WHOI.92-3

SOFAR Float Trajectories from an Experiment to Measurethe Atlantic Cross Equatorial Flow (1989-1990)

by

Phip L. Richardson

Marguerite E. ZemanovicChrstine M. WoodigWilliam J. Schmitz, Jr.

andJames F. Price

--~ Woods Hole Oceanographic InstitutionWoods Hole, Massachusetts 02543-,.- rr

rr--Iõ=-I:i-~3: 0:: 0ai-:E ,.o_rro_0-August 1992

Techncal Report

Funding was provided by the National Science Foundation through Grant Nos.OCE-8521082, OCE-8517375, and OCE-9114656.

Reproduction in whole or in part is permtted for any purose of theUnited States Government. Ths report should be cited as:

Woods Hole Oceanog. Inst. Tech. Rept., WHOI-92-33.

Approved for publication; distribution unlimited.

Approved for Distribution:

James Luyten, airmanDepartment of Physical Oceanography

'j,.!-

~.

Abstract

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 mwere tracked for 21 months in the vicinity of western boundar curents nea 6Nand at several sites in the Atlantic near 11N and along the equator. Thajectories at1800 m show a swift (:; 50 cm/sec), narow (100 km wide) southward-flowing deepwestern boundary curent (DWBC) extending from 7N to the equator. At times(Februar-March 1989) DWBC water tured eastward and flowed along the equa-tor and at other times (August-September 1990) the DWBC crossed the equatorand continued southward. The mean velocity near the equator was eastward fromFebruar 1989 to February 1990 and westward from March 1990 to November 1990.

Thus the cross-equatorial flow in the DWBC appeared to be linked to the directionof equatorial currents which varied over periods of more than a year. No obviousDWBC nor swift equatorial current was observed by 3300 m floats.

Eight-hundred-meter floats revealed a northwestward intermediate level west-ern boundary current although flow patterns were complicated. Three floats thatsignificantly contributed to the northwestward flow looped in anticyclonic eddiesthat translated up the coast at 8 cm/ sec. Six 800 m floats drifted eastward along

the equator between 5S and 6N at a mean velocity of 11 cm/sec; one reached 5Win the Gulf of Guinea, suggesting that the equatorial current extended at least 35-

40° along the equator. Three of these floats reversed direction near the end of thetracking period, implying low frequency fluctuations.

11

Contents

Abstract .

1 Introduction.

2 Methods...

a) Temperature and pressure

b) Groundings . . . . . . . .

c) Float tracking and data processing

3 1800 m Trajectories .........., .

a) Deep Western Boundar Current (DWBC) trajectories.

b) DWBC velocity. . .

c) DWBC recirculation

d) Equatorial currents.

e) DWBC-equatorial current connection

4 3300 m Thajectories

5 800 m Trajectories .

a) Intermediate Western Boundary Current (IWBC) .

b) Anticyclonic eddies . . . . . . .

c) Equatorial curents at 800 m

d) Reversal . . . . . .

e) Southward velocity

f) IWBC-equatorial curent connection at 800 m .

6 Sumar and Conclusions .

Acknowledgments.

References . . . . . .

Appendix A: Sumar Composites of Thajectories .

Appendix B: Plots of Individual Floats . . . . . . .

i

1

2

2

7

9

9

9

15

19

19

21

25

25

25

30

30

30

35

35

35

37

38

39

63

ii

List of Tables

I Sumar of SOFAR float data 4

II A utonomous Listening Station (ALS) moorings 6III Slow sinng rate of SOFAR floats ... . . . . 8

IV Differences between launch position and fist tracked position 10

V Sumar of 1800 m deep western boundar curent observations 18

VI Northwestward intermediate western boundar current at 800 m 29

VII Summar of eddy characteristics estimated from looping float trajec-tories at 800 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

iv

List of Figures

1 Launch locations of SOFAR floats and Autonomous Listening Sta-

tions during Januar-Februar 1989 . . . . . . . . . . . . . . . . .. 3

2 Summar of 1800 m SOFAR float trajectories and overall displace-ment vectors .............................. 12

3 Individual 1800 m float trajectories along the western boundary . 13

4 Summar of 1800 m western boundar current trajectories. . . . 14

5 Segments of 1800 m trajectories of floats that drifted faster than20 cm/ sec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16

6 Average along-boundar velocity, transport, and eddy kinetic en-ergyat 1800 m in the vicinity of the deep western boundary current(DWBC), west of 43W . . . . . . . . . . . . . . . . . . . . . . . . .. 17

7 Profie of velocity (cm/sec) as a function of pressure measured on

17 January 1989 at ON, 30W . . . . . . . . . . . . . . . . . . . . .. 20

8 Individual 1800 m float trajectories along the equator from January

1989 to November 1990 . . . . . . . . . . . . . . . . . . . . . 22

9 Time series of 1800 m eastward velocity along the equator. . 23

10 Schematic diagrams summarzing the 21 months of 1800 m float data 24

11 Summary of 3300 m float trajectories and displacement vectors fromJanuar 1989 to October 1990 ............... 26

12 Summar of 800 m trajectories and displacement vectors . 27

13 Trajectories of four 800 m floats. . . . . . . . . . . . . . . 2814 Thajectories of 800 m floats trapped in eddies and trajectories of the

eddies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3115 Thajectories of six 800 m floats that drifted eastward in equatorial

currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., 34

16 Schematic diagram summarzing the 21 months of 800 m float data. 36

v

VI

i Introduction

This report describes SaFAR float trajectories in the equatorial Atlantic at depthsof 800 m in the Antarctic Intermediate Water and at 1800 m and 3300 m in theNorth Atlantic Deep Water. The fundamental issue investigated is the exchangeof water between the North and South Atlantic Ocean. Water mass propertiesincluding freon imply that deep western boundar current (DWBC) water splitsnear the equator, with part flowing eastward along the equator and par continuingsouthward along the western boundary. It was not known to what extent the tongueof freon lying along the equator near 1700 m is due to advection or to enhancedmixing. Thus a secondar issue investigated is the nature of the connection betweenthe DWBC and flow along the equator.

The DWBC is the major pathway by which cold deep water flows southwardinto the South Atlantic and, eventually, into the Pacific and Indian Oceans. Thewarm upper layer in the Atlantic, including the intermediate water, is thought toflow northward in compensation for the deep water. Schmitz and Richardson (1991)have identified 13 x 106 m3/s of upper level water from the South Atlantic flowingnorthward across, the equator into the Gul Stream. Neither flow had previouslybeen directly measured crossing the equator. This large-scale thermohalne circu-lation results in a northward heat flux through the Atlantic which is importantfor world climate. An improved understanding of the thermohaline circulation andits varability is required in order to design a scheme to measure vaiations in themeridional flux of heat in the oceans and varations in climate.

The results described here are the fist subsurace float trajectories in thisregion. They reveal new information concerning the thermohaline circulation, in-cluding a swift, '" 50 cm/sec, southward-flowing DWBC at 1800 m that at timesfeeds into an eastward equatorial current and at other times crosses the equator

directly. These data provide a first direct measurement of the cross-equatorial flowof deep water and its complex patterns. Some floats at 800 m and 1800 m driftedlong distances along the equator, up to 380 of longitude, and give a fist Lagrangianview of these equatorial currents and their connections to the currents along the

western boundar.

