Currents in large lakes and seas are caused by
various factors - winds, changes in atmospheric pressure, the rotation of the earth, and
the input of large amounts of water via tributaries.
Different methods were used to study the currents in Baikal: bottle post, the drift of
vessels, the drift of fishing nets, free floats (followed by vessels, tracked from the
shore or photographed from the air), various current meters, and automatic recording
instruments. Modelling methods are also used.
In Baikal there are two main kinds
of current: permanent (geostrophic - from the Greek words geo - Earth,
and strophe - a twist, turning about) and drift.
Permanent currents are those that keep their main features over long periods. In
Baikal's waters, these move, as a rule, anticlockwise. For this reason they are
called cyclonical.
In general it can be said that the whole of Baikal is caught up in currents of a
cyclonical type. These currents are divided up into different fields apparently
owing to the peculiarities of form of the Baikal basins.
Speeds of the cyclonical currents measured at different places sometimes reach
18 - 20 centimetres per second [cm/s] and even 50 cm/s. However, countercurrents
and churning are often observed here. So the total (vectorial) speed of the currents
is not very great, up to 8 - 9 cm/s, and often lower than that, up to 4 - 6 cm/s.
Cyclonical currents are characteristic not only of Baikal; they also occur in the
Baltic, Barents and Aegean Seas and in lakes Ladoga and Huron.
Significant distortions in Baikal's currents are brought about by winds. These
cause strong but relatively short-lived drift currents. During a wind, lasting from a
few hours to a few days, movement is caused in the surface layer of water, but this
has its effect on the entire water mass right down to the deep water bottom layers.
The strongest winds which cause these drift currents at Baikal are the
mountain wind, the Kultuk, the Barguzin, Verkhovik and Shelonnik.
The mountain wind (Gora, Gornaya, Gornyak, Sarma, Kharakhaikha) blows
along the western shores of the lake, from the west and north west, and from the
mountainous peaks of the Primorsky and Baikalsky ranges. The Sarma, in the
Olkhon region, is particularly strong. Its gusts are so strong that they can lift roofs
off houses, and turn boats and launches over. In these gusts the wind speed can
reach 40 metres per second [m/s]. Waves whipped up by this wind can be as high
as 2 - 3 metres and in the centre of the lake up to 5.5 metres.
The Kultuk (nizovik) blows from the south-west along the entire length of the
lake. The speed of this wind can get up to 18 - 20 metres a second. When this wind
is blowing the whole lake is disturbed, the height of the waves usually being no
less than two metres, and in the zone where the wind speed is highest can reach
more than 3 metres. The waves caused by the Kultuk take a long time to settle in
North Baikal, here waves of a swell of more than 2.5 metres rock this part of the
lake.
The Barguzin blows from the north-east down the valley of the Barguzin.
Typically this is an autumn and winter wind. In the open sea, it reaches a speed of
18 - 20 metres a second. Storms brought about by the Barguzin whip up waves of
3.5 - 4 metres along the western coast of the lake in the region of Olkhon and
Goloustnaya.
The Verkhovik (Angara), a north and north-easterly wind, blows over the
whole lake, usually in spring and summer. Its speed can reach 18 - 20 metres a
second.
The Shelonnik (Selenga) comes from the south-east. In the summer its speed is
moderate, up to 10 metres a second, and stronger in autumn, up to 20 metres a
second. It reaches its maximum speed at the mouth of the Selenga and quickly
weakens in Baikal's southern basin. Only seldom does it raise storms in this
southern part with waves of more than 2 metres. An important and interesting
question is that of the deep circulation of Baikal's waters.
The possibility of the deep circulation of waters for some reason always
fascinates people, especially the local people. A lot of stories are told about this,
from the most fantastic to the quite authentic. Long since amongst local people
there has been the popular belief that Baikal is connected in its depths with the
ocean. I heard one quite genuine story once from a foreman amongst Baikal
fishermen. I have no doubt about the authenticity of what was told.
On one occasion a team of fishermen put out some two kilometres of sea
netting for the night in the open sea. And, as is usual practice, the fishermen tied
the boats and karbass (a large goods boat) to the end of the nets and drifted
together with them all night around the sea. The night was clear and starry, and
Baikal calm and still. Early in the morning, the foreman, having woken up first,
threw a glance at the whole long stretched out and curving snake-like line of upper
net floats. Everything was all right. But in a short while, taking another look at the
floats, the foreman noticed something wrong. At the far end one after another the
floats were abruptly disappearing under the water and not returning. One after
another, closer and closer to the boats and karbass. There was good reason to be
dumbfounded. And so the last float disappeared under water. The rope connecting
the net and the boats stretched taut, pulling the stern of the karbass down. At last,
waking up to what was happening, the foremen cut through the rope in one stroke.
