UNSW researchers solve mystery of the ocean's shadow zone

What does it take to suspend 180 million cubic kilometres of water below the ocean’s surface for hundreds of years?

According to an international team of researchers led by Dr Casimir de Lavergne, from UNSW’s School of Mathematics and Statistics, the answer lies in the shape of the seabed.

While the research may have answered one question about deep ocean water it has also opened doors to answer more questions that relate to the future impacts of climate change.

Dr Ryan Holmes, Climate Change Research Centre, UNSW

The research has found that the turbulence along the seafloor of the North Pacific cannot lift the bottom water any higher than 2.5km below the surface.

This means that there is a patch of water between about 1 and 2.5km deep that has been sitting stagnant for hundreds of years.

While researchers have known for some time that oldest ocean waters lie deep in the North Pacific Ocean, the South Pacific Ocean and the Indian Ocean, until now they have not been able to pinpoint the exact cause.

Now, factoring in the shape of the sea-bed within their theory, researchers have identified the combination of factors which results in a lack of vertical water circulation

For water to move upwards, deep layers of water must become lighter. This occurs mostly along the seafloor, where turbulence mixes dense waters with lighter ones above, and where heat from inside the Earth enters the ocean. As a result, waters that are in contact with a larger area of the seafloor move upwards faster. Meanwhile, waters that are nearer to the ocean’s surface are moved by wind-driven currents.

In the North Pacific Ocean, the mid-depth layer turns out to be excluded from the overall movement of waters, hence the large pool of quasi-stagnant, isolated waters filling the mid-depths of the basin.

Dr Ryan Holmes, from UNSW’s Climate Change Research Centre, said that the results of this research will help to understand the ocean’s role in climate transitions.

“When this isolated shadow zone traps millennia-old ocean water it also traps nutrients and carbon which impact the ocean’s capacity to influence climate over time,” said Dr Holmes.

“So, while the research may have answered one question about deep ocean water, it has also opened doors to answer more questions that relate to past and future climate variations.”

The full research paper is published in the latest issue of Nature.