The South Pole, Volume 2
by
Roald Amundsen
Part 6 out of 6
extends over to the bank south of Ireland, while the three stations
of the western part lie in the deep sea west of the Porcupine Bank.
[Fig. 2 and caption: Fig. 2. -- The "Fram's" Route from June 20
to July 7, 1910 (given in an unbroken line -- the figures denote
the stations).
The dotted line gives the Frithjof's route, and the squares give five
of the Michael Sars's stations.]
In both parts of this section there are, as shown in Fig. 3, two great
volumes of water, from the surface down to depths greater than 500
metres, which have salinities between 35.4 and 35.5 per mille. They
have also comparatively high temperatures; the isotherm for 10°
C. goes down to a depth of about 500 metres in both these parts.
It is obvious that both these comparatively salt and warm volumes
of water belong to the Gulf Stream. The more westerly of them, at
Stations 11 and 12, and in part 13, in the deep sea to the west of
the Porcupine Bank, is probably in motion towards the north-east
along the outside of this bank and then into Rockall Channel --
between Rockall Bank and the bank to the west of the
[Fig. 3 and caption: Fig. 3. -- Temperature and Salinity in the
"Fram's" Southern Section, June, 1910.]
British Isles -- where a corresponding volume of water, with a somewhat
lower salinity, is found again in the section which was taken a few
weeks later by the Frithjof from Ireland to the west-north-west
across the Rockall Bank. This volume of water has a special interest
for us, since, as will be mentioned later, it forms the main part
of that arm of the Gulf Stream which enters the Norwegian Sea, but
which is gradually cooled on its way and mixed with fresher water,
so that its salinity is constantly decreasing. This fresher water
is evidently derived in great measure directly from precipitation,
which is here in excess of the evaporation from the surface of the sea.
The volume of Gulf Stream water that is seen in the eastern part
(east of Station 10) of the southern Fram section, can only flow
north-eastward to a much less extent, as the Porcupine Bank is
connected with the bank to the west of Ireland by a submarine ridge
(with depths up to about 300 metres), which forms a great obstacle
to such a movement.
The two volumes of Gulf Stream water in the Fram's southern section of
1910 are divided by a volume of water, which lies over the Porcupine
Bank, and has a lower salinity and also a somewhat lower average
temperature. On the bank to the south of Ireland (Stations 1 and 2)
the salinity and average temperature are also comparatively low. The
fact that the water on the banks off the coast has lower salinities,
and in part lower temperatures, than the water outside in the deep sea,
has usually been explained by its being mixed with the coast water,
which is diluted with river water from the land. This explanation may
be correct in a great measure; but, of course, it will not apply to
the water over banks that lie out in the sea, far from any land. It
appears, nevertheless, on the Porcupine Bank, for instance, and,
as we shall see later, on the Rockall Bank, that the water on these
ocean banks is -- in any case in early summer -- colder and less salt
than the surrounding water of the sea. It appears from the Frithjof
section across the Rockall Bank, as well as from the two Fram sections,
that this must be due to precipitation combined with the vertical
currents near the surface, which are produced by the cooling of the
surface of the sea in the course of the winter. For, as the surface
water cools, it becomes heavier than the water immediately below,
and must then sink, while it is replaced by water from below. These
vertical currents extend deeper and deeper as the cooling proceeds in
the course of the winter, and bring about an almost equal temperature
and salinity in the upper waters of the sea during the winter, as far
down as this vertical circulation reaches. But as the precipitation
in these regions is constantly decreasing the salinity of the surface
water, this vertical circulation must bring about a diminution of
salinity in the underlying waters, with which the sinking surface
water is mixed into a homogeneous volume of water. The Frithjof
section in particular seems to show that the vertical circulation in
these regions reaches to a depth of 500 or 600 metres at the close
of the winter. If we consider, then, what must happen over a bank in
the ocean, where the depth is less than this, it is obvious that the
vertical circulation will here be prevented by the bottom from reaching
the depth it otherwise would, and there will be a smaller volume of
water to take part in this circulation and to be mixed with the cooled
and diluted surface water. But as the cooling of the surface and the
precipitation are the same there as in the surrounding regions, the
consequence must be that the whole of this volume of water over the
bank will be colder and less salt than the surrounding waters. And as
this bank water, on account of its lower temperature, is heavier than
the water of the surrounding sea, it will have a tendency to spread
itself outwards along the bottom, and to sink down along the slopes
from the sides of the bank. This obviously contributes to increase
the opposition that such banks offer to the advance of ocean currents,
even when they lie fairly deep.
