TR-193

TECHNICAL REPORT

MAJOR CURRENTS IN THE
NORTH AND SOUTH ATLANTIC OCEANS
BETWEEN 64°N AND 60°S

SEPTEMBER 1967

NAVAL OCEANOGRAPHIC OFFICE
WASHINGTON, D.C. 20390
Price $1.35

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ey

ERRATA
Table 15, change Yucatan Currant to read Yucatan Current

Page 3, Figure 1, add shading as indicated below

Page x,

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FOREWORD

This report is intended to help meet an ever-increasing
need for the identification of the major currents of the
North and South Atlantic Oceans and a comprehensive summary
of their principal characteristics. The information given
herein is based on published reports and directly measured
data, as well as considerable unpublished data, of which a
large amount has recently been obtained. Much of these data
have been analyzed specifically for this report.

This is the first of a planned series of four reports,
the other three to include descriptions of the currents of

the Pacific and Indian Oceans and the polar regions.

Captain, U. S. Navy
Commander
Naval Oceanographic Office

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MAJOR CURRENTS
in the
NORTH and SOUTH
ATLANTIC OCEANS
between

6L°N and 60°S

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Oceanographic Analysis Division
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( By | —
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CONTENTS

Foreword .« « « «

List of Figures . . . « « « « « »
lisigns Git Wales 56 5 60000006
nNGrOGUCt LONG mleilcn tole! lollellentcllelnts
Antilles Curnrenty mien slic) tele lel iel

Atlantic Equatorial Countercurrent
Atlantic Equatorial Undercurrent .
Atlantic North Equatorial Current
Atlantic South Equatorial Current
Azores Current ...

Beene, Guess 6 56 6b 6 0000
ligeialll, @ieaesshs 5G 6 6 65 5 60 5 6
CanaryeCurKenu meee leis) siiel «| ol ce
Cape Breton Current ....e«..
Capel Horny Currenity is) elle is) ele ie
Carsiibheanm Cuzr7enit ma amsM Soi oncitcnnte
WENaleynel OwrasRS 6 56 6 000000
Milenio, GEEINS 56 6 56 00000 0
Guiana Currencies smmcnicnneiicniclnentelle
GuaneanCuirrenicaramcmas cl wel ie) cetieline! le
Che SHARE 86 656 6 60 6 0 610 0
Jutland Currents tere lolte lene te
aAbradoraCucrentcureeenasnielnciisn melee
Labrador Current Extension ....
North Africa Coast Current ....
North Atlantic Current . . . .e.
Norway Coastal Current . ... ee
Poprmneel (GbbaEINS 6 6 6 00000 0
South Atlantic Current .« . . «© e e
Vieng Welerel Wiehe 56 56 Go OOO
Wicatane Current msicilsicnienronlcllcine
Bibliography . . « «© » « «© » « « «

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e e e e e e e e e e e e e e e e e

LIST OF FIGURES

Figure
1 Major Surface Currents, July . « « « « « «© © » © © e
2 Principal Boundaries and Surface and Subsurface
Flow of Antilles Current . . »« « « « « e « © © © @ e
3 Approximate Boundaries of Atlantic Equatorial
Countercurrent by Month . 2. . « »« © » © « « © © « »
h Vertical Section at 13,5°W, April 1961, Atlantic
Equatorial Undercurrent ..-+-++«+s+-+-«+«eee -«
5 Seasonal Fluctuations of Atlantic North Equatorial
Current ° ° e e e e e e e e e e e e e e e e e e e e
6leAtitantic Southebquatorialy Currentayeur-mielselNeiMciientolns
7 Boundaries of Benguela Current and Seasonal Surface
Current Roses e e e e e e e e e e e e e e e e e e e
8 General Flow Pattern between 200 and ]00 Meters,
Benguela Current . . . « © © © © e © © e © © © © © eC
9 Subsurface Current Profile, Benguela Current ....
10 Seasonal Characteristics of Brazil Current .....
11 Boundaries of Southern Part of Canary Current and
Surface Current Rose for January, February, March .
12 Computed Current Speed Profiles, Cape Horn Current .
13 Observed Current Profile, Cape Horn Current ... .»
1h Boundaries of Caribbean Current, Region of Highest
Speed, and Current Profiles. . . « « »« « « «eee
15 Seasonal Boundaries of the Falkland Current ... .
16 Volume Transport between Key West and Havana,

Ne SPOSS), ileraick: Omens 5 6500000000000

viii

10

13

16

19

23

25
25
27

32
35
36

38
he

5

LIST OF FIGURES (cont'd)

Figure

17

18

19

20

21

22

23

2h
25
26

27
28

29
30
31

Percent Frequency of Prevailing Surface Current
by Mean Speed Categories (Summer and Winter),
Florida Current <«- os 2 @

Surface and Subsurface Currents in Straits
Om SHAorida Wat coutaieot terse eltie: ohio elzoltel cobowkedieunestel ts etme

Average Current Speeds at Various Depths along a
Cross Section in the Straits of Florida .......

Surface Current Rose, Guiana Current . .....e«.coee
Boundaries of Guinea Current during January, February,
and March and Percent Frequency of Flow by Prevailing
Directions and Speed Categories ....+-+-«-v+eee

Rhee Gullkfe Sit he am mete oe vets: ote ie) ttciireiite uotitop nel tel Monn euler treline

Precent Frequency of Prevailing Surface Current
from Speed Groups, the Gulf Stream eo ere" See) oF ely te) ee

The Gulf Stream East of 65°W eee “e Veixey- e) ‘e) Vele ie je ie
Subsurface Flow in the Vicinity of the Gulf Stream . .
Boundaries, Surface Current Roses, and Subsurface
Current Profile, Southern Part of Labrador

Current e e e e e e e e e e e e e e e e e e e e e e e

Seasonal Boundaries of Labrador Current Extension ..

Subsurface Current Profiles, North Atlantic
Current,” Ua cen vers. cece ceuisiue: 6 ge caicekeeteld Shere, O tantonmcmbeules

Norway Coastal Current Profile . . « « « »«eeeoe
Seasonal Boundaries of South Atlantic Current ....

Core of Yucatan Current as Determined from GEK
@bservations) (Hebruary—dune) eae scenes entcete 6 6

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50

52

5

57

59
61

65

68

71

76
78
82

89

LIST OF TABLES

Table

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15

Number of observations in prevailing directions by
speed catezonymand™=regdion) —) tere se) 6) ete) (el cel ill oll elie

Subsurface current, summer, 25°930.8'N, 72°32.8'W ...

Percent frequency of speeds by region, northern
Summer (ullys Auguste SCptembexs) eiiciils| toltentontcnicnrcntsitcnnenlte

Percent frequency of prevailing WNW flow by speed
category a e « eo ° © e e e e e e e e e ° e e e e e e e e

Seasonal variations in representative portions of
AtiantalerSoulth, Mquatonritall Curcrcenti yore) tel el tell eile) elton clNeiite

Observations by directions and specified speed ranges . .
Current measurements, March 1966, 299° . ORS SNe

Seasonal percent frequency distribution of
observations by speed categories, mean directions,
and mean speeds for specified regions .« . « « « « « « « e

Percent frequency of prevailing NW flow by speed
Caberory S700 S shee ee ce ch errs les eo eh eo tee et vet tel tote) MontoMmto

Percent of observations by speed categories and direction
during July, August, and September . . « « « © « © © » e

Comparison of surface drift and GEK data ......ee

Percent frequency by 90-degree quadrants of seasonal
wind and current observations between 35° and 5°N,
and «between «5° and WOW ..+ci9¢ 42 telus Kev Hy Ho Bevee nal ne

Directions and speeds during July and August . ....«e

Computed speeds at various depths . . . .« « « © ee « © o

Zz
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Seasonal variations of Yucatan Currant LF SS St as oso tere

Page

11

17

20
Za
28

39

51

53
62

80
81
8h
87

MAJOR CURRENTS IN THE NORTH AND SOUTH ATLANTIC
OCEANS BETWEEN 64°N AND 60°S
Introduction

An examination of existing data sources clearly shows that the
methods utilized to determine the principal characteristics of ocean
currents leave much to be desired. From the many and varied types
of information, currents have been identified on the basis of faunal
zones, increase or decrease in temperature and salinity, changes in
water color, exchange of heat and water vapor with the atmosphere,
cloud cover, mixing, dilution by rain, river discharge, heating and
evaporation, Coriolis force, distribution of organisms, ete. It is
agreed that all these factors, to varying degrees, can help to distin-
guish the currents, but it also appears that the importance of these factors,
when stressed individually, can be greatly exaggerated.

The usual graphic presentation of ocean currents is, at best,
roughly diagrammatic. The entire surface of every oceanic area is in
constant motion, such movement being exceedingly variable in some
regions and relatively stable in others. The currents shown in Figure 1
and described in this report are those where the movement within specified
boundaries exhibits a definite permanent or seasonal flow. The regions
beyond the boundaries of the currents are those where flows, frequently
considered part of a prevailing current, are less defined, characterized
by insufficient data, mainly tidal, under the influence of winds or
river discharge, or variable and turning. The boundaries indicate a
gradual, and not a sharp, change between zones of more persistent

flow and zones of less stable or weaker flow.

The approximate boundaries and the main body of each major current
shown are based on ship drift observations and direct measurements by
instrument, which describe the two main features of the current, namely
direction and speed. Dynamic topography presentations’ are purposely
omitted but mentioned only where they might prove of interest or value
by comparison with results of direct measurements.

The ship drift observations on which many of the descriptions are
based number in the hundreds of thousands and are retained at the
National Oceanographic Data Center and U. S. Naval Oceanographic Office.
These data are basic and most valuable for determining the surface Sues
systems of large areas and major currents of the oceans.

The reliability of ship drift observations has frequently been
questioned. Although external influences, such as wind stress on
ship superstructure, changing sea conditions, draft of vessel, etc., are
included in each observation, the effect of these factors is reduced
in processing the data because only those observations which fall within
certain coding requirements are recorded, and the remaining factors tend
to average out as the number of observations increases within a given
area. For most areas with sufficient observations, a satisfactory
pattern of surface circulation can be derived, and when compared with
other methods of observations, for example drift bottles or even short-
period current meter readings, ship drift is usually preferred.

The number of current meter measurements at sea is negligible, and

readings considered reliable or reasonable are shown where available.

Tt is generally agreed that ocean current information is very
sparse and usually insufficient for most ocean areas. However, as
more and better data become available, the presentations and descrip-

tions in this report can be refined to a greater degree of precision.

©

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NOTE:
THE SHADED REGIONS INDICATE PLACES
WHERE FLOWS ARE LESS DEFINED, OR
CHARACTERIZED BY INSUFFICIENT DATA, OR
MAINLY TIDAL, OR UNDER THE INFLUENCE
OF WINDS OR RIVER DISCHARGE, OR VARIABLE
AND TURNING.

ATLANTIC NORTH EQUATORIAL CURRENT

}— 30°

SOUTH ATLANTIC CURRENT

WEST WIND DRIFT

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Antilles Current

The Antilles Current is probably stronger, larger, and more
persistent than previous descriptions have indicated. Generally,
the surface flow shows little seasonal variation in speed, direction,
and size. The current originates in the vicinity of the Leeward Islands
as part of the Atlantic North Equatorial Current. Figure 2 shows the
outlines of the current. The frequency of sets in the prevailing direc-
tion in the three regions shown averages about 55 percent, the main surface
speed being about 0.6 knot. Table 1 shows the prevailing direction
throughout the year in each area and the total number of surface obser-
vations by area in various speed categories.

