NOAA Technical Memorandum NMFS-NE-170

Interaction of Shelf Water with Warm-Core Rings,

Focusing on the Kinematics and Statistics

of Shelf Water Entrained within Streamers

U. S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Northeast Fisheries Science Center

Woods Hole, Massachusetts

March 2003
155. Food of Northwest Atlantic Fishes and Two Common Species of Squid. By Ray E. Bowman, Charles E. Stillwell, William L.

Michaels, and Marvin D. Grosslein. January 2000. xiv + 138 p., 1 fig., 7 tables, 2 app. NTIS Access. No. PB2000-106735.

156. Proceedings of the Summer Flounder Aging Workshop, 1-2 February 1999, Woods Hole, Massachusetts. By George R.

Bolz, James Patrick Monaghan, Jr., Kathy L. Lang, Randall W. Gregory, and Jay M. Burnett. May 2000. v + 15 p., 5 figs., 5 tables. NTIS

Access. No. PB2000-107403.

157. Contaminant Levels in Muscle of Four Species of Recreational Fish from the New York Bight Apex. By Ashok D. Deshpande,

Andrew F.J. Draxler, Vincent S. Zdanowicz, Mary E. Schrock, Anthony J. Paulson, Thomas W. Finneran, Beth L. Sharack, Kathy Corbo,

Linda Arlen, Elizabeth A. Leimburg, Bruce W. Dockum, Robert A. Pikanowski, Brian May, and Lisa B. Rosman. June 2000. xxii + 99 p.,

6 figs., 80 tables, 3 app., glossary. NTIS Access. No. PB2001-107346.

158. A Framework for Monitoring and Assessing Socioeconomics and Governance of Large Marine Ecosystems. By Jon G.

Sutinen, editor, with contributors (listed alphabetically) Patricia Clay, Christopher L. Dyer, Steven F. Edwards, John Gates, Tom A. Grigalunas,

Timothy Hennessey, Lawrence Juda, Andrew W. Kitts, Philip N. Logan, John J. Poggie, Jr., Barbara Pollard Rountree, Scott R. Steinback,

Eric M. Thunberg, Harold F. Upton, and John B. Walden. August 2000. v + 32 p., 4 figs., 1 table, glossary. NTIS Access. No. PB2001-

106847.

159. An Overview and History of the Food Web Dynamics Program of the Northeast Fisheries Science Center, Woods Hole,

Massachusetts. By Jason S. Link and Frank P. Almeida. October 2000. iv + 60 p., 20 figs., 18 tables, 1 app. NTIS Access. No. PB2001-

103996.

160. Measuring Technical Efficiency and Capacity in Fisheries by Data Envelopment Analysis Using the General Algebraic

Modeling System (GAMS): A Workbook. By John B. Walden and James E. Kirkley. October 2000. iii + 15 p., 9 figs., 5 tables. NTIS

Access. No. PB2001-106502.

161. Demersal Fish and American Lobster Diets in the Lower Hudson - Raritan Estuary. By Frank W. Steimle, Robert A.

Pikanowski, Donald G. McMillan, Christine A. Zetlin, and Stuart J. Wilk. November 2000. vii + 106 p., 24 figs., 51 tables. NTIS Access.

No. PB2002-105456.

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and Steven L. Swartz, with contributions from (listed alphabetically) Neilo B. Barros, Phillip J. Clapham, Timothy V.N. Cole, Carol P. Fairfield,

Larry J. Hansen, Keith D. Mullin, Daniel K. Odell, Debra L. Palka, Marjorie C. Rossman, U.S. Fish and Wildlife Service, Randall S. Wells,

and Cynthia Yeung. November 2000. ix + 303 p., 43 figs., 55 tables, 3 app. NTIS Access. No. PB2001-104091.

163. Essential Fish Habitat Source Document: Red Deepsea Crab, Chaceon (Geryon) quinquedens, Life History and Habitat

Characteristics. By Frank W. Steimle, Christine A. Zetlin, and Sukwoo Chang. January 2001. v + 27 p., 8 figs., 1 table. NTIS Access.

No. PB2001-103542.

164. An Overview of the Social and Economic Survey Administered during Round II of the Northeast Multispecies Fishery

Disaster Assistance Program. By Julia Olson and Patricia M. Clay. December 2001. v + 69 p., 3 figs., 18 tables, 2 app. NTIS Access.

No. PB2002-105406.

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Michael R. Ross, and Scott R. Steinback. December 2001. viii + 129 p., 1 fig., 81 tables, 4 app. NTIS Access. No. PB2002-108348.

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Walpole, Maine, March 13-14, 2001. By William L. Michaels, editor and coconvenor, and Philip Yund, coconvenor. December 2001.

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Oil Spill. By David B. Packer, ed. December 2001. x + 228 p., 54 figs., 58 tables, 6 app. NTIS Access. No. PB2004-106067.

168. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments – 2001. By Gordon T. Waring, Janeen M. Quintal, and

Steven L. Swartz, eds., Phillip J. Clapham, Timothy V.N. Cole, Carol P. Fairfield, Aleta Hohn, Debra L. Palka, Marjorie C. Rossman, U.S.

Fish and Wildlife Service, and Cynthia Yeung, contribs. December 2001. vii + 310 p., 43 figs., 54 tables, 2 app. NTIS Access. No. PB2002-

104350.

169. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments – 2002. By Gordon T. Waring, Janeen M. Quintal, and

Carol P. Fairfield, eds., Phillip J. Clapham, Timothy V.N. Cole, Lance P. Garrison, Georgia Department of Natural Resources, Aleta A. Hohn,

Blair G. Maise, Wayne E. McFee, Debra L. Palka, Patricia E. Rosel, Marjorie C. Rossman, U.S. Fish and Wildlife Service, Frederick W. Wenzel,

and Cynthia Yeung, contribs. September 2002. vii + 318 p., 42 figs., 56 tables, 4 app. NTIS Access. No. PB2003-104167.

Recent Issues in This Series:
U. S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

National Oceanic and Atmospheric Administration

Vice Admiral Conrad C. Lautenbacher, Jr., USN (ret.), Administrator

National Marine Fisheries Service

William T. Hogarth, Assistant Administrator for Fisheries

Northeast Fisheries Science Center

Woods Hole, Massachusetts

March 2003

Ronald J. Schlitz

This series represents a secondary level of scientific publishing. All issues employ

thorough internal scientific review; some issues employ external scientific review.

Reviews are -- by design -- transparent collegial reviews, not anonymous peer reviews.

All issues may be cited in formal scientific communications.

NOAA Technical Memorandum NMFS-NE-170

Interaction of Shelf Water with Warm-Core Rings,

Focusing on the Kinematics and Statistics

of Shelf Water Entrained within Streamers

Postal Address: National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543

E-Mail Address: Ron.Schlitz@noaa.gov
Editorial Notes

Statistical Terms: The NEFSC Editorial Office’s policy on the use of statistical terms in all technical communications is

generally to follow the International Standards Organization’s handbook of statistical methods (i.e., ISO 1981a).

Internet Availability: This issue of the NOAA Technical Memorandum NMFS-NE series is being copublished, i.e., as

both a paper and Web document. The Web document, which will be in HTML (and thus searchable) and PDF formats,

can be accessed at: http://www.nefsc.noaa.gov/nefsc/publications/.

 aISO [International Organization for Standardization]. 1981. ISO standards handbook 3: statistical methods. 2nd ed. Geneva,

Switzerland: ISO; 449 p.
Page iii

Contents

Abstract ......................................................................................................................................................................... v

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

Methods .........................................................................................................................................................................2

Data Collection .....................................................................................................................................................2

Data Analysis ......................................................................................................................................................2

Modeling Dissipation of a Streamer .....................................................................................................................2

Results and Discussion ..................................................................................................................................................3

Structure, Hydrography, and Volume of Entrained Shelf Water in Streamers ......................................................3

Transport of Shelf Water by Streamers and Dissipation in Slope Water .............................................................5

Source of Entrained Water in Streamers ...............................................................................................................6

Dynamic Behavior of Streamers ...........................................................................................................................6

Conclusions ....................................................................................................................................................................7

Acknowledgments ..........................................................................................................................................................8

References Cited .............................................................................................................................................................8

Figures

Figure 1. Positions of all stations during the major cruises of the Warm Core Rings Program .............................. 10

Figure 2a. Positions of stations at which CTD casts were completed on Albatross IV Cruise No. AL 81-11

during 22 September - 6 October 1981 ..................................................................................................... 10

Figure 2b. Positions of stations at which CTD casts were completed on Albatross IV Cruise No. AL 82-04

during 19 April - 4 May 1982 ................................................................................................................... 11

Figure 2c. Positions of stations at which CTD casts were completed on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982 ..................................................................................................................... 11

Figure 3a. Depth of a layer from the surface containing shelf water entrained by Ring 81-E ................................... 12