These conditions, which in many respects are of great importance,
are clearly shown in the two Fram sections and the Frithjof section.
The Northern Fram section went from a point to the north-west of
the Rockall Bank (Station 15), across the northern end of this
bank (Station 16), and across the northern part of the wide channel
(Rockall Channel) between it and Scotland. As might be expected, both
temperature and salinity are lower in this section than in the southern
one, since in the course of their slow northward movement the waters
are cooled, especially by the vertical circulation in winter already
mentioned, and are mixed with water containing less salt, especially
precipitated water. While in the southern section the isotherm for
10° C. went down to 500 metres, it here lies at a depth of between
50 and 25 metres. In the comparatively short distance between the two
sections, the whole volume of water has been cooled between 1° and 2°
C. This represents a great quantity of warmth, and it is chiefly given
off to the air, which is thus warmed over a great area. Water contains
more than 3,000 times as much warmth as the same volume of air at the
same temperature. For example, if 1 cubic metre of water is cooled 1°,
and the whole quantity of warmth thus taken from the water is given
[Fig. 4. -- Temperature and Salinity in the "Fram's" Northern Section,
July 1910]
to the air, it is sufficient to warm more than 3,000 cubic metres of
air 1°, when subjected to the pressure of one atmosphere. In other
words, if the surface water of a region of the sea is cooled 1° to a
depth of 1 metre, the quantity of warmth thus taken from the sea is
sufficient to warm the air of the same region 1° up to a height of much
more than 3,000 metres, since at high altitudes the air is subjected
to less pressure, and consequently a cubic metre there contains
less air than at the sea-level. But it is not a depth of 1 metre of
the Gulf Stream that has been cooled 1° between these two sections;
it is a depth of about 500 metres or more, and it has been cooled
between 1° and 2° C. It will thus be easily understood that this loss
of warmth from the Gulf Stream must have a profound influence on the
temperature of the air over a wide area; we see how it comes about
that warm currents like this are capable of rendering the climate
of countries so much milder, as is the case in Europe; and we see
further how comparatively slight variations in the temperature of the
current from year to year must bring about considerable variations in
the climate; and how we must be in a position to predict these latter
changes when the temperature of the currents becomes the object of
extensive and continuous investigation. It may be hoped that this is
enough to show that far-reaching problems are here in question.
The salinity of the Gulf Stream water decreases considerably between
the Fram's southern and northern sections. While in the former it
was in great part between 35.4 and 35.5 per mille, in the latter it
is throughout not much more than 35.3 per mille. In this section,
also, the waters of the Gulf Stream are divided by an accumulation of
less salt and somewhat colder bank water, which here lies over the
Rockall Bank (Station 16). On the west side of this bank there is
again (Station 15) salter and warmer Gulf Stream water, though not
quite so warm as on the east. From the Frithjof section, a little
farther south, it appears that this western volume of Gulf Stream
water is comparatively small. The investigations of the Fram and the
Frithjof show that the part of the Gulf Stream which penetrates into
the Norwegian Sea comes in the main through the Rockall Channel,
between the Rockall Bank and the bank to the west of the British
Isles; its width in this region is thus considerably less than was
usually supposed. Evidently this is largely due to the influence of
the earth's rotation, whereby currents in the northern hemisphere are
deflected to the right, to a greater degree the farther north they
run. In this way the ocean currents, especially in northern latitudes,
are forced against banks and coasts lying to the right of them, and
frequently follow the edges, where the coast banks slope down to the
deep. The conclusion given above, that the Gulf Stream comes through
the Rockall Channel, is of importance to future investigations;
it shows that an annual investigation of the water of this channel
would certainly contribute in a valuable way to the understanding of
the variations of the climate of Western Europe.
We shall not dwell at greater length here on the results of the Fram's
oceanographical investigations in 1910. Only when the observations
then collected, as well as those of the Frithjof's and Michael Sars's
voyages, have been fully worked out shall we be able to make a complete
survey of what has been accomplished.
Investigations in the South Atlantic, June to August, 1911.
