A "double diffusive" process is one that is driven by the presence
of two different substances which diffuse at different rates. The
salt fingers instability is one
example of such an effect. Another example
is the effect that is observed when a mass of water with a vertical
salinity gradient is cooled or heated from the side.
Consider a region of the ocean with a continuously-varying vertical
salinity gradient (saltier at greater depths, i.e. stable density
stratification) and at uniform temperature. What is the effect of
introducing a cooling influence from the side by, say, an iceberg?
Since heat diffuses so much faster than salt, we may regard the
salinity of any given parcel of water as constant. The water nearest
the iceberg, upon being cooled, becomes dense and begins to fall.
Since the density of the far-field water increases with depth, the
cooled water soon reaches a depth at which its density matches that of
the warmer, saltier water away from the ice, and so its descent is
halted. As parcels of water above this one imitate its motion, it is
displaced horizontally away from the ice.
The net effect is that several horizontal layers of motion develop,
each with nearly uniform density. The relevance of this effect is that
the presence of an iceberg may be felt at surprisingly long distances
away from the iceberg itself.
We demonstrate this effect in the following experiment. If the above
explanation of the physical process is correct, then the thickness of
the layers should depend upon the amount of stratification. We thus
measured the thickness of the layers for different stratifications.
A 50cm by 50cm by 10cm tank was filled with salt water such that
there was a vertical density stratification. The
concentration of the salt was approximately 10% at the bottom and half
that at the top. (This was accomplished via the "double bucket"
technique.) A 30cm by 5cm by 3cm "iceberg" was prepared beforehand by
freezing a solution of dye and water. The iceberg was inserted into the
water along the side of the tank and was fixed into place. As the
layers started to form, potassium permanganate crystals were dropped into
the water to leave streaks by which we could visualize the layers.
Qualitatively, it was observed that the layers formed almost
immediately after placing the ice block into the tank. Also, the
layers extended all the way across the tank (see image and/or movie
The experiment was performed twice, the second time with a smaller
density gradient so as to observe the dependence of the layer thickness
on the density gradient. Our measurements did indicate that the smaller
density gradient led to thicker layers, which is consistent with the
theory explaining the process.
Our experiment sustained a "happy accident". On our second run, we
positioned a small lamp near our tank to provide improved lighting.
We subsequently noticed several double diffusion layers forming that did
not seem to come from the ice block (see the picture below).
They were significantly smaller than the other layers, and were
positioned below the ice, near the light source. The incandescent
bulb acted as a heat source, and these layers were the result of that
heat! This clearly demonstrated how sensitive stratified water is
to side heating or cooling.