The report is divided into two main pars. The fist follows this introductionand summarzes the whole experiment. The second par consists of two appendicesthat show some summary composites of trajectories (Appendix A) and plots ofindividual floats (Appendix B).

i

2 Methods

Durng January and Februar 1989,48 SaFAR floats were launched in the tropicalAtlantic, 14 at 800 m in the intermediate water, 15 at 1800 m and 15 at 3300 min the deep water, and 4 by J. Price as engineering tests of a Bobber float, atdepths near 300 and 650 db (Figue 1, Tables I and II). The floats were traced

acousticaly from January 1989 to November 1990 by means of an aray of sixmoored autonomous listening stations. See Table I for the dates durng whicheach float was tracked. Float tracking is continuing for an additional two yeas.Thity-one of the floats were launched along a line spaning the Atlantic between6N and 11N, with closest spacing between floats near the western boundary offFrench Guiana, where the velocity is swiftest. Seventeen floats were launched alongthe equator in the west, where meridional flow is thought to cross the equator andeastward flow along the equator originates. Thus the whole width of the Atlanticbetween French Guiana and West Africa was instrumented with floats, althoughsparsely in the eastern region.

All but two of the 800 m and 1800 m floats were tracked for the full 21 monthsand were heard out to ranges of 3000 km (Table I). One float (28) entered theCarbbean and another (34) faded after six months. Six of the 3300 m floats werenever heard, two due to a reduced range of around 1000 km there, four due tounexplained failures. The mean trackable lifetime of 3300 m floats was around ayear due to their gradually sinking toward the lower limit of the sound chaneL.Most of the deep floats that were tracked could be heard by at least one listeningstation up to October 1990.

a) Temperature and pressure

All floats except the four Bobbers failed to tranmit correct temperature and pres-sure data after they had equilibrated, and they also failed to activate their buoyancycontrol which keeps them at constant pressure. In order to estimate equilibriumdepths at sea,. two floats at each level were followed acousticaly from the ship asthey sank. The floats at the 800 db level equilbrated at 795 db and 800 db; thoseat the 1800 db level equilibrated at 1825 db and 1770 db. Two deep floats werefollowed down to 2570 db and 2860 db where their telemetry stopped. An extrapo-

lation of their data to equilbrium pressure showed that the floats reached 3255 dband 3250 db. In the following, the three equilibrium pressures wil be referred toas 800 m, 1800 m, and 3300 m, but individual floats could have differed from thesenoiinal depths.

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800

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loat

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9011

0205

.98

45.7

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40.

91-0

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8902

0606

.63

49.6

190

1112

-03.

2636

.43

642

2.60

-1.9

98

(180

0 )

8902

0707

.54

48.7

890

1106

-03.

9536

.37

637

2.53

-2.3

413

(180

0)89

0207

09.2

347

.67

9010

3003

.71

45.6

362

90.

43-1

.16

11(1

800)

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0811

.20

46.3

090

1106

09.4

346

.31

635

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3-0

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0911

.15

40.3

690

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06.9

347

.51

631

- 1.

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8902

1311

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.65

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263

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1211

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25.0

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0.14

3) 3

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m F

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2300

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39.0

290

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5934

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605

0.91

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045

(330

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336

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8910

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36.7

126

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32.9

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1.46

34.1

848

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9

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4232

5589

0206

06.2

549

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8910

2107

.82

50.9

325

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0.83

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300)

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0606

.63

49.6

289

1205

05.5

246

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301

1.44

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840

(330

0)89

0206

07.0

249

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9002

1308

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45.8

237

11.

170.

6036

(330

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07.9

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8909

3009

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51.0

423

5-1

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0.64

29f

(330

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0207

08.8

247

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(330

0)89

0207

09.9

247

.17

43f

(330

0)89

0208

11.2

046

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300)

8902

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8905

2211

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41.1

599

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00.

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f(3

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1311

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32f

(330

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11.8

23.4

0to

tal

57.9

yea

rs

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loca

tion,

then

by

long

itude

.b)

Ini

tial f

loat

pre

ssur

e w

as o

bser

ved

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en f

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r fl

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ures

of o

ther

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par

enth

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last

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eep

to li

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the

east

war

d cu

rren

t jet

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Not

trac

ked

due

to th

e ac

oust

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igna

l bei

ng o

verw

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ed b

y a

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ulta

neou

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igna

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.

,~',

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Table II: Autonomous Listening Station (ALS) Moorings

ALSALS ALS DepthSite # (m)

Launch RecoveryDate Date

yymmdd yymmddLatitude Longitude

deg. N deg. W

A 160A 950

B 161A 815

C 162A 645

D 163A 751

E 164A 751

F 159A 756

890109 901030

890112 901102

890117 901119

890119 901108

890123 901112

890108 901028

13.453

7.845

0.519

-4.711

0.034

6.980

49.260

40.345

30.848

25.667

38.276

51.235

All ALSs functioned normaly except for 159A which faied electronicaly on 890816.

6

To determine the equilibrium pressure of the deep floats, the linear regressionbetween the square of the float's vertical velocity and the pressure was used to es-timate the pressure at the point of zero velocity. Vertical velocity was calculated

from the pressure time series telemetered from the floats as they descended. Thismethod assumed that at any instant the drag force on a float, given by p /2( C DA W2)where CD is the drag coeffcient, A is the area, p is the water density, and W isvertical velocity, is balanced by the negative buoyancy force on the float, which isproportional to its height above equibrium pressure. Calculations using a charac-teristic CTD profile in the tropical Atlantic show that the negative buoyancy of adeep float is approximately linear from its equilbrium pressure up to a pressure ofaround 1000 db. The drag coeffcient of spheres vs Reynold number is nearly con-stant over virtualy the entire range of vertical velocities experienced by the floats

as they descended.

Without active ballasting, SOFAR floats gradually sink due to the slow defor-mation of their pressure housing, which is aluminum for 800 m and 1800 m floatsand glass for 3300 m floats. In order to estimate this sink rate, all avalable histori-cal float data were examned. Ten aluminum floats and five glass floats were foundto give reliable estimates of the long-term sink rate (Table III). The low numberis because (1) most floats actively adjusted their buoyancy to maintain a constantpressure, (2) most floats were ballasted too deep and rose toward their target pres-sure, and (3) many floats were near the Gulf Stream where their pressure vaied intime due to the vertical heaving of the water column, which made estimating thesink rate diffcult.

