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John Huth wrote-

"I think already the current gradient effect of focusing the waves that
Frank mentioned is one distinct culprit, but I don't quite understand how
this would steepen them, although I could be convinced.   I'll let you know
what I find out."

If John returns to my posting [11219], he will find a suggested explanation
which seems at least plausible, although I can not claim it to be certainly
correct. I wrote-

"But when the wavefronts meet these varying currents, they get refracted
accordingly, as the net speed of surface propagation varies. So
unpredictable patches occur, where wavefronts converge and their local
energy adds; others where they diverge and the water becomes smooth. It is,
of course, the adding bits
that matter, and create the danger to small craft.

In just the same way, wave patterns, generated away from the locality of the
Gulf Stream, can get refracted by those powerful local currents when they
meet it, and give rise to characteristic and dangerous wave-patterns, as
they get refracted toward local focus-points.

But all these effects are, in my view, related to local surface velocity
gradients."

It's somewhat similar to the way that sunlight shines through one of those
dimply bathroom windows, and falls on a surface behind it. The varying
slopes of the glass cause the light to refract in different ways, so that at
some points it adds, and concentrates, creating a dappling of the light
pattern.

The disordered surface velocities of the current-flow cause varying
refraction of the waves, giving rise to transient concentrations of energy,
varying with position and time. For a given wavelength of a surface wave,
there is only a limited amount of energy that it can carry before it spills,
and the menace to small-craft is of those spilling seas tumbling aboard.

On top of that, waves generated elsewhere, where surface current is low,
will find that they have to carry an increased energy-density when meeting
an adverse current, which also adds to the tendency to break.

And waves generated elsewhere, which meet a more organised surface gradient
at an angle, such as at the edge of the Gulf Stream, could well be subject
to rather more geometrical bending and focussing, compared with the more
chaotic turbulence that I've been discussing above. That could result in
larger, and more predictable, danger-patches.

If John, or anyone else, comes across a decent explanation of such effects,
on the web or in print, I would be very interested to learn about it.
Somehow, I doubt if that will be a work on theoretical fluid dynamics,
though. But you never know...

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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