major currents in the north and the south atlantic ocean

8/13/2019 Major Currents in the North and the South Atlantic Ocean

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TR-1 93

MFIS

TECHNICAL REPORT

MAJOR CURRENTS IN THENORTH AND SOUTH ATLANTIC OCEANSBETWEEN 64N AND 60S

SEPTEMBER 1967

NAVAL OCEANOGRAPHIC OFFICEWASHINGTON, D. C. 20390

Price $1.35


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This Technical Report Was Prepared byWILLIAM E. BOISVERT

Oceanographic Analysis DivisionMarine Sciences Department

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ERRATAPage x, Table 15, change Yucatan Currant to read Yucatan CurrentPage 3, Figure 1, add shading as indicated below;1fl^

N H


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FOREWORD

This report is intended to help meet an ever-increasingneed for the identification of the major currents of theNorth and South Atlantic Oceans and a comprehensive summaryof their principal characteristics. The information givenherein is based on published reports and directly measureddata, as well as considerable unpublished data, of which alarge amount has recently been obtained. Much of these datahave 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 ofthe Pacific and Indian Oceans and the polar regions.

'L. E. DeCAMP *Captain, U. S. NavyCommanderNaval Oceanographic Office


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MAJOR CURRENTSin the

NORTH and SOUTHATLANTIC OCEANS

between6UN and 60S

William E.^Boisvert \ )Oceanographic Analysis Division


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CONTENTSPage

Foreword iiiList of Figures viiiList of Tables xIntroduction 1Antilles Current 5Atlantic Equatorial Countercurrent 9Atlantic Equatorial Undercurrent 12Atlantic North Equatorial Current 15Atlantic South Equatorial Current .... 18Azores Current 21Benguela Current 22Brazil Current 26Canary Current 31Cape Breton Current 33Cape Horn Current 3UCaribbean Current 37Falkland Current I4IFlorida Current I4.I4.Guiana Current 51Guinea Current 53Gulf Stream 56Jutland Current 66Labrador Current 67Labrador Current Extension 70North Africa Coast Current 73North Atlantic Current 7UNorway Coastal Current ... 77Portugal Current 19South Atlantic Current 81West Wind Drift 81*Yucatan Current 86Bibliography 90


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LIST OF FIGURESFigure Paee

1 Major Surface Currents, July .... 32 Principal Boundaries and Surface and Subsurface

Flow of Antilles Current 73 Approximate Boundaries of Atlantic EquatorialCountercurrent by Month 10k Vertical Section at 13.5W, April 1961, Atlantic

Equatorial Undercurrent 135 Seasonal Fluctuations of Atlantic North EquatorialCurrent 166 Atlantic South Equatorial Current 197 Boundaries of Benguela Current and Seasonal Surface

Current Roses 238 General Flow Pattern between 200 and I|00 Meters,

Benguela Current 259 Subsurface Current Profile, Benguela Current 2 5

10 Seasonal Characteristics of Brazil Current 2711 Boundaries of Southern Part of Canary Current and

Surface Current Rose for January, February, March ... 3212 Computed Current Speed Profiles, Cape Horn Current ... 3513 Observed Current Profile, Cape Horn Current 36Ik Boundaries of Caribbean Current, Region of Highest

Speed, and Current Profiles 3815 Seasonal Boundaries of the Falkland Current ^216 Volume Transport between Key West and Havana,

1952-59, Florida Current il5

vi11


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LIST OF FIGURES (cont'd)Figure Page

17 Percent Frequency of Prevailing Surface Currentby Mean Speed Categories (Summer and Winter),Florida Current U7

18 Surface and Subsurface Currents in Straitsof Florida . *.*., a .


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LIST OF TABLESTable Page

1 Number of observations in prevailing directions byspeed category and region 6

2 Subsurface current, summer, 2$30.8'N, 7232.8'W .... 6

7 Current measurements, March 1966, 29.9S, l40.9W . .8 Seasonal percent frequency distribution of

observations by speed categories, mean directions,and mean speeds for specified regions

