North Brazil Current Rings Experiment : surface drifter data report, November 1998-June 2000

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North Brazil Current Rings Experiment: Sunace Drifter Data ReportNovember 1998 -June 2000

by

Deborah A. Glickson, DavidM. Fratantoni,Christine M. Wooding, and Philip L. Richardson

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Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

July 2000

, Technical ReportFunding was provided by the National Science Foundation under Grant No. OCE-9729765.

Approved for public release; distribution unlimited.

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WHOI-2000-10

North Brazil Current Rings Experient:Surface Drifter Data ReportNovember 1998 - June 2000

by

Deborah A. GlicksonDavid M. Fratantoni

Christine M. WoodingPhilip L. Richardson

July 2000

Technical Report

Funding was provided by the Offce of Naval Researchunder Grant No. OCE-9729765.

Reproduction in whole or in part is permttedfor any purose of the United States Government.

This report should be cited asWoods Hole Oceanog. Inst. Tech. Rept., WHOI-2000-1O.

Approved for public release; distrbution unimted.

Approved for Distrbution:

-- ~7.l"' .A . eATerrence M Joyce, Chai

Departent of Physical Oceanography

Abstract

This data report summarzes 45 surface drfter trajectories collected betweenNovember 1998 and June 2000 as par of the Nort Brazil Current (NC) Rings Experiment.NBC rings have been proposed as one of several important mechanisms for the transport ofSouth Atlantic upper-ocean water across the equatorial-tropical gyre boundar and into theNorth Atlantic subtropical gyre. Such transport is required to complete the meridional

overturning cell in the Atlantic forced by the high-latitude production and southward export ofNorth Atlantic Deep Water. The goal of this program is to obtain, for the first time,comprehensive observations of the NBC retroflection, the NBC ring formation process, andthe physical strcture and propertes of NBC rings as they translate northwestward along thelow-latitude western boundar. A total of 45 drfters were deployed. Twenty-four of these

looped anticyclonically within the five rings identified during this experiment. Seven of thelooping ring drfters entered the Carbbean, while the rest moved northward along the easternflank of the Lesser Antiles.

Front Cover Figure Caption: Drifter trajectories for four of the five rings identified byshipboard surveys during the North Brazil Current Rings Experiment.

11

Table of Contents

Abstract.......................................................................................................................... .............j

1. Introduction.............................................................................................................................1

2. Description of the Drifters... .............. ........... ........ ................. .......... ............ ......... ..................1

3. Drifter Deployment.................................................................................................................2

4. Drifter Performance................................................................................................................ 3

5. Drifter Data Processing and Tracking ......................................................... .......... .................3

6 . Acknowledgments.................................................................................................................. 5

7. References...............................................................................................................................5

Appendix A: Drifter displacements and trajectories................................................................... 7

Appendix B: Drifter trajectories within rings ..........................................................................13

Appendix C: Individual drfter tracks and drfter property plots..............................................21

Appendix D: Surface drfters launched in June 2000.............................................................113

List of Figures

1. Surface drifter duration char ..... ........ ...... .............. ........ ..... ... ......... ............... .........................2

2. Filter used to smooth drfter position and times series data ...................................................3

List of Tables

1. Number of drfters launched during each cruise.....................................................................1

2. Surace drfter summar - launch end positions ...................................................................4

3. Surface drfter summar - launch end positions, June 2000..............................................113

11

iv

1. Introduction

North Brazil Current (NC) rings are large (400 km diameter) anticyclones that pinchoff from the NBC retroflection in the western tropical Atlantic near 8N and translatenorthwestward along the coast of South America toward the Carbbean (Johns et ai., 1990;Didden and Schott, 1993; Richardson et ai., 1994; Fratantoni et ai., 1995). NBC rings havebeen proposed as one of several important mechanisms for the transport of South Atlanticupper-ocean water across the equatorial-tropical gyre boundar and into the Nort Atlanticsubtropical gyre. Such transport is required to complete the meridional overtng cell

