Freemantle / Geraldton Yacht Race
Best recent images
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Currents off WA pertaining to the Fremantle to Geraldton yacht race
George Cresswell
Research Fellow CSIRO
Background
Our work in the past with current meters moored at about 10 m above the bottom in 60 m of water on the continental shelf south of the Abrolhos Islands and near Rottnest Island showed that the Leeuwin Current edges onto the shelf in winter, driving southward flows of a little over ½ knot. After August this does not seem to happen and the shelf waters become influenced more and more by southerly winds which drive northward currents of about ¾ knot - during strong winds. In times of low winds the current was observed to drop to zero and even, during northerlies, reverse.
The northward flow was coined the "Capes Current" by UWA and CSIRO oceanographers Prof Chari Pattiaratchi and Alan Pearce.
Note, that at the depth of the current meters there was no significant effect from the daily sea breeze. Rather, the current responded to a several day cycle in the winds as pressure patterns moved past. Nearer the surface there will probably be a sea breeze effect, maybe close to about 1 knot, driving to the north.
Satellite images and shipboard current measurements - past examples
Summer
Figure 1a shows a satellite sea surface temperature image for 18 Dec 1994 and Figure 1b shows the current vectors that were measured from RV "Franklin" at the time. Note in the image that the thin black line marks the shelf edge, while the thicker black line gives the track taken by the ship. The structure in the image is not particularly dramatic or informative, apart from showing the warm Leeuwin Current coming southward with part of it swinging out to sea and part continuing southward. The cold northward-flowing Capes Current can be seen on the shelf between Capes Naturaliste and Leeuwin. The current vectors in Figure 1b peak at about 0.5 ms-1 , i.e. about 1 knot. Note that the vectors on the shelf and northward and small.
Figure 1a
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Figure 1b
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Winter
Figure 2 shows a satellite image for June 1987 onto which have been superimposed current vectors measured by RV "Franklin". Note how the Leeuwin Current had spread onto the continental shelf (the shelf edge is marked by the thin black dotted line).
Figure 2
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A recent satellite image
The most recent satellite image that we have that is anywhere near useful - because of some holes in the cloud - is for 12 October (Figure 3). With some faith, and taking into account the examples above, it appears that the warm Leeuwin Current is spreading onto the continental shelf. This, of course, would be useful for runs to the south, but a nuisance for a run from Fremantle to Geraldton.
Figure 3