9 Percent frequency of prevailing NW flow by speedcategory10 Percent of observations by speed categories and direction

during July, August, and September c11 Comparison of surface drift and GEK data

113 Percent frequency of speeds by region, northernsummer (July, August, September)h Percent frequency of prevailing WNW flow by speedcategory .....5 Seasonal variations in representative portions ofAtlantic South Equatorial Current6 Observations by directions and specified speed ranges . 21

28

39

12 Percent frequency by 90-degree quadrants of seasonalwind and current observations between 35 and U5N,and between 5 and 10W

13 Directions and speeds during July and August olh Computed speeds at various depths 0115 Seasonal variations of Yucatan Current , 87


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The approximate boundaries and the main body of each major currentshown are based on ship drift observations and direct measurements byinstrument, which describe the two main features of the current, namelydirection and speed. Dynamic topography presentations' are purposelyomitted but mentioned only where they might prove of interest or valueby comparison with results of direct measurements.

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

The reliability of ship drift observations has frequently beenquestioned. Although external influences, such as wind stress onship superstructure, changing sea conditions, draft of vessel, etc., areincluded in each observation, the effect of these factors is reducedin processing the data because only those observations which fall withincertain coding requirements are recorded, and the remaining factors tendto average out as the number of observations increases within a givenarea. For most areas with sufficient observations, a satisfactorypattern 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, andreadings considered reliable or reasonable are shown where available.

It is generally agreed that ocean current information is verysparse and usually insufficient for most ocean areas. However, asmore 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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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 Islandsas part of the Atlantic Worth Equatorial Current. Figure 2 shows theoutlines of the current. The frequency of sets in the prevailing direc-tion in the three regions shown averages about 55 percent, the main surfacespeed being about 0.6 knot. Table 1 shows the prevailing directionthroughout the year in each area and the total number of surface obser-vations by area in various speed categories.

The analysis of about ^2,000 surface observations shows that inRegions A and B, about 80 percent of the observations are between 0.1and 0.9 knot, 13 percent between 1 and 2 knots, and 1 percent over2 knots; in Region C the current is slightly weaker, with 88 percent ofthe observations between 0.1 and 0.9 knot, 6 percent between 1 and 2knots, and less than 1 percent over 2 knots.

Little is known about the subsurface currents. Table 2 wasderived from hundreds of direct meter measurements taken at 10-minute

intervals at 2530.8'W, 7232.8'W between 25 June and 13 July 1965.The data appear to verify the stability of this part of the AntillesCurrent 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 thesame location between 23 January and 8 February I966 indicate that theymay 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 directionwas about the same but the speed was O.k knot; at 500 meters (1,6^-0 feet)the flow was mainly south-southeast at about 0.2 knot; and between 1,000and 3,000 meters (3,28l and 9, $4-2 feet) it was generally south at 0.1knot. At depths of 3,200 and 3,900 meters (10,499 and 12,795 feet) theflow 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 fastestcurrent but near the northern boundary of the Antilles Current, wherethe greatest seasonal change is likely to occur. During winter, whenthe Bermuda high migrates to its maximum south position, the northernboundary of the Antilles Current also moves southward, and the currentsin the region of the direct measurements tend to be more variable.


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Atlantic Equatorial CountercurrentThe Atlantic Equatorial Countercurrent flows eastward between

the west-setting Atlantic North and South Equatorial Currents. Theapproximate boundaries and monthly variations in extent are shown inFigure 3. The surface countercurrent is best defined during Augustand September, when it extends from about 52 to 10 and joins theGuinea Current. In October it narrows and separates into two partsat about 7N, 35 W. The western part, which appears to be a region

where the countercurrent probably sinks and flows eastward beneaththe equatorial currents, gradually diminishes in size to the west-northwest, while the eastern part diminishes to the east-southeastas shown in the lower half of Figure 3. The greatest separationoccurs during March; during April the western part of the countercurrentdisappears, but in May it reappears in the vicinity of 0, 40W Thetwo segments progress west-northwest without too much change in size.They merge at about 6N, 43W during August and continue their flow-eastward uninterrupted through September.