Figure 3b. An AVHRR image for 27 September 1981 showing temperature expression at the surface for shelf

water entrained by Ring 81-E ................................................................................................................... 13

Figure 4a. Depth of a layer from the surface containing shelf water entrained by Ring 82-A .................................. 14

Figure 4b. A series of AVHRR images for 20-24 April 1982 showing temperature expression at the surface

as slope water cyclonically encircles the shelf water entrained by Ring 82-A ........................................ 15

Figure 5a. Depth of a layer from the surface containing shelf water entrained by Ring 82-B during 28 April -

3 May 1982 .............................................................................................................................................. 16

Figure 5b. An AVHRR image for 30 April 1982 showing temperature expression at the surface for shelf

water entrained by Ring 82-B .................................................................................................................. 17

Figure 6a. Depth of a layer from the surface containing shelf water entrained by Ring 82-B during 21-29

June 1982 ................................................................................................................................................. 18

Figure 6b. An AVHRR image for 24 June 1982 showing temperature expression at the surface for shelf water

entrained by Ring 82-B ............................................................................................................................ 19

Figure 7. The average salinity decrease for a cross section within a homogeneous rectangular streamer for

daily time steps calculated using a two-dimensional diffusion model ..................................................... 20

Figure 8a. Cross sections of salinity for an initial layer 60 m thick and 40 km wide after diffusion for 10

days at four different rates for horizontal diffusivity, calculated using a two-dimensional

diffusion model........................................................................................................................................ 21

Figure 8b. Cross sections of salinity for an initial layer 60 m thick and 40 km wide after diffusion for 20

days at four different rates for horizontal diffusivity, calculated using a two-dimensional

diffusion model........................................................................................................................................ 21

Figure 9. Comparison of velocities at depths of 21 and 99 m determined from ADCP measurements, and

at depths of 20 and 100 m relative to 300 m determined from geostrophic calculations, on

concurrent nearly parallel tracks across shelf water entrained by Ring 82-B on 23 June 1982 ................ 22

Figure 10. Temperature and salinity sections from CTD data at Stations 71-74 obtained on Albatross IV

Cruise No. AL 82-07 during 17 June - 2 July 1982 .................................................................................... 23

Figure 11. Temperature and salinity sections from CTD data at Stations 67-69 obtained on Albatross IV

Cruise No. AL 82-07 during 17 June - 2 July 1982 .................................................................................... 24

Figure 12. Temperature and salinity sections from CTD data at Stations 63-65 obtained on Albatross IV

Cruise No. AL 82-07 during 17 June - 2 July 1982 .................................................................................... 25
Page iv

Figure 13a. Temperature section from CTD data at Stations 53-46 obtained on Albatross IV Cruise No. AL

82-07 during 17 June - 2 July 1982 ........................................................................................................... 26

Figure 13b. Salinity section from CTD data at Stations 53-46 obtained on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982 ..................................................................................................................... 27

Figure 14. Temperature and salinity sections from CTD data at Stations 5-9 obtained on Albatross IV Cruise

No. AL 82-04 during 19 April - 4 May 1982 ............................................................................................. 28

Figure 15a. Temperature section from CTD data at Stations 28-24 obtained on Albatross IV Cruise No. AL

82-04 during 19 April - 4 May 1982 .......................................................................................................... 29

Figure 15b. Salinity section from CTD data at Stations 28-24 obtained on Albatross IV Cruise No. AL 82-04

during 19 April - 4 May 1982 ................................................................................................................... 30

Figure 16a. Velocity section from CTD data at Stations 6-11 obtained on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982 ..................................................................................................................... 31

Figure 16b. Salinity section from CTD data at Stations 6-11 obtained on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982 ..................................................................................................................... 32

Tables

Table 1. Estimated volume of shelf water by 10-m depth layer within streamers entrained by Rings 81-E,

82-A, and 82-B twice ............................................................................................................................... 33

Table 2. Geostrophically estimated transport of shelf water by station transect within streamers entrained

by Rings 81-E, 82-A, and 82-B twice ....................................................................................................... 34

Table 3. Comparison of geostrophic and measured transport estimates by salinity classes of shelf water

entrained by Ring 82-B in June 1982 ....................................................................................................... 35

Acronyms

AVHRR = advanced very-high-resolution radiometer

CTD = conductivity-temperature-depth instrument

MOCNESS = multiple-opening/closing net and environmental sensing system

NCSE = Northeast U.S. Continental Shelf Ecosystem

NMFS = National Marine Fisheries Service

NOAA = National Oceanic and Atmospheric Administration

OCS = outer continental shelf

PSU = practical salinity unit

WCR = warm-core ring

WCRP = Warm Core Rings Program

XBT = expendable bathythermograph
Page v

ABSTRACT

Three cruises were completed off the northeastern United States during late 1981 and 1982 in cooperation with the

National Science Foundation-sponsored Warm Core Rings Program in order to study entrainment of shelf water and associated biota by Gulf Stream warm-core rings (i.e., pinched-off meanders of the Gulf Stream which form clockwise/anticyclonic

eddies). Hydrographic data were collected with a conductivity-temperature-depth recorder on board the NOAA R/V Albatross IV during 22 September - 6 October 1981, 19 April - 4 May 1982, and 17 June - 2 July 1982. These hydrographic data

defined the “streamers” of entrained shelf water which were associated with Rings 81-E, 82-A, and 82-B twice, and formed the

basis for the present results and discussion.

The amount of shelf water, delineated by salinity < 34 practical salinity units, varied from 58 km3

 in a nearly isolated

bowl associated with Ring 82-A, to 290 km3 associated with the second visit to Ring 82-B. Geostrophically calculated

transports across sections of the streamers ranged from 488 x 103 m3 sec-1 in the same direction as the azimuthal velocity for

Ring 82-B, to 345 x103

 m3 sec-1 in the opposite direction as the azimuthal velocity for Ring 82-A.

Although each streamer was different in shape and size, a saddle point – a portion of the streamer with a pronounced cross-sectional narrowing – was present within its structure. This saddle point appears to be related to the inshore

periphery of a counterclockwise/cyclonic eddy that develops near the head of the streamer.

Analysis of water properties leads to the conclusion that the source of shelf water for streamers is primarily seaward

of the cold band, present during the stratified season (i.e., April-September), at water depths approximately 100 m. Therefore,

passage of warm-core rings is not likely to have a major direct effect on the biota of the continental shelf, including larvae of

commercially important fish species found between Cape Hatteras and Nova Scotia, since their distributions are generally

concentrated at shallower depths.

Page 1

INTRODUCTION

Understanding variation of biological components is a

focus of the ecological studies of the Northeast U.S. Continental Shelf Ecosystem (NCSE). For populations of commercially important species in the NCSE, a major source of

that variation is recruitment – the number of individuals in

the population that annually survive and grow to reach a

catchable size. Recruitment in commercial fish populations

is highly variable, and the causes of mortality that limit

recruitment are not well understood (Hennemuth et al. 1980).

Advection of eggs and larvae from a preferred habitat

has long been hypothesized as a large source of prerecruit

mortality in commercial fish populations (Iles and Sinclair

1982), capable of causing occasional catastrophic losses

(Walford 1938). One source for advective loss of shelf water from the NCSE is the interaction of warm-core rings

(WCRs) with shelf water at the front separating shelf water

and slope water (i.e., the shelf-slope front or shelfbreak

front). Several authors describe WCRs and associated processes; see the review of Richardson (1983) and the special

series of papers introduced by Joyce (1985). Garfield and

Evans (1987) document statistics for the entrainment of shelf

water, mainly derived from satellite thermal imagery over

the 1979-85 period.

Morgan and Bishop (1977) found a feature with salinity

<34‰ (i.e., a conservative, unambiguous indicator of shelf

water) at least 90 km from the edge of the continental shelf

and to the east of a WCR. However, the precise extent of

the shelf water feature was not determined due to the sparse

distribution of hydrographic stations. They concluded that

a transport of 8.9 x 103

 m3 sec-1 was likely for water with

salinity <34‰.

Smith (1978) speculated on the interaction between a

WCR well south of the Scotian Shelf and fluxes of heat and

salt onto the shelf below 50 m. A volume of entrained shelf

water on the eastern and western sides of the ring was

calculated as 2500 km3

, based on an analysis of an image

derived from an advanced very-high-resolution radiometer

(AVHRR) on board NOAA’s polar-orbiting, environmentalsensing satellites, and on an assumed depth of 50 m for the

entrainment. However, shelf water found west of the ring

was probably caused by a second ring located farther toward the west.

Fornshell and Criess (1979) provided additional evidence for offshore flow from the continental shelf, finding

water with proper temperature and salinity characteristics

on the south side of a ring, and patches of similar water on

the southeast side of it.

