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.
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.).
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.
Load and run the intrusions MPEG movie
See the vertical dye streaks through finite intrusions