The analysis of about 42,000 surface observations shows that in
Regions A and B, about 80 percent of the observations are between 0.1
and 0.9 knot, 13 percent between 1 and 2 knots, and 1 percent over
2 knots; in Region C the current is slightly weaker, with 88 percent of
the observations between 0.1 and 0.9 knot, 6 percent between 1 and 2
knots, and less than 1 percent over 2 knots.

Little is known about the subsurface currents. Table 2 was
derived from hundreds of direct meter measurements taken at 10-minute
intervals at 25°30.8'N, 72°32.8'w between 25 June and 13 July 1965.

The data appear to verify the stability of this part of the Antilles
Current during summer, both in speed and direction at all depths.

Below 800 meters (2,625 feet) a slight clockwise turning is indicated.

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Although only partially analyzed, the direct observations at the
same location between 23 January and 8 February 1966 indicate that they
may differ considerably from the summer subsurface data. At 100 meters
(328 feet) the current varied between south-southeast and south-southwest,
with a mean speed of 0.6 knot; at 200 meters (656 feet) the direction
was about the same but the speed was 0.4 knot; at 500 meters (1,640 feet)
the flow was mainly south-southeast at about 0.2 knot; and between 1,000
and 3,000 meters (3,281 and 9,842 feet) it was generally south at 0.1
knot. At depths of 3,200 and 3,900 meters (10,499 and 12,795 feet) the
flow was predominantly north-northeast at about 0.2 knot. At the bottom,
5,380 meters (17,651 feet), the flow was east-southeast at 0.2 knot.

The current meters were not located in the region of fastest
current but near the northern boundary of the Antilles Current, where
the greatest seasonal change is likely to occur. During winter, when
the Bermuda high migrates to its maximum south position, the northern
boundary of the Antilles Current also moves southward, and the currents

in the region of the direct measurements tend to be more variable.

Atlantic Equatorial Countercurrent

The Atlantic Equatorial Countereurrent flows eastward between
the west-setting Atlantic North and South Equatorial Currents. The
approximate boundaries and monthly variations in extent are shown in
Figure 3. The surface countercurrent is best defined during August
and September, when it extends from about 52° to 10°W and joins the
Guinea Current. In October it narrows and separates into two parts
at about 7°N, 35°W. ‘The western part, which appears to be a region
where the countercurrent probably sinks and flows eastward beneath
the equatorial currents, gradually diminishes in size to the west-~
northwest, while the eastern part diminishes to the east-southeast
as shown in the lower half of Figure 3. The greatest separation
oceurs during March; during April the western part of the countercurrent
disappears, but in May it reappears in the vicinity of 0°, 4O°W. The
two segments progress west-northwest without too much change in size.
They merge at about 6°N, 43°W during August and continue their flow
eastward uninterrupted through September.

Table 3 indicates frequencies of speeds by prevailing direction
during the northern summer, when the countercurrent is most pronounced.
Speeds are stronger but less persistent in the western part of the
countercurrent, as shown by the decreasing frequency of observations
to the east in the higher speed groups of 1.3 to 2.5 knots and
increasing frequency to the east in the lower speed groups of 0.1 to

1.2 knots. Speeds have been observed to exceed 3.0 knots at times.

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REGION PREVATLING MEAN SPEED SPEED CATEGORIES (KNOTS) _ TOTAL

DR (GB) KN Op OROMOn (alee aseilO) 2RO-eR D256 EREO.
West of
45 °W 110 HAO 1.0 2.0 12.5 TK) O.5 55
Between
45° and
20°W O75 0.9 21.0 26.0 10,0 2.5 0.5 60
East of
20°W 095 0.8 30.5 30.5 1a55 ILS =~ 70

Table 3 Percent frequency of speeds by region, northern summer (July, August,
September )

alae

Atlantic Equatorial Undercurrent (Buchanan Undercurrent, Lomonosov
Undercurrent )

The Atlantic Equatorial Undercurrent was discovered in 1886 by the
British oceanographer J. Y. Buchanan. From limited direct observations
the undercurrent appears to be a permanent phenomenon, flowing east at
the Equator during all seasons for a distance at least between 37° and
h°w. Speed of the current has been measured by drogues and anchored
current meters during northern spring and winter, and little seasonal
variation is indicated. Drogue measurements at the Equator near 41°W
showed no undercurrent; however, at 37°, 35°, and 33°W, an east set at
1.5 knots was recorded at depths between 75 and 100 meters (246 and
328 feet). The surface currents observed at these three locations were
moderate westward during northern summer, and weak eastward in winter.

Detailed current measurements obtained at various locations along
the Equator from 30° to }°W disclose a strong deep current with speeds
at times exceeding 2 knots; a remarkable similarity to the Pacific
Equatorial (Cromwell) Undercurrent is indicated. The core of highest
speeds (Figure 4) is at a depth between 40 and 90 meters (131 and 295 feet),
but its limits have not been reliably defined to date.

At O°O09'N, 30°00'W, current meter measurements showed an east set
from the surface to 150 meters (492 feet); at 50 meters (164 feet) the
speed was 1.7 knots, at 100 meters (328 feet) 1.7 mots, and at 150
meters (492 feet) 0.8 knot. The east flow at the surface probably
resulted from surfacing of the undercurrent during a period of weak
winds, not unusual at this latitude. Other observations taken in
February and March 1963 between 30.0° and 27.5°W show the undercurrent

to be strongest and most constant between 0.0° and 0.5°N. Near the

12

LATITUDE

BES
0 0
50 164
B 100 yO
= i
Ww LL
= 150 492
LL
2 200 656
250 820
WEST- SETTING
300 984

QB | O° 1° De

LEGEND
2.0 SPEED (KNOTS)

FIGURE 4 VERTICAL SECTION AT 13.5°W, APRIL 1961,
ATLANTIC EQUATORIAL UNDERCURRENT
13

core at 30 meters' depth (98 feet) the speed ranged between 0.6 and
1.2 knots, averaging 0.9 knot; in the core or its immediate vicinity
at 80 meters (263 feet), the speed ranged between 1.2 and 1.5 knots,
averaging 1.3 knots. At 155 meters (509 feet) the flow appeared very
weak and variable; near 400 meters (i, 312 feet) the flow below the
core was westward at about 0.3 knot. A drogue released at 0°10'N,
13°30'W, depth 55 meters (180 feet), showed the current setting east
at 2.6 kmots.

In view of the exaggerated vertical scale in Figure 4, the under-
current appears to be a continuous flat ribbon flowing east between
outer limits of about 1°N and 1°S beneath the west-setting Atlantic
South Equatorial Current. The volume transport is estimated to be

ho x 10°m3/sec.

1h

Atlantic North Equatorial Current

The broad, slow, west=setting Atlantic North Equatorial Current
is generated mainly by the northeast trade winds. It originates near
the longitude of 26°W between about 15° and 30°N and flows across the
ocean past 60°W, where it forms the Antilles Current north of the West
Indies; the part of the current between 12° and 15°N joins the Guiana
Current and forms the Caribbean Current.

Surface current data show the current to migrate north and south
seasonally; this migration results from the displacement of the Azores
high between about 29°N, 31°W during winter and 34°N, 35°W during
summer. Figure 5 shows the seasonal outlines of the current.

Mean speed differs slightly in different parts of the current;
it appears higher in the southern part and decreases northward. Speeds
are generally lower during winter, when the Atlantic Equatorial Countercurrent
is not evident and the west-setting Atlantic North and South Equatorial
Currents meet at about 9°N.

Table 4 shows the changes in speed of the current in the prevailing
west-northwest direction. From July through December, when the Equatorial
Countercurrent to the south is best defined, higher speeds occur more
frequently. From January through June, particularly during March, April,
and May, when the countercurrent is least evident, lower speeds occur

most frequently.

15

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There are few data available on the subsurface current. Direct
measurements at 16.8°N, 46.3°W showed a general west-southwest flow
during February with the mean speed, almost 0.6 knot near the surface,
decreasing to 0.2 knot at 800 meters. At 12.6°N, 47.6°w, in the
southern part of the current, direct measurements during October showed
a shallow west set to about 20 meters (66 feet); below 20 meters the

flow tended to be south through southeast. Mean speed, about 0.5 knot

at the surface, decreases to about 0.2 knot at 2,800 meters (9,186 feet).

SPEED (KNOTS

0.2 0.5 0.8 Link Aba Lo6 2.2

MONTHS

Jan.
Feb.
Mar.

Apr.
May 35 he 18 4 i ie eS
June

July
Aug.
Sept.
Oct.
Nov.
Dec.

Table 4 Percent frequency of prevailing WNW flow by speed category

17

Atlantic South Equatorial Current

Of the two equatorial currents in the Atlantic, the Atlantic
South Equatorial Current is the stronger and more extensive. ‘The
limits of this current are shown in Figure 6. The major part of
the current is located south of the Equator, the central portion extend-
ing to about 20°S. The light shading shows the northern limit of the
current during July, August, and September, as well as the region of
fastest current where speeds average from 1.0 to 1.5 knots during the
year. The dark shading shows how the current expands during January,
February, and March, when the Atlantic Equatorial Countercurrent
dissipates and is least evident.

Table 5 shows average principal seasonal variations for repre-
sentative areas of the current. In Region A the flow is slightly faster
and more persistent westward during the northern summer; in Region B there
appears to be the least variation in speed and frequency; in Region ¢
there is a slightly greater seasonal change in direction, speeds are
higher during the northern summer, and the percent of observations
setting in the prevailing direction is the same during both seasons.
Although not shown in the table, a total of 640 observations in Region
C shows the percent of observations in the lower speed group between 0.1
and 0.9 knot in the prevailing westward direction to be about 15 more
in the northern winter than in summer, with a significant southwest
component; in the higher speed group between 1 and 2 knots, the
percent of observations is higher during the northern summer by about

5 to 8, prevailing westward.

18

AFRICA

AMERICA

SEE TEXT FOR EXPLANATION

FIGURE 6 ATLANTIC SOUTH EQUATORIAL CURRENT
19

PREVAILING PERCENT MEAN

DIRECTION (°T) FREQUENCY SPEED (KI)

REGION JEM JAS dM JAS JEM JAS
A 285 275 60 70 OA 0.9
B 250 260 65 65 BORG One

@ 270 255 1G) (5 0.6 0.8

Table 5 Seasonal variations in representative portions of
Atlantic South Equatorial Current

20

Azores Current (Southeast Drift Current)

The Azores Current is an inner part of the North Atlantic gyral
that sets between east through south in the general vicinity of the
Azores Islands. It is a slow, but fairly constant southeast branch-
ing of the North Atlantic Current and part of the Gulf Stream. Its
mean speed is only 0.4 knot, and the mean maximum speed computed from
all observations above 1 knot in the prevailing direction is 1.3 knot;
surface current data indicate no discernible seasonal fluctuations.

Table 6, based on over 12,000 observations in the central part
of the current between 30° and 40°N, shows the representative annual
distribution of observations by speed and direction. The largest
number of observations is in the lowest speed range of O.1 to 0.3 knot;
thus, the speed and direction of the current could easily be influenced

for short periods by changing winds.

SPEED RANGES (KNOTS)

1.0- 1.3- 1.6-
Ie alas)” als)

* All other directions average 7.8 percent or less.

Table 6 Observations by direction and specified speed ranges

21

Benguela Current

The Benguela Current, a slow-moving current caused mainly by the
prevailing southeast trades, consists of a coastal part between 25°S
and 0°, which shows considerable irregularity, and a fairly steady
oceanic part shown in Figure 7. Its boundaries change very little
and it is most constant in speed and direction between Cape Aguilhas
and about 25°S. In this region it sets noes about 50 percent
of the time and is strongest from November through July, when speeds
may reach 3 knots at about 35°S, 17°E. North of 25°S the Benguela
Current is slightly more variable in speed and direction; the frequency
of the prevailing northwest set falls slightly below 50 percent,
particularly during August through January.

