Electronics engineer Lindsay Macdonald (left) and oceanographer Ken Ridgway with the deep ocean glider that has been generating substantial profiles of the Tasman Sea. (CSIRO)
Giant water wall discovered riding the ocean currents
Deep-diving ocean 'gliders' have revealed a 200-metre tall, 40 kilometre wide wall of undiluted Bass Strait water travelling hundreds of kilometres to the Indian Ocean.
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Glen Paul: G’day, and welcome to CSIROpod, I’m Glen Paul. If you’ve ever been dumped by a wave in the surf you’ve likely tasted salt water, and you’d be pretty hard pressed to notice any real difference in saltiness if it happened to you at Bondi Beach or Bells Beach, but the salinity of ocean water does vary. Melting ice, flow-in river water, rain, snowfall, evaporation, wind, wave motion, and ocean currents, can all cause differences in salinity.
A good example of this is a recently discovered 200 metre tall, 40 kilometre wide disc of water that travelled undiluted from its origin in Bass Strait for hundreds of kilometres. The wall of water was revealed by deep diving ocean gliders as part of a study by CSIRO Oceanographers and the University of Technology Sydney.
Joining me on the phone to discuss the study is CSIRO Wealth from Oceans Scientist, Ken Ridgway. Now Ken, a giant wall of undiluted water travelling through the ocean sounds pretty amazing, just how undiluted is it, and how does it stay that way and not just mix in?
Ken Ridgway: Well it does change over time, but does retain its signature, basically the temperature and salinity that it had when it was formed, it retains that essentially for months, and even years. So the basic idea is that the surface of the ocean is acted on by wind, which can cause a lot of mixing, especially when the wind is strong, but once the water parcel is below the surface layer that the wind has any influence, there’s not a lot of other mechanisms that can mix it, so mixing does happen, but it’s very slow and happens over a long period of time. So the water that’s formed at the surface, if it does sink and go below the surface, it will retain that signature that it was formed with.
Glen Paul: So could the variation be noticed by marine life, for example a shark notice it in his gills as he swam through it, and think to himself, this doesn’t feel quite right, I think I’ll get out of here?
Ken Ridgway: Well, fish are very sensitive to water conditions, they can utilise how the water varies, and are sensitive to temperature. In the case of the parcels of water that we’ve identified, probably the significant thing is not only do they have a temperature and salinity that’s different from the water around, but they have a high oxygen content, so that’s going to be of considerable interest to fish that can utilise that.
Glen Paul: So are these walls of water a natural occurrence, or are they influenced by other factors?
Ken Ridgway: No, it’s a natural occurrence. It’s no doubt been happening for many thousands of years, so that the water leaving Bass Strait is a phenomenon which we’ve observed, and people have obtained observations in the past, and have identified this process so that in winter in Bass Strait the water cools and becomes more dense, and it reaches a stage where it sinks, it moves off the relatively shallow Bass Strait region, and moves down the Continental Slope into the Tasman Sea. It moves for a vertical distance of some 400 metres, so it’s like a massive undersea waterfall.
Glen Paul: I guess it’s just like a giant bubble of water, of Bass Strait water, moving around under the ocean and heading north?
Ken Ridgway: Well the interesting thing is this process, as I said, has been known about for 20 or 30 years, but what we’ve been able to observe with our new instrument, the ocean glider, is the fact that these bubbles or balloons of water typically they move off north-east Bass Strait and the ocean dynamics means that they’ll move towards the north against the coast of mainland Australia, and just at that region we have large anticlockwise warm core eddies that are associated with the East Australian Current. And in some way these parcels of water caught up by the eddy are attracted to the centre of the eddy and they sit below the bottom of the eddies, then they travel as a complete entity within the eddy and move southwards.
And they settle at sort of a stable depth, and that varies from about 500 metres to 700 metres, so they’re well below the surface, and they have a vertical extent of about 200 metres, and horizontal about 40 kilometres.
Glen Paul: OK. So with the ocean gliders, is this their primary function, to track these walls of water?
Ken Ridgway: No, they’re designed to obtain very high resolution data horizontally, so that from the surface they dive down to about a depth of a thousand metres, and then follow sort of a zigzag motion down and up. They’re very efficient, they run on very small batteries, they change their depth basically by changing their buoyancy, and they can obtain a lot of measurements that are close together.
So we have a dataset which we’ve never really had before. We’ve had individual measurements, which we take them from a ship, they could be possibly very close together, but they’re limited, depending how many times we go out on the ship. If we have other instruments we can get measurements every ten days from a new instrument called an argo float, but these are yet another bit of technology that’s given us an even higher resolution picture of the ocean.
Glen Paul: So can you feed them information once they’re away, or do they just purely operate on a predetermined program?
Ken Ridgway: No, we’re in contact with them, so we control them. Every time they come to the surface, it’s probably ever two or three hours, they communicate via satellite, so basically make a phone call and send their data back to home base, and at that point they can be reprogrammed to follow a different track, or remain on the track they have at that stage. So we can monitor their progress, we know the position at that point, and we can determine whether to keep them going in a particular direction, or change the direction.
Glen Paul: So are the gliders retrievable, unlike the argo float?
Ken Ridgway: We certainly want to retrieve these, and in most cases we do. They’re not an expendable instrument. They’re like a very smart argo float if you like. In this case the particular gliders that we were using in this experiment, they have a duration of about five months, at that point the batteries are basically run down, so at the end of five months we need to have them in a location that we can send out a boat and collect them.
Glen Paul: Well look, it sounds like a great bit of kit, being able to identify these giant walls of water floating around, which in itself is mind boggling stuff. Thank you very much for discussing it with me, Ken.
Ken Ridgway: It’s a pleasure.
Glen Paul: Research Scientist, Ken Ridgway. For more information find us online at www.csiro.au. You can like us on Facebook, or follow us on Twitter at CSIROnews.