The fishermen waited at this place for a long time, hoping that the nets would bob
up, but they waited in vain.
It is difficult to say what this was. A fish? A nerpa? But they float up when
they get into the nets. There are no wales in Baikal. More likely than not one must
suppose that this had been a vertical vortical stream taking the nets down into the
deep.
In the 1920s V.V.Shostakovich expressed the opinion that beneath the active
top 200 metre layer there lies a mass of water that plays no part in the life of the
lake. He wrote that in this body of water "complete calm and motionlessness
reign".
These views were refuted in the thirties by G.Y.Vereshagin who was studying
the lake's thermal regime. He showed that during strong winds, especially from
the north-west, relatively cold deep waters rise to the surface by leeward shores,
coming, judging by their temperature, from depths of up to 600 metres. In the
Barguzin Bay, Vereshagin noted a rising of deep waters near the Svyatoi Nos.
These waters round the Svyatoi Nos peninsular from the south and move further on
towards the Ushkanye Islands.
In 1957, V.A.Tolmachev put forward his opinion on the formation of the
chemical regime of deep waters under the influence of strong vertical circulation.
Following Vereshagin he singled out three hydrochemical zones in the body of
water: an upper dynamic zone having a high concentration of chemical
substances and going down to 500 metres; a middle stagnant zone, where the
component content is minimal, stretching from 500 to 1000 metres; and a deep
water zone, below 1000 metres, in which the concentration of substances varies
with time. The waters of the top and deep zones are involved in mixing, the
mechanisms of which are unknown. Tolmachev made the suggestion that the
surface waters mix with the deep waters by descending the underwater slopes
assisted by the peculiarities of the relief.
Since the 1970s the isotopes oxygen 018, deuterium, (an isotope of hydrogen
with a mass of 2, in combination with oxygen forming "heavy water"), and tritium
(an isotope of hydrogen with a mass of 3) have been used in investigations of the
exchange between different waters and deep circulation in Baikal. The oxygen
isotope and deuterium content in Baikal water is the same as it is in the waters of
Siberian rivers. This prevents its being used as an indicator. It is another matter
with tritium. This is a short-living isotope with a half life of some 12.5 years. In
natural conditions it is formed in the upper layers of the atmosphere under the
impact of cosmic particles (protons and neutrons) on the nuclei of nitrogen and
oxygen. It enters the lake via the atmosphere with rain and river waters. Using
this, it is possible to analyse water circulation. V.N.Soifer, V .S.Brezgunov,
K.K.Votintsev, V.I.Verbolov, M.N.Shimaraev and others have carried out such
research at Baikal.
In a number of places a higher than usual concentration of tritium was found at
different depths in the body of water. This made it possible to draw a conclusion
about the deep circulation of waters, and about the transportation of tritium
enriched river waters into deep water zones. The mechanisms of such mixing,
however, was not clear. It was suggested that river water was transported into the
deeps in compact streams.
Data on the distribution of tritium in Baikal was also discussed by
V.I.Ferronsky, V.A.Polyakov, V.V.Romanov and others. For example, it was
shown that the ratio of tritium in the lake's waters to that coming via rivers and in
atmospheric precipitation depends on the time of water exchange in the lake. The
latter is calculated quite simply: the volume of water in the lake is divided by the
quantity of water input (the "box" model). In Baikal's case it looks like this: 23000
cubic kilometres must be divided by 70.2 cubic kilometres, which comes to 330
years, that is the time it takes for the water in the lake to be renewed completely.
Lake Issyk-Kul has the same time scale for water renewal. It turns out that Baikal
and Issyk-Kul also have the same ratio of tritium in the lake waters to that in
waters entering the lake, 0.31 : 1. On the other hand, according to the ratio of
tritium in atmospheric precipitation and in the lake waters a larger value is obtained
for the exchange of waters, that is, 370 years. But it should be lowered since
Baikal is fed mainly by river waters (82% of all incoming waters). In other
words, a complete renewal of Baikal's waters can take place over 330 years. As
for deep water circulation, this is confirmed by the higher concentration of tritium
(it is true, not always) in the bottom layers. This speaks in favour of one
conclusion, that is, that the circulation of water happens quicker than the half-life
of tritium, or less than 12.5 years.
Recently, researchers have come to the conclusion that tritium enters the rivers
and lake in atmospheric precipitation and that it has anthropogenic origins.