Table 3 indicates frequencies of speeds by prevailing directionduring the northern summer, -when the countercurrent is most pronounced.Speeds are stronger but less persistent in the western part of thecountercurrent, as shown by the decreasing frequency of observationsto the east in the higher speed groups of 1.3 to 2.5 knots andincreasing frequency to the east in the lower speed groups of 0.1 to1.2 knots. Speeds have been observed to exceed 3*0 knots at timeB.


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Guiana Current (North Brazil Current)The Guiana Current is a strong, persistent northwest flow along

the northeast coast of South America between 5S and 12N that may attimes attain a speed of about k knots. It originates mainly from partof the Atlantic South Equatorial Current that branches northwestwardoff Cape Natal and is augmented slightly in its northern part by theAtlantic North Equatorial Current, the weaker of the two equatorialcurrents 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 inTable 9.

The current appears to be somewhat stronger from July throughDecember between 0 and 7N and from January through June between5S and 0 and between 7 and 12N. Strongest speeds during theyear occur between 1 and 6N and may result partly from the consid-erable discharge of the Amazon River.

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

SPEED ( KNOTS')0.2 0.5 0TB 1.1 T.V 1.8 2.2 2.k 3-2 3.7 k.23.8 7.8 11.9 16.0 16.9 16.7 16.2 7.^ 2.1 0.9 0.3

lj.,3 9.k 11.8 ih.-r 14.6 ik.k 16.0 8.8 k.k 1.3 0.3

TOTALOBS.

Il602

8714

9 Percent frequency of prevailing NW flow by speed category

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DIR.(T)

303

303


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10 20 3(PERCENT>

FIGURE 20 SURFACE CURRENT ROSE, GUIANA CURRENT


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Current NDuring the northern summer the Guinea Current begins at aboutas the eastern extension of the well-established Atlantic

Countercurrent. Table 10 shows the constancy of the pre-current within the boundaries shown in Figure 1.

SPEED (KNOTS)MEMSPEED(KNOTS)

frequency(percent).2 0.5 0.8 1.1 1.4 1.8 2.2 2.7 3-2 3.7 >4.o

2.9 4.1 4.0 1.7 2.1 1.5 0.9 0.6 0.3 0.1 1.0 18.23.3 8.4 8.1 7.8 6.7 5-0 3-6 1.7 0.9 0.1 0.1 1.2 45.72.4 3-0 3-3 2.6 1.8 0.9 0.5 0.1 -- ~ 0.9 14.6

other directions 5 percent or less10 Percent of observations by speed categories and directions

during July, August, and September

1,500 observations show the prevailing direction to be east andmean speed 1.2 knots; the table also indicates the general flow tobetween northeast through southeast over 75 percent of the time,

a maximum speed of about 4.0 knots.The Guinea Current appears constant in direction except during

cember through February, when easterly winds reduce the speed andause the current to become variable and at times to reverse; wheneversed, the flow seldom exceeds 1 knot. During the northern winter(January through March) the Atlantic Equatorial Countercurrent is notell established or disappears, and the Guinea Current, mainly


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

The approximate seasonal migration of the southern boundary ofthe Guinea Current by latitude along the meridian 10W is shown asfollows

SEASON LOCATIONNov., Dec, Jan., Feb., Mar.Apr., May, JuneJuly, Aug., Sept., Oct.

Little information is available on subsurface flow. The currentprofile shown in Figure 21 compares favorably with known or typicalcharacteristics of the current in this region.

2 5'Nk .0*n3 5 >N

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REGION 1DIR.

SPEED CATEGORIES (KNOTS) MN.5PD.


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Gulf StreamThe name Gulf Stream was first noted on Benjamin Franklin's

chart of the Gulf Stream (about 1786). It appears to be derived fromthe region between Florida, Cuba, and the Bahama Islands, at one time

called the Gulf of Florida, where the current begins. Some sourcesstate that the Gulf Stream begins off Cape Hatteras, but such a viewhas been found impractical.

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

The Gulf Stream gains its impetus from the large volume of waterthat flows through the Straits of Florida, an amount that is estimatedto be more than 20 times greater per hour than all the fresh waterentering the oceans from all sources such as rivers, runoff, andthawing glaciers.