In the South Atlantic we have the southward Brazil Current on the
American side, and the northward Benguela Current on the African
side. In the southern part of the ocean there is a wide current flowing
from west to east in the west wind belt. And in its northern part,
immediately south of the Equator, the South Equatorial Current flows
from east to west. We have thus in the South Atlantic a vast circle of
currents, with a motion contrary to that of the hands of a clock. The
Fram expedition has now made two full sections across the central
part of the South Atlantic; these sections take in both the Brazil
Current and the Benguela Current, and they lie between the eastward
current on the south and the westward current on the north. This is
the first time that such complete sections have been obtained between
South America and Africa in this part of the ocean. And no doubt a
larger number of stations were taken on the Fram's voyage than have
been taken -- with the same amount of detail -- in the whole South
Atlantic by all previous expeditions put together.
When the Fram left Buenos Aires in June, 1911, the expedition went
eastward through the Brazil Current. The first station was taken
in lat. 36° 18' S. and long. 43° 15' W.; this was on June 17. Her
course was then north-east or east until Station 32 in lat. 20° 30'
S. and long. 8° 10' E.; this station lay in the Benguela Current,
about 800 miles from the coast of Africa, and it was taken on July
22. From there she went in a gentle curve
[Fig. 5 and caption]
past St. Helena and Trinidad back to America. The last station (No. 60)
was taken on August 19 in the Brazil Current in lat. 24° 39' S. and
about long. 40° W.; this station lay about 200 miles south-east of
Rio de Janeiro.
There was an average distance of 100 nautical miles between one station
and the next. At nearly all the stations investigations were made at
the following depths: surface, 5, 10, 25, 50, 100, 150, 200, 250,
300, 400, 500, 750, and 1,000 metres (2.7, 5.4, 13.6, 27.2, 54.5,
81.7, 109, 136.2, 163.5, 218, 272.5, and 545 fathoms). At one or two
of the stations observations were also taken at 1,500 and 2,000 metres
(817.5 and 1,090 fathoms).
The investigations were thus carried out from about the middle of
July to the middle of August, in that part of the southern winter
which corresponds to the period between the middle of
[Fig. 6]
Fig. 6. -- Currents in the South Atlantic (June -- August, 1911).
December and the middle of February in the northern hemisphere We must
first see what the conditions were on the surface in those regions
in the middle of the winter of 1911.
It must be remembered that the currents on the two sides of the
ocean flow in opposite directions. Along the coast of Africa, we have
the Benguela Current, flowing from south to north; on the American
side the Brazil Current flows from the tropics southward. The former
current is therefore comparatively cold and the latter comparatively
warm. This is clearly seen on the chart, which shows the distribution
of temperatures and salinities on the surface. In lat. 20° S. it
was only about 17° C. off the African coast, while it was about 23°
C. off the coast of Brazil.
The salinity depends on the relation between evaporation and the
addition of fresh water. The Benguela Current comes from
[Fig. 7]
Fig. 7. -- Salinities and Temperatures at the Surface in the South
Atlantic (June -- August, 1911) regions where the salinity is
comparatively low; this is due to the acquisition of fresh water in
the Antarctic Ocean, where the evaporation from the surface is small
and the precipitation comparatively large. A part of this fresh water
is also acquired by the sea in the form of icebergs from the Antarctic
Continent. These icebergs melt as they drift about the sea.
Immediately off the African coast there is a belt where the salinity is
under 35 per mille on the surface; farther out in the Benguela Current
the salinity is for the most part between 35 and 36 per mille. As the
water is carried northward by the current, evaporation becomes greater
and greater; the air becomes comparatively warm and dry. Thereby the
salinity is raised. The Benguela Current is then continued westward in
the South Equatorial Current; a part of this afterwards turns to the
north-west, and crosses the Equator into the North Atlantic, where it
joins the North Equatorial Current. This part must thus pass through
the belt of calms in the tropics. In this region falls of rain occur,
heavy enough to decrease the surface salinity again. But the other part
of the South Equatorial Current turns southward along the coast of
Brazil, and is then given the name of the Brazil Current. The volume
of water that passes this way receives at first only small additions
of precipitation; the air is so dry and warm in this region that
the salinity on the surface rises to over 37 per mille. This will
be clearly seen on the chart; the saltest water in the whole South
Atlantic is found in the northern part of the Brazil Current. Farther
to the south in this current the salinity decreases again, as
the water is there mixed with fresher water from the South. The
River La Plata sends out enormous quantities of fresh water into
the ocean. Most of this goes northward, on account of the earth's
rotation; the effect of this is, of course, to deflect the currents
of the southern hemisphere to the left, and those of the northern
hemisphere to the right. Besides the water from the River La Plata,
there is a current flowing northward along the coast of Patagonia --
namely, the Falkland Current. Like the Benguela Current, it brings
water with lower salinities than those of the waters farther north;
therefore, in proportion as the salt water of the Brazil Current
is mixed with the water from the River La Plata and the Falkland
Current, its salinity decreases. These various conditions give the
explanation of the distribution of salinity and temperature that is
seen in the chart.