(MOC) in the Atlantic forced by the high-latitude production and southward export of NortAtlantic Deep Water. The mechanisms that contribute to NBC ring formation and thestrcture and dynamcs of the rings themselves are not well understood. The purose of thisstudy is to obtain, for the first time, comprehensive observations of the NBC retroflection, theNBC ring formation process, and the physical structure, and properties of NBC rings as theytranslate northwestward along the low-latitude western bounda. The goal is to understandthe process of NBC ring generation and to quantify the role of NBC rings in cross-equatorialand cross-gyre transport within the Atlantic MOC. Specific objectives of the drftercomponent of the NBC Rings Experiment are to:

1. Measure and describe the physical strcture of NBC rings after separation from theNBC, including both near-synoptic uantime-evolving temperatue and velocitycharacteristics.

2. Determne characteristic rates of translation, mixing, and decay as NBC rings movenorthwestward toward the Carbbean Sea.

3. Identify the long-term fate of South Atlantic water trapped within the ring core, and

determne the effective intergyre ring transport in several temperature/density

watermass classes.

A total of 45 drfters was launched from RN Seward Johnson durng three NBC RingsExperiment survey cruises (Figure 1 and Table 1). Ten drfters were provided by WHOI and35 were provided by AOML. Five NBC rings were identified during the three cruises.Twenty-four drfters were launched in these rings. Subsurface RAOS floats (not describedhere) were also launched to track the rings and observe their evolution.

Table 1. Number of driters launched durig each crue.

Cruse Beginnng Date Ending DateDrifters Launched

Total Into Rinfls

NBC 98 11-07-98 12-11-98 16 5

NBC 99 02-06-99 03-09-99 16 13NBC 00 01-29-00 02-24-00 13 6

2. Description of the Drifters

The surface drfters used in this study are similar in constrction to the Wodd OceanCirculation Experiment (WOCE) and Tropical Ocean-Global Atmosphere (TOGA)Lagrangian drfters described by Sybrandy and Niier (1991). The drfter consists of a

spherical surface float 35 cm in diameter, a 0.56 cm diameter plastic impregnated wire tether

1

with a 20 cm diameter subsurace float located 275 cm below the surace, and a 644 cm longcylindrcal cloth drogue (92 cm diameter) with circular holes in its sides. The fiberglasssurface float contains a radio transmitter, batteries, antenna, and sensors including athermometer and strain gauge sensor that indicates whether or not the drogue is attached. Thedrogue is centered at a depth of 15 m. The ratio of the drag area of the drogue to the drag areaof the tether and float is approximately 41:1, which results in the drogue's slip though thewater being less than 1 cm/sec in winds of 10 m/sec (Niiler et aI., 1995).

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OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN1998 1999 2000Figure 1. Surface drifter duration chart Drifter numbers are shown to the left of the black line that indicateslifespan. Drifters are listed in order of launch date.

3. Drifter Deployment

To choose suitable locations for drfter and subsurface float deployment, each cruisebegan by mapping the upper-ocean temperature and velocity structure using expendablebathythermographs (XBTs) and a shipboard ADCP. This enabled a detennnation of eachring's center, overall shape and intensity. Surace drfters were then deployed to measure thecirculation in and around NBC rings as they translate and evolve. A single drfter was alsoused to track Ring E's westward translation between consecutive legs of the third cruise.

2

4. Drifter Performance

Table 2 lists the launch and end positions for each drfter, as well as its lifetime. Theaverage lifetime of the NBC drfters up to June 1, 2000 was 168 days. Of the 45 drfters, 7stopped transITtting within a month. As of June 1, 2000, 9 were stil drfting and activelytransIT tting.