The greatest calculated volume transport between 3700' and3820'N along 6830'W is about 137 x 10 m3/sec; however, since thiscross 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 22is a permanent feature, particularly in the 8- to 10-mile-wide axis ofmaximum current speed. The flow prevails throughout the year, withonly minor changes in direction; the speed varies slightly from oneseason to another, being higher during summer and lower during winter.

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The Gulf Stream system's constancy was once shown by a driftbottle carried from Yucatan Channel to Ireland in just under one year.Its most direct path must have been past Florida, along the eastcoast of the United States, and across the North Atlantic ; theminimum speed at which this bottle traveled is computed to be about0.6 knot and in all probability was considerably higher. Figure 23shows the seasonal percent frequency of the prevailing surface currentderived from speed groups ranging from 0.2 to greater than U.O knotsfor specific regions shown in Figure 22

Surface drift currents recorded weekly for a period of two yearsbetween the Bahamas and Hatteras (unpublished data) indicated the meanaxis of the Gulf Stream to pass through 3U36 'N, 7505'W. From Marchthrough August the axis was located south of the mean position atabout 3434'N, in September through November at about 3 ^35 'N, andfrom December through February at 3*+ UO'N. The current was mostconstant north of 3430'N, with 72 percent of all observations rangingbetween 2.0 and 3*9 knots and sets between 031 and 070T; the meandirection was 0^9 . South of 3430*N the speeds were somewhat weaker,and the current tended to be more variable. Of all observations, 63percent ranged between 2.0 and 3.4 knots, the sets lying between 031and 060T; the mean direction was 043T.

The constancy of the current is verified by other surface shipdrift data tabulated in the 1 quadrangle 3^-35N, 75-76W, where aslight seasonal change is evident; in summer, mean speed is higher by0.2 knot and persistency is greater by about h percent than duringwinter. During all months the current sets northeast 77 to 95 percentof the time; mean speed is about 2 knots and maximum speed over 5 knots.

58


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In Region 1, where the current Is most restricted in width, thegreater percentages of observations occur at higher speeds as expected.For example, the constancy of the current throughout the year isshown by 1+0,000 surface drift observations in the region 25-30N,79-80W, which is almost entirely occupied by parts of the FloridaCurrent and the Gulf Stream. About 90 percent of these observationsare of a set directly north at a mean speed of about 3 knots and amaximum speed of 6.5 knots; in each of the other seven directions thenumber of observations averaged about 1 percent or less.

In Regions 2 through 5 the stream widens, and frequencies in thelower speeds are higher. In the northeastern part of the current in38-39N, 64-67W, where the axis of the stream lies, 80 percent of1,650 observations for all months are of an east-northeast set at amean speed of 1.2 knots and a maximum speed of 3*5 knots.

In Figure 2ljA the location of the Gulf Stream as determined by asurvey of surface currents in the easternmost part of the stream inthe spring of i960 is compared with the location as determined fromhistorical drift data for the same region. The results of the surveyhave indicated that the current may extend to the bottom, that a majormeandering pattern occurs east of 63W, that the shapes of theselarge meanders may be influenced by seamounts, and that the meanderingpath of the Gulf Stream changed very little over a period of 10 weeks.

Anomalies in the different types of surface current data aresurprising, particularly since comparisons between results of the twomethods in other areas show reasonable agreement. It is obvious thatvaried interpretations of the Gulf Stream pattern in this region willbe made until additional and more definitive evidence is available.

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LEGENDSUGGESTED SURVEY LOCATIONSREGIONS IN TABLE 1

^- CURRENT FLOWGULF STREAM 2 APRIL TO 15JUNE 1960 (GEK) HISTORICAL DATA (ALL YEARS),5,056 OBS. (SHIP DRIFT)

40N

LEGENDMEAN AXIS OF GULF STREAM8-12 JUNE 1950 ^ CHANGE IN POSITIONSI^OF GULF STREAM WITHIN

A PERIOD OF ABOUT 3WEEKS19-22 JUNE 1950


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In Figure 2i|A, within the dashed lines west of 60W, 77 percentof the observations show a prevailing set toward 072T at a mean speedof 1.1 knots and a maximum speed of about 3.2 knots; east of 60 W,8l percent of the observations show a prevailing set toward 088T at

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

SHIP DRIFT (ALL YEARS)PREVAILING CURRENT

GEK (I960)SHIP

DRIFT OBS. INSAME DIRECTIONSAS GEK OBS.

