Mediterraneun

[andrewlee]

Why does the Med have virtually no tides?

[New Forester]

The moon and the sun pull on the earth.

 

Each bit of ground or water turns under the pull once every day as the earth turns.

 

Each ocean or sea has a natural resonant frequency -- the time it takes a "slop" to travel from one side to the other.  Think of it like the water slopping in a bath.

 

If an ocean or sea has a natural frequency which resonates with the pull of the moon and sun, each little pull makes it slop more (experiment with this in the bath...).   For example, the natural slop of the Atlantic is about 12 hours, which is half a 24 hour day, so it resonates.  However, the slop time of the Med is about 9 hours.  This means that each little pull from the moon and sun hits at just the time to cancel out the previous one.

 

T he effect of this is that the Med (also the Baltic, Arctic and a number of others) only shows very slight tides -- pretty much just the moon's "naked" unresonated pull -- it's half a metre range or less, as opposed to perhaps 10 or 15 m for the Atlantic.

(Cont...)

[New Forester]

(...cont)

The Pacific resonates at about 24 hours.  Pacific tides are complex, but at some times they can be one big tide a day instead of two as in the Atlantic.



More if you want it:   I've simplified the above somewhat, talking about "tugs" by the moon and sun when they are overhead.  In fact there is also a "tug" in the opposite direction when they are exactly on the other side of the earth.  As the earth is facing the other way this tug is also up.  This means that there are in fact two upward tugs each day, not one -- but the resonance works out the same.  If you want this explained further, please post another question....



T he Atlantic is getting wider at about the same rate our fingernails grow -- a few centimetres a year.  If you think about that, its slop must once have been out of phase with the daylength -- and it will go out of phase again in a few tens of millions of years.  So make the most of the tides while they last.