Data on winds, which influence the current, are few. Observations
obtained during a 2-year period along the southwest African coast show
a@ mean direction of about 169°T during all seasons; the mean wind
velocity did not show marked seasonal variations. Southerly winds
appear more persistent with increasing distance from shore, from about
45 percent of the time within 40 miles to about 80 percent of the time
250 miles offshore, and cause significant differences between coastal
and oceanic currents.

At about 25°S the current begins to widen, and the main axis is
located closer to the outer boundary with the more persistent offshore
winds. Near the Equator the current trends toward the west and joins

the Atlantic South Equatorial Current.

22

10°

LEGEND
B ooct.—mar.
[] APR.—SEPT.
MEAN SPEED (KN.)
==
0712.2 MAX. SPEED (KN.)
0.811.8~_ AX. SPEED (KN.)
MEAN SPEED (KN.)
PERCENT CALMS
TOTAL OBS. OCT.—MAR.
PERCENT CALMS
TOTAL O88. | APR—SEPT. 0°

@ LOCATION OF SUBSURFACE
CURRENT PROFILE (SEE FIG.9)

WW INDICATES REGION OF FASTEST CURRENT!
ROSE SCALE (%)

| OCT.—MAR.

| APR.—SEPT.

20°

30°

REGION 2 REGION 1

MAXIMUM SPEED NOT RECORDED. AP

FIGURE 7 BOUNDARIES OF BENGUELA CURRENT AND SEASONAL

SURFACE CURRENT ROSES
23

Observations made between January and April along the southwest
African coast showed winds to be northerly or calm and little evidence
of upwelling. During July, winds were southeabterly with marked
upwelling, which reached a maximum in August or September. Upwelling
is most conspicuous off the southern part of the African coast, where
the water moves north or northwest; the current diminishes rapidly in speed
northward to about 17°S and becomes increasingly variable in direction.
Upwelling reaches to depths of 180 to 350 meters (591 to 1,148 feet) at
28°30'S, 200 to 300 meters (656 to 984 feet) at 25°00'S, 200 to 230
meters (656 to 755 feet) at 23°00'S, and 220 meters (722 feet) at
19°45'S.

Subsurface current data are scarce. Figures 8 and 9 show indirect
and direct measurements, respectively, in the south and north portions
of the current. Counterflows evidently underlie the main surface current.
Dynamic topography at 50 meters(164 feet) relative to 250 meters (820 feet)

shows an east set at 0.5 to 0.8 knot in the vicinity of 4°S, 3°E.

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25

Brazil Current

The Brazil Current is an extension of the Atlantic South Equatorial
Current, which divides between latitudes 7° and 17°S, depending on the
time of year, and flows southwest parallel to the Brazil coast. Off
Uruguay, at about 35°S, it meets the Falkland Current, and the two
currents curve toward the east and join the South Atlantic Current.

The approximate seasonal boundaries of the Brazil Current are
shown in Figure 10. The prevailing flow is Somdn-cemtinmast throughout
the year, but the boundaries and speeds appear to change more signifi-
cantly than most other major currents. The mean speed of the Brazil
Current along its entire length is about 0.6 knot, and its variation in
speed and direction is greater throughout the year than that of the main
part of the Atlantic South Equatorial Current from which it originates.

In the southern winter (July and August), coastal countercurrents
flow along the major part of the Brazil coast, even though the Brazil
Current flows at greater constancy and higher speeds than at other seasons.
The Brazil Current is stronger because the Atlantic South Equatorial
Current is stronger during July and August, with speeds exceeding 2.5
knots off the east coast of South America; the countercurrents result
from the increase in volume of seasonal river discharge of the Rio de la
Plata being augmented partly by-a coastal extension of the Falkland Current.

The hourly measurements of the Brazil Current shown in Table 7
indicate the many changes that can occur in the surface layer. Strong
tidal characteristics are evident in the surface current, which appears
to be influenced also by prevailing northerly and northeasterly winds;

however, the period of observation occurred during the lunar phase of

quadrature at maximum declination, during which the tidal currents in

26

50° 40° 30°W

O°
JAN., FEB., MAR. APR., MAY, JUN.
10°S
®eLOCATION OF
DIRECT CURRENT
MEASUREMENTS
26 AND 27
MARCH 1966 20°
= [ oir. fons] tere |
[ oir, ows SRE msmifsei | olen
s_[221) 07 | Pew] 28.1 1.0
Cw [20.7 [09+ 30

— Sn
DIRECTIONS 48

PNW [11.4 | 0.6 |

ALL OTHER
DIRECTIONS 2”—9”

814 OBS.

OCT., NOV., DEC.

Bale SPEED
<—
[8]
| SW |

El Oe |
ALL OTHER
800 OBS. |DIRECTIONS 4“—8%

BOs 40° 30°

FIGURE 10 SEASONAL CHARACTERISTICS OF BRAZIL CURRENT

27

DEPTH ee

5
DAY| HOURS | SPEED TRECTION | SPEED DIRECTION | SPEED DIRECTION
(KNOT) (am) (KNOT) Cn) (KNOT) (mT)

range range range range
between between between between

mea. mean mean mean

n
0.4 238 0.8 225

e ° e .
°
FWWww Fru FrPWNnNNWN NW FUN

°

WANA DY Fw FUDAN ADADNVWWWwww Fro

“confused :: : om
“readings *: confused:

| readings

e

°

°

OOO OOO OOO ORONO ORONO OOOO OT OOO ee ene)

O.
O
O
0.
O
O
O
O
O.
O.
O.
O
OF
O.
O
QO.
O
O
O
O.
QO.
O.
OF
O.
O.
O.
O.

(OOO ON ON ON OR ORO ORO OOOO ONO ORO OOOO OO eTe)

Table 7 Current measurements, March 1966, 22.9°S, 40.9°W

28

general are weaker than usual, with greatest diurnal inequality. Speeds
inerease with depth and average 1.8 knots at 15 meters (49 feet) and
about 2.0 knots at 30 meters (98 feet).

At the 5-meter depth (16 feet), between 0600 and 1700 on 26 March,
the current had a tendency to rotate counterclockwise at a rate averaging
10°T per hour but showed a general prevailing southwest flow with a
mean speed of 0.4 knot. Between 1800,and 1300 of the following day ,
the current followed a definite rotary counterclockwise pattern and
averaged about 22° per hour, completing a 360° cycle in about 17 hours.
At 1300 a quick reversal occurred from southeast through north to north-
west; thereafter, the counterclockwise rotary movement was again evident
but at a slower rate, averaging about 10° per hour.

At 15 meters there is a tendency for the current to rotate counter-
clockwise but at a slower rate than at the surface, averaging about 12°
per hour between 1800 on the 26th and 0100 on the 27th of March. Confused
readings between 0200 and 0500 may indicate a fast change in the rotary
pattern of flow or more likely a reversal that occurs more slowly, but
about 10 hours sooner than at the surface. After 0600 a continuation
of the counterclockwise rotary pattern is indicated but at a still slower
rate, which averages about 7° per hour; after 1400 the data show a
prevailing flow westward.

At 30 meters (98 feet) the current shows a tendency to rotate
counterclockwise, but the changes in direction are restricted to
fluctuations between southwest and west. Indeterminate readings between
0200 and 0600 correspond to those at 15 meters, and the decrease in

speed may indicate the period when the tidal currents are subject to

29

change in direction. The tendency for the current to prevail south-
westward appears to become stronger with depth, and little change in
direction and decrease in speed may occur for some distance below 30
meters (98 feet) in the Brazil Current.

Tabulated surface current data for representative 5° quadrangles

within particular regions of the current are shown in Figure 10.

30

Canary Current

The Canary Current is that part of the clockwise flow of the
North Atlantic Ocean that sets south off the northwest coast of
Africa, as shown in Figure 1. In the vicinity of the Cape Verde
Islands the current divides, part curving southwestward and joining
the Atlantic North Equatorial Current, and part turning southeastward
into the Guinea Current.

North of 30°N the current has very little seasonal variation;
the flow prevails southward about 40 percent of the time and has a mean
speed of 0.4 knot. Between 30° and 20°N the set becomes more persistent,
and the current prevails southwestward about 55 percent of the time,
with a mean speed of 0.5 knot.

A total of almost 24,000 observations within the area 25°-35°N,
15°-20°W shows that 92 percent of the speed observations are between
O.1 and 0.9 knot, 6 percent between 1.0 and 2.0 knots, and less than
0.5 percent over 2.0 knots. These percentages also apply very closely
to the rose subarea in Figure 11.

The part of the current south of 20°N appears to differ considerably
between winter and summer, as shown in Figures 11 and 1. During July,
August, and September the southern part of the current narrows consid-
erably. During January, February, and March, when the Atlantic Equatorial
Countercurrent is least evident, the Canary Current is wide, extends close
to the African coast as far south as 10°N, and flows into the wide band
of westward flow in the equatorial region. During this period the flow
in the southern part of the current appears more constant; the percent
frequency in the prevailing south-southwest direction is higher, ranging

between 45 and 60 percent, and the mean speed is 0.6 knot.

31

30° 25° 20°W

20°N
MEAN SPEED (KNOTS)
1% CALM
4,624
OBS.
I=
[|S SSS
0 20 40 60
PERCENT
Ops
5°

FIGURE 11 BOUNDARIES OF SOUTHERN PART OF CANARY CURRENT AND
SURFACE CURRENT ROSE FOR JANUARY, FEBRUARY, MARCH
32.

Cape Breton Current

The Cape Breton Current originates in the Gulf of St. Lawrence,
sets southeast in the southwestern half of Cabot Strait, and merges
with the Labrador Current Extension. It may be augmented by a branch
of the constant but tide influenced Gaspé Current to the northwest.

The Cape Breton Current is constant, with 45 to 60 percent of all
surface current observations in the saerraalling direction; mean speed
is 0.7 knot, and maximum speed is between 1.5 and 2.0 knots.

Storms cause the current to vary in direction or at times reverse
for short periods. Direct observations between North Cape and St. Paul's
Island obtained during 5 Bolder: period showed a consistent surface flow
setting about 125°T at a mean speed of 0.6 knot; at a depth of 45 feet
(14 meters) the speed was 0.5 knot, at 330 feet (101 meters) it was
about 0.2 knot, and below this depth it decreased rapidly.

The current appears to be influenced by the tide; observations have
shown that current speeds often are higher during periods when an out-
going ebb tidal current normally occurs. Conversely, the speed of the
prevailing southeast-setting current may be retarded during periods of

the ingoing flood tidal current.

33

Cape Horn Current

The Cape Horn Current sets continuously eastward close to the
tip of South America and enters Drake Passage at about 7O°W ina
150-mile-wide band, with surface speeds observed at up to 2.4 knots.
The set veers north-northeast, and when it crosses longitude 65°W
the current has narrowed to a width of about 85 miles and its speed
has decreased considerably.

The profiles of computed speeds in Figure 12 show the
well-defined limits of the current. Available data indicate a
fairly high rate of transport for this current, averaging about
370 x 10? m2/ir in the weaterm pare and 22) 2107 nO) ment the
eastern part.

The current profile shown in Figure 13 indicates the usual

range of speed and the set that can be expected in Drake Passage.

3h

A B
0 pete ee 0 5
Ges
250 Y 820 820
500 Y 1,640 1,640
an z nD
2 750 Y 2,460 — 2 2460 =
= 1,000 j/ 260K = 3,280
= 1,500 7 4,920 x = 4,920 =
xr ag
=| 2,000 7 6,560 a 6,560 &
Lu
O| 2,500 7 8,200 0 ra 8,200 0
3,000 7 9,840 9,840
3,500 & Yj 11,480 11.480
YU
NOTE CHANGE NOTE CHANGE
IN INTERVAL. IN. INTERVAL.

