Friday, October 08, 2004

Pacific vs. Atlantic, round 3

Following my old memories, I have since dug up some references to the effects of Coriolis forces on ocean water -- there's a phenomenon called "Ekman transport" that can result in elevation differences up to a meter: see, for example,

Related or supporting concepts:
- As the prevailing winds drive the large surface gyres, water is
directed inward to the center of the gyres by Ekman transport.
- Ekman transport creates an elevated sea surface due to the piling
up of the water. The difference in elevation is as much as 1 m (3 ft) > or more.
- Gravity causes water to flow down the slopes of this elevated mound.
The water will be deflected by the Coriolis effect and when
everything is in balance there will be a circular rotation of water
around the mound.

Unfortunately, this doesn't seem to have *anything* to do with the (relatively minor) difference between Atlantic and Pacific average levels. And the only Coriolis-related effect on ocean basins that I can find (on the same Web site) says that currents are stronger, and water piles up a bit, on the _western_ sides of ocean basins. Which could only tend to counteract the observed difference in levels between the eastern side of Pacific and the Western side of the Atlantic...

Oh, well. So much for trusting my memories of old half-learned facts. -- Though of course the fact that my explanation was wrong _still_ doesn't mean that the density explanation is all there is to it -- I'd love to see an extra few sentences explaining how the water that tries to flow down the 20-centimeter average gradient from the Pacific evaporates along the way, and/or mixes with the higher-salt Atlantic and ends up obeying the same gravitational rules as the rest of the water, or whatever is happening.

But unfortunately I don't have nearly enough knowledge or credentials to supply those extra sentences... (No, I'm nothing like an oceanographer -- not even a competent physicist, which might help in evaluating the density explanation. I just know an incomplete story when I see one...)

However, I will venture to add a few facts to my previous amateur speculations: I live 200 km up the Hudson River from the ocean. The Hudson is six feet above sea level here, but its level still goes up and down with the tides (while maintaining that six-foot difference in levels!) So obviously there's nothing strange about a small gradient in water that's apparently flat and unobstructed -- as long as there's a current flow to maintain it, and somewhere for the water to go when it reaches the lowest level.

In the Pacific/Atlantic case, the only place the water could possibly disappear to would be into the air, by evaporation -- which would make sense, since the Atlantic is rumored to be warmer. Unless Coriolis-inspired currents have something to do with the matter after all!

[Now I just need to find a real oceanographer to straighten this all out for me.]


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