The mean sink rate and standard error of aluminum floats was 0.37:l0.05 db/d.No obvious relationship was seen between their sink rate and the pressure level,which suggests that the mean sink rate is appropriate for all depths. The mean rateimplies that the 800 m and 1800 m floats would have sun around 230 m over the 21months discussed here. The mean sink rate of the glass floats was 0.62 :l0.11 db/d,which implies that the 3300 m floats would have sunk around 220 m over their meanlifetime of 12 months. The gradualy decreasing acoustic range observed with the3300 m floats is inferred to be due to their gradual sinking toward the lower limitof the sound channeL.

b) Groundings

A few 1800 m floats on the inshore edge of the DWBC drifted into water shalowerthan their equilibrium depth and probably dragged along the sea floor. One of these(float 10) clearly went aground after 51 days and remained stuck for the rest of the21 months. The speed of a few of these DWBC floats seemed to decrease as they

7

Table III: Slow Sinking Rate of SO FAR Floats

Aluminum floatsFloat ID Pressure (db)

GU 162 2000G U 156 2000LD 62 700G U 167 2000LD 86 1300MO 10 1500LD 51 1300MO 5 1500MO 2 1500LD 65 700

average

Glass floatsFloat ID Pressure (db)

MA 24 2500MA 25 2500MA 22 2500MA 26 2500MA 63 2500average

Days in water278260147120927269585232

Days in water13201240740740320

Sink Rate (db / d)a0.180.360.370.220.410.690.220.480.390.42

0.37:i0.05b

Sink Rate (db/dY0.320.460.740.700.93

0.62:: o.iid

a) Around half of the aluminum floats exhibited a somewhat decreasing sink ratewith time. For these, the slower sink rate is given since this would seem to be thebest estimate of the long term rate. Only floats that san longer than 30 days wereincluded because of this vaable rate. The wal thickness of the alumnum tubeswas 1.59 cm for shalow ones (.. 1000 db) and 1.90 cm for deep ones (:; 1000) db.

b) The standard deviation of values is 0.15 db/d and the standard error is 0.05 db/d.

c) The glass float's sinking rate deviated in curious ways from a constant rate (Resand Gould, personal communication).

d) The standard deviation of values is 0.24 db/d and the standard error is 0.11 db/d.The data imply that the longer a float is in the water the slower its sink rate, whichresults from slower sinking floats taking longer to reach equiibrium pressure thanfaster sinkng ones.

8

drifted landward, probably due to both friction as the floats dragged on the seafloor and reduced near-bottom water velocity.

The abilty of a float to drag upslope along the bottom into water shalowerthan the equilibrium depth can be understood by a simple calculation. Imaginean 1800 m float that is cared upslope along the sea floor to 1300 db where the

float is approximately 0.5 kg negatively buoyant. If we assume that the drag of_ the sea floor on the bottom of a drfting float is equal to this vaue, that the floatremains vertical, and that its drag coeffcient is 1.0, then an average water velocityof", 7 em/see past the float wil provide sufcient drag to force it to drft.

c) Float tracking and data processing

The floats transmitted an 80 sec 250 Hz acoustic signal once per day. Float clockcorrections and positions were calculated from the times of arival of signals receivedat the moored listening stations. Spurious positions were edited manualy, gapsless than 10 days long were linearly interpolated, and the resulting time series weresmoothed by means of a Gaussian shaped filter (of weights 0.054, 0.245, 00403, 0.245,and 0.054) to reduce position errors and tidal and inertial fluctuations. Velocityalong trajectories was calculated at each fial position by means of a cubic splinefunction. The average accuracy of a fi was estimated to be less than 10 km basdon a comparson of float launch locations and fist tracked positions (Table IV).

3 1800 m Thajectories

A sumary plot (Figue 2) of 1800 m trajectories shows strikingly different kinds oftrajectones in different regions. Eight of the fifteen floats drifted southeastwad forvaous lengths of time in a fast (50-60 cm/sec), narow ('" 100 km), deep westernboundar current (DWBC). Five floats drifted long eastward distances, up to 250of longitude, within a few degrees of the equator. Compared to these, the two floatsin the eastern Atlantic near iioN barely moved.

a) DWBC trajectories

The best evidence for a narow, swift DWBC comes from the fist two months,Februar and March 1989, when three floats (10, 14, and 5) drifted southward(Figures 3, 4). Float 10 grounded on the continental slope after 51 days; the two

others reached the equator. Float 14 retured northward and ended up near the

9

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iaOOm TRAJECTORIES~10N ~

5N

a

5S55W 50W 45W 40W 35W 30W 25W

DISPLACEMENT VECTORS

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00

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Figue 2: Sumary of 1800 m SaFAR float trajectories and overal displacementvectors from Januar 1989 to November 1990. Arrowheads are spacd at intervalsof 30 days along trajectories.

12

1800m WESTERN BOUNDARY CURRENT TRAJECTORIES

15N

FLOAT 5

ION

5N

o

40W S5W SOW'

15N

FLOAT 8

10N

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15N

FLOAT 13

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15N

FLOAT 4

10N

'0~"0

5N

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25W 5i5W 50W 45W 40W S5W 30W 25W

Figue 3: Individual 1800 m float trajectories along the western boundary from January

1989 to November 1990. Arrowheads are spaced at 30 day intervals. Upper panels showfloats 5 and 14 launched directly into the DWBC in early February 1989. Middle panels showfloats 2 and 8 that were entrained into the DWBC in January 1990 (5) and March 1990 (2).Lower panels show float 13, which remained in the vicinity of the western boundary fromFebruary 1990 to November 1990, and float 4, which meandered southeastward offshore ofthe mean DWBC.

13

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11 APR - 20 JUL 1990

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20 JUL - 28 OCT 1990

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.

DWBC near 6N in November 1990. Float 5 drfted eastward along the equator to29W (July 1989), and then westward, ending near IN, 40W.

The next three floats (2, 8, and 13) drfted westward from offshore launch

positions and were entrained into the DWBC near 7N. Floats 2 and 8 drftedsouthwad in the DWBC durng Januar-Apri 1990. Float 8 reached the equatorin April, recirculated to the north, was re-entrained into the DWBC in July, crossedthe equator in August, and reached 4S by October, the farthest south of any 1800 mfloat. Float 2 crossed the equator in April 1990, recirculated inshore durng May-July, and then continued southward to 3S at the end. Float 13 entered the DWBCin Februar 1990, where it made numerous loops and reached as far south as 4N byNovember 1990. This float plus float 14 looped in a 200 km diameter cyclonic eddycentered near 4.5N, 46.5W next to the western boundar (July-October 1990).

Three other floats were briefly in the DWBC south of the equator. Float 9,launched in the DWBC near the equator, exited and drifted eastward along theequator. Float 6 drifted near the equator for most of the 21 months, entered theDWBC in October 1990 and drifted south to 2.5S. Float 1, launched on the equatornear 39W, briefly drifted southeastward in the DWBC, then eastward to 14W, thennorth across the equator near 18W. This path showed that a float crossing theequator in the DWBC may return northward again, although the float ended upnear the equator. Out of the six floats in the DWBC, two (2 and 8) crossed theequator within 21 months.

b) DWBC velocity

Most 1800 m floats drfted southeastward paraleling the 1800 m depth contourwhile they were in the DWBC (Figure 5). In order to calculate the cross- andalong-stream characteristics of this current, float positions and velocities were con-verted to distances seaward of the 1800 m contour and velocity components normaland parallel to the contour. The mean velocity and transport of the DWBC asmeasured by 1800 m floats in this coordinate system are shown in Figue 6 and val-ues tabulated in Table V. Only floats west of 43W were included in this compositein order to screen out floats in swift equatorial currents. Figue 6 is noteworthybecause it represents a space (ON-7N) and time (12 months) average that shows thehorizontal structure of this portion of the DWBC. Individual along-boundar veloc-ity vaues peaked at around 55 cm/sec and 10 km bin averages reached 26 cm/sec.

