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    Wind drift, was: DR thread from Nov-Dec '04
    From: Trevor Kenchington
    Date: 2005 Jan 19, 21:05 -0400

    George wrote:
    
    > I can see no reason why wind-drift shouldn't apply to a village duck-pond,
    > though suitably modified to account for its shallowness. I can't call to
    > mind any body of research on village duckponds, however. In my younger days
    > I used to race a dinghy on a reservoir, and we were always conscious (or
    > perhaps just THOUGHT we were conscious) of some wind-driven surface current
    > on a windy day.
    
    
    The problem with wind drift on a duck pond is that there isn't anywhere
    for the water to go to. Before it can move more than a trivial distance,
    it encounters the downwind edge of the pond and can go no further. The
    wind would move some water until it set up the surface (up at the
    downwind side of the pond, down at the upwind). At that point, one of
    two things could happen:
    1: If the pond was deep but the water was all of the same temperature
    (and zero salinity, since this is a pond), the water at the downwind end
    would downwell and then run against the wind, along the bottom of the
    pond, as a countercurrent, producing a circulation of sorts.
    2: In a more realistic shallow pond, or a deep pond with a thermocline,
    there would be nowhere for the countercurrent to go, without excessive
    friction between the water flows, and the set-up would lead to static
    water -- the force of the wind being balanced by gravity trying to drag
    the water back down the surface slope.
    
    Whether Coriolis force would operate to any significant degree in such a
    small-scale system, I rather doubt. Certainly, no Ekman spiral would
    develop. (I don't think they can develop anywhere on the continental
    shelves but I may be wrong.)
    
    George continued:
    
    
    > Because wind-drift is a rather local circulation, being balanced, in the
    > oceans, by a return counter-current a few hundred feet below, there's no
    > call for energy-transfer over thousands of miles, as there is for the
    > buildup of wave energy.
    
    
    As explained in my earlier message (under the subject line "Ekman
    Spiral"), which George will not have seen before writing this later
    message, I explained that there is no such balance. In deep-ocean
    situations, Ekman transport is very powerful and is directed at 90
    degrees to wind direction (once the wind has been blowing steadily for
    long enough).
    
    Wind drift can be a small-scale, local phenomenon. If can also circle
    the planet, as it does in the Southern Ocean. But the particular form of
    wind drift that Doug provided means to correct for is intermediate in
    scale, maybe involving hundreds of miles of open ocean.
    
    Perhaps more to the point, it is the scale of time, rather than area,
    which is important. It takes hours (I think hours, not days) of a steady
    wind to establish an Ekman spiral, with the surface drift having the
    expected orientation and strength relative to the wind. After all, there
    is a whole lot of water to be set in motion by only as much energy as
    the atmosphere bleeds into the ocean through the very slippery
    connection between them.
    
    
    And George again continued:
    
    
    > Jared's other point, about drift depending on how long the wind blows,
    > seems more valid. It must take time for the friction of wind at the surface
    > to transfer energy to the surface current. If the wind were suddenly
    > switched off, how long would it take for the ocean wind-currents to come to
    > rest? Hours? Days? Weeks? Your guess is as good as mine. Perhaps there's a
    > time-lag to be seen between the global wind circulation patterns and the
    > corresponding ocean currents, at times such as the monsoons.
    
    
    As to time to transfer energy: Yes.
    
    As to how long the water continues to move, once set in motion: I do not
    know but it will be quite some time. About all that will stop it is
    friction against other water masses that are not moving with the same
    velocity.
    
    But the reference to ocean currents is a change of topic. Those are
    overwhelmingly geostrophic flows, meaning that they move along pressure
    gradients formed by differences in temperature and salinity (and hence
    density). Longer-term, larger-scale wind fields are a major driver of
    the surface circulation but on a quite different scale to the drifts
    which Doug was talking about and which are tied in to Ekman spirals.
    
    
    Then Bill wrote:
    
    > Lastly, can we count on the wind direction and velocity being identical in
    > both Japan and California?  From the texts I have read wind-induced current
    > can be a relatively local event, e.g. Chesapeake Bay or Great Lakes.  On
    > Lake Michigan we get these wonders called seiches.  High pressure on one end
    > of the lake, low pressure on the other, plus the wind in the right direction
    > and bam, a foot or more "tide" over a matter of hours.  The first time we
    > experienced it was when tied up along the wall in Chicago.  Blew my mind.
    > Perhaps Frank can offer a better explanation of the seiche from the Chicago
    > vantage point.
    
    That is "set-up", not "seiche". A seiche occurs when the wind which set
    the water up dies away and the water sloshes back towards its normal,
    level surface. It will always slosh to far and set up a standing wave,
    with a period that depends on the length and depth of the lake. Careful
    observation at either end of the lake should show a few "high tides" and
    "low tides" over minutes (in a small lake) or many hours (in a big one),
    though to really detect a seiche you need a recording tide gauge since
    the amplitude of the wave rapidly drops away as energy is lost to
    friction between the water and the lake bottom. ("Seiche", by the way,
    is said to have been a local term for this phenomenon on the shores of
    Lake Geneva, before the word was taken up by limnologists and then
    oceanographers.)
    
    And in any water body as tiny as Lake Michigan, pressure will not cause
    a set-up. With a depression centred over Chicago, the air pressure at
    Mackinaw City will be hardly any higher. Thus, the atmosphere will press
    just about equally on all parts of the Lake's surface. It is wind alone
    that sets up the surface of lakes. The North Atlantic is different. It
    is big enough to have an intense depression over Newfoundland and a
    well-developed high over the Azores. Then the sea's surface can be
    forced upwards under the depression by the differential pressure across
    the surface of the basin.
    
    
    Trevor Kenchington
    
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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