130
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
suggested by Martin and Kauffman (1974). This is determined by a specific process of
turbulent entrainment at the pycnocline/freshened layer boundary. In course of laboratory
experiments Kan and Tarnai (1994) described the mechanism of inner waves initiation at the
pycnocline boundary due to velocity shift together with their subsequent collapse due to KelvinHelmgolz instability. This results in formation of vortical structure at the pycnocline/fresh water
boundary (Kan and Tarnai, 1994). The latter favours more effective local penetration of fresh
and warm water from the upper layer to pycnocline represented by more saline and cold water.
In this case maximum supercooling occurs at that level within pycnocline where non-turbulent
freshened water has already penetrated. This happens if loss of heat is more effective than heat
exchange at molecular and molecular-turbulent levels (Krylov and Zatsepin, 1992; Voropayev
et aI., 1995). Buoyancy force hinders fast mixing within pycnocline, whereas part of more
saline and cold water from pycnocline rapidly gets mixed when penetrates into the strongly
turbulent upper layer (through this mechanism of turbulent entrainment). Hence the
supercooling at the pycnocline/upper water layer boundary is considerbly weaker than that
observed within pycnocline. If during the period preceding flood intensive turbulent
entrainment at this boundary is absent, weak supercooling is observed only at the very
boundary between the freshened water layer and pycnocline, and not within pycnocline (Figure
2a). This is in a good accordance with observational data (Martin and Kauffman, 1974).
The presence of supercooled water layers in the pycnocline caused the active formation of
frazil ice in these layers. A bottom - temperature and pressure meter was suspended from cable
at st. LN9610 at the traverse of the Trofimovskaya branch (Figure 1). It remained there from
May 22 to June 11. Thus the period from before through the flood was covered. When the
instrument was recovered, an agglomeration of frazil ice was found on the cable at the depth of
the pycnocline. Its weight was about 1.5-2 kg and its height was about 50-60 cm. It consisted
of transparent chaotically oriented crystals of frazil ice wither which inclusions of sediments
transported by river water were clearly seen (Figure 4). The cable in this case served both as a
nucleus and a place where crystals of frazil ice were trapped.
Thus as a result of the experimental studies in the Lena delta during spring flooding breakup,
the existence of supercooling in the river-sea water contact zone, as well as that of frazil ice
formation in this zone were established.
Discussion
Let us estimate the probability of supercooling in the river/sea water contact zone and the rate of
possible frazil ice formation at the different regimes of the heat-mass exchange through the
pycnocline using the results of laboratory studies.
Features of supercooling in the zone of river-sea water contact
As is known, supercooling of fresh river water from the underlying saline and cold sea water is
the necessary physical condition for frazil ice formation. Stable formation of frazil ice is
possible only when the freshened layer is close to its freezing point (Golovin et aI., 1996;
Krylov and Zatsepin, 1992; Voropayev et aI., 1995,), i.e. when the following condition is
fulfilled:
(1)
Here T I and S I are temperature and salinity of the layer, freshened by river runoff; a=0.055
°C/ppt is the linear relation coefficient between the freezing temperature and salinity. The
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
suggested by Martin and Kauffman (1974). This is determined by a specific process of
turbulent entrainment at the pycnocline/freshened layer boundary. In course of laboratory
experiments Kan and Tarnai (1994) described the mechanism of inner waves initiation at the
pycnocline boundary due to velocity shift together with their subsequent collapse due to KelvinHelmgolz instability. This results in formation of vortical structure at the pycnocline/fresh water
boundary (Kan and Tarnai, 1994). The latter favours more effective local penetration of fresh
and warm water from the upper layer to pycnocline represented by more saline and cold water.
In this case maximum supercooling occurs at that level within pycnocline where non-turbulent
freshened water has already penetrated. This happens if loss of heat is more effective than heat
exchange at molecular and molecular-turbulent levels (Krylov and Zatsepin, 1992; Voropayev
et aI., 1995). Buoyancy force hinders fast mixing within pycnocline, whereas part of more
saline and cold water from pycnocline rapidly gets mixed when penetrates into the strongly
turbulent upper layer (through this mechanism of turbulent entrainment). Hence the
supercooling at the pycnocline/upper water layer boundary is considerbly weaker than that
observed within pycnocline. If during the period preceding flood intensive turbulent
entrainment at this boundary is absent, weak supercooling is observed only at the very
boundary between the freshened water layer and pycnocline, and not within pycnocline (Figure
2a). This is in a good accordance with observational data (Martin and Kauffman, 1974).
The presence of supercooled water layers in the pycnocline caused the active formation of
frazil ice in these layers. A bottom - temperature and pressure meter was suspended from cable
at st. LN9610 at the traverse of the Trofimovskaya branch (Figure 1). It remained there from
May 22 to June 11. Thus the period from before through the flood was covered. When the
instrument was recovered, an agglomeration of frazil ice was found on the cable at the depth of
the pycnocline. Its weight was about 1.5-2 kg and its height was about 50-60 cm. It consisted
of transparent chaotically oriented crystals of frazil ice wither which inclusions of sediments
transported by river water were clearly seen (Figure 4). The cable in this case served both as a
nucleus and a place where crystals of frazil ice were trapped.
Thus as a result of the experimental studies in the Lena delta during spring flooding breakup,
the existence of supercooling in the river-sea water contact zone, as well as that of frazil ice
formation in this zone were established.
Discussion
Let us estimate the probability of supercooling in the river/sea water contact zone and the rate of
possible frazil ice formation at the different regimes of the heat-mass exchange through the
pycnocline using the results of laboratory studies.
Features of supercooling in the zone of river-sea water contact
As is known, supercooling of fresh river water from the underlying saline and cold sea water is
the necessary physical condition for frazil ice formation. Stable formation of frazil ice is
possible only when the freshened layer is close to its freezing point (Golovin et aI., 1996;
Krylov and Zatsepin, 1992; Voropayev et aI., 1995,), i.e. when the following condition is
fulfilled:
(1)
Here T I and S I are temperature and salinity of the layer, freshened by river runoff; a=0.055
°C/ppt is the linear relation coefficient between the freezing temperature and salinity. The