16 January 2002:  Reply by Stuart Godfrey (CSIRO Marine Research) to query on surge up west coast
Hi John,

        Orryorr, me hearties! Oceanographer weighs in. Yes, that is a really
striking feature of the Leeuwin Current -- it does flow directly into the
prevailing wind, in a thin stream along the continental shelf edge (with the
result that, when you are out on a ship measuring it, you are quite
comfortable on either side and get pretty seasick in the middle of it,
because northgoing swells from either side are refracted into it, steepen
and break). The LC is weakest in summer, partly because the northward
(southerly to you!) winds are strongest then.

The reason the Leeuwin Current is there off WA, with no equivalent off the
west coasts of S or N America, or S or N Africa, is that Australia is an
island. Ocean Kelvin waves pass southward from Indonesia, causing
temperatures to be as high as off New Guinea, down almost to NW Cape.
(Temperatures off Peru are as much as 10°C lower, on annual mean, at 75m).
These high temperatures imply high sea levels, relative to a depth of no
motion at (say) 1000m -- there's a sea level head of 40 cm from NW Cape to
the (colder, lower sealevel) waters off Cape Leeuwin, along the shelf edge.
It is this pressure gradient, which cannot be balanced geostrophically, that
drives the LC southward, against the prevailing winds.

Another way to put it: the eastward (onshore) geostrophic flow due to the
aforementioned pressure gradient overwhelms the offshore Ekman drift due to
those winds you show. Thus you hardly ever get upwelling off WA.

        Here Endeth The Oceanography Lesson.

                Regards, and Happy New Year,

                Stuart Godfrey.

John McBride

Stuart... This is wonderful... I Love explanations like that... I have all
sorts of questions; but they have to wait as I have a poor student sitting
at my work-desk waiting for me to discuss his latest PhD calculations.

The student will have to wait a minute... One quick one:  tell me more
about these Kelvin waves coming down from Indonesia.. What do they travel
on? the coast line? the continental shelf? Is there a thermocline type
layer they can go along?

Are they quasi-steady state like the Gill large scale atmospheric
solution, or does the advection occur by a series of shorter term Kelvin
wave events?  can we see them on any operational or mean analyses?  Hmm...
How do they advect temperature... I guess they must be non-linear Kelvin
waves.... Tell me more

back to the sufferin student

JMcB

Stuart Godfrey

Sure John, they are internal Kelvin waves that ride on the thermocline, so
you get a reversal of flow direction at a few hundred meters down. Typical
speed 2.5 m/sec (for lowest internal mode), so it takes about a week to
propagate from Irian Jaya to Fremantle. Thus most of the time you have an
equilibrium, in which nonlinear forces take over from time-dependence; Rory
Thompson of beloved memory did a fine paper on it, showing it to be highly
plausible that bottom friction, associated with southward bottom currents of
60 cm/sec or so near the shelf break, get to balance the longshore pressure
gradient. Just offshore, the vertical shears in the currents are strong
enough to keep Richardson numbers near 0.25, through the top 100m or so.
Stuart