5. Drifter Data Processing and Tracking

Service Argos satellites forwarded the surface drfter positions and transITssions toWHOT via email. At least five data messages had to be received in one pass for the drfterposition to be computed by Argos. Position errors are estimated to be around 300 m. Themost frequently transITtted data message in a pass was used for temperatue, drogue, andvoltage values at each location. Coefficients for conversion of raw data to engineering units

were provided by AOML. The data were linearly interpolated at a six-hour spacing toproduce four records per day. A two-day half-width Gaussian filter (Figure 2a) was appliedto suppress tidal and inertal varabilty. A comparson of the unfitered and fitered data isshown in Figure 2b. Velocities were computed from the fitered position time series using acubic spline function. Temperature measurements were filtered by removing data points morethan 3 standard deviations from the mean, then takng a two day running mean of theremaining data.

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3

Table 2. Surface drifter sury -launch and end positions.

Drifter Launch Launch Position End Position Life Drogue Grounded LaunchedID Date Lon. LaL Lon. LaL (days) off (date) (date) in Rig

01996 9904 1 1 -52.796 8.340 -59.158 23.927 245 D01997 990411 -52.l61 8.318 -19.631 1.795 40102860 9811 17 -49.818 9.160 -57.569 23.556 26902861 981117 -51.463 9.838 -61.79 14.633 79 99020403666 981122 -45.777 6.733 -58.765 8.463 101

03667 981122 -47.519 7.117 -59.531 22.517 556*03668 981124 -47.204 3.152 -47.423 3.439 1

03669 981123 -46.504 4.327 -45.955 13.048 122

04888 981128 -47.755 2.164 -53.969 12.250 5604889 981124 -45.897 5.041 -61.231 14.809 554* 990516 99032005133 981129 -49.387 6.379 -50.570 11.660 158

06207 9811 12 -52.091 10.915 -52.091 10.915 006208 981210 -56.389 9.216 -57.648 8.415 17 A09167 000220 -57.252 10.088 -59.460 13.163 62 000324 E09632 990217 -52.716 8.952 -71.641 22.531 458 C

09633 990214 "57.528 10.039 -67.166 18.460 149 990611 B

09634 990212 -56.842 10.486 -78.406 27.691 474* B

09635 990220 -52.328 9.554 -64.690 38.729 443 C

09636 981206 -50.569 6.506 -61.245 12.988 190 990317 B

09637 981210 -56.263 8.851 -56.673 9.307 7 A09638 981206 -49.536 6.211 -41.048 8.003 62 B

09639 981206 -50.030 6.306 -63.944 27.771 542* B

09898 000220 -56.657 9.469 -60.903 14.618 100* 000403 E12200 990214 -57.223 10.246 -79.860 19.705 319 990908 B

l2201 990220 -52.190 10.034 -61.815 16.262 83 990513 990514 C

12232 990302 -48.357 5.466 -60.532 1 1.294 94 990531 990526 D

15989 990217 -52.734 9.029 -86.104 29.329 469* C

16005 990305 -47.312 2.986 -53.884 13.010 48 D

16006 990219 -52.579 8.741 -64.518 25.844 467* C

16369 990219 -52.432 9.140 -53.687 9.100 9 C

16370 990222 -51.721 6.133 -51.743 6.934 216616 981 11 1 -51.23 12.990 -51.23 12.990 017804 990222 -51.595 6.155 -55.902 10.849 19

17805 990212 -56.846 10.258 -58.189 9.073 18 B

18802 000220 -57.016 9.642 -60.653 14.977 40 0003 17 E18804 000217 -55.381 9.416 -46.053 10.409 8618805 000209 -55.258 9.187 -44.259 16.930 112*

l8810 00022l -56.992 10.055 -58.191 17.011 100* E18811 000220 -56.232 9.332 -61.353 16.333 101 * 000328 000416 E18812 000217 -55.046 9.725 -47.362 11.692 104*