The DWBC was bounded by a flaning counterfow or recirculation; the width ofthe DWBC is 100 km as measured between points of zero velocity, and the widthof the recirculation is at least 600 km.

15

10N o

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11

lS00rn FAST FLOATS

SPEEDS OVER 20 ern/see

5N

~?

I- ~

5855

W50

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Figu

re 5

: Seg

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f 18

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at d

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ed f

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1800m DEEP WESTERN BOUNDARY CURRENT

-10 i-0Q)

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E t.0ALONG- BOUNDARY VELOCITY w

)0 10 5 0t- T"i.U t-O a:-i 20 100w~ a.

(fZ~

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200N0Q) 150II"'NE 1000w 50~w

0-100 0 100 200 300 400 500 600 700

100(fZ 80at- 60~~a: 40w(fCD 20a

0-100 0 100 200 300 400 500 600 700

KILOMETERS FROM 1800m

Figure 6: Average along-boundary velocity, transport, and eddy kinetic energy at 1800 min the vicinity of the deep western boundary current (DWBC), west of 43W. Al availableindividual daily velocities were grouped and averaged in lO-km-wide bins paralel to the1800 m depth contour, which is from the ETOP05 data base. Nine different floats wereused to obtain this composite, which consists of roughly 3000 daily velocity observations.Eight floats drifted in the region of the mean DWBC jet and provided 500 daily observations.Transport per unit depth (103 m3 Is) was obtained by summing, in the seaward direction, theproduct of bin width and the average velocity in each bin. Eddy kinetic energy (cm2 Isec2)-2 -2 -2-2was calculated using 1/2( u' + v' ) where u' and v' are the variances of the velocity vaues

parallel and normal to the 1800 il contour.

17

Table V: Summary of 1800 m Deep Western Boundar Current Observtions,

January 1989 to October 1990; 2S-12N west of 43W

Number MaxumNumber of Velocity Transportof floats observa- Peak (10 km average) Current per unit

il tions in Velocities and standard Width depth (a)Date current current (em/see) error (em/see) (km) (1Q3 m2/s) )

Composite,J an 89-Det 90 8 500 50-60 26 :: 5 (b) 100 14:: 3(b)

1) J an 89-0ct 89 4 124 50-60 39::5 100+ 23 :: 4 (c)

2) Nov 89-Apr 90 3 188 40-50 30::4 100 16:: 3

3) May 90-0ct 90 3 177 30-40 19:: 6 100 8::2

(a) The total transport ofthe upper core ofthe DWBC was estimated to be 14.7 x 106 m3/sby combining the horizontal velocity profile from floats with the vertical profile from acurrent meter array (Colin et al" 1991) that was moored near the center of the DWBCjet as observed at 1800 m. The mooring was located at 6.2N, 51.0W from March 31, 1990to November 18, 1990, a duration of 230 days. The depth of the meters and southwardalong-boundary mean velocities were 800 m, -1.6 cm/sec; 1400 m, 11.4 cm/sec; 2000 m,18.9 cm/sec; and 2700 m, 15.6 cm/sec. The velocity was assumed to be zero at the sea floorat 2800 m. The total transport was calculated from the width and thickness of the DWBCand by assuming it was ellptical in shape.

(b) The standard error of maxmum velocity was estimated from individual observation in10-km bins. The standard error of transport was estimated in two ways: first as listed fromthe average velocities in 10-km bins, second, from the three values of transport over the21 months, which imply a standard error for the composite of around 4 x 103 m2/s.

(c) Floats sampled the DWBC in January-March 1989. A data gap occurred offshore of theDWBC between 90-130 km; the transport is up to the data gap. The width and transportcould have been larger than given here.

18

c) DWBC recirculation

Floats offshore of the DWBC reveal (1) a northwestward recirculation between theDWBC and the Mid-Atlantic Ridge and (2) an inflow to the DWBC in the region 5-ION. Evidence consists off our floats (2,4,8, and 13) launched offshore of the DWBCin latitudes 6-10N. These floats gradualy drifted westwad and were entraied intothe DWBC with a mean velocity of u = -1.5:: 0.5 cm/sec, v = -0.2:: 0.2 cm/sec.In addition, after floats 8 and 14 had reached the equator in the DWBC, theyrecirculated northwestward offshore of the DWBC. In contrast to these floats inthe west, the easternmost floats (7 and 12) near 11N drifted very slowly, implyingvery weak or zero recirculation east of the Mid-Atlantic Ridge. The combined meanvelocity of floats 7 and 12 was u = -0.l1::O.19 cm/sec, v = 0.02::O.21 cm/sec, notsignificantly different from zero. Evidence for recirculation is also seen in Figure 6.Although bin-averaged velocity values fluctuate around zero as a function of distanceoffshore of the DWBC, the integrated alongshore transport gradually decreases inthe offshore direction.

Recirculation velocity and transport were estimated two ways. First the av-erage velocities in 10 km bins (Figure 6) were grouped and averaged which gives amean recirculation speed of 0.61::O.25 cm/sec. Second, all individual float velocitiesin the 90-km to 700-km band seaward of the DWBC (west of 43W) were groupedand averaged which gives a mean northwestward recirculation velocity paralel tothe 1800 m contour of 0.47:: 0.52 cmjsec and inflow velocity toward the DWBCof 0.73:: 0.55 cm/sec. These two mean recirculation velocities suggest that around39% of the 1800 m DWBC recirculated west of the Mid-Atlantic Ridge (which islocated around 1100 km from the 1800 m contour). Assuming that this percentageis representative of volume transport implies that around 5.8 x 106 m3/s of theupper DWBC recirculated, leaving 8.9 x 106 m3/s to cross the equator; however,the estimated standard errors are only slightly smaller than the mean values, so themagnitude of the recirculation is stil uncertain.

d) Equatorial currents

A vertical profie of velocity was measured with a freely fallng velocity profier nearthe equator when the floats were launched there (Figure 7). The profie revealed-awell-developed pattern of alternating eastward and westward currents or jets overthe upper 2200 m. The most prominent eastward jets were (1) the EquatorialUndercurrent, reaching 76 cm/sec at 70 m, (2) a 28 cm/sec jet at 1000 m, and

(3) an 11 cm/sec jet at 2000 m. Four 1800 m floats appeared to be located in thisthird jet, which extended from around 1600 m to 2200 m~ The trajectories imply

19

EQUATORIAL VELOCITY PROFILE (em/see)OON 30° W 17JAN 1989

- 2 0

oo 20 40 60 80

-----..---------------------

1000 ,..,,-r,'.. ..

...."U'-w~:: 2000VlVlW~a.

3000 · FLOAT LAUNCHPRESSURES

4000

Figue 7: Profile of velocity (em/see) as a function of pressure measured on 17 January

1989 at ON, 30W (from Ponte et al., 1990). The profieextended down to within 200 m ofthesea floor. Solid line represents the eatward velocity component; dashed line is northwardvelocity; large dots show nominal float depths at launch.