Bisagni (1983) reported on a direct measurement of flow

from two satellite-tracked buoys, each equipped with a

drogue at 10 m depth and placed at the 106-Mile Dumpsite

(39°N, 72°W). The buoys first moved southwest at <25 cm

sec-1, then anticyclonically at up to 54 cm sec-1 around WCR

80-A. (The nomenclature used in this paper for WCRs follows the scheme of the National Marine Fisheries Service’s

(NMFS’s) Narragansett (Rhode Island) Laboratory: the twodigit year of formation is given first, and the alphabetical

sequence of formation in that year is given second.) Interpretation of thermal images from satellites showed that these

buoys were within shelf water, but always well seaward of

the shelfbreak. Bisagni (1983) estimated the transport of

the feature as 1.5 x 105

 m3 sec-1, assuming a mean speed of 25

cm sec-1, depth of 50 m, and width of 12 km.

During the U.S. Department of Energy’s Shelf Edge Exchange Processes Experiment, WCR 83-D passed through a

linear array of moored instruments across the continental

shelf and slope south of Marthas Vineyard (Churchill et al.

1986). Based on temperature and salinity data from a mooring located at a 125-m depth, and on an expendable

bathythermograph (XBT) section farther to the east that

crossed the eastern half of the ring about 2 wk later, Churchill

et al. (1986) hypothesized that subsurface water from the

cold band encircled the WCR at approximately 50 m. The

minimum temperatures of the water were <10°C over the

shelf and <14°C at the southern end of the ring. Unfortunately, there were no salinity data to distinguish between

shelf water and slope water at the southern end. There was

slope water with similar temperatures nearby on the contoured section.

These various measurements of “streamers” of shelf

water associated with WCRs (i.e., fingers of shelf water

located at the trailing edge of WCRs as they move along

the edge of the continental shelf towards Cape Hatteras)

were mainly fortuitous. However, the National Science Foundation sponsored a specific program – the Warm Core Rings

Program (WCRP) – to study WCRs (Warm-Core Rings Executive Committee 1982; Joyce and Wiebe 1983). The WCRP

provided a unique opportunity to study cooperatively the

interaction of WCRs with shelf water and associated biota.

One purpose for NMFS involvement in the WCRP was to

determine if the passage of a WCR could lead to loss of

early life stages of major commercial fish populations. Such

determination would require a description of the physical

and biological characteristics, as well as the kinematics, of

streamers. A series of interdisciplinary cruises, in conjunction

with the WCRP, was completed between September 1981 and

September 1982 in order to collect the necessary data.

This paper focuses on a description of the physical

characteristics of streamers, as determined primarily from

the oceanographic data. These data, even though collected

during 1981 and 1982, represent the only extensive collection of in-situ information principally directed toward entrainment of shelf water by WCRs. First, the temperature

and salinity characteristics, volume, and spatial structure

of streamers of entrained shelf water are described. Then,

the kinematics of the streamers are discussed, showing the

variability of streamers in time and space. Finally, the primary source for shelf water found in streamers is directly

substantiated.
Page 2

METHODS

DATA COLLECTION

Three cruises on board the NOAA R/V Albatross IV

extensively sampled shelf water streamers associated with

WCRs: 1) 22 September - 6 October 1981, associated with

Ring 81-E (initially mislabeled as Ring 81-D (see Fitzgerald

and Chamberlin (1984) for an explanation), and appearing

as such in Joyce et al. (1984)); 2) 19 April - 4 May 1982,

associated with Rings 82-A and 82-B; and 3) 17 June - 2 July

1982, associated again with Ring 82-B. The locations of all

stations for the three cruises are shown in Figure 1. The

station numbers for those stations are shown in Figures 2a2c. Only a portion of the data collected during the series of

cruises is used here.

Sea surface temperatures, derived from the NOAA-6

and NOAA-7 polar-orbiting, environmental-sensing satellites, were used extensively to guide the sampling at sea.

Graphical interpretations of the processed, high-resolution

images were transmitted from the University of Miami to

the research vessel via the Application Technology Satellite III. A compilation of all images for Ring 82-B is found in

Evans et al. (1984). Colder temperatures and sharp fronts

delimited the shelf water. Later analysis of the images ashore

aided interpretation of the in-situ measurements presented

here.

Temperatures and salinities were sampled with a continuously recording conductivity-temperature-depth instrument (CTD), and with a surface thermosalinograph. Reporting of salinity follows the Practical Salinity Scale, and is

expressed as practical salinity units (PSU). At most stations, a CTD cast was completed to 1000 m or near the

ocean bottom when the depth was <1000 m. Sometimes,

time constraints or special sampling led to shallower casts.

On the return of each cast, water samples were collected for

salinity analysis to compare with the continuous values.

Also, XBTs were used to provide better resolution for the

thermal field.

Eulerian currents and temperatures were measured from

two arrays of moored current meters along the continental

shelf. Positions and schematic views for the two arrays of

current meters are presented in Ramp (1989) and Schlitz et

al. (2001). The two arrays were identical except for their

relative position along the continental shelf break. The first

array, called WCR1, was deployed from 15 April to 1 July

1982, and was centered at roughly 40°N, 70°W (Ramp 1989).

The second array, called WCR2, was deployed during August and September 1982 and was centered at roughly

40°20’N, 67°30’W (Schlitz et al. 2001).

Short-term Lagrangian currents were measured by

drogues placed within streamers of shelf water. Details

about the drifters are found in Schlitz et al. (2001).

 For sampling plankton, especially ichthyoplankton, either a multiple-opening/closing net and environmental sensing system (MOCNESS) or bongo net haul generally followed the CTD cast at each station.

DATA ANALYSIS

To estimate the volume of shelf water in streamers, a

planimetric method was used, based on the isolines shown

in Figures 3a-6a. Each isoline in Figures 3a-6a represents

the depth of a layer measured from the surface to the deepest level in which the salinity is <34 PSU, the value normally

at the inshore side of the shelf-slope front. The surface area

at each 10-m isoline was determined. Two adjoining levels

were averaged, then the value was multiplied by 10 m to

obtain a volume. This computation was done for all levels

containing shelf water.

To estimate the transport of shelf water along each section across the streamers in this study, geostrophic calculations -- relative to the deepest common depth between adjacent stations (generally about 1000 m) -- were used. Joyce

et al. (1992) demonstrated that the azimuthal circulation in a

streamer is generally geostrophic, based on a combined

acoustic Doppler current profiler (ADCP)/“tow-yo” CTD

section across the active streamer associated with Ring 82-

B during June. Transport was estimated by using an average salinity at each depth level between pairs of hydrographic stations, for those stations which effectively described a section across the streamer. When the shelf water

- slope water boundary crossed between paired stations for

which a calculation was made, an underestimate was possible due to the increased gradient at the front. A total of 26

sections across the streamers were used for these calculations; three other sections were across the shelfbreak.

Inspection of satellite imagery by Chamberlin (1981),

Evans et al. (1985), and Garfield and Evans (1987) shows

that the surface expression of streamers is a common feature and can be viewed for extended periods. However, the

balance of mass for continental shelf water and the direct

measurement of shelf water features show that the flow of

shelf water into streamers is not continuous. It is not clear

how long shelf water might remain identifiable within slope

water. Occasionally, shelf water can spiral into the center of

a ring (Evans et al. 1985) and be rapidly mixed. Nevertheless, this is a rare event in the life of a ring. Without this

spiraling into the center of the ring or without interaction

with the Gulf Stream, the mixing of shelf water with surrounding slope water should be an important process to

dissipate the shelf water.

MODELING DISSIPATION OF A STREAMER

To investigate a time scale for dissipation of a streamer

by mixing of shelf water within slope water, a two-dimensional diffusion calculation was performed using an idealized model. The model is based on the diffusion of salt

across the sides and bottom of a rectangular area. Salinity

was chosen to model due to its relatively conservative nature throughout the year (i.e., seasonal variations are relatively small, and a clear contrast between shelf water and

slope water can be defined). Correspondence was also good
Page 3

between the surface thermal gradients and salinity gradients sampled on many crossings of the streamer boundary.

The diffusion model in two dimensions is

(Eq. 1)

where S is salinity (PSU), Kh

 is the horizontal diffusivity

(cm2 sec-1), Kz is the vertical diffusivity (cm2 sec-1), x is the

distance across the region (cm), z is the depth (cm), and t is

the time (sec). In this model, the third spatial dimension is

assumed as infinite (). With boundary conditions of S = 0

at each boundary for simplicity, no diffusion through the

upper boundary, and a constant initial value (S = So) within

the regime, the solution for salinity becomes

(Eq. 2)

where A= ; B= ; C=sin( ); D=sin( );

E= e ; l is width of the streamer, and h is depth

of the streamer. Five parameters are needed for the calculation: So

, l, h, Kh, and Kz. The initial salinity deficit within the

streamer, So, was estimated as 2 PSU based on differences

in the average salinity within the streamer and surrounding

slope water. The width and depth of the streamer, l and h,

were set at 40 km and 60 m, respectively. Horizontal

diffusivity, Kh

, was estimated from the diffusion diagrams

presented by Okubo (1971); for a length of 40 km, it was

calculated as 4 x 105 cm2 sec-1. Vertical diffusivity, Kz, was

varied as 0.1, 1, 5, and 10 cm2 sec-1, following the vertical

diffusivity parameterizations by Schmitt and Olson (1985).

