Output of Harper Simmons global model of semi-diurnal baroclinic tidal generation and propagation. The beam heading from the Maquarie Ridge toward Tasmania motivates the study
Graphic showing the path of the East Australia Current its associated eddies. Citation: Ridgway, K. & Hill, K. (2012). Retrieved from www.oceanclimatechange.org.au [02-02-15]
Anyone who has surfed knows the sheer power of waves, some of which can reach tens of metres high. They seem large and formidable enough. But imagine waves many times bigger than these – some reaching a kilometre or more in height and perhaps hundreds of kilometres in length. Such monsters are not the stuff of science fiction but can be found lurking, far from view, way beneath the ocean surface. Though unseen, undersea waves play a vital role in the transport and distribution of marine sediments and nutrients. They also affect global warming and climate change – and have the wherewithal to overturn ocean oil and gas rigs.
“Deep mixing caused by the internal waves breaking in water more than 500 metres deep is important to predict global ocean circulation, which occurs through density differences between water at the poles and equator,” says environmental engineer Nicole Jones from the University of Western Australia. “If we don’t understand how much internal waves contribute to the deep ocean mixing then we cannot model climate change accurately,” explains Associate Professor Jones. She is participating in an international study of undersea waves off the coast of Tasmania, which is potentially a global hotspot for deep tidal mixing.
To this end, she and her doctoral student Tamara Schlosser will spend most of this month on board a US research vessel, the Roger Revelle, working with fellow scientists from the Scripps Institution of Oceanography, the University of California in San Diego, Oregon State University and University of Alaska. As well as climate modelling, their research is likely to benefit fisheries managers. “Undersea waves are important for transporting deep nutrients, which are trapped in the dark, deep water, into the surface layers where there is enough light for phytoplankton to grow,” Associate Professor Jones says. “This, in turn, influences the productivity of fisheries in the region.”
Not far from where the Sydney-to-Hobart yacht race runs every year, the team will study tidal waves generated off New Zealand’s south coast that break on the Tasmanian continental shelf. Some of these waves are up to 240 kilometres long. Associate Professor Jones, who has conducted similar studies off Western Australia’s North West Shelf, says much remains to be discovered about internal waves, which affect safety aspects of the oil and gas industry and ecosystem management. During their time at sea, the team will deploy moorings, process water samples and analyse data received from a raft of instruments.
Ocean surface waves travel at the interface between heavy water and the light air. This represents a huge difference in density. In the ocean, there is lighter water at the surface and heavier water close to the bottom. These density differences are due to both the temperature (warmer water is lighter) and the amount of salt in the water (saltier water is heavier). Waves travel along these density differences in the ocean just as they travel at the surface. But they have different properties as the density difference is much smaller. Scientists measure undersea waves by assessing the ocean temperature and salinity at different depths. They can see a wave pass by as very cold water from the deep ocean travels up towards the surface and then back down again. Just as surface waves are created by disturbances of some kind, undersea waves are produced by disturbances to various density layers. The surface tide pushing water up and down undersea mountains and slopes is an important factor in creating internal waves.
“This type of internal wave is called an internal tide and its frequency matches that of the surface tide every 12 or 24 hours, depending on its location,” Associate Professor Jones notes. Again, like surface waves, internal waves steepen when they travel into shallow water and break. Imagine, for example, a surface wave breaking at the beach and pushing water up the sand. An internal wave undergoes a similar process but, as it occurs in much deeper water, the effects cannot be observed at the surface – only close to the seafloor. The strong currents and turbulence created by breaking internal waves can impose huge forces on seabed structures. So, it’s important to understand internal waves to ensure that structures such as oil and gas pipelines are designed appropriately.