REGION Dir.(T)

Freq. Speed (kn )(per- Mean Max.cent)

Dir.(or)

Speed range(kn )

Dir. Freq.(T) (per-

cent)ABC

076082090

76 1.2 3.579 1.1 3.285 1.0 2.7

180360l80

1.0-3.01.0-3.01.0-3.0

180 5360 5180 k

Table 11 Comparison of surface drift and GEK dataTable 11 compares the flow determined from ship drift data and from

i960 GEK data for the specified Regions A, B, and C in Figure 2ljA. Theprevailing surface flows indicated by drift observations differconsiderably from those observed by GEK. For example, the total of allsouth-setting drift current observations in Regions A and C averageless than 5 percent, whereas the frequency of the east-setting currentis as high as 85 percent.

The meandering pattern of the current in Figure 2ljA, which hasbeen described as a permanent feature, closely approximates dataobtained in June 1950 (Figure 2l|B); if detailed measurements wereobtained at Locations k, 5, 6, and 7 in Figure 2I4A, they would helpto clarify the apparent complexities of the current. The June 1950data also indicate a cyclonic eddy divorced from the main streamj themain surface flow of the stream probably is located as shown by

62


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historical data within the dashed lines in Figure 2I4A. When a largemeander from the main flow reaches an extreme stage, it may break offinto a large cyclonic eddy to the south, or a large anticyclonic eddyto 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 springof I966 at about 1+1N, 60W in a survey coordinated by the U. S. Coastand Geodetic Survey. Such an eddy at this location tends to verifythat the axis of the Gulf Stream exists at Locations h and 5 inFigure 2I4A.

The sparse data available show the subsurface current to havepermanent features similar to the surface current. Parts A and B ofFigure 22 are composite profiles from data at several stations withinthe same area and show the concentration of direct meter observations

at various depths. Part A, although to the left of the axis, indicatesthat higher speeds occur more frequently during summer; Part B showsthe distribution of speeds where depths are much greater.

Although current measurements have not been obtained near thebottom directly beneath the axis of the Gulf Stream, deepwater observa-tions indicate that the flow is essentially in the same direction fromsurface to bottom; computations show that the current speed at thebottom is probably 10 cm/sec. In i960, direct measurements obtainedin the vicinity of 3830'N, 6U30'W at depths of about 3,000 metersshow a flow of about 0.2 knot in the same direction as the surfacecurrent

Data obtained in 1962 indicate a deep southwest flow near thebottom along the continental slope at depths below 800 meters with


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speeds up to O.k knot; there appears to be a well-defined boundarybetween this current and the northeast-flowing Gulf Stream at deptha few miles to the east (Figure 2$), From previous determinationsof the location of the axis of the Gulf Stream, these observed south-west sets do not appear directly beneath this axis but are probablypart of the coastal current that frequently sets southwest past CapeHatteras. However, in the vicinity of 3500'N, 7U30'W, below 2,300meters, there is also evidence of an opposing southwest flow nearthe bottom.

61*


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76 30' 75W

75

LEGENDSURFACE AXIS OF GULF STREAMSUBSURFACE GULF STREAM FLOW

2000 DEPTH OF OBSERVATIONS (METERS)DEPTH CONTOUR

fc OBSERVED FLOW

35N

30'

- 34

FIGURE 25 SUBSURFACE FLOW IN THE VICINITY OF THE GULF STREAM


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Jutland CurrentThe Jutland Current, narrow and localized off the coast of Denmark

between 830' and 1030'E, originates partly from the resultant counter-clockwise flow in the tidal North Sea. The main cause, however, appearsto he the winds which prevail from south through west to northwest over50 percent of the time throughout the year and the transverse flowsfrom the English coast toward the Skaggerak.

The current retains the characteristics of a major nontidal currentand sets northeast along the northwest coast of Denmark at speedsranging 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 generaldirection as the surface flow. At a depth of l6 feet (5 meters) thespeeds range between 0.6 and 0.8 knot, and at 165 feet ( 50 meters)between 0.1 and O.k knot. It is assumed that speeds continue to decrease

with depth.