Calculations used a grid having horizontal resolution of 2

km and vertical resolution of 3 m, and covered a 20-day

period.

RESULTS AND DISCUSSION

Each ring was at a different stage of maturity and geographic location, and showed various types and intensities

of interactions. The following descriptions of shelf water

associated with each ring are named by the nearby ring.

STRUCTURE, HYDROGRAPHY, AND VOLUME OF

ENTRAINED SHELF WATER IN STREAMERS

Structure and Hydrography

Ring 81-E

Ring 81-E formed about 16 July 1981 (day 197) near

39°N, 63°W, and moved near 40°N, 64°W during mid-September, approximately 120 nm (220 km) southeast of the

Northeast Channel (Fitzgerald and Chamberlin 1984). During 16-24 September, Ring 81-E vigorously interacted with a

Gulf Stream meander, causing major changes in the ring’s

structure (Joyce et al. 1984). After the transformation of the

ring, a survey of most of the streamer was completed between 23 September and 4 October (69-80 days after formation). The position of Ring 81-E during this study’s series

of stations is in the same area as the ring described by

Smith (1978).

The general structure of the streamer is seen in Figure

3a. The thickness of this streamer ranged from >40 m at the

northern end, to 12 m at a saddle point at the southern end.

At the southern end, the shelf water bifurcated, with one

portion with maximum depth of 29 m moving southeastward, and the other portion with maximum depth of 17 m

moving westward along the periphery of the ring. Extension of the western portion was defined at the surface by

data from the thermosalinograph, and below the surface by

XBTs, as the ship moved toward the northwest after Station

68 (Figure 3a). Positions of the boundary were guided by

surface temperatures derived from AVHRR images (Figure

3b).

The streamer associated with Ring 81-E was well away

from the continental shelf. The surface expression of the

shelfbreak front along the continental shelf north of Cape

Hatteras generally separates from close association with

the underlying topography near the southeast corner of

Georges Bank.

Water with Gulf Stream characteristics (i.e., salinity >36

PSU) was measured underneath the shelf water at the southern end. Those measurements confirmed the satellite imagery that showed shelf water moving along the northern side

of the Gulf Stream after interaction between the ring and

Gulf Stream (Figure 3b).

Typical temperature (T) and salinity (S) values for Station 35 at the northern side and for Station 68 at the southern side, in proximity to the Gulf Stream, showed differences in water characteristics (Figure 3a). The values

changed from S = 32.23 PSU and T = 16.8°C on the northern

side, to S = 33.86 PSU and T = 18.4°C on the southern side.

Also, the subsurface temperature minimum in the shelf water was no longer in the water column on the southern side.

The reason for the change in characteristics is not clear,

although different sources for the water in the streamer

seem likely rather than mixing, if the events develop over

approximately 3-5 days.

Ring 82-A

Ring 82-A formed around 15 January 1982 (day 15) about

110 nm (210 km) southeast of Corsair Canyon (Celone and

Price 1985). During April, the ring interacted with a meander

of the Gulf Stream, was transformed, and moved eastward

(Celone and Price 1985). A new ring, Ring 82-D, formed to

the south of Ring 82-A during the last half of April, and

nπz

2h

s

t = Kh

2s

x2

+Kz

2s

z2

cos(mπ) -1 m

-π2

( + )t m2

kh n2kz

 l2 4h2

cos( )-1 n

nπ

2

m=1n=1

m,n odd



S(x,z,t)= 8S0

π2

Σ Σ (A)(B)(C)(D)(E)

mπx

l
Page 4

exchanged water with Ring 82-A, finally destroying Ring

82-A on 2 May 1982 (day 122). At the time of the survey of

the streamer, during 22-27 April (97-102 days after formation), the ring was centered near 39°30’N, 66°30’W, off the

southern side of Georges Bank.

The prominent feature of the streamer associated with

Ring 82-A was a bowl of shelf water, >75 m thick at the

deepest point, nearly separated from the shelf water source

(Figure 4a). The series of satellite images during the survey

period (Figure 4b) showed slope water moving counterclockwise/cyclonically around the northern side of the

streamer. It finally surrounded shelf water at the surface,

agreeing with the station and surface thermosalinograph

data collected earlier during the process of separation.

The temperature and salinity values for a station within

the center of the entrained shelf water, Station 8, and at the

shelf end of the connecting neck, Station 27 (Figure 4a),

both showed a subsurface temperature minimum in shelf

water, which was cooled during winter and then became

isolated by vernal warming. Temperature and salinity within

the streamer, T = 5.51°C and S = 32.89 PSU, were in the range

of nearby shelf water.

Ring 82-B – First Visit

Ring 82-B was formed on 14 February 1982 (day 45)

near 40°N, 68°25W, to the west of the New England Seamount Chain and farther west than most rings previously

monitored (Celone and Price 1985), including Ring 82-A.

Movement was westward until the end of March when the

motion stalled between Block Canyon and Hudson Canyon

where the orientation of the continental shelf changes from

east-west to northeast-southwest. During this period no

interaction occurred with other rings or the Gulf Stream.

The streamer was surveyed between 28 April and 3 May

1982 (73-78 days after formation). During this time, the

shelf water within the streamer was generally in a layer <30

m thick over slope water, except for a layer >50 m thick in the

northeast corner (Figure 5a). Although the entire streamer

was not sampled due to time constraints, indications from

satellite imagery were that the shallow layer extended further to the west along the southern side of the ring (Figure

5b).

Temperatures and salinities for one station in the northeast corner of the streamer, Station 39, and at the southern

end of the area sampled, Station 59 (Figure 5a), both showed

salinity near the transition between shelf water and slope

water (S = 33.73 PSU for Station 39 and S = 33.99 PSU for

Station 59). The surface temperature at the southern station was about 0.5°C higher.

Again, there was no way to resolve from these data the

different sources for the streamer and the local diffusion of

salt and heat into the streamer causing property changes.

The lack of a subsurface temperature minimum and the salinities near 34 PSU indicate a relatively quiescent period

without significant interaction with the nearby shelf. Also,

current vectors were alongshelf on 29 April, potentially

blocking the source of shelf water (see Ramp (1986), Figure

20).

The greater depth of water with salinity <34 PSU at

Stations 34 and 39 (Figure 5a) can be interpreted as a new

active outbreak of shelf water into slope water. This interpretation is supported by the thermal imagery on 30 April

and 1 May (Figure 5b), and also by the daily current vectors

with offshore directions (see Ramp (1986), Figure 21).

Ring 82-B – Second Visit

Between early May and the last half of June, Ring 82-B

moved along the continental slope to a position southeast

of Delaware Bay. An extensive survey of the streamer (including a closely sampled section in cooperation with other

vessels) and adjoining waters over the continental shelf

was carried out between 21 and 29 June (127-135 days after

formation).

The greatest horizontal extent at the surface (approximately 90 nm, 145 km), depth of a layer with salinity <34 PSU

(>80 m at Stations 65, 20, and 21), and volume of entrained

shelf water were found in this streamer (Figures 2c and 6a).

A prominent saddle point was observed where the layer of

shelf water was only 20 m thick, as in Ring 81-E, and previously in Ring 81-B.

At Station 28 at the southern end of the streamer (Figure 6a), the surface temperature was slightly warmer (T =

17.04 vs. 16.64°C), and the surface salinity was slightly

higher (S = 32.87 vs. 32.67 PSU) than at Station 65 at the 200-

m isobath near the center of the streamer. An AVHRR image

for a time during the CTD stations is shown in Figure 6b. At

the subsurface temperature minimum, the water at Station

28 was both slightly cooler and slightly fresher than that at

Station 65. Surface temperature and salinity values for Stations 65, 20, and 21 (Figures 2c and 6a) were T = 16.64, 16.27,

and 15.87°C, and S = 32.67, 32.73, and 32.83 PSU, respectively. The temperatures at the transition from shelf water

at those three stations were T = 9.10, 9.42, and 9.50°C, respectively. These data suggest rapid movement from the

continental shelf without significant mixing.

Volume of Entrained Shelf Water

The estimated volume for each streamer is presented in

Table 1. The values were between 58 km3

 for the nearly

detached streamer associated with Ring 82-A, and 290 km3

for the streamer associated with Ring 82-B during June.