Between the two long lines of section there is a distance of
between ten and fifteen degrees of latitude. There is, therefore,
a considerable difference in temperature. In the southern section
the average surface temperature at Stations 1 to 26 (June 17 to
July 17) was 17.9° C.; in the northern section at Stations 36 to 60
(July 26 to August 19) it was 21.6° C. There was thus a difference
of 3.7° C. If all the stations had been taken simultaneously, the
difference would have been somewhat greater; the northern section
was, of course, taken later in the winter, and the temperatures were
therefore proportionally lower than in the southern section. The
difference corresponds fairly accurately with that which Kr:ummel
has calculated from previous observations.
We must now look at the conditions below the surface in that part of
the South Atlantic which was investigated by the Fram Expedition.
The observations show in the first place that both temperatures and
salinities at every one of the stations give the same values from
the surface downward to somewhere between 75 and 150 metres (40.8 and
81.7 fathoms). This equalization of temperature and salinity is due to
the vertical currents produced by cooling in winter; we shall return
to it later. But below these depths the temperatures and salinities
decrease rather rapidly for some distance.
The conditions of temperature at 400 metres (218 fathoms) below the
surface are shown in the next little chart. This chart is based on
the Fram Expedition, and, as regards the other parts of the ocean, on
Schott's comparison of the results of previous expeditions. It will
be seen that the Fram's observations agree very well with previous
soundings, but are much more detailed.
The chart shows clearly that it is much warmer at 400 metres (218
fathoms) in the central part of the South Atlantic than either farther
north -- nearer the Equator -- or farther south. On the Equator
there is a fairly large area where the temperature is only 7° or 8°
C. at 400 metres, whereas in lats. 2O° to 30° S. there are large
regions where it is above 12° C.; sometimes above 13° C., or even
14°C. South of lat. 30° S. the temperature decreases again rapidly;
in the chart no lines are drawn for temperatures below 8° C., as we
have not sufficient observations to show the course of these lines
properly. But we know that the temperature at 400 metres sinks to
about 0° C. in the Antarctic Ocean.
[Fig. 8]
Fig. 8. -- Temperatures (Centigrade) at a Depth of 400 Metres
(218 Fathoms).
At these depths, then, we find the warmest water within the region
investigated by the Fram. If we now compare the distribution of
temperature at 400 metres with the chart of currents in the South
Atlantic, we see that the warm region lies in the centre of the great
circulation of which mention was made above. We see that there are
high temperatures on the left-hand side of the currents, and low on the
right-hand side. This, again, is an effect of the earth's rotation, for
the high temperatures mean as a rule that the water is comparatively
light, and the low that it is comparatively heavy. Now, the effect
of the earth's rotation in the southern hemisphere is that the light
(warm) water from above is forced somewhat down on the left-hand side
of the current, and that the heavy (cold) water from below is raised
somewhat. In the northern hemisphere the contrary is the case. This
explains the cold water at a depth of 400 metres on the Equator; it
also explains the fact that the water immediately off the coasts of
Africa and South America is considerably colder than farther out in the
ocean. We now have data for studying the relation between the currents
and the distribution of warmth in the volumes of water in a way which
affords valuable information as to the movements themselves. The
material collected by the Fram will doubtless be of considerable
importance in this way when it has been finally worked out.
Below 400 metres (218 fathoms) the temperature further decreases
everywhere in the South Atlantic, at first rapidly to a depth
between 500 and 1,000 metres (272.5 and 545 fathoms), afterwards very
slowly. It is possible, however, that at the greatest depths it rises
a little again, but this will only be a question of hundredths, or,
in any case, very few tenths of a degree.