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Labrador CurrentThe Labrador Current, originating from cold arctic water flowing

southeast through Davis Strait at speeds of 0.2 to 0.5 knot and froma westward branching of the warmer West Greenland Current, setssoutheast along the Continental Shelf of the Canadian coast

At Hudson Strait, part of the current sets into the strait alongits north shore. The outflow of fresh water along the south shore ofthe strait from the large land area surrounding the Hudson Bay and

Hudson Strait region augments the part of the current flowing alongthe Labrador coast . The current also appears to he influenced bysurface outflow from inlets and fiords along the Labrador coast

The Lahrador Current usually is described as being more persistentover the narrow Continental Shelf than elsewhere; however, there mayhe seasonal fluctuations in the strength and volume of the current,depending on the amount of fresh water discharge and runoff along thecoast during the spring and variations in speed and direction resultingfrom tidal influences. The prevailing current, on the hasis of movementof hergs and surface drift observations, appears to extend some distanceoffshore. The mean speed is ah out 0.5 knot, hut current speeds at timesmay reach 1.5 or 2.0 knots.

The surface current roses in Figure 26 show the distribution ofobservations in the southern part of the current and the slight seasonalfluctuations. Calms average about 3 percent.


80/112FIGURE 26 BOUNDARIES, SURFACE CURRENT ROSES, AND SUBSURFACE

CURRENT PROFILE, SOUTHERN PART OF LABRADOR CURRENT


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Little information is available on subsurface currents. Thespeed of the southward flow decreases with depth, usually rangingbetween 0.2 and O.k knot at 30 meters (98 feet) and around 0.1 knotat 500 meters (1,6^-0 feet). The current profile in Figure 26 showsthe computed speed of the prevailing southward flow between thesurface and 200 meters (656 feet).


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Labrador Current ExtensionThe current setting southwest along the northeast coast of the

United States has no designated name and is therefore referred to asthe Labrador Current Extension in this publication. This coastalcurrent originates from part of the Labrador Current flowing clock-wise around the southeast tip of Newfoundland. Its speeds are fairlyconstant throughout the year and average about 0.6 knot. Thegreatest seasonal fluctuation appears to be in the width of thecurrent as shown in Figure 27; the current is widest during winterbetween Newfoundland and Cape Cod. Southwest of Cape Cod to CapeHatteras 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 betweenCape Cod and Long Island in July and August. The current in someplaces encroaches on tidal regions. For example, meter measurementsat 39.7N, 72.7 obtained in April i960 indicated a prevailing south-west set about 100 percent of the time to a depth of 50 feet (15 meters);speeds ranged from 0.2 to 0.5 knot and averaged 0A knot. About 215observations over a period of 293 hours at a depth of 100 feet (31 meters)showed a rotary tidal current; speeds ranged from 0.1 to 0.7 knot andaveraged O.k knot but did not seem to vary significantly with springsand neaps

In the widest part of the current east of J0\I during winter, theprevailing direction was west-southwest; about 50 percent of theobservations showed speeds between 0.1 and 0.9 knot, about k percentbetween 1.0 and 1.9 knots, and some between 2.0 and 2.5 knots.

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Q. out 59N, 10E and follows

the coast of Norway, as shown in Figure 1. It originates mainly fromOslofjord outflow, counterclockwise return flow of the Jutland Currentwithin the Skaggerak, and outflow from the Kattegat . The currentextends to ah out 20 miles in width. Speeds are strongest off thesoutheast coast of Norway, where they frequently range between 1 and 2knots . Along the remainder of the coast the current gradually weakens

and may widen to almost 30 miles at about 63N, where it joins theNorway Current; south of 62 N the current speed usually ranges betweenO.k 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 inFigure 29 shows the usual range of speed and maximum speed observed.

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SPEED (KNOTS)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

25

50

2 100

xiQ_LLJQ

400

800

1600

LATITUDE 60.5NLONGITUDE 4.7EMONTH JULYDEPTH OF BOTTOM APPROX. 350 M. RELIABILITY GOODMETHOD EKMAN METER

LETTERS SHOW DIRECTION OF FLOW (TOWARD).

