Michael Tobis 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.)
[...]
>
> 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.)
Hmmm.
It's commonly portrayed that way, but AIUI the input of work (done
against gravity) that really "drives" the circulation is actually the
mixing that takes place over the wider ocean basin, not the convection
at the northern end. This mixing is driven by tidal dissipation and
wind-driven currents over the ocean topography (preferentially over
ridges and around the edges of basins). Geothermal input may also play a
significant part, expecially in localised areas (heating from below is
very efficient at mixing fluids).
> 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.
I think it's only to be expected - the Gulf Stream / North Atlantic
Drift / THC are basically the same thing in the public consciousness,
probably in many non-specialist scientists' minds too.
James (not a bona-fide oceanographer by any means, but I have played one...)
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