Remote Sensing Project

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Eddies were first described off SE Australia by Bruce Hamon of CSIRO in 1960, although Capt Cook and J. Lort Stokes had earlier been influenced by currents that would have been due to eddies. Hamon's lead was followed by defence oceanographers Paul Scully Power, Carl Nilsson and John Andrews and CSIRO oceanographers John Church, Fred Boland, David Tranter and George Cresswell - and others.

The eddies form from the pinch-off of meanders of the East Australian Current. They are also called warm-core eddies because at depth, particularly, they are much warmer than the outside waters - for example at 250 m depth their temperature could be 19\060C, while outside 12\060C could be expected.

The eddies have lifetimes of at least 18 months. A common diameter is 200 km. Their edge speeds are about 3 knots. They do not rotate as solid bodies: at the edge a complete rotation may take 5 days; in nearer the centre a rotation is 2 days. The current speed does not decrease very much down to 200 m, but from there down to 1 km it does decrease considerably.

Given that the eddies are 200 km across and about 1 km thick, they are like very thin uncooked pancakes rotating and twisting themselves up.

Eddies can be elliptical as well as circular, particularly after they have collided with the edge of the continental shelf. When elliptical, the flow conserves angular momentum in that the axis from the centre to the perimeter sweeps equal areas in equal times - fast flow for a short axis; slow flow for a long one.

Eddies can be reabsorbed by the EAC, get recharged with new warm surface water from the Coral Sea, and then re-emerge.

Eddies can merge with one another in a process that involves a several-week waltz as they go around with the distance between them decreasing. Some of their waters are lost, but the result is a bigger eddy with waters from the heavier eddy on top of some of the waters of the lighter eddy. Many years back we studied "Leo" and "Maria" as they joined to become "Mario". During the process the current speeds at the edge of the merging eddy pair increased to 4 knots.

Because eddies are warmer and lighter than the surrounding waters their surfaces are almost 1 m higher than those waters. As the "hill" tries to flatten itself it feels the earth's rotation and is forced to rotate, just the same way as atmospheric highs rotate.

There are satellites with radars that can measure the sea surface topography with a precision of better than 10 cm. These easily detect the bumps and hollows in the sea surface off eastern Australia.

In storms when southerlies stir up big waves that propagate into the strong currents, e.g. the 1993 Sydney to Hobart race, the waves steepen and grow as their wavelength shortens. Work by South African oceanographers suggests that the increase in height in these conditions can be as much as 50%.

Work on eddies and the EAC makes use of tools such as the research vessel Franklin, satellite tracked drifters,and moored instruments, along with satellite measurements of temperature, colour, topography and surface roughness.