RIVERS IN THE SEA
“Sinuous or straight, now rapid and now slow.”--COWPER.
“As winds that coerce the sea.”--C. G. ROSSETTI.
Actual Rivers in the Ocean; distinct streams of water, flowing over a
bed of water, with banks of water. Not merely one or two such rivers,
but scores of them, hundreds of them, great and small, in all parts of
the world.
Chief perhaps in importance is the Gulf Stream, that vast flood which
pours out of the Gulf of Mexico, and acts as a winter heating apparatus
for the west of Europe. Though by no means the largest of ocean
streams, it is one of the most useful to man.
After quitting the Gulf, it hurries at speed through the Straits of
Florida; then spreads out into a river, about fifty miles wide and over
two thousand feet deep, journeying at a rate of some sixty miles in
twelve hours.
For a while it hugs the American coast; but, happily for Europe, it
forsakes this friend of its youth, and wanders to the north-eastward
across the Atlantic.
To call it a “river” is no mere fiction of speech. Near Halifax the
separation between warm and cold water is so sharp, that those on board
a ship may know what latitude they have reached, on entering or leaving
the stream, by simply dipping a bucket in the water and taking the
temperature. Literally the Gulf Stream is a warm river, flowing over a
bed of cold water, with cold-water banks.
So far as Cape Hatteras the stream clings to its early friend; and
after that the American coast knows it no more, being left to the
mercies of a very different acquaintance. An icy stream flows southward
from the far north, clinging to the coast of North America, while we
in western Europe benefit by the presence of the warm current which
travels over to us. This stream has been described as forming a “cold
wall” to the mild Gulf Stream.
Fan-like, the Gulf Stream spreads as it journeys, growing gradually
wider and wider, shallower and shallower, cooler and cooler--yet the
last so slowly that, even off the coast of Scotland, water nine
hundred fathoms deep is found to be at 40° F.
How strongly this mass of warm water affects the air above it is well
known to sailors. When passing from the stream to the outside ocean, or
from the ocean to the stream, they often change in a few hours from a
warm to a cool or from a cool to a warm climate. The atmosphere is ever
ready to tune its mood sympathetically to that of the ocean over which
it sweeps.
We know well in Great Britain that our soft south-west breezes seldom
fail to bring us warmth; though we do not always remember the debt that
we owe to our friend the Gulf Stream.
But for the immense stores of heat carried northward, and given over
to us, our Island climate would be different indeed from what it now
is. That is why our fellow-subjects in eastern and central Canada,
living no farther from the equator than we do ourselves, suffer from
an intensity of cold in winter which we never endure. It is difficult
to realise that parts of ice-bound Labrador and of Canada, where the
thermometer often drops to 40° below zero, are no farther north than
London and Paris; while Newfoundland lies actually more to the south
than Erin’s green Isle.
Turning to the Pacific Ocean, we find there a corresponding river,
again flowing to the north-east. Just as the Gulf Stream wanders across
the Atlantic, so this river wanders over the Pacific, carrying stores
of tropical warmth to opposite coasts. At its quickest, it is less
rapid than the Gulf Stream, and about three times as wide. It too, as
it journeys, becomes gradually broader, shallower, slower, colder.
This “Kuro Sivo” or “Black Stream,” so named from its dark colour,
flows outside Japan, and then strikes freely for the northern coasts of
North America. And because of its work as a winter heating apparatus
in Alaska, the humming bird is found at a latitude which, on the
other side of the American Continent, means, not the play and whirr
of humming-birds in a soft air, but the disporting of walruses among
ice-floes.
As in the Atlantic, so in the Pacific, the warm northward-travelling
current is balanced by a cold southward-travelling current. The Arctic
stream of the Pacific is not so marked as that of the Atlantic, perhaps
partly because of the much shallower outlet from the Arctic Ocean;
still it is quite chilly enough in its effects upon the Siberian
climate. Here again the cold stream acts as a “wall” to the warm river
flowing the other way.
More reasons than one may help to explain why these two currents slant
off to the eastward instead of pouring due north. The shape of the
various coast-lines has something to do with it; also the presence
of ridges and hollows in the ocean-beds, and the resistance of other
contending currents. A river, either on land or in the sea, will always
travel where it finds least opposition.
One main cause, however, is the whirl of our Earth upon its axis. This,
which greatly affects the directions of prevailing winds, alters also
the lines followed by ocean rivers.
A current starting from near the equator for the north shares in the
rapid rush of the Earth’s surface, which at the equator spins eastward
at a rate of about one thousand miles each hour. As the volume of
water gets farther north, it reaches parts of the Earth which are
whirling more slowly, while it has not lost much of its own eastward
whirl. A sideways flow is the result, changing the northward into a
north-eastward direction.