Each estimate, except for Ring 82-A, was a minimum

since there was an area at the southern end containing shelf

water that was not sampled. By comparing the shelf water

determined from hydrography with the available satellite

images for the same time, an underestimate of volume by

10-15% seems likely, but caution must be used since the

subsurface structure of a streamer, which is highly variable
Page 5

in these four cases, must be extrapolated from the surveyed

region.

Variability in Streamer Structure and Volume of

Entrainment

Description of the hydrographic properties of shelfwater streamers provides one view of the variability in the

structure and volume of entrained water in these streamers

(Figures 3a-6a). Each streamer had a region where the depth

of shelf water was at least 40 m, and for the streamer associated with Ring 82-B during June, the depth reached >80 m

both near the shelfbreak and at the southern end.

In each streamer, there was a saddle point where the

layer of shelf water reached a minimum thickness within the

streamer. This saddle point can be striking as in Ring 82-B

during June, or relatively subtle as in Ring 82-B during April,

but each such saddle point indicates variations in the flow

of shelf water associated with WCRs.

The encircling of shelf water by slope water (i.e., thinning of the depth at the saddle point to zero) as seen in the

streamer associated with Ring 82-A is the first known report

of the isolation of a relatively large mass (30 nm (55 km) in

diameter, 60 km3

 in volume) of largely unaltered shelf water

within slope water, based on data collected in-situ. There

have been other indications of streamers and eddies at the

shelf-slope front causing losses of shelf water, but at smaller

scales (Houghton et al. 1986; Garvine et al. 1988).

TRANSPORT OF SHELF WATER BY STREAMERS

AND DISSIPATION IN SLOPE WATER

Transport

The overall mean transport of shelf water away from

the continental shelf (i.e., for those 21 sections with transport in the same direction as the near-surface circulation of

the associated WCR) was 165 x 103

 m3 sec-1, with a standard

deviation of 149 x 103

 m3 sec-1. The overall mean transport

for the 19 sections with transport of shelf water in the opposite direction as the near-surface circulation of an associated WCR was 62 x 103

 m3 sec-1, with a standard deviation of

90 x 103 m3 sec-1. Table 2 shows offshore transport across

sections ranging from 0 to 488 x 103 m3 sec-1. Onshore transports ranged from 0 to 344 x 103

 m3 sec-1. The streamer

associated with Ring 82-B during June was the most active

in removing shelf water.

Dissipation

Based on the two-dimensional model for salt diffusion,

the average salinity within the streamer for Kz

 = 1 cm2 sec-1

was halved, but did not quite reach the So/e value (0.74), at

14 days (Figure 7). The average salinity for Kz

 = 5 cm2 sec-1

was halved just before 8 days, and reached the So/e value at

12 days. Note that for a streamer with a given width and

depth, these time scales are valid for any initial salinity

value within the boundaries, since the decay depends only

on the values of Kh

 and Kz (Equation 2).

Cross sections of the salinity differences are shown at

10 and 20 days in Figure 8. Several points become clear by

using a salinity deficit of 1 PSU as a conservative indicator

of the recognizable streamer. At 10 days, there was water

with shelf characteristics clearly seen at the surface for all

values of Kz

 from 0.1 to 10 cm2 sec-1. The width of shelf

water at the surface was at least one-half of the original

dimension for all values of Kz

. The depth of shelf water was

at least two-thirds of the original dimension for Kz = 5 cm2

sec-1, and was one-half of the original dimension for Kz = 10

cm2

 sec-1. At 20 days, this streamer was still recognizable at

the surface for values up to Kz = 5 cm2 sec-1. Calculations

for both Kz = 0.1 and 1 cm2 sec-1 showed a substantial section of shelf water that was 24 km in width and at least 44 m

in depth, little changed in 20 days from the initial condition.

The calculation for diffusion of salt indicates that a

streamer of shelf water relatively isolated within slope water would remain cohesive for 2-3 wk in the absence of advective shearing processes or strong air-sea interaction with

mixing. Moreover, this streamer could be viewed from satellites even when the original cross section of the streamer

has been eroded to a large extent, thereby complicating

interpretations based solely or primarily on thermal imagery.

Geostrophic Considerations

It was assumed that the balance of geostrophic forces

was the cause for movement, and thus the basis for calculating the velocity and transport, of streamers. Any highfrequency vertical variations of the density field could not

be detected based on the sampling scheme, and are an unknown source of error.

One comparison between calculated geostrophic velocities and directly measured velocities was possible. The

section sampled by Joyce et al. (1992) was concurrent and

nearly parallel with Stations 17-25 sampled on 23 June 1982

in this study (Figures 2c (inset) and 9). Calculated velocities for this study were plotted for the 20- and 100-m levels

to the same scale as the measured velocities in the Joyce et

al. (1992) study. (Shallow CTD casts were required in the

Joyce et al. (1992) study in order to keep up with the second

ship as it crossed the streamer.) Vertical shear compares

well, changing signs at Station 21 from a subsurface maximum west of the station to a surface maximum east. The

lower velocities and occasional reversals in the geostrophic

calculation can be at least partially explained by the shallow

level of no motion (300 m) for this section, by the inherent

averaging in the geostrophic calculations, by the orientation of stations that are not perpendicular to the measured

current, and by the spatial variations within the streamer.
Page 6

Transport by salinity classes estimated by these geostrophic calculations (Table 3) can also be compared with

those direct measurements obtained by Joyce et al. (1992).

The transport of shelf water (S < 34 PSU) calculated geostrophically for Stations 17-25, 408 x 103

 m3 sec-1, is 32 x 103

m3 sec-1 higher than that obtained by direct measurement.

This difference could be partially caused by low salinity

water not being sampled at the western end of the Joyce et

al. (1992) section. The difference of 9% between these

methods, in spite of the shallow reference level and slightly

different sections, gives confidence that other transport

calculations are reasonable for an overall characterization.

The deficit in geostrophic transport for salinities between

34 and 35 PSU appears to be caused by the greater measured velocities at 100 m and the greater intrusion of water

with salinity >34 PSU at the western end of the Joyce et al.

(1992) section. In spite of the range of potential errors, the

differences between geostrophic calculations and direct

measurements are considered reasonable, and geostrophic

calculations are considered acceptable for estimating velocity and transport of entrained shelf water.

SOURCE OF ENTRAINED WATER IN STREAMERS

Various modeling and observational studies using current meters (e.g., Beardsley et al. 1985; Ramp 1986; Wang

1992) indicate that the waters of the continental shelf are

generally buffered from the passage of WCRs. Yet significant amounts of shelf water (Table 1) were found to be

entrained within the streamers associated with this study’s

rings. The hydrographic data collected during this study’s

series of cruises provided sufficient spatial resolution to

examine the proximate source of water for the streamers.

The most prominent characteristic of the Middle Atlantic Bight during the stratified season (i.e., April-September)

is the continuous subsurface band of cold water found along

the outer continental shelf (OCS), with its core found along

the bottom between 50 and 80 m (Houghton et al. 1982).

The low temperature and associated low salinity of cold

band water are distinct, permitting the unambiguous identification of cold band water if present within a streamer.

Alternatively, the absence of cold band characteristics in a

streamer requires a source seaward of the cold band for

active streamers.

The data collected for the streamer associated with Ring

82-B during June were examined for the presence or absence of cold band characteristics in order to determine the

sources of shelf water within the streamer. The temperature

and salinity structure on the shelf adjacent to the streamer

associated with Ring 82-B during June is shown for three

sections which were nearly perpendicular to the local

bathymetry (Figures 2c, 6a, and 10-12). The cold band is

prominent, with temperatures below 7°C and salinities generally between 32.75 and 33.00 PSU. Note that in the surface layer, salinities below 33.00 PSU extended further seaward only with temperatures above 8°C. A section across

the streamer (Figure 13b) showed that only two stations off

the continental shelf, Stations 48 and 50 (Figures 2c and

6a), had water with a subsurface salinity minimum below

33.00 PSU. At Station 48, there were two occurrences of

water, each more than 3 m thick, separated from each other

by about 5 m, and located about 23 and 35 m of depth.

Temperatures of 9.0-9.5°C were associated with the upper

occurrence, and 8.0-8.5°C with the lower occurrence. At

Station 50, there was an occurrence of water about 2 m

thick, located between about 28 and 30 m of depth, with a

temperature of 7°C. The outcome is that the source of shelf

water for the streamer is above the OCS, generally seaward

of the cold band. The plume-like structure of the cold band

detached from the bottom may occasionally contribute, but

in small volume, according to these data. This inference

contrasts with the inference of Churchill et al. (1986).

Other data also support this present inference. For the

streamer associated with Ring 82-B during April, the salinity was entirely above 33.00 PSU except for a surface layer

at Station 34 (Figures 2b and 5a) that was 7 m thick. For the

nearly-isolated bowl of shelf water associated with Ring 82-

A, there was considerable water with temperature below

6°C and salinity below 33.00 PSU (Figures 2b, 4a, and 14).

