Meridional Overturning Circulation vs. Thermohaline Circulation?

[theworld]

What is the difference between Meridional Overturning Circulation and Thermohaline Circulation?

Is it correct to say:

THC is a principle - basically just the fact that colder and/or saltier water sinks because it's denser than warmer or less salty water. And that if some other factor such as, say, a heat source at the bottom, or wind-driven turbulence at the top, is applied, the temperature and/or salt gradients result in circulation.

And you could observe THC in a fish tank if everything was controlled and measured just right.

MOC is an actual specific phenomenon that we observe in oceans: a particular pattern followed by currents such as the Gulf Stream (and others?) which is driven by wind, THC, heat from the atmosphere, perhaps some other factors.

MOC couldn't be observed in a fish tank because it describes currents in actual places - it's not a principle you can demonstrate, as THC is.

...did I get this right?

 

[billiards]

Wikipedia says they are the same thing but I personally have never heard of MOC and I've done courses in ocean ciculation (maybe it's an american thing?). I actually get a feeling wikipedia means that it is the same thing as what i call the thermohaline conveyer, which is a convenient(simplified) way to picture global ocean circulation.

temp and salt gradients are obviously important, so are pressure gradients (which under baroclinic conditions can amount to the same thing as temp and salt gradients), coriolis acceleration etc..

wind does some funny things, look into Ekman and his spirals and pumping mechanisms. In order to understand the gulf stream you need a firm grounding in equatorial currents, which are too complex to go into here, it's not simply the wind, pressure gradients are very important ...best of luck with this, an interesting subject..

 

[theworld]

>>Wikipedia says they are the same thing but I personally have never heard of MOC and I've done courses in ocean ciculation

just curious - do you mean as an instructor or as a student?

>>(maybe it's an american thing?).

That's a really interesting possibility- thanks for bringing it up. I'll start paying attention to where the people are who are using it.

>>I actually get a feeling wikipedia means that it is the same thing as what i call the thermohaline conveyer, which is a convenient(simplified) way to picture global ocean circulation.

Yes - that's how it's used in the popular media and by some scientists. Other scientists define it more narrowly -- not that they disagree on the physical oceanography fact, only on how specific the term THC should be.

My conclusions, based on reading articles by the big names in the field and talking to a respected oceanographer.

1) Both terms are defined more broadly or narrowly by different scientists. Not due to disagreements about the science, only looseness about what shorthand name to give things.

2) When scientists communicate with each other, there's no ambiguity about the terms because they are talking/writing in detail about some particular measurements, experiments, processes or models. In those situations the contexts makes the meaning obvious.

3) Before the internet, blogs and wikipedia, terminology like this would have had more time to gel in the scientific community before picked up by the popular media or interested non-oceanographers.

4) In 2002 journal Science published an article called "What is the thermohaline circulation?" It's *unheard of* for Science articles to have plain-English what's-this-all-about titles -- so we can take it as a sign that things are still 'in progress'.

 

[billiards]

Well I took a course last year as a student (so I'm a little rusty, it's coming back now though), I was thinking about doing a PhD in it but I decided to finish my 4th year first and then get a job instead.

Sounds to me like you've looked into this and I don't disagree with any of your conclusions, to be honest it was one of the things I never even thought about (i.e. terminology wise) and I guess that's because my lecturer never introduced us to any ambiguity in that regard.