18813 000219 -55.783 9.280 -49.801 9.441 6218815 000216 -56.249 8.862 -60.889 13.913 104* 000510 . 000510 E18817 000217 -55.768 9.191 -59.475 13.175 66 000305 00041518818 000215 -55.422 9.264 -48.734 14.406 106*18819 000215 -55.419 9.261 -46.368 13.147 97* still alive as of June 1,2000

4

6. Acknowledgments

The assistance of the captain and crew of the RN Seward Johnson is gratefullyacknowledged. We thank the paricipants of the three NBC Ring cruises for their assistancein launching drfters. Thirty-five drfters were provided for launch by the Global Drifter

Center of NOAAlAOML. We thank Mark Swenson, Doug Wilson, and Silvia Garoli fortheir cooperation in acquiring, deploying, and tracking these drfters., Financial support forthis research program was provided by the National Science Foundation through Grant No.OCE-9729765.

7. References

Didden, N. and F. Schott, 1993. Eddies in the North Brazil Current retroflection regionobserved by Geosat altimetry. J. Geophys. Res., 98(CLL), 20121-20131.

Fratantoni, D.M., W.E. Johns and T.L. Townsend, 1995. Rings of the North Brazil Curent:Their structure and behavior inferred from observations and a numerical simulation.J. Geophys. Res., LOO(C6), 10633-10654.

Johns, W.E., T.N. Lee, F.A Schott, R.J. Zantopp and R.H. Evans, 1990. The Nort BrazilCurrent retroflection: Seasonal structure and eddy varability. J. Geophys. Res., 95 (C12),22103-22120.

Niiler, P.P., AL. Sybrandy, K. Bi, P.M. Poulain, and D. Bitterman, 1995. Measurements ofthe water-following capability of holey-sock and TRISTAR drfters. Deep-Sea Res. I, 42,1951-1964.

Richardson, P.L., G.B. Hufford, R. Limeburner, and W.S. Brown, 1994. North Brazil Currentretroflection eddies. J.Geophys.Res., 99(C3), 5081-5093.

Sybrandy, AL. and P.P. Niiler, 1991. The WOCErrOGA Lagrangian drfter constrctionmanual. SIO Ref. 91/6, WOCE Rep. 63, 58 pp. Scripps Institution of Oceanography, La Jolla,CA

5

6

Appendi A:Drifter displacements and trajectories.

The following figures present drfter vector displacements and trajectories for alldrfters launched in the NBC Rings Experiment. Launch positions are marked with a plussign and end positions with a circle-dot symbol and the drfter il. Drifter tracks are solid

black lines. A dashed line represents the 200 m isobath, and dotted lines are isobaths in 2000m increments. Both the displacements and trajectories are shown first at a small-scale,encompassing most of the drfter trajectories, and then at a larger scale, showing only theCarbbean and NBC area.

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Appendi B:Drifter trajectories within rings.

This section presents diagrams of the drfter trajectories for each ring identified in thisstudy. Time ticks (small filled circles) are shown every two days, and the date ('yydd') isappended to small open circles on the first and fifteenth day of each month (unless removedfor clarty). Individual drfter trajectories are not labeled, but a list of drfters shown is

provided in the upper right-hand comer of each figure. Drifter launch and end positions aremarked as in Appendix A, as are isobaths. Drifter tracks are solid black lines, unlessindicated otherwise.

13

Drifters launched into NBC Ring A, 199820N

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Appendix C:Individual drifter tracks and drifter property plots.

The following section presents individual drfter trajectories and temperature, position,and velocity time series measurements. The trajectories are presented at a large, flexible scale(top) to show detail, and at a fixed scale (bottom) for comparson to other drfters in andaround the study area. All features are marked as in Appendices A and B, although time ticksand isobaths (except at 200 m) have been omitted from the fixed scale plots.