20

that this equatorial jet extended at least 20_30 north and south of the equator(Figues 3, 8) and around 250 longitudinaly.

Three floats launched near the equator (1, 6, and 9) plus another (5) that

peeled off from the DWBC into the equatorial band drfted long distances in this1800 m curent. Float 1 drfted along 2S-3S from 39W to 14W, a distance of

2750 km over 310 days at a mean velocity of 10 cm/sec. Float 6 drfted eastwardalong 1N-3N from 42W to 27W and then bac to 40W, where it turned and headedsouth in the DWBC.

Three of the floats (5, 6, and 9) that fist drfted eatward along the equatortured and then drfted back westward along the equator. Grouping al avaable ve-

locities into a large equatorial box, 2.5S-2.5N, 20W-40W, and caculating monthlymean velocity values (Figure 9) shows the mean flow near 1800 m was 4.1 cm/seceastward from Februar 1989 to February 1990, and then 4.6 cm/sec westward from

March 1990 to October 1990. These values include float 15, which differed by slowlydrifting westward over the 21 months. A confrmation of the time varation of theequatorial current system is sen in a second velocity profie near ON, 30W in June1991 that showed a westward current from 1600 m to 2000 m where the profiestopped (Böning and Schott, 1992).

In sumary, of the floats that were launched on the equator (1, 6, 9, and 15)or that drfted there in the DWBC (2, 5, 8, and 14), one (14) recirculated, two (2and 8) crossed the equator in the DWBC, and one (6) entered the DWBC from theequator, leaving four near the equator at the end of tracking in November 1990.

e) DWBC-equatorial current connection

The 1800 m trajectories show that when the equatorial current was going eastward,some of the DWB C water turned and flowed eastward along the equator (floats 5, 9).Eventually, after about a year, the equatorial current reversed and flowed westward

(floats 5, 6, and 9). When the westward equatorial current reached the westernboundar, some of the equatorial current turned southward and entered the DWBCsouth of the equator (float 6). At this time two of the DWBC floats (2 and 8)crossed the equator. Thus the pattern of cross-equatorial flow in the DWBC seemsto be coupled with the direction of equatorial curents. An implication is that the

equatorial currents act as a temporar reservoir for DWBC water, storing it ineastward flow and releasing it in westward flow. Virtually al net cross-equatorialflow occurred in the west near the boundary, except for temporary crossings by

floats farther east trapped in higher frequency motion within a few degrees of theequator. A. schematic diagram of the inferred general circulation of upper NorthAtlantic Deep Water is given in Figure 10.

21

1800m TRAJECTORIES ALONG THE EQUATOR

5N

o

5S50W 45W 40W 35W 30W

5N

o

5S50W 45W' 40W 35W 30W

5N

ø

. ...-.

o

5S50W 45W 40W 35W 30W

FLOAT 9

-

25W l5W20W

FLOAT 6

-

25W 20W l5W

FLOAT 1

25W 20W l5W

Figue 8: Individual 1800 m float trajectories along the equator from January 1989to November 1990. Arrowheads are spaced at 30-day intervas.

22

iow

iow

iow

8

~6

Q) ~ E

4.s )- .-

2U 9 w

~;:

0~

0 e: ~-2

li -c-4

w

1800m EQUATORIAL FLOATS.

10 -6 -8F

M A

M J

J A

S 0

NO

J F

M A

M J

J A

S 0

1989 1990

Fig

ure

9: T

ime

serie

s of

180

0 m

eas

twar

d ve

loci

ty a

long

the

equa

tor,

cal

cula

ted

by g

roup

ing

indi

vidu

al v

eloc

ityva

lues

in a

box

who

se li

mits

are

20-

40W

, 2.5

S-2.

5N. P

lotte

d va

lues

are

mon

thly

ave

rage

s. O

n av

erag

e th

ere

wer

eap

prox

imat

ely

four

flo

ats

and

120

daily

obs

erva

tions

per

mon

th in

the

box.

200N., .:. Il.,...

100N

1 800 m '.. ,....

00

..~,. ."q;

, '.':.'... C4'L

:¡¡~" ... QlQå~

Reversing Equatorial Currents

10°8600W 50° 400 20° 100W

200N., .:

1 800 m '.. ,. Il. ., ......lOON

10°5

00

600W 50° 400 30° 20° 100W

Figure 10: Schematic diagrams summarizing the 21 months of 1800 m float data.The width of currents is roughly proportional to estimated transport: 15 x 106 m3/s

in the DWBC north of the equator, 6 x 106 m3/s in the recirculation there, and9 x 106 m3/s in the DWBC south of the equator.

24

4 3300 m Trajectories

The 3300 m float trajectories look very different from the 1800 m ones (Figue 11).No obvious DWBC is seen, which was surrising because the 3300 m floats werelaunched near the western boundary in the lower North Atlantic Deep Water. Fourtrajectories (floats 36, 39, 40, and 42) were obtained near 7N, 50W from Februar1989 to Februar 1990, but none of these looks like those in the DWBC at 1800 m.Thus the evidence from the 3300 m floats suggests that there was either no DWBC ora very weak one at this depth, with the core of lower deep water located signcantlybelow 3300 m.

Three of the four equatorial floats alo drfted rather erratically without anyindication of being in the DWBC. Their mean velocity was u = -0.04:l0.39 cm/sec,v = 0.30:l 0.42 cm/sec. The one exception, float 30, drfted southeastward at1.4 cm/sec over topography shalower than 3000 m. In addition, it was trackedlonger than the others, implying it was probably shallower than they were. Forthese reasons, we think this float was in the upper part of the North Atlantic DeepWater and therefore unepresentative of velocity at 3300 m. We conclude that thereis no evidence of a prominent DWBC at 3300 m.

The 3300 m floats near the equator did not drift far eastward, as the 1800 mfloats did. This lack of significant flow along the equator at 3300 m agrees with theequatorial velocity profile (Figure 7) that showed weak flow below 2200 m.

5 800 m Trajectories

a) Intermediate Western Boundary Current (IWBC)

Two 800 m floats launched near 6N (22,23) and a third near the equator (28) clearlytranslated in a mean northwestward direction along the boundar (Figures 12, 13and Table VI) in the inferred direction taken by Antarctic Intermediate Water.One of these (28) probably entered the Caribbean through the Grenada Passagein April 1990. In addition, a 650 m bobber float (B63) launched near the equatortranlated up the boundary. The mea velocity of these four floats was 3.5 :l0.8 cm/sec toward 3070, where the standard error was calculated from the fourvelocity values. Several other 800 m floats (20, 21, 25, and 26) translated eastwardand southeastward in counterfows. Floats 19 and 26 drifted southward and theneastward long distances in equatorial currents. These floats suggest that some ofthe water in a countercurrent offshore of the IWBC fed into the equatorial current.

25

15N

10N

.3300m

TRAJECTORIES..

5N. I

..0

- .

0

5855W

15N

10N .

50W 45W 40W 35W 30W 25W

... .~ 11,~44."'_ c:'. ~

3300mDISPLACEMENT

VECTORS

5N

""~ =

- .