LEGEND

STATION LOCATION

r

= APPROXIMATE BOUNDARY

OF CAPE HORN CURRENT
LIMITS OF CROSS SECTION
CURRENT SPEED (CM/SEC.)

FIGURE 12 COMPUTED CURRENT SPEED PROFILES, CAPE HORN CURRENT

35

DEPTH (METERS)

SPEED (KNOTS)
ge OF27, O45 06) 05S EO Ie 2a 4am came: Olme2e2. 24

25 82
50 164
100 328
200 656
400 1312
800 LEGEND 2824
RANGE OF OBSERVED SPEED
PREVAILING DIRECTION
Bo DEPTH OF OBSERVATION ue
1600 MEAN POSITION:
LAT. 56°43’S, LONG. 66°09’W
MARCH 1963-64
MEASURED BY BUOYANT FLOATS
3200 DEPTH OF BOTTOM 3780 M 10496
CUE
6400 20992

FIGURE 13 OBSERVED CURRENT PROFILE, CAPE HORN CURRENT

36

DEPTH (FEET)

Caribbean Current

The Caribbean Current, seldom mentioned and given little consideration
in the literature, is clearly shown by available data to be among the most
persistent and well-defined major currents. Figure 14 shows the boundaries
of this current, which sets west throughout the year between 65 and 75
percent of the time; mean speed is 0.9 knot, and maximum speed at times
is about 3.5 knots. The band within which highest speeds occur during any
month also is shown in Figure 14; this band, where speeds average 1.1
knots 75 to 80 percent of the time, originates from the fast coastal
Guiana Current and is located in the southern part of the Caribbean Current.
Current speed over Rosalind Bank (16.5°N, 80.5°W) is strong, averaging
1.2 knots; however, this region is not included in the main band of highest
speed, and the swift flow over the bank appears to be a funneling of the
slower prevailing flow from the east.

Table 8 shows the frequency by speed category during winter (January,
February, and March) and summer (July, August, and September) in the four
regions shown in Figure 1+. ‘The flow in the prevailing direction is very
consistent, being located in a steady trade-wind region, and there is
little variation between seasons. Because of the limiting topography of
the region, the Caribbean Current, unlike currents in the open ocean, has
the basic characteristic of a one-way flow through a channel. Frequency
in other than prevailing directions ranges only between 2 and 6 percent.

Little is known about subsurface currents. Profiles in Figure lh

for two locations seem to agree with the known features of the current.

37

20°

15°

10°

CURRENT BOUNDARIES

DEPTH (METERS)

0.4 06 08
0

25

50

100

200

400

800

1600

SPEED (KNOTS)
1.0

1A 2)

© (WNW)
he

LATITUDE
LONGITUDE

MONTH

DEPTH OF BOTTOM
RELIABLITY

METHOD

SPEED (KNOTS)

ope 0.2 0.4 0.6 0.8 1.0 1.4

1.2

1.6 1.6
0

1.8 2.0 2.2
0

82
LEGEND
SOLID LINE INDICATES MEAN
SPEED. 164
DASHED LINE INDICATES MAXIMUM
SPEED.
DOTS INDICATE DEPTHS OF
OBSERVATIONS. 328
LETTERS SHOW DIRECTION OF
FLOW (TOWARD).
PARENTHESES INDICATE APPROXI-
MATION OR PROBABLE . 656
T MEANS TIDAL.
1312
17.0N BOTTOM
1,300 M LONGITUDE 61.1W
FAIR MONTH FEBRUARY 2624
PILLSBURY DEPTH OF BOTTOM 550 M.
RELIABILITY GOOD
METHOD PILLSBURY
5248

5248
CURRENT PROFILES

1600

FIGURE 14 BOUNDARIES OF CARIBBEAN CURRENT, REGION OF HIGHEST
SPEEDS, AND CURRENT PROFILES

38

DEPTH (FEET)

SUOTSeL peftyToeds azoxy gypeeds ues pue
‘suotjoertp usem ‘seTsoZe7e0 pesds Aq suoTzeAresqo JO uoTInqEIISsTp Kousnbearzzy queozed Teuoseses Q eTqeL

HI OMSTT 399 x

Le
aaa
fe
ie

e le)
SLONY) SHLYOOMLVO CHedS xNOIDHY

39

Profile A shows a southwest flow of the surface layer, which probably
results from the current through Windward Passage; below 150 meters the
flow is west-northwest. On the other hand, Profile B in St. Lucia Passage
indicates the predominance of the wind-induced surface layer setting

northwest, with probable tidal effects below 50 meters (164 feet).

0

Falkland Current (Malvin Current )

The Falkland Current originates mainly from the Cape Horn Current
in the northern part of Drake Passage. After passing through the
strait, the current sets north between the continent and the Falkland
Islands and follows the coast of South America until it joins the
Brazil Current at about 36°S near the entrance to Rio de la Plata.
Figure 15 shows the monthly changes in the northern limit of the
current northeast of the mouth of Rio de la Plata. These changes
appear to result mainly from the volume of seasonal river discharge,
which is greatest during May and June; during this period the river
outflow has been reported to have extended a considerable distance
eastward, at times as far as 37°W, or a distance of about 1,200
nautical miles.

Within 20 nautical miles of the coast, tidal currents predominate.
Between 30 and 50 nautical miles from the coast the current is stronger
and may reach speeds to 3 knots off headlands; however, southerly sets
have been observed under the influence of northerly winds. Beyond
50 nautical miles of the coast the Falkland Current usually sets
north-northeast. The current has been described as easily influenced
by, and changing direction with, the wind; however, there are few
storms to affect the current, and wind speeds seldom exceed 25 knots,
most frequently averaging 13 knots. The constancy of the current
during the northern summer of July and August is indicated by 55 per-
cent of the observations setting 025° at a mean speed of 0.8 knot.
Strongest southerly winds cause a maximum speed of about 2 knots.

Fewer observations available during the northern winter months of

January and February indicate some change in the current. Primarily,

mn

SOx c 30°S
z a,
: /
5
3 35°
‘ |
iw /
© /
3 /

36
JFMAMJJASOND SI
MONTHS
MONTHLY CHANGES IN_ THE
NORTHERN LIMIT OF THE FALK-

LAND CURRENT 40°
45°
50°
re
eee
She
/
VA —— JANUARY AND FEBRUARY
—— JULY AND AUGUST
55°
60° 50°

FIGURE 15 SEASONAL BOUNDARIES OF THE FALKLAND CURRENT
42

the current is narrower, the west boundary is located farther offshore
throughout its length, and the northern limit appears farthest south
between 35° and 36°S as shown in Figure 15. The constancy of the
current is somewhat greater, with about 70 percent of the observations
prevailing north-northeast, but the mean speed is 0.6 knot, 0.2 knot
lower than that during July and August. Maximum speed during January

and February is about 1.5 knots.

43

Florida Current

The Florida Current sets through the Straits of Florida from the
Gulf of Mexico to the Atlantic Ocean. It shows a gradual increase
in speed and persistency as it flows northeast and then north along
the Florida coast; the current flows over a broad coastal plateau at
depths usually less than 3,000 feet (914 meters). The volume trans-
port of water through the strait between Key West and Havana is shown
in Figure 16.

In summer the part of the surface current south of 25°N moves
farther south of its mean position, with a@ mean speed of 2.0 knots
and a maximum speed of about 6.0 knots; the part of the current north
of 25°N moves farther west of its mean position, with a mean speed of
2.9 knots and a maximum speed of 6.5 knots. In winter the shift of
position is in the opposite direction, and speeds are somewhat less by

about 0.2 to O.5 knot.

The flow prevails throughout the year, with no significant changes

in direction; the speed, however, varies slightly from one season to
another. Figure 17 shows the seasonal percent frequency of the pre-
vailing surface current by speed groups for the specific regions
indicated in Figure 18. In Region 1 the current is less restricted,
and the higher frequencies are between 1.0 and 2.0 knots; in Region 2
the current narrows, and the higher frequencies average about 2.2
knots; in Region 3, the narrowest part of the current, the highest
frequencies are greater than 2.2 knots. In all three regions the per-
cent frequency during winter is higher in the lower speed groups,

whereas the frequency in summer is higher in the higher speed groups.

SHLNOW
f qd N O S Vv f if Ww Vv Ww

[tt a a a i Ol
(7167S 1‘ 188s) (ors) (969) (89Z) (v¥Z) (918) (vz9) (v98) (OVE) (918) (Z16)

SUNOH Zz
JO SdOlydd YOs
NOILVAYISEO JO
SYUNOH IVLOL(Z16)

90 L) LYOdSNVUL JWNIOA

(‘33S/—W

FIGURE 16 VOLUME TRANSPORT BETWEEN KEY WEST AND HAVANA,

1952—59, FLORIDA CURRENT

45

Fluctuations in current speed can occur under the influence of
tide-producing forces, with maximum speeds occurring daily about 9
hours before upper or lower transit of the moon over the local
meridian. The mean speed also appears to increase in some regions
and decrease in others after maximum north and south lunar declina-
tions. The current in the Miami-Cat Cay region is partly out of phase
with astronomical forces; mean maximum speeds of 2.8 knots occur
about 3 days after neap tides, and mean maximum speeds slightly below
2.5 knots occur at spring tides. Regions A, B, C, D on Figure 18
show the gradual change in surface speed across the axis of the
current.

Current meter measurements are sparse, but results of such
observations are shown in Figure 18; Insets 1 and 2, composite current
profiles in the axis of the Florida Current, show the differences in
speed between April and August. Figure 19 shows average speeds of
the Florida Current at various depths and distances from shore.

Subsurface flow is similar to that shown for the surface except
where submarine topography affects the direction of flow; the mean
position of the axis of the Florida Current is located to the left of
the deepest part of the channel and generally coincides with the 100-
fathom contour around the southeast coast of Florida.

Subsurface currents at depths less than 350 meters (1,148 feet)
may flow over parts of the Florida reefs, while water at greater
depths flow through the strait. In shallow depths of the Cay Sal
region, the flow is generally northward, but at depths greater than
100 meters (328 feet) the flow follows the direction of Nicholas and

Santaren Channels.

46

SPEED CATEGORIES (KNOTS)
O22 1055 0.8 1.1 1.4 1.8 DP Proll 372 3.7 >4.0

OCTOBER—MARCH (WINTER) ===
APRIL—SEPTEMBER (SUMMER) —- ——

~< 5,049 OBS

PERCENT FREQUENCY

12,012 OBS

i 2,282 OBS
of

REGION 3

SEE FIGURE 18 FOR LOCATION OF REGIONS

FIGURE 17 PERCENT FREQUENCY OF PREVAILING SURFACE CURRENT BY
MEAN SPEED CATEGORIES (SUMMER AND WINTER), FLORIDA CURRENT
} 47

Speeds over 2.0 knots may occur between Miami and Cat Cay at a
depth of 300 meters (984 feet). Speeds over 1.0 knot have been
observed at 200 meters (656 feet) between Key West and Havana.
Photographs of current ripple marks on the floor of the northern
part of the strait in depths over 800 meters (2,625 feet) indicate a
current generally flowing northward at about O.5 knot, with some

southward flow also possible but not verified.

