The South Pole, Volume 2 by Roald Amundsen

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The Fram's southerly section, from Station 1 to 13 (see Fig. 3)
is divided into two parts at Station 10, on the Porcupine Bank,
south-west of Ireland. The eastern part, between Stations 1 and 10,
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 10deg.
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
10deg. 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 1deg. and 2deg.
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 1deg.,
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 1deg., when subjected to the pressure of one atmosphere. In other
words, if the surface water of a region of the sea is cooled 1deg. 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 1deg. 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 1deg. between these two sections;
it is a depth of about 500 metres or more, and it has been cooled
between 1deg. and 2deg. 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. 36deg. 18' S. and long. 43deg. 15' W.; this was on June 17. Her
course was then north-east or east until Station 32 in lat. 20deg. 30'
S. and long. 8deg. 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. 24deg. 39' S. and
about long. 40deg. 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. 20deg. S. it
was only about 17deg. C. off the African coast, while it was about 23deg.
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 17.9deg. C.; in the northern section at Stations 36 to 60
(July 26 to August 19) it was 21.6deg. C. There was thus a difference
of 3.7deg. 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 7deg. or 8deg.
C. at 400 metres, whereas in lats. 2Odeg. to 30deg. S. there are large
regions where it is above 12deg. C.; sometimes above 13deg. C., or even
14deg.C. South of lat. 30deg. S. the temperature decreases again rapidly;
in the chart no lines are drawn for temperatures below 8deg. 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 0deg. 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 0deg. and 3deg. 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. 25deg. and 35deg. 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 0deg. 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 0deg. C. But in the West African Deep the temperature is
about 2deg. C. higher; we find there the same temperatures of between 2deg.
and 2.5deg. 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 5deg.
and 7deg. 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

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