But a stream starting from the far north for the south is affected in
the opposite way. Near the north pole the Earth’s surface hardly moves
at all; and the southward-flowing current, being weighted with northern
inertia, takes a contrary course to the current flowing north. It lags
more and more _behind_ the faster-revolving surface, and so wanders
westward instead of eastward. Or, if prevented by the land from so
doing, it hugs the coast which hinders it.
So the _pull_ of the two great streams in the Atlantic is exactly
opposed, each to the other. That of the Gulf Stream is towards the
east; that of the Labrador Stream is towards the west; and the resolute
manner in which the two refuse to mingle may be partly due to this fact.
If our Earth could be made to change the present whirl from west
to east, and to revolve instead from east to west, those two great
currents would alter their directions. The Gulf Stream would hug the
American coasts, and the Labrador would find its way over to Europe.
Then the British Isles in winter would know a temperature of 30° or 40°
below zero, and the Canadians would experience soft damp winters and
moderate summers. Perhaps they would no more welcome the exchange than
we should.
Then, too, the Black Stream would cling to the Asiatic side,
transforming the climate of western Siberia, and the cold Arctic river
would put a speedy end to humming-birds in Alaska. But abundance of ice
floes would soon be awaiting the walruses which would have to emigrate
from the other side of the continent.
A good deal of discussion has been held as to whether we do truly owe
our mildness to the Gulf Stream alone, or whether that Gulf Stream is
merely part of a general northward movement of Atlantic waters from the
tropics towards the pole. The question has been warmly contested; and
no doubt on both sides, as generally is the case, truth has been mixed
with error.
The Gulf Stream cannot be viewed as a separate entity. Its very birth
in the Gulf of Mexico depends on a great mass of water ever flowing
from the south-east into the Caribbean Sea. Since so much water pours
in, the same volume must pour out, and as it does so it gains the name
of the “Gulf Stream.”
But after quitting the Gulf of Mexico the stream does not exist alone.
It becomes a leading part of the North Atlantic circulation. The whole
surface of that ocean is slowly turning round and round--“whirling as
if stirred in the direction of the hands of a watch,”[1] and the Gulf
Stream occupies one side or more of the vast _maelström_. In the centre
of this revolving mass of water lies a district where the motion is
slight, and at that centre floats an enormous collection of drift and
sea-weed called the Sargasso Sea.
[1] Dr. H. R. Mill.
Suppose we pour some water in a large basin, drop into it a handful of
small leaves and chips, and make the whole spin gently with one hand.
We shall then see how the chips and leaves will collect at the centre,
and will float there, almost stationary. That is what happens, on a
large scale, in the North Atlantic Ocean.
Other Oceans also have this steady circular movement, not of the whole
body of water, but of the surface-water, down to a greater or less
depth--precisely how deep one cannot say; it lessens gradually with
increasing depth. The same is found also in the South Atlantic, in the
North and South Pacific, and doubtless in the Indian Ocean.
To find its direction in any part, one need only lay a watch, face
upwards, upon a map of the North Atlantic or the North Pacific. In
both cases the water travels round _with_ the watch-hands. In the
South Atlantic and the South Pacific the flow is just in the opposite
direction.
So both the Gulf Stream and the Black Stream are merely parts of a
big oceanic whirlpool. Each ocean on Earth has its own system of
circulation; and that system is part of a world-wide system. The waters
are in perpetual and complicated motion. Streams pour incessantly
hither and thither, to north and south, to east and west.
Two vast streams, known as the Equatorial Currents, nominally pour
round the world, but really are best seen in the open Pacific, where
for long distances no land meddles with their career. They flow
steadily westward, one to the north and one to the south of the equator.
Between them flows a reverse stream, called the “Equatorial Counter
Current.” If a certain amount of water travels north, an equal
quantity must travel south. Or if, as in this case, so much water
runs in a westerly direction, a corresponding amount has to run in an
easterly direction. Water may be marvellously piled up here or there,
by influences of land or of wind, but it cannot remain piled up,
without efforts on the part of the ocean to restore the equilibrium.
Were the whole Earth covered by a single unbroken sheet of water,
these drift currents might circle round and round the globe for ever,
undisturbed in their working. But the Earth has lands as well as
oceans; and when a current strikes a coast its course is altered, part
at least being turned in a fresh direction.
Much discussion has taken place about the causes which bring these
great Drift Currents into being.
Disturbing elements, many in number, may have had a hand in the matter.
So soon as one part of the sea-surface becomes warmer than another
part, movements are set going; the heavy cold water sinks, the light
warm water rises, and streams are started from the one place to the
other. There are also countless rivers pouring into the ocean, each
helping to upset its equilibrium. Heavy downpours of rain raise the
level of the sea here or there, inducing more currents.
But these are lesser causes. It is now recognised, as a fact beyond
question, that the main power in starting and sustaining ocean-currents
is that of Wind.