4,8

SPEED (KNOTS)

0.0 1.0 2.0 3.0 4.0 0.0 1.0 2.0 3.0 4.0 5.0
) )
200 300 91
Ps a
fa, 408 2 — 600 183 =
cs iG re
Ww
Bure 26°00'N, = = 900 asesan, [274 &
5 79°50'W - = 79° 48’ =
4 800 i 8 1200 ant 366 =
3 STATIONS oa STATIONS &
1000 APRIL 1500 UGUSTINN A
1200 1800 549
BOTTOM ABOUT 1,100 FT.
2100 640
BOTTOM ABOUT 1,900 FT.
RANGES OF SPEED OBSERVED BY SHORT PERIOD
MEASUREMENTS ON THREE SEPARATE DAYS
82° 81° 80° 79°W

27°N

LEGEND
——— BOUNDARIES OF THE FLORIDA
CURRENT

——— CURRENT AXIS AND PREVAILING
CURRENT DIRECTION IN EACH

REGION
© MEAN POSITIONS OF COMPOSITE
PROFILES

@——e LOCATION OF CURRENT CROSS
SECTION IN FIGURE 19

TRANCE OF SPEED| MEAN SPEED
(KNOTS) (KNOTS)

24°

Zam

FIGURE 18 SURFACE AND SUBSURFACE CURRENTS IN STRAITS OF FLORIDA
49

valdOld JO SlIVaLS FHL NI NOMOAS SSOUD
V SNOW SH1d3d SNOIVA LV Sdd4ddS LNSYUND JAOVASAV 6l ANOS!

(STIW IWDILAVN) JONVISIC

zy (BO %- Be Ce Gh Ol ta wh

yl6 : REIS ‘

ZEL
is}
m
3 69S
=
=
“1 99€
rs)
&

esi

0

00 ———— =

ee emeieeeer ae ieee ca “fe] OOP = Ts
Bylo de Sa NNO ee ys 00%
Se a ea ee SN RA a oe “y3

CF Bees SSS ee Be ie Ee ¥ %
o —— aS SSS 5
= 07 SS ee ee 0% =
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= _— =
Zoe oe

(on 4 : OV
M.61.6Z M9008
“NSE.ST “NSE ST

(1344) Hld3a

50

Guiana Current (North Brazil Current)

The Guiana Current is a strong, persistent northwest flow along
the northeast coast of South America between 5°S and 12°N that may at
times attain a speed of about 4 knots. It originates mainly from part
of the Atlantic South Equatorial Current that branches northwestward
off Cape Natal and is augmented slightly in its northern part by the
Atlantic North Equatorial Current, the weaker of the two equatorial
currents in the Atlantic. The direction of the current remains con-
stant throughout its length most of the year with a frequency of about
85 percent, and any variations are primarily in its speed as shown in
Table 9.

The current appears to be somewhat stronger from July through
December between O° and 7°N and from January through June between
5°S and O° and between 7° and 12°N. Strongest speeds during the
year occur between 1° and 6°N and may result partly from the consid-
erable discharge of the Amazon River.

The approximate boundaries of the Guiana Current are shown in
Figure 1; the surface current rose in Figure 20 shows the prevailing
flow and the slight variations during two 6-month periods for the

entire length of the current.

MONTHS SPEED (KNOTS TOTAL | DIR.
Sane es 7 2 | OBS. on

B18 Tg Ads Stes Oped Sells Tf ees ee (atte Bade Casio W3 11602 | 303

July
Seeeuein 2 Gh aa) aie aun cy Wh TGs) tenis) ele aloe) Ons) | tye ||, St0ls

December

Table 9 Percent frequency of prevailing NW flow by speed category

51

LEGEND

E JAN.—JUNE
U JULY—DEC.

0.5 CALMS JAN.—JUNE (PERCENT)
0 CALMS JULY—DEC. (PERCENT)
9

1
0.

MEAN SPEED (KNOTS)

0.9

10 20
PERCENT

FIGURE 20 SURFACE CURRENT ROSE, GUIANA CURRENT
52

Guinea Current (
During the northern summer the Guinea Current begins at about
1)°w as the eastern extension of the well-established Atlantic

Equatorial Countercurrent. Table 10 shows the constancy of the pre-

vailing current within the boundaries shown in Figure l.

Pe ee eee

SPEED (KNOTS SPEED |FREQUENCY
0.2 0.5 O.8 In Th aS 22 247 S42 S076 Sh.0 | (aons)) || (SaRonpe
fos [a

To 7

All other directions 5 percent or less

Table 10 Percent of observations by speed categories and directions
during July, August, and September

Almost 1,500 observations show the prevailing direction to be east and

the mean speed 1.2 knots; the table also indicates the general flow to

be between northeast through southeast over 75 percent of the time,

with a maximum speed of about 4.0 knots.

The Guinea Current appears constant in direction except during
December through February, when easterly winds reduce the speed and
cause the current to become variable and at times to reverse; when
reversed, the flow seldom exceeds 1 knot. During the northern winter
(January through March) the Atlantic Equatorial Countercurrent is not

well established or disappears, and the Guinea Current, mainly

53

influenced by the Canary Current, widens considerably between 10°
and 20°W. Figure 21 shows the approximate winter boundaries and
the variations of the current by percent of observations in the
prevailing directions and mean speeds.

The approximate seasonal migration of the southern boundary of

the Guinea Current by latitude along the meridian 10°W is shown as
follows: |

SEASON LOCATION
Nov., Dec., Jane, Feb», Mar.

2.5N
Apr., May, June 4 .O°N
July, Aug., Sept., Oct. 3.5°N

Little information is available on subsurface flow. The current
profile shown in Figure 21 compares favorably with known or typical

characteristics of the current in this region.

5h

REGION 1 REGION 2

[SPEED CATEGORIES (KNOTS) Jun |Z,
Bs elma

o>

LOCATION OF
CURRENT PROFILE

REGION 1
2,289 OBS

2,036 OBS

° °
20° 10° 0 10
SPEED (KNOTS)
LON OS2 OFA OG OFSie Oe eZ E4,
0 5

LEGEND

82 Mee imag SBEED

—— MEAN OBSERVED

25

— — MAXIMUM
NE CURRENT SET (TOWARD)
e DEPTH OF OBSERVATION

50 164

100 328

® 200 656 —
ii rr
tu ra
2 o
a =
é 1312 is
eS 400
e oWSW
800 WSW 2624
LATITUDE 04.0°N
LONGITUDE 000.9°W
1600 MONTH NOV. 5248
WIND DIRECTION SSW
DEPTH OF BOTTOM 4,600 M
RELIABILITY GOOD
METHOD EKMAN
3200 RAETER 10496

Oy
MMMM] | @YTq@#]@q V]@TMt

FIGURE 21 BOUNDARIES OF GUINEA CURRENT DURING JANUARY, FEBRUARY,
AND MARCH AND PERCENT FREQUENCY OF FLOW BY PRE-
VAILING DIRECTIONS le) SPEED CATEGORIES

Gulf Stream

The name “Gulf Stream" was first noted on Benjamin Franklin's
chart of the Gulf Stream (about 1786). It appears to be derived from
the region between Florida, Cuba, and the Bahama Islands, at one time
called the Gulf of Florida, where the current begins. Some sources
state that the Gulf Stream begins off Cape Hatteras, but such a view
has been found impractical.

The major part of the Gulf Stream is a well-defined swift current
which begins north of Grand Bahama Island, where the Florida and
Antilles Currents meet, and extends northeastward to about 40°N, 63°W.
The approximate boundaries of this current, based upon thousands of
surface ship drift observations, are shown in Figure 22.

The Gulf Stream gains its impetus from the large volume of water
that flows through the Straits of Florida, an amount that is estimated
to be more than 20 times greater per hour than all the fresh water
entering the oceans from all sources such as rivers, runoff, and
thawing glaciers.

The greatest calculated volume transport between 37°00' and
38°20'N along 68°30'W is about 137 x 10° m>/sec; however, since this
cross section extends only about half the width of the Gulf Stream,
the actual total volume transport may be double that amount.

Available data indicate that the Gulf Stream as shown in Figure 22
is a permanent feature, particularly in the 8- to 10-mile-wide axis of
maximum current speed. The flow prevails throughout the year, with
only minor changes in direction; the speed varies slightly from one

season to another, being higher during summer and lower during winter.

(SYaLaW) Hid3G

cl

0

Wvddls JIND AHL 22 JNO

*S3dO13AN3
JLVWIXOUddV JYW SNOIOIY GIGVHS
*JLON

Moo 99 ‘NoG 8E
NOILISOd NVAW

ISNONV GNV ANNE

SNOILVIS S$

(SYaL3W) Hid3ad

09 OS OF Of OZ OL OO g
(SLONX) G33dS

|
oS OF O€ OZ OL OO Of OZ OL OO

(SLON™) aaadS V

(1344) Hid3aq

(1334) Hldaa

SNOILVAdASHO LNawwND

S31dOUd ALISOdWOD
JHL JO NOILISOd NVAW ©

NOI93a HOV] NI NOIL
-D3vIG LNAWND ONITMIVAId
JHL GNV SIXV WV3ULS 3H. —<—

WVadalS
JINS AHL JO SALWVGNNOG —— ——

GQN4931

Se © ¥y NOIO3Y ee
qg a

—
§ NOISY

a=

oS9 oOL

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57

The Gulf Stream system's constancy was once shown by a drift
bottle carried from Yucatan Channel to Ireland in just under one year.
Its most direct path must have been past Florida, along the east
coast of the United States, and across the North Atlantic; the
minimum speed at which this bottle traveled is computed to be about
0.6 knot and in all probability was considerably higher. Figure 23
shows the seasonal percent frequency of the prevailing surface current
derived from speed groups ranging from 0.2 to greater than 4.0 knots
for specific regions shown in Figure 22.

Surface drift currents recorded weekly for a period of two years
between the Bahamas and Hatteras (unpublished data) indicated the mean
axis of the Gulf Stream to pass through 34°36'N, 75°05'W. From March
through August the axis was located south of the mean position at
about 34°34'N, in September through November at about 34°35'N, and
from December through February at 34°L0'N. The current was most
constant north of 34°30'N, with 72 percent of all observations ranging
between 2.0 and 3.9 knots and sets between 031° and 070°r: the mean
direction was o49°T, South of 34°30 'N the speeds were somewhat weaker,
and the current tended to be more variable. Of all observaticns, 63
percent ranged between 2.0 and 3.4 knots, the sets lying between Osue
and 060°T ; the mean direction was 043°r.

The constancy of the current is verified by other surface ship
drift data tabulated in the 1° quadrangle 34°-35°N, 75°-76°W, where a
slight seasonal change is evident; in summer, mean speed is higher by
O.2 knot and persistency is greater by apoue 4 percent than during
winter. During all months the current sets northeast 77 to 95 percent

of the time; mean speed is about 2 knots and maximum speed over 5 knots.

58

WvddlS SIND FHL ‘SdNOYSD aazadS WOU
IN3dYND JDVAUINS ONIIVAINd JO ADNINOAYS LNAOYId EZ IYNO!S

CV GS Bee vil 8°0 (40)
(SLON») GaadS

“TWAUALNI GadS
NI JONVHD SJLVSIGNI ONIGVHS
LN3YdND ADVAYNS 4O
NOIMDIaIG ONMIWAId JN
‘das “ONY “AINF ———
VW “dad “Nvf ———

GQNI931

CAV aes CRC CNC VA EO ROMEECHO

CV

cE
(SLON») aa3adS

(o)

«wy

(e)

( LNJDudd ) ADNINOIAS

59

In Region 1, where the current is most restricted in width, the
greater percentages of observations occur at higher speeds as expected.
For example, the constancy of the current throughout the year is
shown by 40,000 surface drift observations in the region 25°-30°N,
79°-80°W, which is almost entirely occupied by parts of the Florida
Current and the Gulf Stream. About 90 percent of these observations
are of a set directly north at a mean speed of about 3 knots and a
maximum speed of 6.5 knots; in each of the other seven directions the
number of observations averaged about 1 percent or less.

