CSIRO Marine Research

An article written by George Cresswell
(for Australian Yachting Magazine, Dec 2001)
Oceanographer
CSIRO Marine Research
Hobart TAS 7001
October 2001

You can bet your first at-sea breakfast that when the gun goes off on Boxing Day, there will an array of East Australian Current meanders and eddies between you and Hobart, with some just waiting to be exploited. So, how do you go about it? Well, at CSIRO we have been studying the EAC and its eddies for decades and……we still get tricked. But we are getting better and I’d like to impart some of our knowledge, give some detailed examples, and introduce you to the type of information that we provide to the competitors via the Internet. I might say that when you pass through the Heads, having farewelled the fruit bats in the fig trees at Rushcutters Bay, you will enter a regime where the speeds in the EAC and the eddies that it spawns can reach up to 4 knots; where contrary storm winds and currents can conspire to generate waves that are large, steep, and chaotic. Your appreciation of your first breakfast may suffer.

Introduction to the East Australian Current

The EAC is a strong western boundary current like the Gulf Stream. Its source is in Coral Sea, which has waters that are warm and light, standing almost a metre higher than the colder denser waters of the Tasman Sea (Figure 1). Currents tend to follow contours of sea-surface elevation - the slowly changing contours, not the transients due to waves and swell - and so the EAC follows the boundary between the Coral and Tasman Seas. Just as winds are fastest where pressure changes are greatest, so currents are strongest where the slope of the sea surface is steepest.

The boundary between the Coral and Tasman Seas does not, however, simply lie east-west. At its western end a 100 mile-wide, somewhat bumpy, ridge extends southward, sometimes as far as Bass Strait. A dramatic feature of the EAC system is its surface jet of warm water that is perhaps 100 m thick and some tens of miles wide. It runs hundreds of miles southward on the western side of the ridge - fastest where the ridge is steepest, a balance of pressure gradient and Coriolis (earth’s rotation) forces. Wherever the EAC jet meanders out to sea and back it spins up a small cold clockwise eddy in the "elbow" that is formed. The jet can spread across the continental shelf, but its speed decreases quickly from its maximum a few miles seaward of the shelf edge. Ultimately, the jet U-turns until it is running northward of the eastern side of the ridge. It may then split so that part recirculates while the remainder wanders eastward in the general direction of New Zealand.

South of the ridge are 'warm-core' eddies 100 miles in diameter that rotate anticlockwise and can have lifetimes of over a year. These are isolated 'highs' in the sea-surface topography that are formed when occasional migrations of the ridge to the south are followed by retractions to the north. At those times the ridge can pinch off and spawn two or three new or rejuvenated eddies.

The eddies may be several degrees warmer at the surface than the surrounding waters. At depth though, say 250 m down, the temperature differential can increase to almost 10°C.

The eddies are not generally encircled by the EAC jet. However, at times a saddle is formed in the sea surface topography between the ridge and an eddy (a high in the sea surface) and the jet can then reach across to the eddy and encircle it. The process can serve to charge up the potential energy of the eddy by supplying it with new warm water, thereby increasing the height of its sea surface. Eddies can also coalesce with one another in a process that may involve a three-week waltz and courtship. The resulting eddy is larger than either of the two that contributed to it.

The speeds in an eddy increase more or less linearly from the centre out to the edge. There they are over 2 knots, but they decrease quickly beyond that. Much higher speeds occur in an eddy when its wanderings bring it in to strike against the continental shelf edge (Figure 2). This distorts it into an ellipse, around which the flow appears to conserve angular momentum - high speeds at the minor axes; low speeds at the major axes - and speeds of 3-4 knots may occur.

Sitting out from southern NSW, these warm anticlockwise eddies are favourable for yachts proceeding southward. However, between the southern end of the ridge and the eddy immediately to the south -- or between warm eddies -- there can be a cold clockwise eddy that produces northward currents of 1-2 knots just off the shelf edge. Moving closer to the coast at these places will reduce the effect of the unfavourable current.