82

164

328 rr

xiQ_LLI

656 O

1312

2624

5248

FIGURE 29 NORWAY COASTAL CURRENT PROFILE


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Portugal Current

The prevailing southward flow off the Atlantic coasts of Spain andPortugal is known as the Portugal Current and is part of the generalclockwise circulation in the North Atlantic Ocean. It is a slow-movingcurrent that averages only about 0.5 knot during both winter and summer;maximum speed seldom exceeds 2.0 knots north of U0N and 2.5 knots southof U0W.

The current is easily influenced by winds. An interesting relation-ship based on almost 25,000 surface current drift observations is shownin Table 12. The table compares wind and current observations in theregion between 35 and U5N, and between 5 and 10W within the limitsof the current shown in Figure 1; it shows the wind and current to bemost constant during summer, when both set in the same general directionat least 50 percent of the time. The higher percent of observations ofsouth sets during summer probably indicates that the current is atstrength during this period and less influenced by short -period changesin the wind. The percent of observations in other directions is some-what less than in winter and indicates that the wind may cause thecurrent to set in any direction for short periods at any time of theyear and that the flow may even reverse during persistent southerly winds,

During winter the current still shows a prevailing south set butwith a smaller proportion of observations; the percent distribution inthe other directions based on 8 points of the compass ranges between 8and 13 percent, with the higher percentages north of ^0W.


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The current west of 10W has a mean speed of 0.5 knot and may attimes exceed 2.0 knots in the prevailing south direction. In thisregion 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 ofobservations in the other directions averages about 9 percent.

Little is known about subsurface flow. Currents computed in Juneat 4l.5N, l4.5W show that the flow is generally south-southeast atabout 0.4 knot near the surface; speeds gradually decrease to about0.1 knot near the bottom at about 5>300 meters (17,388 feet).

REGIONWINTER SUMMER

Wind Current Wind CurrentDir.(from)

Per-cent

Dir.(toward

Per-cent

Dir.(from)

Per-cent

Dir. Per-(toward ) cent

kO-k5N SE-SWNW-NE 36kl

NW-NESE-SW

3hkl

SE-SWNW-NE 2750

NW-NE 23SE-SW 59

35-40N SE-SWNW-NE 25kl

NW-NESE-SW

26k6

SE-SWNW-NE 1850

NW-NE 18SE-SW 56

Table 12 Percent frequency by 90-degree quadrants of seasonal wind andcurrent observations between 35 and 45N, and between 5 and10W

80


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South Atlantic CurrentThe South Atlantic Current, counterpart of the North Atlantic

Current, appears to originate mainly from the Brazil Current andpartly from the northernmost flow of the West Wind Drift westof ^0W. Although surface data are limited, Table 13 shows themain features of the current during July and August in fourspecified regions shown in Figure 30. The current is under theInfluence of the prevailing westerly trade winds; the constancyand speed increase from the northern boundary to ah out 40S, wherethe current converges with the West Wind Drift. Maximum speed inRegions A and B is ahout 1.5 knots and in Region C ahout 2.0 knots.REGION

(See Fig. 30)PREVAILING DIR.

(T)FREQUENCY(percent)

MEM SPEED(knots)

TOTAL 0BS.

A 095 ^5 0.5 300B 080 50 0.6 2V7C 090 55 0.7 239

D Insufficient data] 30 of 3^ observations show setsevenly distributed from 315 clockwise to 135 , witha resultant flow of 0^5 at a mean speed of 0.6 knot.

Tahle 13 Directions and speeds during July and AugustThe data for January and February show the greatest number of

observations with predominant east sets and indicate the region ofthe South Atlantic Current to be mainly south of 30S and between0 and about 30W. During these months the region of variable orindeterminate flow between the Atlantic South Equatorial Current andthe South Atlantic Current appears to widen and migrate southward.The major portion of the South Atlantic Current narrows and becomes more


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Z>< oz(V o;< Z>Oas

1

CO

1

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major currents in the north and the south atlantic ocean

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