In Regions 2 through 5 the stream widens, and frequencies in the
lower speeds are higher. In the northeastern part of the current in
38°-39°N, 64°-67°W, where the axis of the stream lies, 80 percent of
1,650 observations for all months are of an east-northeast set ata
mean speed of 1.2 knots and a maximum speed of 3.5 knots.

In Figure 2]),A the location of the Gulf Stream as determined by a
survey of surface currents in the easternmost part of the stream in
the spring of 1960 is compared with the location as determined from
historical drift data for the same region. The results of the survey
have indicated that the current may extend to the bottom, that a major
meandering pattern occurs east of 63°W, that the shapes of these
Jarge meanders may be influenced by seamounts, and that the meandering
path of the Gulf Stream changed very little over a period of 10 weeks.

Anomalies in the different types of surface current data are
surprising, particularly since comparisons between results of the two
methods in other areas show reasonable agreement. It is obvious that
varied interpretations of the Gulf Stream pattern in this region will

be made until additional and more definitive evidence is available.

60

LEGEND
SUGGESTED SURVEY LOCATIONS

REGIONS IN TABLE 1
CURRENT FLOW

GULF STREAM 2 APRIL TO 15
JUNE 1960 (GEK)

HISTORICAL DATA (ALL YEARS),
5,056 OBS. (SHIP DRIFT)

LEGEND

s— MEAN AXIS OF GULF STREAM
OF GULF STREAM WITHIN

yD 19—22 JUNE 1950 A PERIOD OF ABOUT 3
WEEKS

40°N

Yue %y
2 “/p

5)"

65° 60°

FIGURE 24 THE GULF STREAM EAST OF 65°W
61

In Figure 2hA , within the dashed lines west of 60°W, 77 percent
of the observations show a prevailing set toward 072°T at a mean speed
of 1.1 knots and a maximum speed of about 3.2 knots; east of 60°,

81 percent of the observations show a prevailing set toward 088°T at

a mean speed of 1.0 knot and a maximum speed of about 2.9 knots.

SHIP
DRIFT OBS. IN
SAME DIRECTIONS
AS GEK OBS.

SHIP DRIFT (ALL YEARS)
PREVAILING CURRENT

GEK (1960)

REGION Dire Speed range

Dir. Freqe
(°T) (kn )

Dir. Freq. Speed (kn )
(9T) (per-

(°T) (per- Mean Maxe
cent )

Table 11 Comparison of surface drift and GEK data

Table 11 compares the flow determined from ship drift data and from
1960 GEK data for the specified Regions A, B, and C in Figure 2A. The
prevailing surface flows indicated by drift observations differ
considerably from those observed by GEK. For example, the total of all
south-setting drift current observations in Regions A and C average
less than 5 percent, whereas the frequency of the east-setting current
is as high as 85 percent.

The meandering pattern of the current in Figure 2)A, which has
been described as a permanent feature, closely approximates data
obtained in June 1950 (Figure 2B); if detailed measurements were
obtained at Locations 4, 5, 6, and 7 in Figure 2A, they would help
to clarify the apparent complexities of the current. The June 1950
data also indicate a cyclonic eddy divorced from the main stream; the

main surface flow of the stream probably is located as shown by

62

historical data within the dashed lines in Figure 2A. When a large
meander from the main flow reaches an extreme stage, it may break off
into a large cyclonic eddy to the south, or a large anticyclonic eddy
to the north. This condition would appear to satisfy most interpre-
tations to date of the Gulf Stream in this region; in fact, a clock-
wise eddy about 50 miles in diameter has been studied in the spring
of 1966 at about 41°N, 60°W in a survey coordinated by the U. S. Coast
and Geodetic Surveye Such an eddy at this location tends to verify
that the axis of the Gulf Stream exists at Locations 4 and 5 in
Figure 2)A.

The sparse data available show the subsurface current to have
permanent features similar to the surface current. Parts A and B of
Figure 22 are composite profiles from data at several stations within
the same area and show the concentration of direct meter observations
at various depths. Part A, although to the left of the axis, indicates
that higher speeds occur more frequently during summer; Part B shows
the distribution of speeds where depths are much greater.

Although current measurements have not been obtained near the
bottom directly beneath the axis of the Gulf Stream, deepwater observa-
tions indicate that the flow is essentially in the same direction from
surface to bottom; computations show that the current speed at the
bottom is probably 10 cm/sec. In 1960, direct measurements obtained
in the vicinity of 38°30'N, 64°30'W at depths of about 3,000 meters
show a flow of about 0.2 knot in the same direction as the surface
current.

Data obtained in 1962 indicate a deep southwest flow near the

bottom along the continental slope at depths below 800 meters with

63

speeds up to O.4 knot; there appears to be a well-defined boundary
between this current and the northeast-flowing Gulf Stream at depth

a few miles to the east (Figure 25). From previous determinations

of the location of the axis of the Gulf Stream, these observed south—
west sets do not appear directly beneath this axis but are probably
part of the coastal current that frequently sets southwest past Cape
Hatteras. However, in the vicinity of 35°00'N, 74°30'W, below 2,300
meters, there is also evidence of an opposing southwest flow near

the bottom.

6h

35°N

SO

3Ae

LEGEND
SURFACE AXIS OF GULF STREAM

SUBSURFACE GULF STREAM FLOW
2000 DEPTH OF OBSERVATIONS (METERS)

—-—-— DEPTH CONTOUR

—— pe OBSERVED FLOW

FIGURE 25 SUBSURFACE FLOW IN THE VICINITY OF THE GULF STREAM
(AFTER BARRETT AND WEBSTER 1962) (SEE FUGLISTER, 1963.)

Jutland Current

The Jutland Current, narrow and localized off the coast of Denmark
petween 8°30' and 10°30'E, originates partly from the resultant counter-=
clockwise flow in the tidal North Sea. The main cause, however, appears
to be the winds which prevail from south through west to northwest over
50 percent of the time throughout the year and the transverse flows
from the English coast toward the Skaggerak.

The current retains the characteristics of a major nontidal current
and sets northeast along the northwest coast of Denmark at speeds
ranging between 1.5 and 2.0 knots 75 to 100 percent of the time.

Limited data show that the subsurface flow is in the same general
direction as the surface flow. At a depth of 16 feet (5 meters) the
speeds range between 0.6 and 0.8 knot, and at 165 feet (50 meters)
between O.1 and 0.4 knot. It is assumed that speeds continue to decrease

with depth.

66

Labrador Current

The Labrador Current, originating from cold arctic water flowing
southeast through Davis Strait at speeds of 0.2 to 0.5 knot and from
a westward branching of the warmer West Greenland Current, sets
southeast along the Continental Shelf of the Canadian coast.

At Hudson Strait, part of the current sets into the strait along
its north shore. The outflow of fresh water along the south shore of
the strait from the large land area surrounding the Hudson Bay and
Hudson Strait region augments the part of the current flowing along
the Labrador coast. The current also appears to be influenced by
surface outflow from inlets and fiords along the Labrador coast.

The Labrador Current usually is described as being more persistent
over the narrow Continental Shelf than elsewhere; however, there may
be seasonal fluctuations in the strength and volume of the current,
depending on the amount of fresh water discharge and runoff along the
coast during the spring and variations in speed and direction resulting
from tidal influences. The prevailing current, on the basis of movement
of bergs and surface drift observations, appears to extend some distance
offshore. The mean speed is about 0.5 knot, but current speeds at times
may reach 1.5 or 2.0 knots.

The surface current roses in Figure 26 show thé distribution of
observations in the southern part of the current and the slight seasonal

Fluctuations. Calms average about 3 percent.

67

50°W
55°N

LEGEND
YEAR ROUND

B winter
[] SUMMER
0.5 MEAN SPEED (KNOTS)

IN ROSES, CURRENT FLOWS
FROM CENTER OF CIRCLE.

@ LOCATION OF SUBSURFACE PROFILE
NO DATA

ROSE SCALE (%)
30_4

|

|

igen
SPEED (KNOTS)

0002 04 06 08

LATITUDE 49°N
LONGITUDE 52°W

50° MONTH _ SEPT. | 50°
25 RELIABILITY FAIR 82| REGION B
SET SOUTH

2

ti 50 164,

=

po x

= o

fin] Ww

2100 3280 |

200 656 |
)

400 1312 Ie

55° 50°

FIGURE 26 BOUNDARIES, SURFACE CURRENT ROSES, AND SUBSURFACE

CURRENT PROFILE, SOUTHERN PART OF LABRADOR CURRENT
68

Little information is available on subsurface currents. The
speed of the southward flow decreases with depth, usually ranging
between 0.2 and 0.4 knot at 30 meters (98 feet) and around 0.1 knot
at 500 meters (1,640 feet). The current profile in Figure 26 shows
the computed speed of the prevailing southward flow between the

surface and 200 meters (656 feet).

69

Labrador Current Extension

The current setting southwest along the northeast coast of the
United States has no designated name and is therefore referred to as
the Labrador Current Extension in this publication. This coastal
current originates from part of the Labrador Current flowing clock-
wise around the southeast tip of Newfoundland. Its speeds are fairly
constant throughout the year and average about 0.6 knot. The
greatest seasonal fluctuation appears to be in the width of the
current as shown in Figure 273 the current is widest during winter
between Newfoundland and Cape Cod. Southwest of Cape Cod to Cape
Hatteras the current shows very little seasonal change.

The current narrows considerably during summer and flows closest.
to shore in the vicinity of Cape Sable, Nova Scotia and between
Cape Cod and Long Island in July and August. The current in some
places encroaches on tidal regions. For example, meter measurements
at 39.7°N, 72-7°W obtained in April 1960 indicated a prevailing south-
west set about 100 percent of the time to a depth of 50 feet (Gu meters );
speeds ranged from 0.2 to 0.5 knot and averaged O.4 knot. About 215
observations over a period of 293 hours at a depth of 100 feet (31 meters )
showed a rotary tidal current; speeds ranged from O.1 to O.7 knot and
averaged O.4 knot but did not seem to vary significantly with springs
and neaps.

In the widest part of the current east of 7O°W during winter, the
prevailing direction was west-southwest; about 50 percent of the
observations showed speeds between O.1 and 0.9 knot, about 4 percent

between 1.0 and 1.9 knots, and some between 2.0 and 2.5 knots.

70

NOISNALX4 LNJYIND YOAUVAAVT JO SAIWVGNNO IVWNOSVAS 22 AYNSI4S

YSLNIM ONIWNG AYVGNNOd YsLNO ADVASAV -——
YaWWNS ONRNG AYVGNNO YLNO JDVASAV ——

MO14d JYOHSYVAN YO WAIL ATLNVNIWOG3ad
GN3i941

N.S

M.OS cis a OY) ofl) oOL

71

In the region of least fluctuation west of 70°W during winter,
the prevailing direction was south; 30 percent of the observations
showed speeds between 0.1 and 0.9 knot, about 10 percent between
1.0 and 1.9 knots, 0.5 percent between 2.0 and 2.9 knots, and some

between 3.0 and 4.0 knots.

72

North Africa Coast Current

The most permanent current in the Mediterranean Sea sets east
along the African coast from the Strait of Gibraltar to the Strait
of Sicily. The stability of the current is indicated by the
proportion of calm (no current) observations, which averages less
than 1 percent. The current is most constant after it passes
through the Strait of Gibraltar; in this region, west of 3°W, 65
percent of all observations show a set east, with a mean speed of
1.1 knots and a mean maximum speed of 3.5 knots.

Although the current is weaker between 3°W and 11°E, it remains
constant, the speed averaging O.7 knot throughout its length and its
maximum speed being about 2.5 knots. Of almost 54,000 observations,
50 to 60 percent are in the prevailing direction. West of O° longitude
the higher percentages of observations in the prevailing direction
occur most frequently from April through September; east of this
meridian they occur most frequently from October through March.