However the source of the water was still from the OCS, as

shown in Figure 15 for Stations 24-28 in Figure 2b, since

stratification from vernal warming formed a cold band having minimum temperatures below 4.5°C and salinities below

32.75 PSU. For the streamer associated with Ring 81-E, the

data are consistent with a source above the OCS and generally seaward of the cold band, but the interpretation is

equivocal because both the ring and interacting shelf water

are well seaward of the continental shelf.

DYNAMIC BEHAVIOR OF STREAMERS

Each survey of streamers associated with WCRs seemed

to give a unique result except for the observation of a saddle

point at some distance along the streamer. The presence of

a nearby continental shelfbreak was not required. For example, the streamer associated with Ring 81-E during September 1981 was well away from the continental slope, yet

entrained at least 131 km3

 of shelf water. The strength of

interaction, defined by the amount of shelf water within

slope water, did not depend on the age, and therefore energy, of the ring. For example, the most intense observed

streamer was during late June 1982 associated with Ring 82-

B after this ring had traversed along the continental slope

from its formation south of Nantucket to southeast of Delaware Bay, about 130 days after formation. Also as an example, significant interaction occurred with a small ring,

Ring 82-A, within a week of its demise.

The idealized results of Wang (1992) were used to merge

the seemingly disparate descriptions of this study’s streamers. In Wang (1992), a series of models generate results

with which to examine the interaction of an eddy with topography resembling a continental slope. The first model
Page 7

is based on a step in topography (i.e., small escarpment)

and point vortex under quasigeostrophic conditions, which

is then expanded to a finite step and vortex. The fluid in the

model is assumed to be barotropic and frictionless in a rotating coordinate system with a rigid lid. Contour dynamics

are used in the solutions to the model, giving similar principal results in both cases: 1) ageostrophic nonlinear advection of fluid across the escarpment, 2) formation of a cyclonic eddy containing shelf water offshore, and 3) excitation of waves trapped against the escarpment. The second

model, which incorporates a semispectral primitive equation model developed by Haidvogel et al. (1991), uses the

same basic equations, including bottom friction, but with

three different cross-shelf exponential topographies, in order to generate results on the f and ß-planes. Wang (1992)

concluded that there were “no fundamental differences in

shelf/slope responses and cross-slope exchanges between

the results of these two models.” The initial tongue of

entrained shelf water with high potential vorticity evolves

into a narrow streamer and eddy (Wang 1992). The eddy at

the head of the streamer is formed from fluid with cyclonic

vorticity continually entering after being compressed between the vortex and the anticyclonic field north of the

tongue. The saddle point between the eddy and the rest of

the streamer is formed at the point of greatest compression

just inshore of the eddy. For example, Ring 82-B during

June has a deep pool of low-salinity water at the southern

end, and a narrowing toward the continental shelf (Figure

6). The cyclonic circulation of the eddy then continues

constricting the streamer, eventually detaching as in Ring

82-A (Figure 4).

Cyclonic eddies located within entrained shelf water

have been interpreted from satellite imagery (e.g., Evans et

al. 1985; Ramp 1986), and those located northeast of WCRs

have been interpreted from ADCP data (Kennelly et al. 1985).

This study’s data and the modeling by Wang (1992) provide evidence that not only were curvilinear streamers and

cyclonic eddies dynamically linked with entrainment, but

also those eddies northeast of the ring. For example, the

second lobe of shelf water to the northeast of Ring 82-B

during April (Figure 5a) corresponds to the evolution of a

cyclonic eddy.

The extent to which the assumption of a barotropic fluid

by Wang (1992) describes the observed entrainment could

be problematic, depending on the vertical scale chosen for

the comparison. To illustrate, consider Ring 82-B; it was

distinguishable at a depth of 2000 m during June after moving along the continental slope for about 4 mo (e.g., Kennelly

et al. 1985). Figure 16a shows the geostrophic velocity for

the 0-300 m depth range, relative to approximately 1000 m,

for Stations 6-11 (Figures 2c and 6a) across the streamer

associated with Ring 82-B during June. Within the depth

range corresponding to shelf water, 65 m (Figure 16b), the

ring could be considered radially barotropic (i.e., little vertical shear at a given distance from the center). Although the

velocity is somewhat intensified near the surface for the

region with shelf water, below 100 m, all velocities are <10

cm sec-1, and vertical shear in the velocity field above 1000

m is small. Therefore, by choosing a vertical scale of approximately 300 m, qualitative comparisons for the interaction between a barotropic vortex and shelf water seems reasonable, if not complete, for the level of comparison presented here.

CONCLUSIONS

Data have been analyzed to describe the interaction

between WCRs (i.e., 81-E, 82-A, and 82-B during April and

June) and shelf water, leading to entrainment of shelf water

along the trailing edge of WCRs as they move generally

southwest in slope water. In-situ data of this resolution

have not been described before.

The short-term estimated volumes of entrained shelf

water (i.e., salinities < 34 PSU) were 58-290 km3

. An underestimate of 10-15% was likely for all but the nearly enclosed

bowl of shelf water associated with Ring 82-A, since the

entrainment was not circumscriptively sampled in other

cases.

Geostrophically calculated transports of shelf water in the

direction of the anticyclonic circulation of an associated WCR

were 0-488 x 103

 m3 sec-1 (mean = 165 x 103 m3 sec-1; standard

deviation = 149 x 103 m3 sec-1). Transports in the opposite

direction were 0-345 x 103 m3 sec-1 (mean = 62 x 103 m3 sec-1;

standard deviation = 90 x 103 m3 sec-1). These transport

estimates are thought to be reasonable, based on a comparison between geostrophic calculations and ADCP/”towyo” CTD measurements taken concurrently during a twoship crossing of an active streamer during June 1982 associated with Ring 82-B. Changes in the direction of flow are

probably a consequence of cyclonic eddies that develop

within entrained water.

Analysis of temperature and salinity sections indicates

that the shelf water supplying streamers comes primarily

from the outer edge of the continental shelf, seaward of the

cold band. Since the source of shelf water for streamers is

distributed along the shelfbreak for considerable distances

rather than being limited to an area nearby, the majority of

the continental shelf is, therefore, isolated from direct influence by passage of rings. This finding confirms results of

prior studies which measured in detail the temperature and

salinity structure and currents across the shelf as WCRs

passed.

If the mean seaward extent of the cold band is the 100-

m contour (Houghton et al. 1982), and if the mean position

of the shelf-slope front is anchored at the 100-m contour

and intersects the surface 40 km seaward (Beardsley et al.

1985), then the ratio of the volume (km3

) of shelf water in a

streamer to the length (km) of shelfbreak supplying water to

the streamer is 2:1. This ratio results in a range of 29-145 km

along the shelfbreak for supplying shelf water to the streamers in this study. It therefore seems plausible to use calculated volumes of streamers to determine the lengths of

shelfbreak affected by entrainment.
Page 8

Being able to consistently and generally interpret the

data with modeling is an encouraging, but problematic, accomplishment, given the ideal assumptions of the model --

particularly a barotropic ocean -- and the variability of the

strictly nonsynoptic hydrographic data. One puzzling aspect, at least, remains: the streamer associated with Ring

81-E during September 1981 (Figure 3a). Since the presence

of an adjacent continental shelf was apparently not required

to initiate this streamer, the model by Wang (1992) cannot

directly apply in this case. Therefore, other processes should

be important in explaining the initiation of entrainment by

WCRs at different stages in the history of those rings.

One possible explanation for this “continental-shelffree” initiation of entrainment are growing frontal waves

which are due to horizontal-shear instability on the northern periphery of a WCR, and which then “break” to subsequently become entrained by that ring (Ramp et al. 1983).

Such frontal waves were observed for a number of days on

the periphery of Ring 82-B based on AVHRR images, but

the final fate of those waves was not observed due to clouds.

The position of the horizontal-shear instability was close to

the anticyclonic advection of an eddy on the periphery of

Ring 82-B, as interpreted from sequential satellite images

(Kennelly et al. 1985, Figure 8b). Also, the AVHRR image

for Ring 82-B on 30 April 1982 (Figure 5b) supports those

observations.

Conceptually, a combination of curvilinear streamers

on the periphery, and cyclonic eddies which may remain

coherent for 2-3 wk, can describe the observed structure for

entrainment by WCRs. Evidence from this study’s data

and from the modeling supports this concept. Clearly, further investigation is needed to understand the initiation

and evolution of these streamers.

