The existence of currents in certain localities was known at a very
early date, and navigators in their voyages to the new world soon
discovered the Gulf Stream and other currents of the Atlantic. The
first current charts were published more than two hundred years ago.
Theories were soon advanced to explain the causes, one group of
scientific men attributing the origin of currents to differences of
level produced by an unequal distribution of atmospheric pressure over
the oceans, another set connecting the tidal phenomena with the cause
of ocean currents, and still another finding in the rotation of the
earth a sufficient reason for their existence. The polar origin of the
cold deep water found in low latitudes has long been considered
probable, and has given rise to a theory of a general oceanic
circulation in a vertical and horizontal direction, produced by
differences of temperature and density. Recent theoretical
investigations, however, seem to indicate that these causes alone are
incapable of producing currents, and, to-day, the theory that the winds
are mainly responsible for all current movements very largely
predominates. Benjamin Franklin was probably the first who recognized
in the trade winds the cause of the westerly set in the tropics, and
Rennel soon after made the division of drift and stream currents. The
objections which have appeared against the wind theory have been met
with the reply that the present state of oceanic movements is the
result of the work done by the winds in countless thousands of years.
Current phenomena is briefly summarized as follows by one of the latest
authorities on the subject:
1. The greater portion of the current movement of the ocean must be
regarded as a drift, produced by the prevailing winds, whose mean
direction and force are the measures for the mean set and velocity of
the current.
2. Another group of currents, and in fact a fraction of all currents,
consists of compensating or supply streams, created by the necessity of
replacing the drifted water in the windward portion of the drift
region.
3. A third group results from drifts deflected by the configuration of
the coasts; these which are denominated free currents, quickly pass
into compensating streams.
4. The deflecting force of the rotation of the earth is considered as
of subordinate importance, but may have some influence on currents that
are wholly or in part compensating or free.
Late investigations of the Gulf Stream by the U. S. Coast Survey give
interesting facts in regard to that notable current.
A satisfactory explanation of the cause of the stream has not yet been
found, but many believe, with Franklin, that the powerful trade drift
entering the Gulf of Mexico through the broad channel between Yucatan
and Cuba presses the water as a strong current through Florida Strait,
where the stream is turned to the northward along the coast. Since 1850
American naval officers have added greatly to our knowledge of the
characteristics of this stream, particularly within the last decade,
during which notable investigations have been carried on by Commanders
Bartlett and Sigsbee and Lieut. Pillsbury, U. S. N., under the
direction of the U. S. Coast Survey, and by Lieutenant Commander
Tanner, U. S. N., in the Fish Commission steamer Albatross.
Of special importance are the valuable and interesting results in
regard to tidal action in the stream obtained by Lieut. Pillsbury, U.
S. N., in the Coast Survey steamer Blake, from observations begun by
him in 1885 at the narrowest part of Florida Strait, between Fowey
Rocks and Gun Cay (Bah.), and continued since between Rebecca Shoal and
Cuba, and between Yucatan and Cape San Antonio (Cuba), and off Cape
Hatteras.
During the past year Lieut. Pillsbury extended the field of operations
to the passages between the islands encircling the Caribbean Sea, and
in order to study the Atlantic flow outside the limits of the trade
drift a station was to have been occupied about 700 miles to the
north-east of Barbados; this, however, was unfortunately prevented by
bad weather.
The deductions from the observations in Florida Strait showed very
clearly a _daily_ and a _monthly_ variation in the velocity of the
stream, the former having a range of 2½ knots, and reaching a maximum
on the average about 9^h 9^m before and 3^h 37^m after the moon's upper
transit, and the monthly variation reaching its maximum about two days
after the maximum declination of the moon. The variations in this
section were found greater on the western than on the eastern side of
the strait, and the axis of the stream, or position of strongest
surface flow, was located by Lieutenant Pillsbury 11½ miles east of
Fowey Rocks, and, farther north, about 17 miles east of Jupiter Light.
The average surface current at this section was 3-3/5 knots, the
maximum 5¼ knots, and the minimum 1¾ knots per hour. The results also
indicate that when the current is at its maximum the surface flow is
faster than at any depth below it, but when at its minimum the velocity
at a depth of 15 fathoms or even down to 65 fathoms is greater than at
the surface, and that there is at times a current running south along
the bottom in all parts of the stream except on the extreme eastern
side.
The results of the investigations in 1887 and 1888 have not yet been
published, but from information kindly furnished by the authorities of
the Coast Survey, I am able to give a brief outline of the more
prominent facts ascertained.
In the section between Rebecca Shoal and Cuba the daily variation in
velocity was found as prominent as in Florida Strait, the mean time of
eight maxima corresponding to 9^h 18^m before, and that of three maxima
to 3^h 25^m after the moon's transit. The axis of the stream in this
section was found near the center of the current prism, and the flow
was easterly and inclined on either side toward the axis. The axis
seemed to occupy a higher level than other parts of the stream, and
this appears to be borne out by the fact that about half the number of
the current bottles thrown out in Florida Strait on the west side of
the axis were recovered along the east coast of Florida, while of those
thrown out east of the axis not a single one was heard from. As a rule
it was found that the stronger the current the more constant the
direction and the deeper the stratum. Remarkable fluctuations in the
flow near the axis were noted, the velocity increasing sometimes one
knot in ten or fifteen minutes, and then as suddenly decreasing again.