It is known from previous investigations in the South Atlantic, that
the waters at the greatest depths, several thousand metres below the
surface, have a temperature of between 0° and 3° C. Along the whole
Atlantic, from the extreme north (near Iceland) to the extreme south,
there runs a ridge about half-way between Europe and Africa on the
one side, and the two American continents on the other. A little
to the north of the Equator there is a slight elevation across the
ocean floor between South America and Africa. Farther south (between
lats. 25° and 35° S.) another irregular ridge runs across between these
continents. We therefore have four deep regions in the South Atlantic,
two on the west (the Brazilian Deep and the Argentine Deep) and two
on the east (the West African Deep and the South African Deep). Now
it has been found that the "bottom water" in these great deeps -- the
bottom lies more than 5,000 metres (2,725 fathoms) below the surface --
is not always the same. In the two western deeps, off South America,
the temperature is only a little above 0° C. We find about the same
temperatures in the South African Deep, and farther eastward in a
belt that is continued round the whole earth. To the south, between
this belt and Antarctica, the temperature of the great deeps is much
lower, below 0° C. But in the West African Deep the temperature is
about 2° C. higher; we find there the same temperatures of between 2°
and 2.5° C. as are found everywhere in the deepest parts of the North
Atlantic. The explanation of this must be that the bottom water in
the western part of the South Atlantic comes from the south, while
in the north-eastern part it comes from the north. This is connected
with the earth's rotation, which has a tendency to deflect currents
to the left in the southern hemisphere. The bottom water coming from
the south goes to the left -- that is, to the South American side;
that which comes from the north also goes to the left -- that is,
to the African side.
The salinity also decreases from the surface downward to 600 to 800
metres (about 300 to 400 fathoms), where it is only a little over
34 per mille, but under 34.5 per mille; lower down it rises to about
34.7 per mille in the bottom water that comes from the south, and to
about 34.9 per mille in that which comes from the North Atlantic.
We mentioned that the Benguela Current is colder and less salt at the
surface than the Brazil Current. The same thing is found in those parts
of the currents that lie below the surface. This is clearly shown in
Fig. 9, which gives the distribution of temperature at Station 32 in
the Benguela Current, and at Station 60 in the Brazil Current; at the
various depths down to 500 metres (272.5 fathoms) it was between 5°
and 7° C. colder in the former than in the latter. Deeper down the
difference becomes less, and at 1,000 metres (545 fathoms) there was
only a difference of one or two tenths of a degree.
Fig. 10 shows a corresponding difference in salinities; in the first
200 metres below the surface the water was about
[Fig. 9.]
Fig. 9. -- Temperatures at Station 32 (In the Benguela Current, July
22, 1911), and at Station 6O (In the Brazil Current, August 19, 1911).
1 per mille more saline in the Brazil Current than in the Benguela
Current. Both these currents are confined to the upper waters;
the former probably goes down to a depth of about 1,000 metres (545
fathoms), while the latter does not reach a depth of much more than 500
metres. Below the two currents the conditions are fairly homogeneous,
and there is no difference worth mentioning in the salinities.
The conditions between the surface and a depth of 1,000 metres along
the two main lines of course are clearly shown in the two sections
(Figs. 11 and l2). In these the isotherms for every second degree are
drawn in broken lines. Lines connecting points with the same salinity
(isohalins) are drawn unbroken, and, in addition, salinities above
35 per mille are shown by shading. Above is a series of figures,
giving the numbers of the stations. To understand
[Fig. 10 and caption]
the sections rightly it must be borne in mind that the vertical scale
is 2,000 times greater than the horizontal.
Many of the conditions we have already mentioned are clearly apparent
in the sections: the small variations between the surface and a depth
of about 100 metres at each station; the decrease of temperature and
salinity as the depth increases; the high values both of temperature
and salinity in the western part as compared with the eastern. We
see from the sections how nearly the isotherms and isohalins follow
each other. Thus, where the temperature is 12° C., the water almost
invariably has a salinity very near 35 per mille. This water at 12°
C., with a salinity of 35 per mille, is found in the western part
of the area (in the Brazil Current) at a depth of 500 to 600 metres,
but in the eastern part (in the Benguela Current) no deeper than 200
to 250 metres (109 to 136 fathoms).
We see further in both sections, and especially in the southern one,
that the isotherms and isohalins often have an undulating course,
since the conditions at one station may be different from those at the
neighbouring stations. To point to one or two examples: at Station 19
the water a few hundred metres down was comparatively warm; it was,
for instance, 12° C. at about 470 metres (256 fathoms) at this station;
while the same temperature was found at about 340 metres (185 fathoms)
at both the neighbouring stations, 18 and 20. At Station 2 it was
relatively cold, as cold as it was a few hundred metres deeper down
at Stations 1 and 3.
