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calendar   Wednesday - April 06, 2011

Larnin me some stuff

ISOPYCNAL ADVECTION


Whereas convection means the vertical movement or circulation of a fluid (air, liquid, magma), advection means the horizontal movement.

Similar to the other iso-s (isotherm - areas of equal temperature, isobar - areas of equal pressure, etc), an isopycnal is an area of equal density. Well, potential density actually, which is what the actual density of the isopycnic (no, that isn’t an outdoors lunch for one) substance would be if it were at a standard pressure and temperature. And to be completely correct, any “iso” is merely the mapped line that shows the edge of any such area, but common usage allows the line term to be used to refer to the whole thing.

We all know that pure water is incompressible; it’s liquid state density does not change when subject to pressure or temperature changes except when it is at the very edge of state transition (about to freeze or boil), when it’s density decreases. This is not the case for salt water. The more salt in the water - it’s salinity - the denser it is, given equal temperatures. Surprise surprise, while the salinity of the oceans are fairly constant, the salinity of the various seas around the edges can vary significantly. This is because many of them are fed by freshwater rivers. The Baltic Sea and the Caspian Sea are much less salty than the Atlantic Ocean. So there can be areas of less dense less salty water that float over areas of more dense more salty water. These areas can be measured and plotted, and because they tend to layer they are called haloclines. “halo” from halide, meaning salt. “cline”, meaning layer.

There are several different clines involved with large bodies of water, such as thermoclines and chemoclines. And yes, there is such a thing as a pycnocline; water stratified into layers based on density.

A rough example of isopycnal advection would be to take a gallon of well mixed paint and to pour it out on the dining table. Because the density of the table top is much greater than the density of the paint, the paint flows out to the sides, horizontally. Don’t try this example at home, or you’ll be having your own isopicnic in the doghouse for a long time.

I have no idea how I got there, but I found myself reading the Wiki page on the Black Sea. The Black Sea is an odd place, a relatively shallow (7200’ max) brackish lake with a significant underwater shelf mostly on the north side. It is only slightly connected to the world’s oceans via the narrow Straits of Bosporus at Istanbul. Fed by several of the major European rivers, the Black Sea has lots of isopycnal advection. The fresh water comes in at the surface, floats only around the sides of the sea, and sneaks out into the Aegean and from there into the Mediterranean via the Bosporus, and then the tiniest sea in the world called the Sea of Marmara, and then through that little passage there by the Dardanelles (the Hellespont) at the feet of ancient Troy. Meanwhile, while the fresh water is flowing out, salt water is flowing in. The Aegean Sea is a bit of an isolated zone in the northeast corner of the Mediterranean, so because of less mixing due to gentler currents it is slightly less salty to begin with. Quite a bit of mixing goes on in the Marmara and it’s two connecting rivers, so the salt water that does get back to the Black Sea is considerably less salty and sinks right to the bottom. And it stays there. Forever. The Black Sea is one of those few bodies of water that is actually anoxic (oxygen limited), and this is all due to the isopycnal advection. Nicely aerated fresh water comes in, and supports abundant life, but the deeper you go the deader it gets. A special kind of bacteria - an extremophile - lives at the medium depths (~200m) right at the pycnocline, and eats up all the remaining nutrients and uses up all the remaining oxygen. It poops out hydrogen sulfide, which sinks into the local abyss. So not only is the deep water of the Black Sea devoid of oxygen and life, it’s also poisonous. If there were a massive earthquake or a big meteor strike that brought those deep waters to the surface, the gas released could kill half the continent. And it’s all completely natural and humans had nothing at all to do with it.

Because the Black Sea has only one drainage point and a small one at that, it can be thought of as one of the world’s largest lakes. Actually, it has been a lake in the geologically recent past, more recently than the last Ice Age. Human dwellings have been found in a number of areas on the northern shelf, so what was once dry land is now 100m underwater. Anyway, the term for lakes where the surface water does not mix with the deeper water is meromictic, and that applies here as well. So water comes in, flows on top due to isopycnal advection, and flows out, and the end result is a meromictic body of water.

Ok, time for recess!!


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Posted by Drew458   United States  on 04/06/2011 at 10:01 AM   
Filed Under: • Education •  
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