~ - ,¿'" c: .-

o

5855W 50W 45W 40W 35W 30W 25W

Figure 11: Smnmary of 3300 m float trajectories and displacement vectors fromJanuary 1989 to October 1990. Arrowheads are spaced at 3D-day intervas.

26

20N

15N

10N

5N

o

5S

10SB5W BOW 55W 50W 45W 40W 35W 30W 25W 20W 15W iow 5W

20N

15N

10N -

5N

~j,. ~

f¡

II,\i, ." ..

~'~~"

,"'.._..'~"'"

,..-.._~,...-

26

o-+

24

5S

10SB5W BOW 55W 50W 45W 40W 35W 30W 25W 20W L5W LOW 5W

Figure 12: Summary of 800 m trajectories and displacement vectors from January1989 to November 1990. Float 34 was tracked up to July 25, 1989 when it stoppedbeing heard, and float 28 was tracked up to April 22 when it is inferred to haveentered the Caribbean. Four Bobber (B) floats at shallower depths were included;three of these were short records. Arrowheads are spaced at 3D-day intervals.

27

i5N

iON

iON

800m

WE

ST

ER

N B

OU

ND

AR

Y C

UR

RE

NT

TR

AJE

CT

OR

IES

5N

i5N

iON

iON5N

00

2828

60W

55W

50W

45W

40W

60W

55W

50W

45W

40W

t' 00i5

N r

; i '

, Ii

II

~i5

NFL

OA

T 2

8

FLO

AT

22

5N

FLO

AT

23

5N

o 2860

W55

W50

W45

W40

W

o 2860

W55

W50

W45

W40

W

Figu

re 1

3: T

haje

ctor

ies

of f

our

800

m f

loat

s th

at d

rift

ed, o

n av

erag

e, n

orth

wes

twar

d al

ong

the

wes

tern

bou

ndar

y.Floats 22, 28, and B63 (at 650 m) looped in

antic

yclo

nes

that

tran

slat

ed n

orth

wes

twar

d.

Table VI: Northwestward Intermediate Western Boundary Current (IWBC) at 800 m

MaxmumVeloeity Transport

N umber of (20 km average) per unit

observations and standard Width depthin IWBC error (em/see) (km) (103 m2/s)

I All Floatsincluding 1833 7.1 :I 3.8 300 5.8:1 1.8 (a)

Bobbers

II No Loopers (b) 1606 4.5 :I 3.6 300 2.7 :I 1.6

III No Bobbers (e) 1091 6.8 :I 3.8 180 4.2 :I 1.6

iv No Bobbersor Loopers 782 4.5 :I 3.6 160 2.2 :I 1.6

(a) Float velocity values were grouped in 20-km bins as a function of distance seawardof the 800 m depth contour. Only floats within the rectangle extending from 4N-13N,

43W-55W were included in order to screen out floats in equatorial currents. The north-westward transport in the IWBC was estimated by integrating the mean velocity in 20-kmbins seaward from the western boundary to the point of maxmum transport, near a dis-tance of 200-300 km from the 800 m contour. The standard error of IWBC transport wasestimated from the different 20-km mean velocity values within the region of the IWBC.Total transport did not vary much when the size of the rectangle 4N-13N, 43W-55W wasvaried, except for increasing somewhat as the western edge was shifted farther to the westto include more of the northwestward going trajectories.

(b) Loopers are floats looping in anticyclonic eddies (see Table VII).

(c) Two Bobber floats located near 650 m were included in I and II.

29

b) Anticyclonic eddies

Three of the four floats (22, 28, and B63) that drifted northwestward the farthestlooped for vaous amounts of time in three different anticyclonic eddies as the eddiestranlated up the western boundary (Figue 14, Table VII). The mean velocity ofthe eddies, which was northwestward at 8.1 :: 1.0 cm/sec, contributed signficatly

to the mean velocity and transport in the IWBC (Table VI). Approximately 50% ofthe total northward transport in the IWBC was accounted for by the measurementsof these floats looping in a.ticyclones.

c) :Equatorial currents at 800 m

Most visually striking of the 800 m trajectories (Figures 12, 15) is the long eatwarddrift of floats in a band from around 5S to 6N. These floats equilibrated at a depthnear the top of an eatward equatorial jet that had a peak speed of 28 cm/sec

and thickness of 500-600 m (Figure 7). The floats apparently descended into thejet and were carried eastward by it. Most remarkable is the broad width, '" 1 1 0 inlatitude, of the dominantly eastward equatorial curents. Peak speed along eastwardtrajectories was '" 30 cm/sec, and the average eastward velocity calculated bygrouping al eastbound floats in the box 5S-6N, 5W--0W was 1O.6:: 0.9 cm/sec.Coupling this value with the 110 width and 500 m thickness gives an eastwardtransport per unit width of 128 x 103 m2/s and a volume tranport of 64 x 106 m3/s.Of course, regions of westward flow could be embedded in the eastward current,which would reduce the mean velocity, and a few are seen. Stil, the trajectoriesimply that very large amounts of water can flow in equatorial currents at this depth.

Six different floats drifted eastward in the equatorial currents; two of thesedrifted southward into the equatorial band and then eastward. Float 24, launchedon the equator at 30W, went 260 east along 0-2S to 5W, the farthest east of anyfloat. This float was ballasted to equilibrate near 1125 m, near the center of thejet (Figure 7). Assuming that the eastward jet began near the western boundaryimplies that the current extended coherently eastward about 380 of longitude, to atleast 5W.

d) Reversal

Three of the four floats (24, 26, and 31) that were stil in the 5S-6N band at the endof the 21 months reversed direction shortly before the end. The fourth float (19)looked as if it had just stopped near 5N, 26W, and was perhaps about to reversedirection. This reversal of the equatorial current is inferred to be primarly a tempo-

30

13N

10N

5N

o

BOW 55W 50W 45W 40W

Figure 14: A) Trajectories of 800 m floats trapped in eddies as inferred from loopingtrajectories (see Table VII). In each case shown, a float made at least two consecu-tive loops in the same direction, implying it was trapped in an eddy. Anticyclone Awas tracked by floats 28 and B63 almost continuously from Januar 25, 1989 toJanuary 22, 1990. An early meander in the trajectory of 28 was found to be a loopwhen the mean translation of this float was subtracted from the trajectory.

31

13N

10N

5N

o

----'¡l: EDDY TRAJECTORIESi,,,

" ,""'" ." _..,'1t D"t,"---'------"'-C~~

'........,

,,,"" A2,-',"

'~'-," ,'- .,,\"

\,,",--,...._-,

\,,,,,

60W 55W 50W 45W 40W

Figure 14: B) Trajectories of the eddies, estimated from the looping float trajecto-nes.

32

Tab

le V

II: S

umm

ary

of E

ddy

Cha

ract

eris

tics

Est

imat

ed f

rom

Loo

ping

Flo

at T

raje

ctor

ies

at 8

00 m

Num

ber

Per

iod

ofSw

irl

Mea

n V

eloc

ity

Dur

atio

nof

Rot

atio

nV

eloc

ityD

iam

eter

( em

/see

)E

KE

Floa

tD

ates

( da

ys)

Loo

ps(

days

)(e

m/s

ee)

(km

)u

v(c

m2/

sec2

)

Ant

icyc

lone

s

Al

28A

89 01 25 - 89 05 16

111

2.8

40-1

5.8

173

-7.8

1.5

125

A2

B63

89 06 21 - 89 08 27

672.

