Step-Like Vertical Structure Formation Due to Turbulent Mixing
of Initially Continuous Density Gradients
A. Zatsepin 1 , S. Dikarev 1 , S. Poyarkov 1 , N. Sheremet 1 , I. Dmitrenko2, P. Golovin 2
and H. Kassens 3
(1) P.P. Shirshov Instute of Oceanology, 23 Krasikova, 117851 Moscow, Russia
(2) State Research Center - Arctic and Antarctic Research Institute, 38 Bering St., 199226 St. Petersburg,
Russia
(3) GEOMAR Forschungszentrum fur marine Geowissenschaften, Wischhof5trasse 1-3, D 24248 Kiel,
Germany
Received 3 March 1997 and accepted in revised form 9 February 1998
Abstract - The results of a simple laboratory experiments on the stirring of continuously
stratified fluid by oscillating vertical rods are described and analyzed. It is discovered that, if
turbulent stirring is rather weak, the strong linear stratification is transformed into a steplike structure during each experimental run. This structure consists of nearly homogeneous
layers separated by thin density interfaces. The initial average thickness of the layers
depends quasi-inversely on the buoyancy frequency and is growing with time during the
experiment. Thus, the number of layers decreases with time mainly because of merging of
layers. The analysis of our laboratory results combined with the analysis of previous studies
give strong support for the suggestion that the "staircase" structure formation may be not
an exotic phenomenon in the shallow summer pycnocline of Arctic seas under the influence
of drifting ice floes (Golovin et. ai, 1996). The disintegration of such a pycnocline into a
series of turbulent layers separated by thin density interfaces may enhance the vertical
transport of sediments and increase the rate of frazil ice formation (Krylov and Zatsepin,
1992).
Introdution
In the polar Arctic seas a shallow (3-10 m) and strong (10-25 sigma-t units) halo-pycnocline is
formed during the summer due to ice melting processes and/or river run-off. The drifting ice
floes often produce a considerable velocity shear and turbulent mixing across such a
pycnocline. On the base of ship observations in Kara sea, it was shown (Golovin et aI., 1996)
that during these events the vertical density structure of the shallow pycnocline may be changed
from "monolith" to the "step-like" form. Such changes in density structure should influence the
exchange of properties between the upper and the lower layers, particularly, the heat, salt and
sediment fluxes across the pycnocline zone. As a result, the rates of surface and underwater
(frazil) ice formation (or melting) may be also changed.
It was suggested that the transformation of the initially continuous density stratification of the
halocline into a series of homogeneous sublayers divided by extremely sharp density interfaces
is due to the turbulence instability mechanism in strongly stratified fluid (Phillips, 1972;
Posmentier, 1977). The qualitative explanation of this instability mechanism is based on the
nonlinear dependence of vertical buoyancy flux on the density gradient in turbulent stratified
flow. If the stratification is strong (Richardson number is enough high), the slight local
enhancement of the density gradient considerably reduces the turbulent exchange coefficient, so
that the buoyancy flux in this region is decreased, The local decrease of the buoyancy flux will
tend to further increase of the density gradient. So the perturbation will amplify. As the result of
the amplification of small disturbances (both positive and negative) initially continuous
stratification may disintegrate into a series of quasi-homogeneous layers separated by sharp
density interfaces. If the stratification is rather weak (Richardson number is below the critical
In: Kassens, H., H.A. Bauch, I. Dmitrenko, H. Eicken, H.-W. Hubberten, M. Melles, J. Thiede and L. Timokhov (eds.)
Land-Ocean Systems in the Siberian Arctic: Dynamics and History. Springer-Verlag, Berlin, 1999,93-99.
of Initially Continuous Density Gradients
A. Zatsepin 1 , S. Dikarev 1 , S. Poyarkov 1 , N. Sheremet 1 , I. Dmitrenko2, P. Golovin 2
and H. Kassens 3
(1) P.P. Shirshov Instute of Oceanology, 23 Krasikova, 117851 Moscow, Russia
(2) State Research Center - Arctic and Antarctic Research Institute, 38 Bering St., 199226 St. Petersburg,
Russia
(3) GEOMAR Forschungszentrum fur marine Geowissenschaften, Wischhof5trasse 1-3, D 24248 Kiel,
Germany
Received 3 March 1997 and accepted in revised form 9 February 1998
Abstract - The results of a simple laboratory experiments on the stirring of continuously
stratified fluid by oscillating vertical rods are described and analyzed. It is discovered that, if
turbulent stirring is rather weak, the strong linear stratification is transformed into a steplike structure during each experimental run. This structure consists of nearly homogeneous
layers separated by thin density interfaces. The initial average thickness of the layers
depends quasi-inversely on the buoyancy frequency and is growing with time during the
experiment. Thus, the number of layers decreases with time mainly because of merging of
layers. The analysis of our laboratory results combined with the analysis of previous studies
give strong support for the suggestion that the "staircase" structure formation may be not
an exotic phenomenon in the shallow summer pycnocline of Arctic seas under the influence
of drifting ice floes (Golovin et. ai, 1996). The disintegration of such a pycnocline into a
series of turbulent layers separated by thin density interfaces may enhance the vertical
transport of sediments and increase the rate of frazil ice formation (Krylov and Zatsepin,
1992).
Introdution
In the polar Arctic seas a shallow (3-10 m) and strong (10-25 sigma-t units) halo-pycnocline is
formed during the summer due to ice melting processes and/or river run-off. The drifting ice
floes often produce a considerable velocity shear and turbulent mixing across such a
pycnocline. On the base of ship observations in Kara sea, it was shown (Golovin et aI., 1996)
that during these events the vertical density structure of the shallow pycnocline may be changed
from "monolith" to the "step-like" form. Such changes in density structure should influence the
exchange of properties between the upper and the lower layers, particularly, the heat, salt and
sediment fluxes across the pycnocline zone. As a result, the rates of surface and underwater
(frazil) ice formation (or melting) may be also changed.
It was suggested that the transformation of the initially continuous density stratification of the
halocline into a series of homogeneous sublayers divided by extremely sharp density interfaces
is due to the turbulence instability mechanism in strongly stratified fluid (Phillips, 1972;
Posmentier, 1977). The qualitative explanation of this instability mechanism is based on the
nonlinear dependence of vertical buoyancy flux on the density gradient in turbulent stratified
flow. If the stratification is strong (Richardson number is enough high), the slight local
enhancement of the density gradient considerably reduces the turbulent exchange coefficient, so
that the buoyancy flux in this region is decreased, The local decrease of the buoyancy flux will
tend to further increase of the density gradient. So the perturbation will amplify. As the result of
the amplification of small disturbances (both positive and negative) initially continuous
stratification may disintegrate into a series of quasi-homogeneous layers separated by sharp
density interfaces. If the stratification is rather weak (Richardson number is below the critical
In: Kassens, H., H.A. Bauch, I. Dmitrenko, H. Eicken, H.-W. Hubberten, M. Melles, J. Thiede and L. Timokhov (eds.)
Land-Ocean Systems in the Siberian Arctic: Dynamics and History. Springer-Verlag, Berlin, 1999,93-99.