that have affected the travel of the bath toys. The North Pacific Gyre has currents rotating clockwise around a high pressure zone and forms the dominant gyre of the North Pacific. Above the North Pacific Gyre, the Sub-Polar or Aleutian Gyre rotates anti-clockwise around a low pressure zone. (Figure 3)
Figure 3. Major currents and gyres of the North Pacific – A crop into an image from the Wikimedia Commons: http://en.wikipedia. org/wiki/File:Ocean_currents_ 1943_(borderless)3.png. Original source: Ocean Currents and Sea Ice from Atlas of World Maps , United States Army Service Forces, Army Specialized Training Division. Army Service Forces Manual M-101 (1943).
These surface gyres provide the major long term (months to years) average movement of surface currents and the bath toys, but details are important for each of the individual toys. Winds change direction and force constantly, which will affect the hour to hour tracks of each toy. Since the toys ride high in the water, at least when new, they can also be pushed directly by the wind.
The more of the toy that floats above the water, the more sail area the wind has to act upon. The more the wind pushes the toy directly, the faster the toy will move and the more the toy’s direction of movement will be closer to the true direction in which the wind is blowing. The toy will also respond to changes in wind direction rapidly when floating high in the water.
Changes in the local wind field also affect the surface currents making them divert from the average current path either in speed or direction or both. These changes in wind and current create turbulence and eddies in a range of energy and size scales and change the paths of each individual toy.
It is also likely that the toys obtained their freedom from their packaging over a period of several hours. It may not matter for the average current and gyre analysis, but it could make a difference for a floating toy whether it turns north or south at the end of the North Pacific Drift current. If it turns north does it continue to cycle in the sub-polar gyre or wash ashore on Chickagof Island or Shimaya Island? Or maybe it travels into and through the Bering Sea and on to the Arctic?
Maybe due to the timing of the release and the small variations in wind and current, the toy will turn south at the end of a loop through the North Pacific Drift current. It could then wash ashore in Washington State as several did in 1995. Perhaps having turned south it will remain afloat all the way through California and enter the North Equatorial current, which will carry it south of Hawaii, to the Philippines and Japan.
Anywhere along the way, another storm or variation in the wind could break it out of the gyre loop and into another current system, perhaps to visit Australia, or it could travel deeper into the interior of the gyre where it may remain for an eternity slowly breaking up and degrading to smaller pieces, new flotsam trapped in the most infamous of the garbage patches.
As for why these floating toys – and countless other accidental floats such as Nike shoes, basketballs, and even computer monitors – are important, it comes down to money and time. Scientists have been performing drifter experiments for quite some time now and in general there are two basic options: cheap and plentiful, or expensive and sparse.
The first option is to use cheap drifters, usually a message in a bottle, and release hundreds of them of them hoping for decent returns (usually 2-3%). The advantage is that drifters are cheap, so many more of them can be used. Ship time is something that can be done relatively cheaply (1-2 days) or it can be piggybacked with other missions that are paying the boat time for longer cruises to more remote starting points. The downside is low rate of return (1-3%) even when rewards are offered.
The second option is to use instrumented drifters or hi-visibility drifters and track them by boat or air. The instrumented drifters are far more expensive so fewer can be used for a given budget, though these drifters can also do oceanographic sampling (temperature, salinity, etc.) and either store the data or phone it home.
Accidental drifters releases like the bath time toys, however, when the time and location of their release is known, can provide literally 10s to 100s of thousands of drifters. Even with a 2% return rate, this is still thousands of data points that can be analyzed to continue to refine our surface ocean models which are used to model and predict oil spill flow, fisheries recruitment, and plankton distribution and transport.
About the Author: Eric Heupel worked in Satellite communications, computers and design for many years, but returned to school to pursue his early love of marine sciences. He is curently working on his Masters in Biological Oceanography at the University of Connecticut’s Avery Point Campus. Eric also blogs at The Other 95%, photo-blogs at Larval Images, has a personal site at Eclectic Echoes, he tweets as @eclecticechoes, and his images are available on Flickr.
The views expressed are those of the author and are not necessarily those of Scientific American.
Related at Scientific American:
Slabs, Sneakers, Gyres and the Grotesque By Matthew Garcia
Overboard: 28,000 toys and one man, lost at sea By Lindsey Hoshaw
A True Duck Hunt: interview with Donovan Hohn By David Manly
How does a floating plastic duckie end up where it does? By Eric Heupel
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