There are no subsurface current data available, but the flow

probably is eastward, with speeds decreasing with depth.

73

North Atlantic Current (North Atlantic Drift)

The North Atlantic Current originates from extensions of the

Gulf Stream and the Labrador Current near the edge of the Grand Bank.

As the current fans outward and widens into a northeast through east

set, it decreases sharply in speed and persistence. Some influence

of the Gulf Stream is noticeable near the extreme southwest boundary
of the current, where a narrow band sets east along the “43°N parallel; |
this flow is stronger and more constant than the current on either

side, and its width, speed, and persistence diminish to about 29°W.

A total of almost 49,000 surface drift observations in the
region 45°-50°N, 15°-35°W show a general prevailing set east by
northeast, usually ranging between 40 and 45 percent of the time at
a mean speed of 0.4 knot during both summer and winter; speeds seldom
exceed 1.2 knots. In this region about 92 percent of all observations
are between 0.1 and 0.9 knot, 4 percent between 1 and 2 knots, and only
0.2 percent over 2 knots; calms average almost 4 percent.

In the region 55°-60°N, 10°-25°W a total of over 3,600 observations
also show the current to have a mean speed of O.4 knot but to prevail
northeastward at a smaller frequency (between 25 and 40 percent). ‘The
current is only slightly stronger and more persistent in the western
part of this region than in the eastern part; 90 percent of all
observations are between 0.1 and 0.9 knot, 4 percent between 1 and 2 knots,

and less than 0.5 percent over 2 knots; almost 6 percent are calms.

7

,

The information presented here appears to verify prior
descriptions of the North Atlantic Current as a sluggish, slow
moving flow that can easily be influenced by opposing winds. It
is further corroborated by observations of about 6 and 9 percent
frequency in other directions than the prevailing set. Conversely,
strong augmenting winds may strengthen the prevailing flow,
particularly in the surface layer.

Direct measurements are sparse in this part of the ocean;
the current profiles in Figure 28 show speeds and directions for
specified depths at various locations. Profile 7 is a composite
of seven different current profiles in the same vicinity and shows

that the flow at all depths is fairly stable in speed and direction.

75

3 4

MEAN SPEED (KNOTS)
UO) Weed WL! OW: 2tn Sie OG)
0 te)

1 2

MEAN SPEED (KNOTS) MEAN SPEED (KNOTS)
0.0 0.2 04 06 08 0.6 08
) )

MEAN SPEED (KNOTS)

0.2 04 06 08 1.0
te) i}

® (ENE)

\

(ENE}s

DEPTH (FEET)
DEPTH (METERS)
DEPTH (FEET)

DEPTH (FEET)
DEPTH (METERS)
DEPTH (METERS)

DEPTH (FEET)

DEPTH (METERS)

iS)
ro}
o

NEe BOTTOM —]|

MMMM BOTTOM
WCM!|MMM!|!|™’'! BOTTOM

CHM

BOTTOM een
MM LAT, 62.8°N Se 78) LAT, 60.6°N
LONG. 4.8°E LONG. 6.4°W oy
JULY APRIL LONG. 9.3°W
LAT. 62.3°N USSR METER APRIL
EKMAN METER
FONG AU ZiE (ANCHORED) USSR METER
MAY (ANCHORED)
DROGUE

5 6 7
MEAN SPEED (KNOTS) MEAN SPEED (KNOTS) SPEED (KNOTS)

0.2 0.4 0.6 08 1.0 0.0 0.2 04 06 08
to} (o) ie} 0

© 00° of «©

|

MINIMUM MAXIMUM.

NE ee eo cece 08 te

DEPTH (METERS)
DEPTH (FEET)

DEPTH (METERS)
DEPTH (FEET)

DEPTH (METERS)
DEPTH (FEET)

LAT. 61.0°N
LONG. 12.8°W
APRIL VARIABLI °

LAT. 55.2°N MM@@E@@@C@@@@@@C@@@@@@E@C@C0T@CCMs

USSR METER
(ANCHORED) LONG. 23.1°W
OCTOBER

USSR METER
BOTTOM (ANCHORED)
MUMdMtéir

LAT. 61.2°N AUGUST
LONG. 2.3°W EKMAN METER

BOTTOM
Ml

LEGEND
LETTERS SHOW MEAN DIRECTION OF
FLOW (TOWARD) AT SPECIFIED DEPTHS.
DOTS INDICATE DEPTHS OF OBSERVA-
TIONS.
SHADING INDICATES TIDAL REGION.
PROFILE 7 IS A COMPOSITE.

FIGURE 28 SUBSURFACE CURRENT PROFILES, NORTH ATLANTIC CURRENT
76

Norway Coastal Current

The Norway Coastal Current begins at about 59°N, 10°E and follows
the coast of Norway, as shown in Figure 1. It originates mainly from
Oslofjord outflow, counterclockwise return flow of the Jutland Current
within the Skaggerak, and outflow from the Kattegat. The current
extends to about 20 miles in width. Speeds are strongest off the
southeast coast of Norway, where they frequently range between 1 and 2
knots. Along the remainder of the coast the current gradually weakens
and may widen to almost 30 miles at about 63°N, where it joins the
Norway Current; south of 62°N the current speed usually ranges between
O.4 and 0.9 knot. Speeds are generally stronger in spring and summer,
when the flow is augmented by increased discharge from the fiords.

Although few data are available, the July current profile in

Figure 29 shows the usual range of speed and maximum speed observed.

77

DEPTH (METERS)

25

50

100

200

400

800

1600

SPEED (KNOTS)
0.0 0.2 04 06 08 1.0

Nw hee

MMMMMMMe@TJ|V@E@])YN|]M|@$@PEeE/CWWMMM@@=MMMMe-

LATITUDE 60.5°N
LONGITUDE A.7°E

MONTH JULY

DEPTH OF BOTTOM APPROX. 350 M.
RELIABILITY — GOOD

METHOD EKMAN METER

LETTERS SHOW DIRECTION OF FLOW (TOWARD).

FIGURE 29 NORWAY COASTAL CURRENT PROFILE
78

2 eet Vs 2.0
0

82

164

328

1312

2624

5248

DEPTH (FEET)

Portugal Current

The prevailing southward flow off the Atlantic coasts of Spain and
Portugal is known as the Portugal Current and is part of the general
clockwise circulation in the North Atlantic Ocean. It is a slow-moving
current that averages only about 0.5 knot during both winter and summer;
maximum speed seldom exceeds 2.0 knots north of 4O°N and 2.5 knots south
of 4OON.

The current is easily influenced by winds. An interesting relation-
ship based on almost 25,000 surface current drift observations is shown
in Table 12. The table compares wind and current observations in the
region between 35° and 45°N, and between 5° and 10°W within the limits
of the current shown in Figure 1; it shows the wind and current to be
most constant during summer, when both set in the same general direction
at least 50 percent of the time. The higher percent of observations of
south sets during summer probably indicates that the current is at
strength during this period and less influenced by short-period changes
in the wind. The percent of observations in other directions is some-
what less than in winter and indicates that the wind may cause the
current to set in any direction for short periods at any time of the
year and that the flow may even reverse during persistent southerly winds.

During winter the current still shows a prevailing south set but
with a smaller proportion of observations; the percent distribution in
the other directions based on 8 points of the compass ranges between 8

and 13 percent, with the higher percentages north of 40°N.

19

The current west of 10°W has a mean speed of O.5 knot and may at
times exceed 2.0 knots in the prevailing south direction. In this
region also, there is no significant seasonal change in current speed,
but the percent distribution of observations during summer is higher
(55 percent) than during winter (45 percent). The distribution of
observations in the other directions averages about 9 percent.

Little is known about subsurface flow. Currents computed in June
at 41.5°N, 14.5°W show that the flow is generally south-southeast at
about O.4 knot near the surface; speeds gradually decrease to about

0.1 knot near the bottom at about 5,300 meters (17,388 feet).

Dir. Per- Dir. Per-= Dir. Per- Dir. Per-
(from) cent | (toward) cent | (from) cent | (toward) cent

4O°-45°N | SE-SW 36 NW-NE 34 SE-SW 27 NW-NE 23
NW-NE Ta SE-SW Tal NW-NE 50 SE-SW 59

35°-40°N | SE-SW 25 NW-NE 26 SE-SW 18 NW-NE 18
NW-NE Tal SE-SW h6 NW-NE 50 SE-SW 56

Table 12 Percent frequency by 90-degree quadrants of seasonal wind and
current observations between 35° and L5ON, and between 5° and
10°w

80

South Atlantic Current

The South Atlantic Current, counterpart of the North Atlantic
Current, appears to originate mainly from the Brazil Current and
partly from the northernmost flow of the West Wind Drift west
of 4O°W. Although surface data are limited, Table 13 shows the
main features of the current during July and August in four
specified regions shown in Figure 30. The current is under the
influence of the prevailing westerly trade winds; the constancy
and speed increase from the northern boundary to about 40°S, where
the current converges with the West Wind Drift. Maximum speed in

Regions A and B is about 1.5 knots and in Region C about 2.0 knots.

REGION PREVAILING DIR. FREQUENCY MEAN SPEED TOTAL OBS.
(See Fig. 30) Gr) (percent) (knots )

Insufficient datas; 30 of 34 observations show sets
evenly distributed from 315° clockwise to 135°, with
a resultant flow of O45° at a mean speed of 0.6 knot.

Table 13 Directions and speeds during July and August

The data for January and February show the greatest number of
observations with predominant east sets and indicate the region of
the South Atlantic Current to be mainly south of 30°S and between
O° and about 30°W. During these months the region of variable or
indeterminate flow between the Atlantic South Equatorial Current and
the South Atlantic Current appears to widen and migrate southward.

The major portion of the South Atlantic Current narrows and becomes more

81

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82

constant. About 60 percent of the 661 observations within the
boundary of the current in January and February show a prevailing
set east at a mean speed of 0.7 knot; maximum speed is about 2.0
knots. Figure 30 shows the differences in extent of the current
between winter and summer.

Subsurface current data are negligible. Direct measurements
inside the north boundary at 28.1°S, 19.3°W by the METEOR during
13 to 15 August 1925 at depths of 30, 500, and 2,500 meters
(98, 1,640, and 8,202 feet) showed sets in all directions but
primarily southward. More than likely this flow is entrainment
between the South Atlantic Current and the Atlantic South Equatorial

Current,

83

West Wind Drift (Great Eastward Drift)

The West Wind Drift, described by the Soviets as the Great Eastward
Drift, is the largest ocean current on earth; in the Southern Hemisphere,
it is a barrier between the warm water of the lower latitudes and the
cold water flowing around the Antarctic. The West Wind Drift, like other
large ocean currents complex in origin, is a homogeneous flow comprising
a thick layer of water from surface to great depths which cannot be
regarded strictly as a drift or wind-driven current. The fairly steady,
large-scale, and deep flow is believed to originate mainly as the result
of the combined action of friction and gravity forces generated by the
westerly winds that prevail in this region.

The speed of the West Wind Drift increases considerably toward
Drake Passage, where it averages up to 0.7 knot, a value about three
times higher than in the open ocean. Current speeds decrease slightly
with depth, and the average speed of the surface flow is only about
twice that at 2,000 feet (610 meters). An analysis of oceanographic
observations at various locations around Antarctica shows the following
computed mean speeds at various depths. These figures, shown in Table 14,
may be considered lower than the actual speeds but nevertheless can be

used as a general description of the currents.

