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Australian Academy of Science, Becker House, Canberra. Friday 16 December 1994
Dr Peter Craig
My talk fits very much into focus 1 of LOICZ. I am going to be looking at the influence of the large-scale ocean circulation on Australia's continental shelf and coastal zone, so I have started with this picture (Overhead 1) which is taken from a 1985 cover of "Nature". From the perspective of this audience, it shows us the geographical situation that Australia is in. Australia is very much a dot in the middle of three major oceans: the Pacific, the Southern Ocean and the Indian Ocean. Any attempt to describe fluxes in the coastal zone is obviously going to have to take account of the impact of this vast water mass which surrounds Australia. What I am going to be talking about is the influence of the large ocean circulation on our coastal zone, the data that we have to support our description of this influence and the way in which we might incorporate these data into our research programs.
The next slide (Overhead 2) is just a cartoon which you should not take too literally: it shows the current systems around the Australian coastline.
In particular I want you to focus on the eastern coast where you see the southward flowing East Australian Current. The East Australian Current, as John Parslow already mentioned, is a classic western boundary current, like the Gulf Stream. It is the closure of the huge anti-clockwise gyre in the South Pacific Basin. So, the East Australian Current is dependent on circulation in the whole of the South Pacific. To understand it, we need to also understand the circulation of the Pacific.
The situation on the west coast is a little different. Here, we also have a poleward-flowing warm current, but this one is unique in the world's oceans. That is because the Indian Ocean is unique amongst the oceans, in the sense that it does not have a Northern Hemisphere basin. It also has a source of warm water flowing through from the Pacific Ocean to the Indian Ocean. What we end up with is this flow of warm water down the west coast, down the pressure gradient. This is the Leeuwin Current. Interestingly, the Leeuwin Current flows for only half of the year. During summertime, the prevailing southerly winds actually blow the current to a halt.
I have a couple of satellite images of these currents. This (Overhead 3) is the southern part of the Leeuwin Current as it flows down around the south-west part of our continent. We are looking at sea-surface temperature -- the darker the water the higher its temperature. It is important to note that the currents are by no means a straight linear flow like a river. This is a highly variable system and you can see in its image the evidence of eddying motion that we will have to take into account if we are looking at impacts on the coastal zone.
Similarly, for the East Australian Current, here (Overhead 4) is a nice image of the flow down past Sydney on the continental shelf. You can again see eddying motion quite clearly in this image. Another thing that you should note is just how close to the shoreline this current encroaches. It is likely to have a substantial influence on waters in the coastal zone.