V.S. Lapin and V.I.Lapkanov have determined that 160 grammes of tritium are
contained in Baikal, 144 g. of which, i.e. 90%, is the product of nuclear explosions
made in the atmosphere over the period 1956 - 1962. A decrease in the tritium
content of Baikal's waters will take place over the next 50 years.
Results have also been achieved in the study of the circulation by using
coloured substances. In the autumn of 1984, K.A.Korotenko and A.L.Sukhov
studied the currents and transportation of matter in Central Baikal using the point
dye method. They pointed out that Central Baikal is notable for its intense mixing
of water masses and complex current pattern. In one experiment off Cape Khoboi,
at the northern end of the island of Olkhon, the quick descent of a coloured patch
was observed which was accounted for by a current of warm waters here from the
Maloye Morye and consequent sinking of cold waters of the open part of the lake.
A systematic study of the temperature regime of deep waters is also being
carried out. In 1984 M.M.Liubimtsev and A.L.Sukhov determined an
anti-cyclonical deep water current along the south-eastern shore of South Baikal
appearing beneath the temperature jump layer. On the surface here there is the
usual circulation current, that is, the water moves in an anticlockwise direction,
while at depth it moves in the opposite, clockwise, direction.
K.K.Votintsev came to an interesting conclusion about the deep circulation of
waters. He drew attention to the dissolved oxygen content in the waters. Baikal's
waters are very oxygen rich. At the surface the oxygen content is near normal,
and in summer the surface waters are even oversaturated in oxygen, that is, up to
as much as 115 - 120% saturation. The oxygen content decreases with depth, but
even in the deepest parts of the lake it is never below 75 - 70%. This incredible
feature is found only in Baikal. In Lake Tanganyika, for example, even at a depth
of 70 - 130 metres the water lacks oxygen, and at depths of 200 - 300 metres
there is a constant hydrogen sulphide content. Oxygen enters the deep waters of
Baikal only from the surface where its concentration is maintained by exchange
with the atmosphere and photosynthesis. The oxygen content was analysed at
different depths along a line from Listvyanka to Tankhoi. Down to a depth of 200
metres a springtime maximum and autumn minimum in the oxygen distribution is
observed. Lower down (to 1400 metres) top maximums appear: a winter
(December to February) and summer one (July - August). The 750 metre horizon
is an exception, however, with one maximum occurring (from November to
February).
The pattern of oxygen distribution indicates its build up in the deep waters as
a result of their constant mixing with surface waters. Such mixing is particularly
strong in autumn, when storms rage at Baikal before the ice forms.
Votintsev came to the conclusion that vertical mixing of the waters in Baikal
takes place yearly. In 1991, R.F.Weiss (USA), E.C.Carmack (Canada) and
V.M.Koropalov (Russia) published the results of a study of the temperature of the
water mass of Baikal from the surface to the bottom, and the distribution of
Freons, oxygen, phosphates, nitrates, and silica in the journal "Nature" (GB). The
distribution of the Freon (CFC-12, difluorinechloromethane) is particularly
interesting. The Freon, as is well known, is of purely anthropogenic origin. It is
filling more and more the waters of the world's oceans, rivers and lakes, and
eroding away the ozone layer in the planet's ionosphere, removing its defence
against ultra-violet rays. Its distribution in Baikal's waters reflects the speed of
exchange of surface and deep waters and enables us to determine their
approximate age. The youngest - about one year - are the waters of the top 200
metre layer. The water 'ages' with depth and in the 800 to 1200 metre layers they
are some 12 - 16 years old. At the very bottom, in the deepest parts, they are
again younger (up to 8 - 9 years), which indicates the existence of mechanisms
that take water from the surface layers to great depths directly.
The distribution of the Freon in Baikal's water indicates, in the opinion of
researchers, deep circulation of surface waters that renew the bottom layers of
water. The exchange of waters of all layers below 200 metres takes 8 years.
In 1993, M.N.Shimaraev, N.G.Granin and A.A Zhdanov (Russia) published the
results of research into the temperature of the lake's water in June in the journal
"Limnology and Oceanography" (USA): The waters of the open part of the lake
remain relatively cold. It is the coastal waters that warm up. And convection is
caused in these waters when the temperature threshold of +4°C, that is,
when water is at its most dense, is reached. But this is still not deep water
exchange. The gradual warming of the coastal waters brings about a so called
'thermobar', during which the compact part of the water body under the effect of
compression move downwards and gradually reach the bottom. In this way the deep waters
are diluted by the surface waters. The process of deep convection can occur all over
the lake; it depends on the climate regulating it and on the temperature of the lake's
waters. During the deep circulation of surface waters sudden vortical currents can
arise, plunging down into the deeps, into Baikal's abyss. |
Currents