These undulating curves of the isotherms and isohalins are familiar to
us in the Norwegian Sea, where they have been shown in most sections
taken in recent years. They may be explained in more than one way. They
may be due to actual waves, which are transmitted through the central
waters of the sea. Many things go to show that such waves may actually
occur far below the surface, in which case they must attain great
dimensions; they must, indeed, be more than 100 metres high at times,
and yet -- fortunately -- they are not felt on the surface. In the
Norwegian Sea we have frequently found these wave-like rises and
falls. Or the curves may be due to differences in the rapidity and
direction of the currents. Here the earth's rotation comes into play,
since, as mentioned above, it causes zones of water to be depressed
on one side and raised on the other; and the degree of force with
which this takes place is dependent on the rapidity of the current
and on the geographical latitude. The effect is slight in the tropics,
but great in high latitudes. This, so far as it goes, agrees with the
[Fig. 11 and captions]
fact that the curves of the isotherms and isohalins are more marked
in the more southerly of our two sections than in the more northerly
one, which lies 10 or 15 degrees nearer the Equator.
But the probability is that the curves are due to the formation of
eddies in the currents. In an eddy the light and warm water will be
depressed to greater depths if the eddy goes contrary to the hands
of a clock and is situated in the southern hemisphere. We appear to
have such an eddy around Station 19, for example. Around Station 2 an
eddy appears to be going the other way; that is, the same way as the
hands of a clock. On the chart of currents we have indicated some of
these eddies from the observations of the distribution of salinity
and temperature made by the Fram Expedition.
While this, then, is the probable explanation of the irregularities
shown by the lines of the sections, it is not impossible that they
may be due to other conditions, such as, for instance, the submarine
waves alluded to above. Another possibility is that they may be a
consequence of variations in the rapidity of the current, produced,
for instance, by wind. The periodical variations caused by the tides
will hardly be an adequate explanation of what happens here, although
during Murray and Hjort's Atlantic Expedition in the Michael Sars (in
1910), and recently during Nansen's voyage to the Arctic Ocean in the
Veslemöy (in 1912), the existence of tidal currents in the open ocean
was proved. It may be hoped that the further examination of the Fram
material will make these matters clearer. But however this may be, it
is interesting to establish the fact that in so great and deep an ocean
as the South Atlantic very considerable variations of this kind may
occur between points which lie near together and in the same current.
As we have already mentioned in passing, the observations show that
the same temperatures and salinities as are found at the surface are
continued downward almost unchanged to a depth of between 75 and 150
metres; on an average it is about 100 metres. This is a typical winter
condition, and is due to the vertical circulation already mentioned,
which is caused by the surface water being cooled in winter,
thus becoming heavier than the water below, so that it must sink
and give place to lighter water which rises. In this way the upper
zones of water become mixed, and acquire almost equal temperatures
and salinities. It thus appears that the vertical currents reached a
depth of about 100 metres in July, 1911, in the central part of the
South Atlantic. This cooling of the water is a gain to the air, and
what happens is that not only the surface gives off warmth to the air,
but also the sub-surface waters, to as great a depth as is reached by
the vertical circulation. This makes it a question of enormous values.
This state of things is clearly apparent in the sections, where
the isotherms and isohalins run vertically for some way below
the surface. It is also clearly seen when we draw the curves of
distribution of salinity and temperature at the different stations, as
we have done in the two diagrams for Stations 32 and 60 (Fig. 9). The
temperatures had fallen several degrees at the surface at the time
the Fram's investigations were made. And if we are to judge from the
general appearance of the station curves, and from the form they
usually assume in summer in these regions, we shall arrive at the
conclusion that the whole volume of water from the surface down to
a depth of 100 metres must be cooled on an average about 2° C.
As already pointed out, a simple calculation gives the following:
if a cubic metre of water is cooled 1° C., and the whole quantity
of warmth thus taken from the water is given to the air, it will be
sufficient to warm more than 3,000 cubic metres of air 1° C. A few
figures will give an impression of what this means. The region lying
between lats. 15° and 35° S. and between South America and Africa --
roughly speaking, the region investigated by the Fram Expedition --
has an area of 13,000,000 square kilometres. We may now assume that
this part of the ocean gave off so much warmth to the air that a
zone of water 100 metres in depth was thereby cooled on an average 2°
C. This zone of water weighs about 1.5 trillion kilogrammes, and the
quantity of warmth given off thus corresponds to about 2.5 trillion
great calories.
