The Deep Ocean Conveyer Belt
and its Relationship to the Gulf Stream

T   
he new movie coming out next week, "The Day After Tomorrow", has generated a lot of gloom and doom prognostications in magazines, television and on the internet.

According to the "experts", if the North Polar ice sheet melts and mixes with the waters of the North Atlantic, the resulting density changes may cause the deep ocean conveyer belt to stop ... in turn causing a worldwide disruption of weather patterns. Chief among them is the stoppage or reversal of the Gulf Stream which brings warm equatorial waters to the north thereby warming Europe which would otherwise be as cold as Siberia.

The general theory of ocean currents is that there are only two important factors involved ... salinity and temperature (the thermohaline cycle). Thus, water near the equator evaporates from the hot sun there and leaves the salt behind. The excess salt makes the water there denser than other ocean waters ... but ... it is also warmer which tends to make it expand and therefore be less dense. When it flows northward, it cools, giving up its heat to the air which warms Europe. But this makes it more dense ... which, when coupled with its excess salt content, causes it to sink in the North Atlantic. It then flows hither and yon all over the planet in a current that is sometimes in the deep ocean and sometimes on top. The essential information is that this stream has been all mapped out and is not in dispute (at least I found no basic dispute on this point on the internet).

Here are some good links I found while rumaging around for facts:

http://www.physicalgeography.net/fundamentals/8q_1.html
http://earth.usc.edu/%7Egeol150/variability/deepocean.html
http://earth.usc.edu/~stott/Catalina/Oceans.html (This page is very good)
http://earth.usc.edu/~stott/Catalina/Deepwater.html
http://science.nasa.gov/headlines/y2000/ast15jun_2.htm
http://www.grida.no/climate/vital/32.htm
http://science.nasa.gov/headlines/y2000/ast15jun_2.htm ... about tidal energy in the deep ocean

Implicit in this general theory is the causal influence of the deep ocean current on the Gulf Stream ... and we will get to that shortly. But first ...

Why hasn't the conveyer belt been upset already?

Navy subs that regularly poke their noses through the Arctic pack ice report that in the last decade the ice up there has gone from an average of 12 feet thick to an average depth of 4 feet. That's means the ice sheet is 2/3 melted already. Well ... is there as yet no indication of any change in the conveyer? I have heard of none so far. Perhaps the conveyer will run for another decade or two strictly on its own built up momentum?

And, while we're at it ... if the 12 feet of ice melts up there ... why wouldn't it get salty from the underlying Arctic Ocean which is about a thousand times that depth? Wouldn't twelve feet of sweetwater get somewhat brackish lying on top of 12000 feet of saltwater? If the salt is 12 feet away and the melted water has, say, 400 miles to get out of the Arctic Ocean, wouldn't the salt migrate by simple difusion into the freshwater melt? What do you think? Could a calumn of freshwater stay fresh on top of a column of saltwater two miles deep at around 3o centigrade for weeks on end? Would it not then enter the upper Atlantic with about the same salinity as waters that came from the Gulf?

The factors listed in the creation of ocean currents are ...

  • Salinity
  • Temperature
  • Wind
  • Coriolis Forces

In my web search, no where was it listed ... as a cause of ocean currents ... the tidal effect of the moon. At best, I found a link which found that around 30% of the energy was "dissipated" in the deep ocean primarily at the mid-ocean ridges (that's the NASA link above). And that gave no indication that there was any involvement of tides in the ocean currents themselves.

Yet, lunar tides are undoubtedly the prime generator of the Gulf Stream.

The tidal effect does not "just" dissipate energy ... it slows the angular momentum of the Earth and in doing so increases the angular momentum of the moon relative to the earth so that the angular momentum of the the Earth-Moon system is conserved.

It works like this ...

The moon causes a tidal bulge on the earth. As the earth spins it displaces the bulge in the direction of its spin. The tidal bulge caused by the moon is NEVER directly centered on a line connecting the centers of the earth and moon. It is always displaced to some extent due to the earth's rotation.

The moon is attracted more to this bulge so that the attraction of the moon to the earth is not directly on the line connecting the center of mass of the earth and moon. This effect "gives a couple" as Eddington would say ... and the result is that the moon has a force vector pushing it along faster in its direction of motion around the earth which is the same as the earth's rotation direction (prograde). Thus, the moon ever climbs higher in its orbit and was in times past much closer to the earth.

By "reaction", the tidal bulge is pulled back by the moon in the same direction it appears to be going anyway, i.e. there is force vector on the tidal bulge opposite to that on the moon in orbit. The moon is pulling the tidal bulge as a current which runs into the land masses on the west side of the oceans. There is a tide on both sides but the "tidal current" only runs into the west side subtracting momentum from the earth ... slowing the earth's rotation and lengthening the day. Eventually, the earth and moon will always present the same face to each other just as the moon always presents the same face to us now.

The point should be stressed here that this is a genuine equatorial current ... though very wide, and ... in the Atlantic ... it "breaks" into two streams on the easternmost shore of South America. That is why there are rotating eddies in the middle of all the oceans. Even if the oceans were isohalinic and isothermic ... there would still be rotating currents in the oceans from this tidal effect alone ... and they would rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. The other effects merely reinforce this current or go against it temporarily. The question must be one of "how much" each aspect contributes to ocean currents.

Coriolis forces are not the "cause" of the circular currents. If Coriolis was causal, the eddies would flow counterclockwise in the northern hemisphere (just like hurricanes) ... but they don't ... they flow clockwise ... hence the Coriolis forces act on a pre-existing current.

So, a fifth force must be added to the above list which should read:

  • Salinity
  • Temperature
  • Wind
  • Coriolis Forces
  • Tidal Forces

And I would guess that the tidal force, as I have described it, is the most influencial of the lot. If so, it should be understood that the Gulf Stream would not cease to flow regardless of the cessation or reversal of flow of the deep ocean current. It would simply do something else while the Gulf Stream continued its regular flow bringing warm water to Europe. Other significant changes might occur but the cooling of Europe would not occur by this mechanism recently suggested by oceanographers.

This is a consequence of standard physical theory and cannot be denied by oceanographers lest they wish to do battle with physicists over elementary physics. Nothing radical is being proposed here.



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