626

-17.

712

5-4

.66.

815

8

A3

28B

89 08 25 - 90 01 22

150

13.7

11-2

4.0

72-7

.43.

528

8

~ ~B

22B

90 01 21 - 90 03 14

527.

57

-16.

932

-9.9

6.1

144

C22

A89 02 08 - 89 04 23

743.

025

- 11. 7

79-5

.02.

868

Cyc

lone

D23

B90 04 30 - 90 07 15

762.

926

7.6

551.

50.

129

The

mea

n tr

ansl

atio

n ve

loci

ty o

f th

e an

ticyc

lone

s fr

om th

e 5

indi

vidu

al e

stim

ates

is u

= -

6.9

:f 1

.0 e

m/s

ee, v

= 4

.2:f

1.0

em

/see

or

8.1 em/see toward 3010.

The

num

ber

of lo

ops

was

est

imat

ed v

isua

lly a

nd u

sed

to c

alcu

late

the

peri

od o

f ro

tatio

n. S

wir

l vel

ocity

was

est

imat

ed a

s be

ing

equa

lto

the

root

mea

n sq

uare

(R

MS)

vel

ocity

of

a fl

oat a

bout

its

mea

n ve

loci

ty. D

iam

eter

(D

) of

the

loop

s w

as e

stim

ated

fro

m th

e m

ean

period of rotation (T) and mean swirl velocity (Ve) with the relation D = (VeT)/,Tr. The mean translation velocity of each eddy was

estim

ated

by

calc

ulat

ing

the

mea

n ve

loci

ty o

f ea

ch f

loat

. Edd

y K

inet

ic E

nerg

y (E

KE

) w

as e

stim

ated

fro

m th

e av

erag

e of

the

u an

dv

velo

city

var

ianc

es a

bout

thei

r re

spec

tive

mea

n ve

loci

ty v

alue

s.

15N

10N

~ .¡

10S 50

W

5N o 5S

45W

25W

iow

5W20

W15

W40

W35

W30

W

Figure 15: Trajectories of six 800 m floats that drifted eastward in equatorial currents. Float 24, which was

balla

sted

to li

e ne

ar th

e ce

nter

of t

he e

astw

ard

jet (

Fig

ure

7), e

quili

brat

ed n

ear

1125

m.

ral change because the 800 m floats would have stil been well within the equatorialjet seen on Figue 7. The one deeper float (24) would have been near 1350 m whenit reversed, close to the lower limit of the jet. The mean westward velocity of thewestbound trajectories in the box 5W--0W, 5S-6N was 7.8:l 1.4 cm/sec, roughlyequal to the eastbound velocities.

e) Southward velocity

Three of the five 800 m floats launched near the equator (16, 24, and 34) plus twoothers launched near 9N (26) and 11N (19) drifted on average southward betweenBrazil and Africa. This can be seen in the southward til of their trajectories anddisplacement vectors (Figure 12). The mean southward velocity of these five floatswas 1.3:: 0.3 cm/sec, with the standard error estimated from the five individualmean velocity vaues. Although the mean southward velocity of all observations inthe box 5W -40W, 5S-6N was 0.8:: 1.0 cm/ sec, not significantly different from zero,the southward trend of the trajectories and displacement vectors suggests that thesouthward velocity might be of importance.

f) IWBC-equatorial current connection at 800 m

A schematic diagram of the inferred mean circulation at 800 m is shown in Figure 16.The IWBC is interpreted to be continuous along the boundar with par feedinginto the equatorial current. Since the eastward equatorial current only reversed

near the end of the 21 months, the mean circulation shown here does not include awestward equatorial current. If the westward flow persists for the same duration asthe eastward flow, then a schematic of the longer term circulation at 800 m mightlook like Figure 10 with its arows reversed in direction.

:'1~

i,

r¡

6 Summary and Conclusions

SaFAR floats have given a fist Lagrangian view of flow in the upper core of theDWBC, its connection to equatorial currents, and its cross-equatorial flow. TheDWBC at 1800 m was found to be a narow, 100 km wide jet, flowing with peakspeeds of 55 cm/sec and peak average (10 km bin) speeds of 26 cm/sec. Roughly39% of its 14.7 x 106 m3/s transport recirculated between the current and theMid-Atlantic Ridge, leaving around 9 x 106 m3/s to cross the equator. At times

DWBC water flowed eastward along the equator long distances. At other times,when the equatorial current was westward, the DWBC crossed the equator, joined

35

~ 0)

200N

. ,,i 'IL" . .

800

m'.. ~

. ,.

lOO

N

Rev

ersi

ng E

quat

oria

lC

urre

nts

00

100S

600W

50°

400

300

200

100W

Figu

re 1

6: S

chem

atic

dia

gram

sum

mar

zing

the

21 m

onth

s of

800

m f

loat

dat

a. T

he d

irec

t con

nect

ion

betw

een

the

IWB

C a

nd th

e eq

uato

rial

cur

rent

was

not

obv

ious

fro

m th

e tr

ajec

tori

es b

ecau

se th

e IW

BC

flo

ats

wen

t nor

thw

est-

war

d an

d th

e eq

uato

rial

flo

ats

wen

t eas

twar

d. T

he s

chem

atic

sho

wn

here

is th

us b

ased

par

tially

on

a co

nsid

erat

ion

of c

ontin

uity

.

by flow turing south from the equator. Thus the equatorial curent sems toserve as a temporar reservoir for DWBC water. Variations in the DWBC and itscross-equatorial flow seem to be linked to low-frequency vaations of the equatorial

current. The inferred period of the vaations is around three years basd on the21 months of data. The longer term drft of the floats over an additional two yearshould provide a better picture of the varations of these curents and the longer-term fate of DWBC water.

The 3300 m float trajectories look very different from the 1800 m ones; noindication of a DWBC was observed and the mean velocity was slow. We concludethat these floats were located in a low velocity layer separating the upper and lowercores of both freon and velocity in the DWBC. Thus the DWBC is very dierentoff Northeast Brazil than off Abaco, where a single southward flowing jet extendedfrom around 1000 m to the sea floor near 4700 m.

Most visually striking of the 800 m trajectories is the long eastward drift offloats between 5S-6N, which suggests large transports. The reversal in directionof several floats near the end of the 21 months implies that the flow vared with aperiod of around three years. Large southward transport in the equatorial band issuggested by the southward tilt of five trajectories there. If the equatorial currentsare tilted on average, their reversal could also cause a reversal of transport, whichimplies that the cross-equatorial flow could have large low-frequency varations atthis depth.

At 800 m, a northwestward-flowing IWBC was observed north of the equa-tor, bounded in the offshore direction by counterflow which fed into the equatorialcurrent from as far north as UN. Around half of the transport per unit depth inthe IWBC consisted of a series of three anticyclonic eddies that translated up theboundar. One was tracked all the way from the equator to 1 IN. North of 7N theanticyclones are inferred to be subsurface manifestations of

North Brazil Current

retroflection eddies.

