Remote Sensing Project
Prepared SST imagery for the Telstra Sydney to Hobart Yacht Race - 1997
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Despite the fact that we have had no cloud-free images in the past two days, and that new warm water (temperature greater than 25°C) from the Coral Sea is flooding southward in the East Australian Current (EAC) to confuse matters, I still think that eddies A and B are still important.
Click here to see these eddies identified and click here for more information about eddies of the East Australian Current. Find out more about the EAC by clicking here.
Glimpses through the cloud cover suggest that the EAC has flooded across the top of A, perhaps as a layer many tens of metres thick, but that this flow is starting to be turned anticlockwise by the eddy.
Part of the EAC has broken southward from eddy A and then turned SW, eventually running into the NW quadrant of eddy B. It is likely that it will be wrapped anticlockwise around eddy B.
Over the next few days the warm surface temperature "signatures" of A and B should become a little clearer than what they are now. This means that an increase of temperature to exceed 25°C should confirm, along with the GPS, that an eddy has been encountered. Checking our updated images on the web up to race time (we stop adding images to the web on Boxing Day) may help you follow the evolution of these eddies.
I expect eddy A to move a little to the SSW. If this happens then yachts could expect current assistance at the shelf edge down to Jervis Bay. This could amount to almost 3 knots - but if I'm wrong and the eddy moves further out to sea instead, then the current will be much less.
The shelf edge currents between JB and Eden will probably be unfavourable by about 1 knot. This is based on the currents that we have seen between eddies in the past. The current should be less unfavourable in across the shelf towards the coast.
While eddies in the position of eddy B often move out to sea, this is not always the case. If B stays where it is then the shelf edge currents will be up near the 3 knots maximum that we measured there with our ship Franklin at the start of this month. These favourable currents will decrease in speed in across the shelf.
Eddy C, earlier this month at 40°S, 149°30'E, is too small and now too far to the east to play a role in the race. Currents around it should be about 1 knot.
The shape of the cold plume of water from NE Tasmania suggests slow northward flow on the continental shelf. There may be advantages in seeing if the currents at the shelf edge are favourable.
Safe sailing - with some excitement !
Cheers .....George Cresswell
Earlier this month it appeared that the yachts would have the option of trying for current assistance of up to several knots on the western sides of anticlockwise eddies A & B, and over a knot from eddy C. The eddies were easy to identify in satellite sea temperature images.
During the month the pattern has become more complicated with the East Australian Current flooding the tops of eddies A and B with 26 deg C water that would have been in the Coral Sea a couple of weeks ago. The eddy pattern is juggling itself in response to the jolt by the new warm water and we are trying to understand this new pattern.
The images for 21 December show the EAC heading SE from near Sydney, probably being steered by eddy A with its centre beneath cloud at about 34°30'S, 153°30'E.
The EAC then turns to the SW at 36°S, approaching the shelf edge near 36°30'S. Turning southward it cuts across the top of eddy B with its centre at 38°S, 152°30'E.
Along the shelf edge between Wollongong and Eden the currents will not be favourable. In fact, they could be about 1 knot northward due to the clockwise eddy that seems to have formed inshore of the right angle turn taken by the EAC at 36°S.
The edge of eddy B just seaward of the shelf edge is very favourable for the yachts and 3 knots could be expected. This will decrease in across the shelf. The SW perimeter of eddy B can be seen at 39°S, 150°E.
Cloud has prevented us from following eddy C since we saw it in the 2 December image.
From 27 November to 3 December 1997 RV "Franklin" travelled southward along the continental shelf edge from Fraser Island to Bass Strait and then on to St Helens, and finally Hobart. We measured currents from near the surface down to 200 m with the ship's acoustic Doppler current profiler (ADCP). Only the near-surface currents concern us here.
The only cloud-free image that we have from around this time is from 2 December. I will discuss this image and the current measurements. I want to identify the features (meanders, anticlockwise eddies) that will be important for the Sydney to Hobart race. Having done that I hope to be able to follow these features in successive images up to race time. The features will move and evolve: eddies, for example, can move up to 20 miles per day and they can be distorted into ellipses by collisions with the edge of the shelf. Sometimes two eddies can join to become one. Sometimes an eddy can be reabsorbed into the parent East Australian Current (EAC). In the end I hope that I can say things like: "That was a 3 knot eddy. Now it is located 30 miles to the SE" - or whatever.
The 2 December image shows the EAC with temperature 25°C running southward to 33°S and then turning to the east. The current speeds that we measured near and several miles seaward of the shelf edge were up to 2.5 knots. We may have been inshore of the maximum current.
Part of the EAC continued southwestward angling onto the shelf at around 33° 30's at over 1 knot. This flow then wrapped itself around a very large and elongated eddy A. This eddy has a surface temperature of 24°C. On the outer edge of the shelf we measured current speeds of up to 3 knots between Sydney and the Shoalhaven River. We dropped out of the eddy when we were off Jervis Bay.
From Jervis Bay to Eden the flow at the shelf edge was coastward at about 3/4 knot. We were experiencing strong southerly winds and they would have influenced the surface currents. It is worth commenting that between eddies at other occasions we have observed northward currents of about 1 knot at the shelf edge.
Eddy B does not have a particularly strong surface temperature signature (19°C versus 17°C), but as we clipped its edge we encountered predominantly southward currents of almost 3 knots. The perpendiculars to their direction intersected at the eddy centre on the image: about 38° 15'S, 151° 15'E.
Proceeding on course 190° we crossed just to the west of the centre of eddy C (surface temperature 17°C), although not by design - we were on our way to the 2000 m isobath off St Helens. North of the centre the current reached to over 1 knot to the west, while southwest of the centre the current was up to 1.5 knots to the SE . I expect that the current would have been slightly higher where eddy C pressed up against the shelf off Flinders Island.
The currents on and off the shelf from St Helens to Hobart were less than a knot and probably driven by the wind.
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°C, while outside 12°C 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.
For more information email:George.Cresswell@marine.csiro.au