On Oct 20, 4:19 pm, "Michael Tobis" <[EMAIL PROTECTED]> wrote:
> I'm not sure why this should be directed at me in particular, though I
> don't know if we have any bona fide oceanographers here. I'll give it
> a try.
>
> There are fundamentally very different mechanisms moving water
> horizontally and moving it vertically in the ocean. (Actually, the
> same can be said for any stratified fluid.)
I noticed in one of the popular books I read on climate change ("Winds
of Change" I think) the author said that some scientist don't like
the term "thermohaline circulation" because it includes a circulation
mechanism in the name of actual ocean circulation pattern. They
prefer a mechanism-neutral name.
The wiki says:
"The thermohaline circulation is sometimes called the ocean conveyor
belt, the great ocean conveyer, the global conveyor belt, or, most
commonly, the meridional overturning circulation (often abbreviated as
MOC)."
>
> As long as the planet rotates and is heated by the sun, there will be
> trade winds and middle latitude westerlies. As long as this is the
> case, water in the ocean basins except right at the equator and
> pole(s) will be pulled westward. As long as this is the case, poleward
> jets will form at the western edge of the ocean basins. The North
> Pacific example is called the Kuroshio and the North Atlantic one is
> called the Gulf Stream. For various geometric reasons these are the
> strongest ones, and will remain so until the continents move or the
> oceans freeze or boil.
>
> If you drop a rubber duck into the ocean at Cape Hatteras, it will
> scoot up the coast, past Nantucket and up to Newfoundland, then might
> will slowly meander past Reykjavik, Galway Bay and Lisbon, and start
> meandering westward from around Marrakesh. Its leisurely journey to
> the Bahamas will take several years, at which point it would speed up
> again, and rush homeward to North Carolina.
>
> The romantic duck has followed the wind-driven circulation of which
> the Gulf Stream is a salient feature. Marrakesh and Nantucket and
> Galway (and the rubber duck) will have long since crumbled to dust
> before anything interrupts this broad pattern. This broad circulaiton
> on abouty the timescale of a decade is the dominant pattern in each of
> the five major ocean basins (FIve? you ask. Yes, I say, N. Atlantic,
> S. Atlantic, N. Pacific, S. Pacific and S. Indian; this does not
> describe the equatorial region or the Arctic.)
>
> While this pattern won't be interrupted, it may be disrupted. Here is
> where our life gets complicated.
>
> The process described above describes what happens to the top fifth of
> the ocean, above a layer called the "thermocline". That water is warm
> and buoyant enough that it comes into contact with the atmosphere part
> of the time (in its circuit around the basin).
>
> Now this wind-driven pattern, it turns out, does not penetrate to the
> depths of the ocean (poetically known as the "abyss"). So what happens
> to the abyssal waters? Do they just sit there entirely stagnant,
> mixing with the upper ocean very slowly through diffusion? No, there
> is a process which is much slower than the wind-driven circulation and
> much faster than diffusion which forms the connection between the
> surface waters and the abyss. This is the thermohaline circulation, or
> THC to its friends.
>
> It is driven by ice phase changes. When ice forms, of course the
> temperature is near freezing. Because pure water freezes better than
> salt water does, removing heat from seawater at the freezing point
> makes water saltier. The ice does not incorporate most of the salt.
> This cold, salty water becomes dense enough to sink to the abyss. This
> happens, naturally, at the fringes of the Arctic and the Antarctic,
> during their respective winters. Under present conditions, all the
> northern hemisphere action in this business happens in the Atlantic.
> This drives the famous conveyor belt cartoon that you have probably
> seen a few times by now; it's important to note that it is a very
> crude representation and the truth is vastly more complicated than
> that (and more complicated than the wind-driven circulation, though I
> have also simplified that a bit.)
>
> This is a completely different mechanism, and it can indeed fail.
> There are at least two ways this could happen. First, winter ice could
> quit forming, reducing brine formation. Second, a huge influx of fresh
> water may appear, diluting the brine and preventing it from sinking.
>
> The consequences of such a change are hard to intuit, and we rely on
> models or on paleoclimate evidence.
>
> There is some paleoclimate indication (at 12.5 Ka) of a
> dilution-driven shutdown of the THC during the last deglaciation, as a
> result of the failure of an ice dam and the abrupt draining of Lake
> Agassiz, whose remnants still form the Great Lakes. (Mr. Gore's movie
> mentions the ice escaping through the ST Lawrence, which is an actuall
> error, rather than an 'error'; current evidence has the great flushing
> event escaping northward to the Arctic.) This is the leading
> hypothesis for the cause of the Younger Dryas cooling event which
> lasted over 1000 years. As is fairly well known, Glaciers (which had
> not entirely retreated) re-advanced across Britain during this time
> and much of Europe returned to much colder conditions.
>
> Models tend to agree that a large influx of cool, fresh water into the
> North Atlantic leads to a slowdown in the THC. This in turn leads to a
> cooling of the NOrth ATlantic, as the fraction of cold water sinking
> to the bottom declines. This in turn leads to a colling of Europe,
> with two causes; first a mixing of cold water into the Gulf Stream jet
> and a somewhat southward deflection of it. Quite possibly more
> important (I've seen a paper arguing the case, can't recall where) is
> simply that Europe just finds itself downwind of colder water.
>
> There is no evidence of the Gulf Stream "shutting down" at any time
> since there has been an Atlantic Ocean, and there is very little
> chance of such a thing happening. It would be interesting to trace the
> ortigins of this widespread misunderstanding.
>
> mt
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