Drifter property plots show temperatue, longitude, latitude, u (east) velocity, v (north)velocity, and velocity stick diagrams. The lower x-axis marks the number of days in thedrfter record. Measurements were interpolated to six hour intervals. Temperature

measurements were filtered by removing data points more than 3 standard deviations from themean, then takng a two day running mean of the remaining data. The upper x-axis shows aGregorian calendar corresponding to the record day in the lower x -axis. Thin double lines

mark the day where the drogue fell off before the drfter stopped transmitting.

21

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Appendi D:Surface drifters launched in June 2000.

Four surface drfters were launched durng the NBC Rings Experiment mooringrecovery cruise in June 2000 (Table 3). This appendix presents trajectories and time seriesmeasurements from the thee drfters that transmitted data. A composite plot showing alldrfter trajectories is presented first, followed by individual drfter trajectories and propertyplots. The presentation format follows that of the previous appendices. Temperatue anddrogue data for the June 2000 drfters were unavailable for inclusion in ths summar.

Table 3. Surface driter sury - launch and end positions, June 2000.

Drifter Launch Launch Position End Position

il Date Lon. Lat. Lon. Lat.Life (days)

l2203 000610 -47.342 3.581 -32.540 5.876 52*17277 000610 -46.4 10 4.670 -47.777 2.134 2217352 000611 -45.670 5.364 -27.721 5.103 52*

* still alive as ofJuly 31, 2000

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DOCUMENT LmRAYDistrbution List/or Technical Report Exchange -July 1998

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50272-101

REPORT DOCUMENTATION 11. REPORT NO.PAGE WHOI-2000-104. Title and Subtite

North Brazil Current Rings Experiment: Surface Drifter Data ReportNovember 1998 - June 2000

12.

3. Recipient's Accession No.

5. Report DateJuly 2000

6.

7. Author(s) Deborah A. Glickson, David M. Fratantoni, Christine M. Wooding, Philp L. Richardson 8. Performing Organiztion Rept. No.

WHOI-200-109. Performing Organization Name and Address 10. Projecask!ork Unit No.

Woods Hole Oceanographic InstitutionWoods Hole, Massachusetts 02543

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

(C) OCE-9729765

(G)

12. Sponsoring Organizaion Name and Addres

National Science Foundation

13. Type of Report & Period Covered

Technca Report

14.

15. Supplementary Notes

Ths report should be cited as: Woods Hole Oceanog. Inst. Tech. Rept., WHOI-2000-10.

16. Abstract (Limit: 20 words)

This data report summarzes 45 surace drfter trjectories collected between November 1998 and June 2000 as par of the NorthBrazil Curent (NC) Rings Experiment. NBC rings have been proposed as one of several important mechanisms for thetransport of South Atlantic upper-ocean water across the equatorial-tropical gye boundar and into the North Atlantic subtropicalgyre. Such transport is required to complete the meridional overting cell in the Atlantic forced by the high-latitude productionand southward export of North Atlantic Deep Water. The goal of this program is to obtain, for the first time, comprehensiveobservations of the NBC retroflection, the NBC ring fonnation process, and the physical strctue and properties of NBC rings asthey translate northwestward along the low-latitude western boundar. A total of 45 drfters were deployed. Twenty-four of theselooped anticyclonically within the five rings identified durng this experiment. Seven of the looping ring drfters entered the

Carbbean, while the rest moved northward along the eastern flank of the Lesser Antiles.

17. Document Analysis a. DescriptorsTropical Atlantic circulationMesoscae rigs

satellte-tracked drers

b. Identifiers/Open-Ended Terms

c. COSATI Field/Group

18. Availabilty Statement

Approved for public release; distribution unlimited.

19. Seurity Class (This Report)

UNCLASSIFIED21. No. of Pages

12920. Security Class (This Page) 22. Price

(See ANSI-Z39.18) Se Instructions on Reverse OPTIONAL FORM 272 (4-77(Formerly NTIS'35)Department of Commerce

Documents

North Brazil Current Rings Experiment : surface drifter data report, November 1998-June 2000