It has been calculated that the whole atmosphere of the earth
weighs 5.27 trillion kilogrammes, and it will require something
over 1 trillion great calories to warm the whole of this mass of
air 1°C. From this it follows that the quantity of warmth which,
according to our calculation, is given off to the air from that part
of the South Atlantic lying between lats. 15° and 35° S., will be
sufficient to warm the whole atmosphere of the earth about 2° C., and
this is only a comparatively small part of the ocean. These figures
give one a powerful impression of the important part played by the
sea in relation to the air. The sea stores up warmth when it absorbs
the rays of the sun; it gives off warmth again when the cold season
comes. We may compare it with earthenware stoves, which continue to
warm our rooms long after the fire in them has gone out. In a similar
way the sea keeps the earth warm long after summer has gone and the
sun's rays have lost their power.
Now it is a familiar fact that the average temperature of the air for
the whole year is a little lower than that of the sea; in winter it
is, as a rule, considerably lower. The sea endeavours to raise the
temperature of the air; therefore, the warmer the sea is, the higher
the temperature of the air will rise. It is not surprising, then,
that after several years' investigations in the Norwegian Sea we
have found that the winter in Northern Europe is milder than usual
when the water of the Norwegian Sea contains more than the average
amount of warmth. This is perfectly natural. But we ought now to be
able to go a step farther and say beforehand whether the winter air
will be warmer or colder than the normal after determining the amount
of warmth in the sea.
It has thus been shown that the amount of warmth in that part of the
ocean which we call the Norwegian Sea varies from year to year. It
was shown by the Atlantic Expedition of the Michael Sars in 1910 that
the central part of the North Atlantic was considerably colder in 1910
than in 1873, when the Challenger Expedition made investigations there;
but the temperatures in 1910
[Fig. 13]
Fig. 13. -- Temperatures at one of the "Fram's" and one of the
"Challenger's" Stations, to the South of the South Equatorial Current
were about the same as those of 1876, when the Challenger was on her
way back to England.
We can now make similar comparisons as regards the South Atlantic. In
1876 the Challenger took a number of stations in about the same region
as was investigated by the Fram. The Challenger's Station 339 at the
end of March, 1876, lies near the point where the Fram's Station 44
was taken at the beginning of August, 1911. Both these stations lay in
about lat. 17.5° S., approximately half-way between Africa and South
America -- that is, in the region where a relatively slack current
runs westward, to the south of the South Equatorial Current. We
can note the difference in Fig. 13, which shows the distribution
of temperature at the two stations. The Challenger's station was
taken during the autumn and the Fram's during the winter. It was
therefore over 3° C. warmer at the surface in March, 1876, than in
August, 1911. The curve for the Challenger station shows the usual
distribution of temperature immediately below the surface in summer;
the temperature falls constantly from the surface downward. At the
Fram's station we see the typical winter conditions; we there find the
same temperature from the surface to a depth of 100 metres, on account
of cooling and vertical circulation. In summer, at the beginning of
the year 1911, the temperature curve for the Fram's station would
have taken about the same form as the other curve; but it would have
shown higher temperatures, as it does in the deeper zones, from 100
metres down to about 500 metres. For we see that in these zones it
was throughout 1° C. or so warmer in 1911 than in 1876; that is to
say, there was a much greater store of warmth in this part of the
ocean in 1911 than in 1876. May not the result of this have been
that the air in this region, and also in the east of South America
and the west of Africa, was warmer during the winter of 1911 than
during that of 1876? We have not sufficient data to be able to say
with certainty whether this difference in the amount of warmth in the
two years applied generally to the whole ocean, or only to that part
which surrounds the position of the station; but if it was general,
we ought probably to be able to find a corresponding difference in
the climate of the neighbouring regions. Between 500 and 800 metres
(272 and 486 fathoms) the temperatures were exactly the same in
both years, and at 900 and 1,000 metres (490 and 545 fathoms) there
was only a difference of two or three tenths of a degree. In these
deeper parts of the ocean the conditions are probably very similar;
we have there no variations worth mentioning, because the warming of
the surface and sub-surface waters by the sun has no effect there,
unless, indeed, the currents at these depths may vary so
[Fig. 14]
Fig. 14. -- Temperatures at one of the "Fram's" and one of the
"Valdivia's" Stations, in the Benguela Current. Much that there may
be a warm current one year and a cold one another year. But this is
improbable out in the middle of the ocean.
