Australian oceanographers and mathematicians have joined forces to track down those responsible for the floating garbage islands in the world’s oceans.
The best known here in Australia is the Great Pacific Garbage Patch between Hawaii and California where pieces of plastic outweigh plankton in that part of the ocean and pose risks to fish, turtles and birds.
Scientists believe the garbage patch is but one of at least five, each located in the centre of large ocean currents called gyres which suck in and trap floating debris.
Researchers from the University of New South Wales have created a new mathematical model which could help determine who’s to blame for each garbage patch, a difficult task for a system as complex and massive as the ocean.
“In some cases, you can have a country far away from a garbage patch that’s unexpectedly contributing directly to the patch,” said Gary Froyland, a mathematician at UNSW.
For example, the ocean debris from Madagascar and Mozambique would most likely flow into the south Atlantic, even though the two countries’ coastlines border the Indian Ocean.
Erik van Sebille, an oceanographer, says the new model could also help determine how quickly garbage leaks from one patch into another.
“We can use the new model to explore, for example, how quickly trash from Australia ends up in the north Pacific,” he says.
At the heart of the researchers’ work on the origins and fate of floating rubbish lies a bigger question: how well do the ocean’s surface waters mix?
Fast-moving ocean currents form due to winds, differences in water temperatures, salinity gradients across the globe and the forces caused by the spinning Earth.
Currents stir ocean waters but they also serve as barriers which minimise mixing between different ocean regions, much like the blast of fast-moving air at the entrance of an air-conditioned store keeps the cold inside air from mixing with the warm outside air.
Froyland, van Sebille and their UNSW colleague Robyn Stuart divided the oceans into seven regions whose waters mix very little.
Their approach borrowed mathematical methods from a field known as ergodic theory, which has been used to partition interconnected systems such as the internet, computer chips and human society.
The analysis revealed the underlying structure of the ocean without getting bogged down in complex simulations.
“Instead of using a supercomputer to move zillions of water particles around on the ocean surface, we have built a compact network model that captures the essentials of how the different parts of the ocean are connected,” says Froyland.
According to the new model, parts of the Pacific and Indian oceans are actually most closely coupled to the south Atlantic, while another sliver of the Indian Ocean really belongs in the south Pacific.
“The take-home message from our work is that we have redefined the borders of the ocean basins according to how the water moves,” says van Sebille.
The researchers describe the model in a paper published in the journal Chaos.
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