Thermohaline Intrusions


To demonstrate the growth of thermohaline intrusions driven by double-diffusive vertical fluxes.

What Happens:

A long tank is stratified with a vertical gradient of sugar solution on the left, and salt solution on the right, with equal densities at the same level on either side. When the barrier separating the two tank halves is removed and the small internal wave disturbances die out, a series of intrusive layers forms and grows.

The layers are thicker near the bottom, where the salt/sugar contrast is larger, and the speed of advance of the layers is proportional to their thickness.

Physics of the Phenomenon:

The layers are driven by the density changes that double-diffusive fluxes create. The feedback cycle is the following:

1. Lateral advection in alternating directions draws sugar solution (In the ocean this would be warm, salty water.) above salt solution (In the ocean this would be cool fresher water.), creating a vertical stratification suitable for "sugar" fingers.

2. The vertical density flux of the "sugar" fingers causes the sugar layers to become less dense, and the salty layers to become more dense, as each layer continues to advect.

3. The changes in density in (2.) above cause the sugary layers to rise as they advect to the right, and the salty layers to fall as they advect to the left. This creates the systematic tilt from lower left to upper right that can be seen.

4. The slight density anomaly of each layer, combined with the tilt, creates pressure perturbations that drive the advective motions in step (1.).

Of course, between each finger region is a diffusive region (sugar solution below salt soution), and if the density flux in the diffusive regions were to dominate, the layers would slope the opposite way. This was rarely observed in the laboratory experiments.


Turner, J.S., Buoyancy Effects in Fluids, Cambridge University Press, Cambridge, U.K.368 pp.,1973.

Ruddick, B.R., 1992. Intrusive mixing in a Mediterranean salt lens -- intrusion slopes and dynamical mechanisms. J. Phys. Oceanogr. 22(11) 1274-1285.

Ruddick, B., D. Hebert, 1988. The mixing of Meddy "Sharon". Small-scale Mixing in the Ocean, Elsevier Oceanography Series, Vol. 46, pp. 249-262, J.C.J. Nihoul and B. M. Jamart, Eds.

Ruddick, B. and J.S. Turner, 1979. The vertical length scale of double-diffusive intrusions. Deep-Sea Res. 26A, 903-913.


Movie and text - Barry Ruddick
Digitization of movie - Dave Hebert

Load and run the intrusions MPEG movie

See the vertical dye streaks through finite intrusions