Lieutenant Pillsbury attributes this, however, to a serpentine movement
of the maximum flow, which would sometimes strike the station occupied
by the Blake. The edge of the stream was found at about 30 miles south
of Rebecca Shoal light-house.
Between Yucatan and Cape San Antonio the stream was found flowing about
north, and the line of maximum velocity corresponds on the average to
10^h before and to 2^h 20^m after the moon's transit. The excessive
variations were like those in Florida Strait, on the west side of the
stream, and the maximum velocity of 6¼ knots was found about 5 miles
off the 100-fathom line of Yucatan Bank. The eastern edge of the stream
lies about 20 miles west of Cape San Antonio, and between this edge and
the island, eddy currents exist. At the time the easternmost station in
this section was first occupied, the declination of the moon was low
and the set of the surface current north-easterly. At a high south
declination of the moon the surface current was found south-easterly in
direction, and east or south-east below the surface. The normal flow
below the surface was in each case from the Gulf into the Caribbean
Sea, and this makes it probable that the station was situated inshore
of the average limit of the stream. On Cape San Antonio Bank the
currents are tidal, flood running northward and ebb southward. On the
Yucatan Bank the currents were also tidal, but as the edge of the bank
is approached the stronger flow of the Gulf Stream predominates. The
monthly variation in velocity, which was found clearly defined at the
first two sections occupied, appeared at this section to be obliterated
by anomalies not existing at the former.
Off Cape Hatteras the Blake accomplished the remarkable feat of
remaining at anchor in 1,852 fathoms, and this with a surface current
of over 4 knots. Two stations were occupied, and similar variations in
velocity were observed as at the other stations. The notable feature at
this station was the discovery of tidal action beneath the Gulf Stream,
the currents at 200 fathoms depth changing their direction very
regularly, the average current flowing about S. S. E. ½ E. for 7 hours
and N. N. W. ½ W. for a little over 5 hours.
The first section investigated in 1888 was in the equatorial drift
between Tobago and Barbados, where seven stations were occupied. The
axis of the stream was found west of the middle, or nearer the South
American shore, and the average direction was towards the north. At
none of the stations did the current set in the direction of the wind,
although the trades were blowing at all times with a force of from 2 to
7. The daily variation was also here very pronounced, the average time
of maximum flow occurring about 5^h 56^m after the moon's transit. At
65 and 130 fathoms depth the current, at three of the stations
occupied, was north-westerly; at one south-easterly. The velocity at
130 fathoms was greater than at 65 fathoms, and greater at the surface
than at 15 and 30 fathoms.
At all of the three stations between Grenada and Trinidad tidal action
was observed, with deflections due to local influences.
The passage between Santa Lucia and St. Vincent appears to be in the
line of the equatorial stream. At each of the five stations in this
passage tidal action was pronounced, the currents setting in and out of
the Caribbean Sea at some depth. The daily variation in this passage
reaches a maximum at about 6^h 3^m after the moon's transit, and a
minimum when the moon is on the meridian. The currents entering the
Caribbean Sea through this passage are but 100 fathoms in depth, but
there is probably an almost equal volume flowing out below that depth.
Between the Windward Islands the currents flow generally westward, but
tidal action is everywhere apparent.
To the east of Desirade the currents at all observed depths have a
northerly direction, fluctuating between about N. E. by E. to N. W. by
N.
In the eastern part of the Anegada Passage the surface current flows
into the Caribbean Sea in directions varying between S. S. W. and S.
E., but the submarine current down to 130 fathoms flows in a direction
lying between north and east.
In the more western part of the passage the currents are more complex,
apparently on account of the greater variations in depth in the
vicinity of the station occupied.
In the Mona Passage no regular currents were perceptible. Between Mona
and Puerto Rico the currents observed set out of the Caribbean Sea,
varying in direction from about W. by N. to E. N. E., except at 65
fathoms depth, where there appeared to be an inward flow. On the
western side of the passage, near Santo Domingo, the direction of the
currents was between S. S. E. and S. W. by W. But few observations
could be taken on account of unfavorable weather.
In the Windward Passage, on the western side the currents from the
surface down to 130 fathoms set in the directions lying in the S. E.
quadrant, and at 200 fathoms the direction changed to W. by S. On the
eastern side the surface current varied between E. N. E. and E. S. E.,
with about ½ knot velocity. Variations in the direction similar in
extent characterized also the subsurface currents in the middle and on
the eastern side of the passage.
The average of the observations at these three stations gives but a
small volume of water passing in either direction.
In the old Bahama Channel, at the station north of Cayo Romano (island
off the north coast of Cuba) the currents at and near the surface set
south of east; at 65 fathoms, however, the direction varies from about
N. W. to E. The deeper current of great volume flowed continually to
the north of west with a velocity of over 1½ knots at depths of 130 and
200 fathoms.
Outside the Bahamas, to the north of Great Abaco, a slight current
flows about N. W. on the surface and down to 30 fathoms; at 65 fathoms
depth the direction changes to a point more westerly, and at 130
fathoms to a point more easterly than the set of the surface current.
The maximum in the daily variation at this station occurs about 12^h
after the moon's transit.
The observations so far as completed by Lieutenant Pillsbury furnish
the most valuable data we have at present concerning the Gulf Stream,
and it is hoped that further investigation and the analytical treatment
of these observations will clearly develop the dynamic laws involved
and lead us to a correct theory of current phenomena in general.