Near the coast the influence of both wind-driven currents and so-called coastal trapped waves may swamp the effects of the EAC and eddies that are strongest off the continental shelf. The waves are produced by wind forcing hundreds of miles away and they have been observed to propagate anti-clockwise around the southern part of the continent. They have wavelengths of many hundreds of miles, periods of a few days to a week, and they drive alternating alongshore currents of about 1/2 knot and lower and raise sea level by about 20 cm.

Examples

This section will attempt to get across some of the ideas above by taking a couple of "textbook" examples of ship and satellite measurements of the EAC system. To this end I spent quite a few days and nights plotting and replotting data from two of my voyages on the research vessel Franklin.

The first voyage covered the region from Sydney to Bateman’s Bay and a couple of hundred miles seaward between 29 November and 5 December 1989. The key data sources are a satellite sea temperature image (Figure 3) and the currents measured by an acoustic instrument on the ship (Figure 4). The image shows that the warm EAC was running southward past Sydney with its inner edge spreading across the continental shelf and into the coast. The EAC appeared to take a half loop around an eddy centred ESE of Jervis Bay. The cloud in the top right of the image obscures the NE quadrant of the eddy. The situation at Jervis Bay is a little confusing because from it extends a band of cloud to the SE and a 40-mile long spur of cold upwelled water to the south. The upwelling may be result of the EAC driving water across the bottom at a gentle angle in to the coast. Similar upwellings occur south of Sugarloaf Point, Smoky Cape and Cape Byron. Perhaps the headlands constrict and accelerate the EAC where it flows on the shelf, with the result that the upwelling action is enhanced.

The current vectors measured by the ship show that as the EAC joined the eddy it accelerated from a maximum of about 2 knots off Sydney to 4 knots off Jervis Bay and Batemans Bay. The two lines NE and SE from the eddy centre have maxima of 2-3 knots. Note that 1989 was early days for the Global Positioning System, with the network of satellites being incomplete. This meant that the accurate positions needed for our current calculations were only available for half a dozen hours at a time — hence the incomplete picture of the vectors.

The paired current component and temperature diagrams for the Sydney, Jervis Bay and Bateman’s Bay lines (Figures 5-7) reveal that they were influenced in quite different ways by the EAC: Off Sydney the warm, relatively strong EAC reached in near the coast, even though the maximum along the outbound line that we have plotted here was only about 2 knots.

Off Jervis Bay the current was near-zero at the mouth of the Bay, but it steeply ramped up to a maximum of almost 4 knots in the warm EAC jet near the shelf edge. The cold spike on the inner shelf on the Jervis Bay line marks the cold spur seen in the image.

Off Bateman’s Bay the currents were southward but weak all across the shelf, then weakly northward, and it was only about 15 miles out from the shelf, beyond the spur of cold upwelled water, that the warm EAC reached its 4 knot maximum. The limited GPS "window" cut off further measurements.

On the second voyage, in late November 1997, the ship was taken along the shelf edge from Fraser Island, where, incidentally, the EAC touched 4.5 knots, to southern NSW and then along the rhumb line to St Helens.

Skies were cloudy during the passage down the NSW shelf edge to St Helens (29 November to 1 December), so for this discussion I have had to use a satellite image from 22 November (Figure 8). The pattern would have changed, but not greatly, in the intervening ten days to the time of the voyage. The image shows the warm EAC running southward beyond the shelf edge to about Woolongong, where turns sharply out to sea. They was an anticlockwise eddy centred east of Gabo Island and another in the bight east of Bass Strait.

The plot of the current vectors (Figure 9a ) shows the strong currents on the western side of the EAC near Sydney and the anticlockwise eddy out from Gabo Island. There was westward flow onto the shelf from roughly Ulladulla to Eden. En route to Tasmania the ship bisected an eddy that was centred at 40°S. The strong southward vectors on the western side of the EAC and the Gabo eddy are difficult to distinguish in the vector diagram. However, in the plot of the north-south current component (Figure 9c ) we can see that these currents were about 3 knots.

