80
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
fronts? The most simple mechanism for the formation of the secondary thermoclinic discharge
front relates to the isopycnic convergence of the solar-heated surface water beneath the river
water lens. The lighter fresh water flows onto saline sea water. This mechanism is probably
most important when the river discharge extends far northward from the river mouths. This
must be accompanied by earlier ice melting in the same area in order for the water to have
enough time to be heated (as occurred in the summer of 1995, for example). Obviously, the
presence of a flaw polynya in these regions during winter and spring will increase heat
accumulation (Zakharov, 1966), and thus warmer water will be "sucked" under the river water
lens.
Another mechanism is connected with the possible deflection of the thermocline under the
influence of frontal convergence in those cases when temperature does not influence the density
field formation. This mechanism may be regarded as a passive admixture (Kuz'mina, 1980;
MacVean and Woods, 1980). This process can be the most efficient when isotherms are
initially inclined to isopycnals. This mechanism probably occurs within a short distance from
the river mouth regions. It is most efficient during years when the main front-forming
processes occur at a relatively small distance from the mouth regions (for example, the summer
of 1994). Both mechanisms form the secondary thermoclinic front beneath the upper baroclinic
one in the zone of frontal convergence.
An anomalous lowering of the upper well-heated quasi-thermal layer can result from the
development of this type of convergence circulation at the front. The seasonal thermocline
under the river water lens is located between 25 and 27 m, reaching the main pycnocline
(Figure 3A), though its usual depth corresponds to the depth of the seasonal pycnocline (5-7
m). As the autumn progresses, radiative cooling, and wave and wind driven mixing occur, and
the upper water layer is quickly cooled. However, a thicker heated water layer remains "buried"
at depths of 10-12 to 25-27 m (Figures 3B, 5B). This leads to the generation of a thick
anomalous warm water interlayer at the northern periphery of the zone of river water spreading
during summer. Its stability in autumn results from the coincidence of the upper warm layer
boundary with the position of the seasonal pycnocline.
A significant redistribution of oceanographic characteristics occurs under the influence of
convergent processes in the region of the discharge fronts and under the lens of river water
(Figure 6). The lowest concentration of dissolved silicon (less than 320 mg/l) was recorded in
the intermediate layer directly under the outflow axes (Kassens and Dmitrenko, 1995; Kassens
et aI., 1997). In the surface and bottom layers, silicon concentration was 650 and 980 mg/l
respectively. These layers coincided spatially with oxygen maxima, with enhanced chlorophyll
a fluorescence, which reached values typical of the sea surface, and with the absolute maxima
of the light transmission coefficients in water. These vertical distribution patterns typical of the
river water outflow zone were observed both in summer 1994 and autumn 1995. It is obvious
that water yielding such characteristics must originate at the surface. It is formed outside the
river outflow zone (low values of dissolved silicon and high transparency). Then, as a result of
frontal convergence in the spring and summer, it sinks, thus forming a sufficiently thick
intermediate water layer with anomalous hydrochemical and hydro-optical characteristics
(Figure 6).
Influence of the surface intermediate layer on the formation of ice-hydrological
conditions in the autumn-winter season
The existence of a warm (up to 4°C) and thick (up to 20 m) water layer is expected to
significantly alter ice formation conditions and subsequent ice growth processes in regions
affected by river discharge. The heat flux from this layer to the surface can probably limit ice
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
fronts? The most simple mechanism for the formation of the secondary thermoclinic discharge
front relates to the isopycnic convergence of the solar-heated surface water beneath the river
water lens. The lighter fresh water flows onto saline sea water. This mechanism is probably
most important when the river discharge extends far northward from the river mouths. This
must be accompanied by earlier ice melting in the same area in order for the water to have
enough time to be heated (as occurred in the summer of 1995, for example). Obviously, the
presence of a flaw polynya in these regions during winter and spring will increase heat
accumulation (Zakharov, 1966), and thus warmer water will be "sucked" under the river water
lens.
Another mechanism is connected with the possible deflection of the thermocline under the
influence of frontal convergence in those cases when temperature does not influence the density
field formation. This mechanism may be regarded as a passive admixture (Kuz'mina, 1980;
MacVean and Woods, 1980). This process can be the most efficient when isotherms are
initially inclined to isopycnals. This mechanism probably occurs within a short distance from
the river mouth regions. It is most efficient during years when the main front-forming
processes occur at a relatively small distance from the mouth regions (for example, the summer
of 1994). Both mechanisms form the secondary thermoclinic front beneath the upper baroclinic
one in the zone of frontal convergence.
An anomalous lowering of the upper well-heated quasi-thermal layer can result from the
development of this type of convergence circulation at the front. The seasonal thermocline
under the river water lens is located between 25 and 27 m, reaching the main pycnocline
(Figure 3A), though its usual depth corresponds to the depth of the seasonal pycnocline (5-7
m). As the autumn progresses, radiative cooling, and wave and wind driven mixing occur, and
the upper water layer is quickly cooled. However, a thicker heated water layer remains "buried"
at depths of 10-12 to 25-27 m (Figures 3B, 5B). This leads to the generation of a thick
anomalous warm water interlayer at the northern periphery of the zone of river water spreading
during summer. Its stability in autumn results from the coincidence of the upper warm layer
boundary with the position of the seasonal pycnocline.
A significant redistribution of oceanographic characteristics occurs under the influence of
convergent processes in the region of the discharge fronts and under the lens of river water
(Figure 6). The lowest concentration of dissolved silicon (less than 320 mg/l) was recorded in
the intermediate layer directly under the outflow axes (Kassens and Dmitrenko, 1995; Kassens
et aI., 1997). In the surface and bottom layers, silicon concentration was 650 and 980 mg/l
respectively. These layers coincided spatially with oxygen maxima, with enhanced chlorophyll
a fluorescence, which reached values typical of the sea surface, and with the absolute maxima
of the light transmission coefficients in water. These vertical distribution patterns typical of the
river water outflow zone were observed both in summer 1994 and autumn 1995. It is obvious
that water yielding such characteristics must originate at the surface. It is formed outside the
river outflow zone (low values of dissolved silicon and high transparency). Then, as a result of
frontal convergence in the spring and summer, it sinks, thus forming a sufficiently thick
intermediate water layer with anomalous hydrochemical and hydro-optical characteristics
(Figure 6).
Influence of the surface intermediate layer on the formation of ice-hydrological
conditions in the autumn-winter season
The existence of a warm (up to 4°C) and thick (up to 20 m) water layer is expected to
significantly alter ice formation conditions and subsequent ice growth processes in regions
affected by river discharge. The heat flux from this layer to the surface can probably limit ice