A major goal of this study was to examine the possibility that passage of WCRs could affect recruitment of commercially important fish species during the larval stage. Distributions of larval fish have been portrayed by many authors for the continental shelf between Cape Hatteras and

Nova Scotia (e.g., Grosslein and Azarovitz 1982; Morse et

al. 1987). These portrayals show that the maxima in the

general distributions of larvae are well inshore of the 200-m

isobath, although bias may occur due to limited sampling

near the offshore boundaries. Since the direct measurements and analyses of water masses in this study indicate

that the onshore influence of WCRs is about the 100-m

isobath, then the potential for significant mortality caused

by entrainment of shelf water, as a single process, seems

unlikely. Earlier modeling by Flierl and Wroblewski (1985)

and statistical analysis by Myers and Drinkwater (1989)

had linked WCRs with reduced recruitment of fish. Flierl

and Wroblewski (1985) recognized, however, that the results of their model, averaged across the shelf and also

vertically, would not be applicable if entrainment were limited near the shelf break.

Detailed analysis of the biological samples collected

concurrently during this study’s series of cruises was not

possible except for the analysis by Le Blanc (1986). Le

Blanc (1986) reported that larvae of fish species normally

spawned on the continental shelf were sampled in the entrained shelf water associated with Ring 82-B during June.

A quantitative assessment of the impact upon those species’ populations was not attempted, though. Unfortunately,

other biological samples for the series of cruises were lost.

ACKNOWLEDGMENTS

These data were originally collected as part of the WarmCore Experiment in 1981 and 1982. Thanks go to all who

participated during the series of cruises, particularly the

officers and crew of the Albatross IV. Their dedication to the

success of data collection made these analyses possible.

REFERENCES CITED

Bisagni, J.J. 1983. Lagrangian current measurements within

the eastern margin of a warm-core Gulf Stream ring. J.

Phys. Oceanogr. 13:709-715.

Beardsley, R.C.; Chapman, D.C.; Brink, K.H.; Ramp, S.R.; Schlitz,

R. 1985. The Nantucket Shoals Flux Experiment (NSFE79).

Part I: a basic description of the current and temperature

variability. J. Phys. Oceanogr. 15:713-748.

Celone, P.J.; Price, C.A. 1985. Anticyclonic warm-core Gulf

Stream rings off the northeastern United States during

1982. Ann. Biol. 39:19-23.

Chamberlin, J.L. 1981. Application of satellite infrared data

to analysis of ocean frontal movements and water mass

interactions off the Northeast coast. NAFO [Northwest Atl. Fish. Org.] SCR [Sci. Counc. Res.] Doc. 81/IX/

123; 15 p.

Churchill, J.H.; Cornillon, P.C.; Milkowski, G.W. 1986. A cyclonic eddy and shelf-slope water exchange associated

with a Gulf Stream warm-core ring. J. Geophys. Res.

91:9615-9623.

Evans, R.; Baker, K.; Brown, O.; Smith, R.; Hooker, S.; Olson,

D.; Warm Core Rings Program Service Office. 1984. Satellite images of warm core ring 82-B sea surface temperature

and a chronological record of major physical events affecting ring structure. Unpublished document. Available

from: R.J. Schlitz, National Marine Fisheries Serv., Woods

Hole, MA.

Evans, R.H.; Baker, K.S.; Brown, O.B.; Smith, R.C. 1985. Chronology of warm-core ring 82B. J. Geophys. Res. 90:8803-

8811.

Fitzgerald, J.L.; Chamberlin, J.L. 1984. Anticyclonic warm-core

Gulf Stream rings off the northeastern United States in

1981. Ann. Biol. 38:29-33.

Flierl, G.R.; Wroblewski, J.S. 1985. The possible influence of

warm core Gulf Stream rings upon shelf water larval fish

distribution. Fish. Bull. (Wash., D.C.) 83:313-330.

Fornshell, J.A.; Criess, W.A. 1979. Anticyclonic eddy observations in the slope water aboard CGC Evergreen. J.

Phys. Oceanogr. 9:992-1000.
Page 9

Garfield, N., III; Evans, D.L. 1987. Shelf water entrainment by

Gulf Stream warm-core rings. J. Geophys. Res. 92:13003-

13012.

Garvine, R.W.; Wong, K.-C.; Gawarkiewicz, G.G.; McCarthy,

R.K. 1988. The morphology of shelfbreak eddies. J.

Geophys. Res. 93:15593-15607.

Grosslein, M.D.; Azarovitz, T.R. 1982. Fish distribution. MESA

(Mar. Ecosyst. Anal.) N.Y. Bight Atlas Monogr. 15; 182 p.

Haidvogel, D.B.; Wilkin, J.L.; Young, R.E. 1991. A semi-spectral primitive equation ocean circulation model using vertical sigma and orthogonal curvilinear horizontal coordinates. J. Comput. Phys. 94:151-185.

Houghton, R.W.; Olson, D.B.; Celone, P.J. 1986. Observations

of an anticyclonic eddy near the continental shelf break

south of New England. J. Phys. Oceanogr. 16:60-71.

Houghton, R.W.; Schlitz, R.; Beardsley, R.C.; Butman, B.;

Chamberlin, J.L. 1982. The Middle Atlantic Bight cold

pool: evolution of the temperature structure during summer 1979. J. Phys. Oceanogr. 12:1019-1029.

Iles, T.D.; Sinclair, M. 1982. Atlantic herring: stock discreteness and abundance. Science 215:627-633.

Joyce, T.M. 1985. Gulf Stream warm-core ring collection: an

introduction. J. Geophys. Res. 90:8801-8802.

Joyce, T.; Backus, R.; Baker, K.; Blackwelder, P.; Brown, O.;

Cowles, T.; Evans, R.; Fryxell, G.; Mountain, D.; Olson, D.;

Schlitz, R.; Schmitt, R.; Smith, P.; Smith, R.; Wiebe, P. 1984.

Rapid evolution of a Gulf Stream warm-core ring. Nature

308:837-840.

Joyce, T.M.; Bishop, J.K.B.; Brown, O.B. 1992. Observations

of off-shore shelf-water transport induced by a warmcore ring. Deep-Sea Res. 39(suppl.):S97-S113.

Joyce, T.; Wiebe, P. 1983. Warm-core rings of the Gulf Stream.

Oceanus 26:34-44.

Kennelly, M.A.; Evans, R.H.; Joyce, T.M. 1985. Small-scale

cyclones on the periphery of a Gulf Stream warm-core

ring. J. Geophys. Res. 90:8845-8857.

Le Blanc, P.R. 1986. The distribution and abundance of

larval and juvenile fishes in shelf water entrained by

warm-core ring 82-B and contiguous water masses.

[M.S. thesis.] Dartmouth, MA: Southeastern Massachusetts Univ.; 143 p.

Morgan, C.W.; Bishop, J.M. 1977. An example of Gulf Stream

eddy-induced water exchange in the Mid-Atlantic Bight.

J. Phys. Oceanogr. 7:472-479.

Morse, W.W.; Fahay, M.P.; Smith, W.G. 1987. MARMAP

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[Natl. Ocean. Atmos. Admin.] Tech. Memo. NMFS [Natl.

Mar. Fish. Serv.]-F/NEC-47; 215 p.

Myers, R.A.; Drinkwater, K. 1989. The influence of Gulf Stream

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Ramp, S.R.. 1986. The interaction of warm core rings with the

shelf water and shelf/slope front south of New England.

[Ph.D. thesis.] Narragansett, RI: Univ. of Rhode Island;

137 p.

Ramp, S.R. 1989. Moored observations of current and temperature on the shelf and upper slope near ring 82B. J.

Geophys. Res. 94:18071-18087.

Ramp, S.R.; Beardsley, R.C.; Legekis, R. 1983. An observation

of frontal wave development on a shelf-slope/warm core

ring front near the shelf break south of New England. J.

Phys. Oceanogr. 13:907-912.

Richardson, P.L. 1983. Gulf Stream rings. In: Robinson, A.R.,

ed. Eddies in marine science. New York, NY: SpringerVerlag; p. 19-64.

Schlitz, R.J.; Manning, J.P.; Smith, K.W. 2001. Structure and

transport of alongshelf currents across the southern flank

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II 48:341-372.

Schmitt, R.W.; Olson, D.B. 1985. Wintertime convection in

warm-core rings: thermocline ventilation and the formation of mesoscale lenses. J. Geophys. Res. 90:8823-8837.

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[Mass. Inst. Technol./Woods Hole Oceanogr. Inst.] Joint

Program in Oceanography; 216 p.

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835.
Page 10

Figure 1. Positions of all stations during the major cruises of the Warm Core Rings Program. (Cruise numbers are proximate to the stations

of the respective cruises. Squares represent positions at which CTD casts were completed. Depth contours are in meters.)

Figure 2a. Positions of stations at which CTD casts were completed on Albatross IV

Cruise No. AL 81-11 during 22 September - 6 October 1981. (Depth contours are in meters.)
Page 11

Figure 2c. Positions of stations at which CTD casts were completed on

Albatross IV Cruise No. AL 82-07 during 17 June - 2 July

1982. (Inset shows closeup view of Stations 17-37. Depth

contours are in meters.)