APPROXIMATE DEPTH MEAN COMPUTED VELOCITIES
meters feet cm/sec knot
O O 720 < 0.2
50 165 50) Opal
200 660 4.0 << Ooi
600 1,970 B20) Oem:
1,000 3,300 20D Ooal
2,000 6,560 Lo) @oal
2,500 8,200 Om Qo dl

Table 14 Computed speeds at various depths
8h

The amount of water carried by this current is indicated by the
computed average annual volume transport of about 4,445,000 x 107 m3
in Drake Passage and about 6,000,000 x 10? m3 between South Africa and
Antarctica.

Because of variations in the strength of the wind, the northern
boundary of the West Wind Drift at about 40°S is not well defined.
Observations in this area show a significant flow toward the northeast
and north where the West Wind Drift joins the east-setting South Atlantic

Current.

85

Yucatan Current

The Yucatan Current is that part of the circulation in the
Gulf of Mexico and Caribbean Sea which passes through Yucatan
Strait between 18° and 26°N and has a predominant north-northwesterly
set. It extends from the Caribbean north of Honduras to the north
edge of Campeche Bank and toward the Mississippi Delta.

The outstanding feature of this current is its westward
intensification, which occurs most noticeably in the region of
maximum current strength, about 40 to 60 miles wide between about
21° and 22°N.

Seasonal changes occur as shown in Table 15. West of 86°W the
current is strongest and most constant in April, May, and June. The
current weakens in August and September and is weakest during October,
November, and December. It again becomes stronger in January, February,
and March. East of 86°W the current is considerably weaker, as indicated
by the lower mean speeds and smaller percent frequency of observations
in the prevailing direction.

The strong current in the strait appears to depend little upon
the constriction of the strait, and although a definite seasonal change
is indicated, variations in speed may occur at any time of the year.
For example, a number of observations made in October 1961 did not
exceed 2 knots, and the core, much less clearly defined, was 10 to 20
miles farther east in about 300 fathoms (549 meters); in October 1959,
speeds up to 4 knots were recorded, with the core clearly marked at
100 fathoms (183 meters). In May, drogue measurements 30 miles north

of the strait showed surface current speeds of about 3.5 knots,

PREVAILING | SPEED | PERCENT

FREQUENCY
Ke)

PREP

°
FOoW

De fet Df

BO BEY WV E> BH° Say

So") 8S" 87" BE ES> Gals

Table 15. Seasonal variations of Yucatan Current

87

and 12 days later the speed was only about 1.0 knot.

When the current is strong the core is narrow and farther west,
being located close to the 100-fathom (183-meter) curve. When it is
weakest during winter, the core is broader and lies 10 to 20 nautical
miles east of the 100-fathom curve.

Figure 31 shows the core of the current as determined from GEK
observations; the surface current component is N and NNW, coinciding
with the prevailing data in Table 15. The lines along which
measurements were taken are limited to where speeds are 1.0 knot
and greater; the width of the current was observed to be about 65
miles.

Little is known about subsurface currents. Recent works have
indicated that maximum speeds may occur below the surface near the
middle of the current to depths of 300 meters (984 feet). At 500
meters (1,640 feet) in the middle of the strait, east of the current
axis, the currents determined from dynamic computations show a range

in speed between 0.5 and 0.9 knot.

88

23%

22°

eo)

SPEED (KNOTS)
_ ND

B7/~ 86°W

LEGEND
—— 100-FATHOM CURVE

Yyy CORE OF CURRENT,
Zi

BOUNDARY AT 2.5

KNOTS

A-B LINE ALONG WHICH
MEASUREMENTS WERE
MADE

O POINT OF MAXIMUM
SPEED

's

N—w
°

°

\Y

SPEED (KNOTS)

(eo) —
oO

|

S$

Dpe

FIGURE 31 CORE OF YUCATAN CURRENT AS DETERMINED FROM GEK

OBSERVATIONS (FEB.—JUNE)
89

BIBLIOGRAPHY

Boltovskoy, E.
The Malvin Current (a study on the basis of the investigation
of Foraminifera). Republica Argentina, Secretaria de Marina,
Servicio de Hidrografia Naval, Publico H.1015, Buenos Aires,

1959.

Cochrane, J- D.
Equatorial Currents of the Western Atlantic. Oceanography and
Meteorology of the Gulf of Mexico, Progress Report A and M
Project 286, Ref. 65-171, Texas A and M University, College
Station, Texas, 1965.

Yucatan Current. Oceanography and Meteorology of the Gulf of
Mexico, Annual Report 1 May 1962 - 30 April 1963, A and M
Project 286, Ref. 63-18A, Texas A and M University, College
Station, Texas, 1963.

Eskin, L. Ie
Contribution to the Study of the Water and Thermal Balance of
Drake Passage. Soviet Antarctic Expeditions Information Bulletin,
Arctic and Antarctic Research Institute, Vol. 2, Elsevier Publish-
ing Company, New York, 196).

Farquharson, Wo I.
Tides, Tidal Streams and Currents in the Gulf of St. Lawrence.
Marine Sciences Branch, Department of Mines and Technical
Surveys, Ottawa, 1962.

Fuglister, F. C.
Gulf Stream. Transactions American Geophysical Union, Vol. 44,
No. 2, June 1963.

Gulf Stream 60. Woods Hole Oceanographic Institution, Ref. 64-h,
January 196).

Fuglister, F. C. and Le V. Worthington
Some Results of a Multiple Ship Survey of the Gulf Stream.
aHSILabbIS! Sh INESILP ~ ILO Si. -

Helland-Hansen, B. and F. Nansen
The Norwegian Sea, Its Physical Oceanography Based Upon the
Norwegian Researches 1900-1904. Report on Norwegian Fishery
and Marine-Investigations, Vol. 2, No. 2, Bergen, 1909.

90

seein Cr Ole
A Study of the Northern Part of the Labrador Current. Bulletin
of the National Research Council, No. 61, Washington, D. C.,
July 1927.

Leipper, D. and L- Capurro
Continuation of Surface and Deep Water Current Measurements in
The Antarctic Ocean (Drake Passage). Bulletin of the U. S.
Antaretic Projects Office, Vol. 5, No. 10, September 1963-June 196.

Maksimov, I. Ve, Dr.
Currents in the Bellingshausen Sea Region. Soviet Antarctic
Expeditions Information Bulletin, The Admiral Makarov College of
tecnee Engineering, Vol. 2, Elsevier Publishing Company, New York,
1964.

Malkus, W. and K. Johnson
A Drift Study of the Gulf Stream Atlantic Cruise 198 and Caryn
Cruise 78. Woods Hole Oceanographic Institution, Ref. 54-67,
1954. Unpublished.

Mosby, H.
Current Measurements in the Faeroe-Shetland Channel 1960 and 1961.
Tables, NATO Subcommittee on Oceanographic Research, Bergen, May
1962.

Mosby, H.
Current Measurements in the Norwegian Sea and in the North Sea,
1923, 1924, 1928, 1929. Tables, NATO Subcommittee on
Oceanographic Research, Bergen, March 1963.

Parr, A- E.
On the Longitudinal Variations in the Dynamic Elevation of the
Surface of the Caribbean Current. Bulletins of the Bingham
Oceanographic Collection, Vol. oF Art. 2, New Haven, Conn., 1937-

Ponomarenko, G. P.
Discovery of a Deep Countercurrent at the Equator in Atlantic
Ocean on Research Vessel MIKHATL LOMONOSOV. Okeanologicheskiye
Issledovaniya, No. 13, 1965.

Saelen, OQ. H.
Studies in the Norwegian Atlantic Current, Part II: Investigations
during the years 1954-59 in an area west of Stad. Geofysiske
Publikasjoner, Geophysica Norvegica, Vol. 23, No. 6, March 1963,
Oslo, 1963.

91

Schubert, O. Ve
Ergebnisse der Strommessungen und der ozeanografischen
Serienmessungen auf den beiden Ankerstationen der zweiten
Teilfahrt. Aus Den Wissenschaftlichen Ergebnissen der
Deutschen Nordatlantischen Expedition 1937 und 1938. 1.
Lieferung, Annalen der Hydrographie und Maritimen Meteorologie,
Januar-Beheft, 194).

Smith, E. H., Soule, F. M. and O. Mosby
The MARION and GENERAL GREENE Expeditions to Davis Strait
and Labrador Sea 1928, 1931, 1933, 1934, 1935. U.S. Treasury
Department, Coast Guard Bulletin No. 19, Scientific Results,
Part 2, Physical Oceanography, Washington, 1937.

Staleup, M. C. and We. G. Metcalf
Direct Measurements of the Atlantic Equatorial Undercurrent.
Journal of Marine Research, Yol. 24, No. 1, Sears Foundation
for Marine Research, Bingham Oceanographic Laboratory, Yale
University, New Haven, Conn., January 1966.

Stander, G.- H.
The Benguela Current off South West Africa. The Pilchard of
South West Africa, Administration of South West Africa Marine
Research Laboratory, Investigational Report No. 12, 1964.

U. Se Department of Commerce, Coast and Geodetic Survey
Coast and Geodetic Survey Drift Bottle Program. Washington
Seience Center, Rockville, Maryland, April 1965.

Ue. S. Department of Commerce, Coast and Geodetic Survey
Tides and Tidal Currents Southern Brazil and Argentina.
Report No. 46, Washington, August 194}.

USSR
IGY Cruise Data. EKVATOR A-1 Stations 67, 69, 71, 28 March to

28 April 1958 and A-2 Station 174, 29 Oct. to 2 Nov. 1958.
Unpublished.

92

@

Unclassified
. Security Classification

DOCUMENT CONTROL DATA-R&D

4 (Security classification of title, body of abstract and indexing annotation must’be entered when the overall report is classified
1. ORIGINATING ACTIVITY (Corporate author) 2a. REPORT SECURITY CLASSIFICATION
Naval Oceanographic Office Unclassified

Washington, D. C. 20390

i
|
| 4. DESCRIPTIVE NOTES (Type of report and inclusive dates)

Technical Report (TR-19

9 5- AUTHOR(S) (First name, middle initial, last name)

William E. Boisvert

8a. CONTRACT OR GRANT NO. 98. ORIGINATOR’S REPORT NUMBER(S)

None
b. PROJECT NO. TR-193
None

9b. OTHER REPORT NO(S) (Any other numbers that may be assigned
this report)

None
#10. DISTRIBUTION STATEMENT
U. S. Government agencies may obtain copies of this report directly from DDC.
Other DDC users shall request through: Commander, Naval Oceanographic Office,
Washington, D. C. 20390, ATTN: Code 40

i1- SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY

Naval Oceanographic Office

13. ABSTRACT

An examination of existing data sources clearly shows that the methods
utilized to determine the principal characteristics of ocean currents leave
much to be desired. From the many and varied types of information, currents
have been identified on the basis of faunal zones, increase or decrease in
temperature or salinity, changes in water color, exchange of heat and water
vapor with the atmosphere, cloud cover, mixing, dilution by rain, river
discharge, heating and evaporation, Coriolis force, distribution of organisms,
etc. It is agreed that all these factors, to varying degrees, can help to
distinguish the currents, but it also appears that the importance of these
factors, when stressed individually, can be greatly exaggerated.

The currents shown in Figure 1 and described in this report are those
where the movement within specified boundaries exhibits a definite permanent
or seasonal flow.

The approximate boundaries and the main body of each major current
shown are based on ship drift observations and direct measurements by
instrument, which describe the two main features of the current, namely
direction and speed.

DD ..2"".1473 (Pace 1) ~

S/N 0101-807-6801 Security Classification

. Unclassified
i Security Classification

KEY WORDS

Currents
Countercurrents
Undercurrents
Drift

DD ©..1473 (eco

Unclassified
(PAGE 2)

Security Classification

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