In the neighbourhood of the African coast, on the other hand, it looks
as if there may be considerable variations even in the deeper zones
below 500 metres (272 fathoms). During the Valdivia Expedition in 1898
a station (No. 82) was taken in the Benguela Current in the middle of
October, not far from the point at which the Fram's Station 31 lay. The
temperature curves from here show that it was much warmer (over 1.5°
C.) in 1898 than in 1911 in the zones between 500 and 800 metres
(272 and 486 fathoms). Probably the currents may vary considerably
here. But in the upper waters of the Benguela Current itself, from the
surface down to 150 metres, it was considerably warmer in 1911 than
in 1898; this difference corresponds to that which we found in the
previous comparison of the Challenger's and Fram's stations of 1876
and 1911. Between 200 and 400 metres (109 and 218 fathoms) there was
no difference between 1898 and 1911; nor was there at 1,000 metres
(545 fathoms).
In 1906 some investigations of the eastern part of the South Atlantic
were conducted by the Planet. In the middle of March a station was
taken (No. 25) not far from St. Helena and in the neighbourhood of the
Fram's Station 39, at the end of July, 1911. Here, also, we find great
variations; it was much warmer in 1911 than in 1906, apart from the
winter cooling by vertical circulation of the sub-surface waters. At
a depth of only 100 metres (54.5 fathoms) it was 2° C. warmer in 1911
than in 1906; at 400 metres (218 fathoms) the difference was over 1°,
and even at 800 metres (486 fathoms) it was about 0.75° C. warmer in
1911 than in 1906. At 1,000 metres (545 fathoms) the difference was
only 0.3°.
From the Planet's station we also have problems of salinity,
determined by modern methods. It appears that the salinities at the
Planet station, in any case to a depth of 400 metres, were lower, and
in part much lower, than those of the Fram Expedition. At 100 metres
the difference was even greater than 0.5 per mille; this is a great
deal in the same region of open sea. Now, it must be remembered that
the current in the neighbourhood of St. Helena may be regarded as a
continuation of the Benguela Current, which comes from the south and
has relatively low salinities. It looks, therefore, as if there were
yearly variations of salinity in these
[Fig. 15]
Fig. 15. -- Temperatures at the "Planet's" Station 25, and the "Fram's"
Station 39 -- Both in the Neighbourhood of St. Helena
[Fig. 16]
Fig. 16. -- Salinities at the "Planet's" Station 25 (March 19, 1906)
And the "Fram's" Station 39 (July 29, 1911).
regions. This may either be due to corresponding variations in the
Benguela Current -- partly because the relation between
precipitation and evaporation may vary in different years, and partly
because there may be variations in the acquisition of less saline
water from the Antarctic Ocean. Or it may be due to the
Benguela Current in the neighbourhood of St. Helena having
a larger admixture of the warm and salt water to the west of it in
one year than in another. In either case we may expect a
relatively low salinity (as in 1906 as compared with 1911) to be
accompanied by a relatively low temperature, such as we have
found by a comparison of the Planet's observations with those of
the Fram.
We require a larger and more complete material for comparison; but even
that which is here referred to shows that there may be considerable
yearly variations both in the important, relatively cold Benguela
Current, and in the currents in other parts of the South Atlantic. It
is a substantial result of the observations made on the Fram's voyage
that they give us an idea of great annual variations in so important a
region as the South Atlantic Ocean. When the whole material has been
further examined it will be seen whether it may also contribute to
an understanding of the climatic conditions of the nearest countries,
where there is a large population, and where, in consequence, a more
accurate knowledge of the variations of climate will have more than
a mere scientific interest.
NOTES
[1] -- Named after Dr. Nansen's daughter. -- Tr.
[2] -- A vessel sailing continuously to the eastward puts the clock
on every day, one hour for every fifteen degrees of longitude; one
sailing westward puts it back in the same way. In long. 180° one
of them has gone twelve hours forward, the other twelve hours back;
the difference is thus twenty-four hours. In changing the longitude,
therefore, one has to change the date, so that, in passing from east
to west longitude, one will have the same day twice over, and in
passing from west to east longitude a day must be missed.
[3] -- For the benefit of those who know what a buntline on a sail is,
I may remark that besides the usual topsail buntlines we had six extra
buntlines round the whole sail, so that when it was clewed up it was,
so to speak, made fast. We got the sail clewed up without its going to
pieces, but it took us over an hour. We had to take this precaution,
of having so many buntlines, as we were short-handed.
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