Acknowledgments

FUnds were provided by National Science Foundation grants OCE-8521082, OCE-8517375, and OCE-9114656. J. R. Valdes, R. Tavares, B. Guest and G. Tupperwere in charge of the SOFAR floats and listening stations which were launchedfrom the RV Oceanus and RV Iselin. M. Zemanovic and C. Wooding tracked thefloats, generated figures, and calculated statistics. R. Goldsmith created the routineto calculate distance and direction of a float from topographic contours. G. Hufford

37

helped with the graphics and made a video of the trajectories which helped us tointerpret them. B. Gafron typed the manuscript.

References

Böning, C. W., and F. A. Schott, 1992. The WOCE model in the equatorialAtlantic: deep currents and the equatorial salnity tongue. Unpublished

manuscript.

Colin, C., J. M. Bore, R. Chuchla, and D. Corre, 1991. Programme Noe, Resultatsde courantometrie. Centre ORSTOM de Cayenne: Documents ScientifiquesNo. O.P.IV 1991, 54 pp.

Ponte, R. M., J. Luyten, and P. L. Richardson, 1990. Equatorial deep jets in theAtlantic Ocean. Deep-Sea Res., 37, 711-713.

Schmitz, W. J., Jr., and P. L. Richardson, 1991. On the sources of the FloridaCurrent. Deep-Sea Res., 38, Suppl. 1, S379-S409.

Uchupi, E., 1971. Bathymetric atlas of the Atlantic, Caribbean, and Gulf ofMexico. Woods Hole Oceanog. Inst. Tech. Rept., WHOI-71-72, 10 pp.

38

Appendix A: Summary Composites of Trajectories

The following figues include: (1) sumares of al trajectories and displacementvectors at eac depth (6 figues), (2) sumares of eastbound and westbound floatsnear the equator at 800 m (floats 16, 19,24,26,31,34) and 1800 m (floats 1,5,6,9)(4 figues), and (3) threemonth composites of al floats at each depth (12 figues).

39

~to

~0T"

~toT"

~~0N

.ri...... ~l !'" to.. N

~0C0

rJ ~~I- to~ C"0E-U ~~ 0I- ~.c~E- ~S to0 ~0CO ~0

to

~toto

~0to

~to

Z Z Z Z torJ rJ0 to 0 to 0 to 0N T" ,. ,.

40

~i.

~0..

~i...

1~

~~ 0C0 C\

.~-.... ~l !'".. i.C\

rJ ~~ 0

C"a~1

E-u ~~~ i.C"

E-Z~ ~~ 0~ -.U~l- ~~ i.rJ -.i-~S ~0 00 i.co

~i.i.

~0to

~i.Z Z Z Z

CDrJ if

0 i. 0 i. 0 LO 0C\ .. .. ..

41

~a..

~i...

. ~o. a

N.. 0

~- :

~i.N

~~a

rn Cl~~p:0E-U ~~ i.I- Cl~~E-

:: ~aa aco ~..

~i.~

~ai.

~in

Z Z Zin

rnL. a in a in.. ..

42

~o..

~LO..

... ~0. 0

N.. 0

¡~

. :

~LON

m~0E-U ~ ~~ N 0~ ,~ .0. _. D C"

f- CD

Z~:: ~~ LO

U C"

~~~mi- ~~ 08

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61

62

Appendix B: Plots of Individual Floats

The followig figures are ordered by increasing depth into four groups: (1) Bobbers,(2) 800 m floats, (3) 1800 m floats, and (4) 3300 m floats. Three plots are includedfor eac float: a common-area trajectory plot with arowheads spaced at intervas of30 days, a trajectory enlargement showing daily positions and dates every 30 days,and velocity vectors and eastward and northwad velocity components. Maximumand minimum pressures are added for Bobber floats.

63

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TROPICAL ATLANTIC B 62

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DOCUMENT LIBRAY

March 11, 1991

Attn: Stella Sanchez-WadeDocuments SectionScripps Institution of OceanographyLibrary, Mail Code C-075CLa Jolla, CA 92093Hancock Library of Biology &

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Mac 90-32

50272-101

REPORT DOCUMENT A TION ì 1. REPORT NO.PAGE WHOI-92-33 T23. Recipient's Accession No.

SOFAR Float Trajectories from an Experiment to Measure the Atlantic Cross Equatorial Flow(1989-1990)

5. Repor DateAugust, 19924. Title and Subtitle

6.

7. Author(s) Philp L. Richardson, Marguerite E. Zemanovic, Christine M. Wooing,Wilia J. Schmitz, Jr and James F. Prce

9. Perfonning Organization Name and Address

8. Performing Organization Rept. No.WH0I92-33

10. ProjectfaskIork Unit No.

The Woos Hole Oceaographic InstitutionWoods Hole, Massachusett 02543

11. Contract(C) or Grant(G) No.

(C) OCE-8521082, OCE-8517375,

(G) OCE-9114656

12. Sponsoring Organization Name and Address13. Type of Report & Period Covered

Funding was provided by the National Science Foundation.Technica Report

14.

15. Supplementary Notes

This report should be cited as: Woods Hole Oceanog. Inst. Tech. Rept., WHOI-92-33.

16. Abstract (Limit: 200 words)

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity ofwestern bounday currents near 6N and at several sites in the Atlantic near I1N and along the equator. Trajectories at 1800 m show aswift (::50 cm/sec), narow (100 km wide) southward-flowing deep western bounda current (DWBC) extending from 7N to theequator. At times (Februar-March 1989) DWBC water turned eastward and flowed along the equator and at other times (August-September 1990) the DWBC crossed the equator and continued southward. The mean velocity near the equator was eastward fromFebruar 1989 to Februar 1990 and westward from March 1990 to November 1990. Thus the cross-equatorial flow in the DWBCappeared to be linked to the direction of equatorial currents which vared over periods of more than a year. No obvious DWBC norswift equatorial current was observed by 3300 m floats.

Eight-hundred-meter floats revealed a northwestward intermediate level western bounday current although flow patterns werecomplicated. Three floats that significantly contrbuted to the northwestward flow loope in anticyclonic eddies that translated upthe coast at 8 em/sec. Six 800 m floats drifted eastward along the equator between 5S and 6N at a mean velocity of 11 em/see; onereached 5W in the Gulf of Guinea, suggesting tliat the equatorial current extended at least 35-400 along the equator. Three of thesefloats reversed direction near the end of the tracking period, implying low frequency fluctuations.

17. Document Analysis a. Descriptors

1. SOF AR floats2. Equatorial curents3. deep western bounda current

b. Identifiers/Open-Ended Terms

c. COSATI Field/Group

18. Availability Statement 19. Security Class (This Report)

UNCLASSIFIED21. No. of Pages

197Approved for publication; distribution unlimited. 20. Security Class (This Page) 22. Price

(See ANSI-Z39.18)See Instructions on Reverse OPTIONAL FORM 272 (4-77)

(Formerly NTIS-35)nAn::rtmPnt of CommArr.p

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