Is there any relationship between southward current strength and sea temperature along the voyage track as it passed NSW? It is worth taking a look, so I plotted temperature versus latitude (Figure 9b), ran a straight line through it, and did a subtraction to come up with an anomaly. This anomaly is plotted as a red line in Figure 9c and, yes, if you are in warm water then you are probably getting a lift southward. I might say that I had to move the fitted straight line left and right a little to maximise the visual similarity of the temperature anomaly and the southward current. This only holds when one is running along the NSW shelf edge and not in the open water south of Gabo, because there, rather than grazing the western side of an eddy, the rhumb line usually goes right across the centre of any eddy that happens to be there.

Final comments

Well, we’ve reached the end -- for now. Hopefully readers will be able to access and better interpret the satellite images and other information that we put on our Sydney to Hobart yacht race web page: Ocean Conditions and Yacht Racing

I’d like to finish with the earliest descriptions of, first, an encounter with the East Australian Current on HMS Endeavour and, second, being caught by an eddy in a slow sailing ship (HMS Beagle):

"Winds southerly, a fresh gale," wrote James Cook in his log at sunset on 15 May 1770 as he neared Cape Byron. Seeking more searoom for the night, he headed offshore until,"having increased our soundings to 78 fathoms, we wore and lay with her head in shore until 5 o'clock a.m., when we made sail. At daylight we were surprised by finding ourselves farther to the southward than we were in the evening, and yet it had blown strong all night". We would not be surprised if the current that he encountered was about 3 knots. He had earlier logged a southgoing current of less than a knot as he coasted northward along NSW.

From J. Lort Stokes, R.N. Commander, HMS Beagle, 1838: "We sailed from Hobarton on the 19th of July and carried a strong fair wind to within a few days' sail of Sydney, when we experienced a current that set us 40 miles S.E. in 24 hours; this was the more extraordinary as we did not feel it before, and scarcely afterwards; and our course being parallel to the shore, was not likely to have brought us suddenly within the influence of the currents said to prevail along the coast. The ship's position was 40 miles east of Jervis Bay when we first met it".

Figure Captions

[click on images for lager view]

Figure 1 A map of sea surface topography and inferred currents prepared by Madeleine Cahill of CSIRO from measurements with radar altimeters on US and European satellites. These can "see through" cloud. The resolution of the altimeters is about the length of one’s little finger. The scale for the surface currents is in m/s; 0.51 m/s equals 1 knot. . The bumps and hollows in this map are analogous to atmospheric pressure highs and lows: the sea surface gradient and the earth’s rotation contrive to drive currents anticlockwise around the bumps and clockwise around the hollows.

Figure 2 The tracks of three satellite-tracked drifters in a warm eddy in April/May 1981. The collision of the eddy with the continental shelf edge distorts it into an ellipse and the current on the western side of the eddy has accelerated from low speeds up to 2.5 knots. The white markers on the tracks give successive noon positions.

Figure 3 A satellite sea surface temperature image from 2 December 1989. Clouds are white. There is a temperature scale on the top of the image and a latitude-longitude grid and the continental shelf edge have been added. Note the warm EAC jet that curves around the eddy that is centred out from Jervis Bay. The coastal waters of NSW are cold and there is a spur of upwelled water emanating from near Jervis Bay.

Figure 4 Current vectors measured with the Acoustic Doppler Current Profiler (ADCP) on RV Franklin.

Figures 5-7 Sea surface temperature and the north-south velocity component measured by Franklin along the Sydney, Jervis Bay and Bateman’s Bay lines. The locations of the 100 m and 200 m depth contours are marked in green.

Figure 8 A composite satellite sea surface temperature image from 21/22 November 1997. Note the EAC jet turning offshore out from Jervis Bay and the warm eddies out from Eden and Flinders Island.

Figure 9a Current vectors measured by Franklin as it was taken along the shelf edge of NSW and then along the rhumb line to St Helens.

Figure 9b The sea surface temperatures measured by Franklin along the path in Figure 9a. The dashed line is thought to approximate the latitude effect on temperature.

Figure 9c The north-south current components and the temperature anomaly, which is the difference between the measured temperature and the dashed line in Figure 9b.

Updated: 13/12/04