Figure 2b. Positions of stations at which CTD casts were completed on Albatross IV Cruise No. AL 82-

04 during 19 April - 4 May 1982. (Depth contours are in meters.)
Page 12

Figure 3a. Depth of a layer from the surface (portrayed with 10-m contours) containing shelf water

(salinity 34 PSU) entrained by Ring 81-E. (Data were collected on Albatross IV Cruise No. AL

81-11 during 23 September - 4 October 1981.)
Page 13

Figure 3b. An AVHRR image for 27 September 1981 showing temperature expression at the surface for shelf water entrained by Ring 81-E.

(Clouds are present over the western half of the ring.)
Page 14

Figure 4a. Depth of a layer from the surface (portrayed with 10-m contours) containing shelf water (salinity 34 PSU) entrained by

Ring 82-A. (Data were collected on Albatross IV Cruise No. AL 82-04 during 22-27 April 1982.)
Page 15

Figure 4b. A series of AVHRR images for 20-24 April 1982 showing temperature expression at the surface as slope water cyclonically encircles

the shelf water entrained by Ring 82-A. (The warm water at the southwest corner was caused by a large meander of the Gulf Stream

which destroyed Ring 81-E and formed Ring 82-D.)
Page 16

Figure 5a. Depth of a layer from the surface (portrayed with 10-m contours) containing shelf water (salinity 34 PSU) entrained by Ring 82-

B. (Data were collected on Albatross IV Cruise No. AL 82-04 during 28 April - 3 May 1982.)
Page 17

Figure 5b. An AVHRR image for 30 April 1982 showing temperature expression at the surface for shelf water entrained by Ring 82-B. (Note

that the undulations at the boundary between shelf water and Ring 82-B show some characteristics of frontal waves.)
Page 18

Figure 6a. Depth of a layer from the surface (portrayed with 10-m contours) containing shelf water (salinity 34 PSU) entrained by Ring

82-B. (Data were collected on Albatross IV Cruise No. AL 82-04 during 21-29 June 1982.)
Page 19

Figure 6b. An AVHRR image for 24 June 1982 showing temperature expression at the surface for shelf water entrained by Ring 82-B. (The

northern part of the entrainment is partially obscured by clouds and overall contrast of temperatures at the surface is decreased from

earlier in the year due to vernal warming.)
Page 20

Figure 7. The average salinity decrease for a cross section within a homogeneous rectangular streamer for daily time steps calculated using a two-dimensional diffusion

model. (Vertical diffusivity, Kz, was varied as 0.1, 1, 5, and 10 cm2 sec-1, and horizontal diffusivity, Kh, was maintained at 4 x 105 cm2 sec-1. The line at average

salinity was 0.74 (2 x e-1).)
Page 21 Figure 8a. Cross sections of salinity for an initial layer 60 m thick and 40 km wide after diffusion for 10 days at four different rates (0.1, 1, 5, and 10 cm2 sec-1) for horizontal diffusivity, calculated using a two-dimensional diffusion model. Figure 8b. Cross sections of salinity for an initial layer 60 m thick and 40 km wide after diffusion for 20 days at four different rates (0.1, 1, 5, and 10 cm2 sec-1) for horizontal diffusivity, calculated using a two-dimensional diffusion model.
Page 22

Figure 9. Comparison of velocities (m sec-1) at depths of 21 and 99 m (represented by and , respectively) determined from ADCP

measurements, and at depths of 20 and 100 m (represented by x and , respectively) relative to 300 m determined from geostrophic

calculations, on concurrent nearly parallel tracks across shelf water entrained by Ring 82-B on 23 June 1982. (Positions of CTD

casts are shown as dotted circles. Modified from Joyce et al. 1992)
Page 23

Figure 10. Temperature (C) and salinity (PSU) sections from CTD data at Stations 71-74 obtained on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations

are found in Figure 2c.)
Page 24

Figure 11. Temperature (C) and salinity (PSU) sections from CTD data at Stations 67-69 obtained on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations

are found in Figure 2c.)
Page 25

Figure 12. Temperature (C) and salinity (PSU) sections from CTD data at Stations 63-65 obtained on Albatross IV Cruise No. AL 82-07

during 17 June - 2 July 1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations

are found in Figure 2c.)
Page 26

Figure 13a. Temperature (C) section from CTD data at Stations 53-46 obtained on Albatross IV Cruise No. AL 82-07 during 17 June - 2 July

1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2c.)
Page 27

Figure 13b. Salinity (PSU) section from CTD data at Stations 53-46 obtained on Albatross IV Cruise No. AL 82-07 during 17 June - 2 July 1982.

(Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2c.)
Page 28

Figure 14. Temperature (C) and salinity (PSU) sections from CTD data at Stations 5-9 obtained on Albatross IV Cruise No. AL 82-04 during 19 April - 4 May 1982.

(Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2b.)
Page 29

Figure 15a. Temperature (C) section from CTD data at Stations 28-24 obtained on Albatross IV Cruise No. AL 82-04 during 19 April - 4 May

1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2b.)
Page 30

Figure 15b. Salinity (PSU) section from CTD data at Stations 28-24 obtained on Albatross IV Cruise No. AL 82-04 during 19 April - 4 May

1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2b.)
Page 31

Figure 16a. Velocity (cm sec-1) section from CTD data at Stations 6-11 obtained on Albatross IV Cruise No. AL 82-07 during 17 June - 2 July

1982. (Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2c.

Geostrophic velocity is calculated relative to a level of no motion at approximately 1000 m, and shown for the top 300 m.

Illustrated is the extent to which the distribution of velocity can be considered barotropic for comparison with a model including a

barotropic vortex.)
Page 32

Figure 16b. Salinity section from CTD data at Stations 6-11 obtained on Albatross IV Cruise No. AL 82-07 during 17 June - 2 July 1982.

(Depths are in meters and distance along the section is in kilometers. The positions of the stations are found in Figure 2c.)
Page 33

Table 1. Estimated volume (km3) of shelf water (34 PSU) by 10-m depth layer within streamers entrained by Rings 81-E,

82-A, and 82-B twice. (Volumes were calculated using a planimetric method.)

Depth

(m)

Ring

81-E 82-A 82-B (May-June) 82-B (June-July)

Layer  Layer  Layer  Layer 

00000

80 19 49 71

10 80 19 49 71

54 13 38 54

20 134 32 87 125

30 10 27 43

30 164 42 114 168

13 8 17 38

40 177 50 131 206

3 5 8 34

50 180 55 139 240

<1 2 27

60 181 57 267

1 16

70 58 283

6

80 289

1

90 290
Page 34

Table 2. Geostrophically estimated transport (103 m3 sec-1 in both the same and opposite direction as the ring’s swirl)

of shelf water (34 PSU) by station transect within streamers entrained by Rings 81-E, 82-A, and 82-B

twice.

Ring Stations

Transport (103

 m3 sec-1) Relative to Ring

Swirl Direction Lowest Average

Salinity Class

(PSU) Same Opposite

81-E 65-68 3.5 0 33.75-34.00

60-65 10.9 0.8 33.75-34.00

59-56 0 11.5 33.50-33.75

54-56 0 0 -

54-50 38.6 0 33.50-33.75

46-50 22.6 80.8 33.50-33.75

46-40 163.7 0 33.50-33.75

30-37 277.4 223.2 32.50-32.75

82-A 5-2 61.7 27.0 32.75-33.00

5-9 263.5 14.4 32.75-33.00

15-11 88.2 344.6 33.25-33.50

16-22 33.4 51.1 33.25-33.50

82-B (April-May) 60-56 38.8 0 33.50-33.75

50-55 0 0 -

50-47 0 0 -

44-47 0 54.0 33.75-34.00

43-37 32.7 42.5 33.50-33.75

30-36 45.6 18.2 32.75-33.00

[61-64] 52.6 73.8 32.75-33.00

82-B (June-July) 6-1 399.8 1.7 33.00-33.25

6-11 305.5 5.0 32.75-33.00

16-12 487.9 9.2 33.00-33.25

17-25 407.6 46.6 32.50-32.75

38-45 109.2 12.8 33.25-33.50

53-46 212.3 175.6 32.25-32.50

81-74 239.7 37.5 32.25-32.50

82-88 214.4 17.2 32.50-32.75

[54-58] 47.0 35.2 31.50-31.75

[71-74] 0 141.8 31.75-32.00

Mean 164.6 61.8

Std. Dev. 148.5 90.2
Page 35

Table 3. Comparison of geostrophic and measured transport estimates (103 m3 sec-1) by salinity class of shelf water

entrained by Ring 82-B in June 1982. (Comparisons were based on geostrophic calculations for Stations

17-25 and on a nearby section directly measured (ADCP/"tow-yo" CTD) by Joyce et al. (1992).)

Estimation

Method

Transport (103

 m3 sec-1) by Salinity Class (PSU)

<33 33-34 34-35

CTD/Geostrophy 41 367 230

ADCP/CTD 22 354 481

Difference 19 13 -251

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