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.)
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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