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John Huth asked about any influence of ocean current on wave shape, as
follows-

"I'm trying to track down some information on the detection of currents in
the open ocean.


I have three sources that discuss the following phenomenon - seasoned
navigators are able to tell the set of the current by looking at the shape
of waves - they tend to steepen up in the direction of the motion of the
current.   So far, I have three sources 1.) A short mention of this in
Joshua Slocum's Sailing Alone Around the World, 2.) First hand interview
with a Micronesian sailor in The Last Navigator, and 3.) Discussion and
interviews by David Lewis in We the Navigators.


The issue that I'm having some difficulty in understanding how wave shapes
could be affected by current in the open ocean.   If everything is in a
moving frame of reference together, I wouldn't expect any effect on the
shape of waves.   The only explanation I could come up with is that there's
a stationary layer not too far under that is creating a kind of drag effect
which would cause the waves to steepen up.


So - anyone have any information on this phenomenon, or sources that make
conjectures about it?   Any other anecdotal experiences with this?"

==============================

Responses from Jeremy and Frank confirn that surface currents can affect the
shape of waves, and I can add my own ha'porth, below, to confirm this from
my own experiences. However, all those accounts relate entirely to local
currents, constricted by the nearby presence of land, or shallow waters.
However, John was asking about currents in the open ocean, in (presumably)
deep waters, without the presence of nearby land to complicate matters.

I am sceptical about such claims, that mariners could detect such currents
from a change in wave shape, in the absence of any way in which the ocean
surface can "know" the way in which it's moving with respect to the ocean
floor, thousands of feet below. In John's words "If everything is in a
moving frame of reference together, I wouldn't expect any effect on the
shape of waves." It would be unphysical to expect anything else (which is no
more than an elaborate way of saying that I can't conceive of a mechanism).
If ocean currents were confined to a thin surface layer, flowing above a
bulk of stationary water, such effects might occur, but as Frank has pointed
out, ocean currents go deep down.

I would dismiss such notions as romantic claims of mystical powers, to which
David Lewis was prone, seasoned and competent navigator though he was.

===========================

Small craft sailors, in the waters around Britain, seething with strong
tidal currents, get to know well the areas they should keep clear of,
especially in wind-against-tide conditions, when waves tend to pile up and
form breakers, in what are called "overfalls". These dancing, pointy-topped,
waves are often given local picturesque names, such as "the dancing
maidens", "the merry men of Mey", "the Bishops and Clerks" (pointy hats),
the "washerwoman's race"; in rough conditions they can be a menace to small
craft. I attribute them all to the situation when waves, initially
travelling over a uniform body of water, meet suddenly with local changes in
surface velocity.

Particularly, this can occur when ocean waves meet a local ebb from a river
or body of inland water, particularly if there's a bar to enhance
surface-water flow. That at the Columbia River, in the US, is famous.
Another effect can combine with that of contrary current: surface waves in
shoal water travel more slowly than in deep water, which alone causes waves
to break as a beach is approached.

The effect of waves breaking against a counter-current was demonstrated to
me years ago at the entrance to the Morbihan "inland sea", in Brittany,
through the entrance of which is a powerful tidal current. With my small
craft safely anchored within, I was watching a spring ebb, from a small hill
near the entrance, on a pleasant day with a fresh westerly breeze. Small
waves were rolling in from the West across Quiberon Bay, which isn't exposed
directly to the ocean. In the zone where those waves met the jet of
ebb-tide, fanning out from the Morbihan entrance, they became steeper and
steeper. Eventually, at a well-defined spot, at which the water speed was
presumably equal to the propagation speed of those waves, they stopped,
locked in place. At that point was a ferocious mass of tumbling breakers,
where all the energy was being dissipated. And just a few feet further in,
the water was like a millpond, absolutely flat-calm. Perhaps, I should say,
like a mill-stream; for it was flowing out so fast that the wave energy
could not travel against it. These effects must happen at many places, but I
have never seen it so clear-cut as on that day.

There are several spots on the Channel coast of England, where a rocky
headland point runs down into the sea, then the rocky reef continues
underwater for some way out, such as at Portland Bill, St. Alban's Head, and
Start Point. Overfalls occur when a strong tide runs past them, especially
when the wind is blowing so as to create waves which are travelling the
oppposite way.

There seem to be two mechanisms at work here. The bottom-current, when it
meets the reef, gets deflected upwards, and mixes with the surface current
in an irregular pattern of transient swirls and "kelds" (smooth "oily"
patches). And then, just downsream of the headland, an eddy usually occurs
toward the shore, giving rise to a back-cuurent which circles around to
rejoin the main stream, creating further non-uniformity in the surface
current as it does so. All this results in local surface currents that are
all over the place, but on their own, in flat-calm conditions, do not give
rise to much in the way of waves.

But now consider a contrary wind, bringing a pattern of waves across this
area of disordered flow. Until arriving at the headland, surface currents
were rather uniform, and the wave energy uniformly distributed. 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
dinverge 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. In the deep ocean, such effects might conceivably occur where,
say, an equatotial current and a similar counter-current run side-by side,
creating a velocity gradient. However, such currents are an order of
magnitude weaker than the streams we have been discussing, and their
boundaries are generally unsharp. Perhaps a perceptive navigator might be
able to discern the presence of such a boundary, or convergence, but in my
view it would be more likely to result from observation of surface
temperature or colour, or floating debris, than from wave behaviour..

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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