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
by Ronald
J. Schlitz
National Marine Fisheries Serv., 166 Water St., Woods Hole, MA 02543
Print
publication date March 2003;
web version posted September 30, 2004
Citation: Schlitz RJ. 2003. Interaction of shelf water with warm-core rings, focusing on th kinetics and statistics of shelf water entrained within streamers. NOAA Tech Memo NMFS NE 170; 35 p.
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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.
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, environmental-sensing 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 two-digit 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.
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 2a-2c. 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 between the surface thermal gradients and
salinity gradients sampled on many crossings of the streamer boundary.
The diffusion model in two dimensions is
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
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
where
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 39N, 63W, and moved
near 40N, 64W 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 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 40N, 6825W,
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 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 shelf-water 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 high-frequency 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 (Figure 2c (inset) and Figure 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.
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 (Figure 2c, Figure 6a, and Figure 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 (Figure 2b and Figure 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 (Figure 2b, Figure 4a, and Figure 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
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 -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 5)
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 16 shows the geostrophic velocity for the 0-300
m depth range, relative to approximately 1000 m, for Stations 6-11 (Figure 2c and Figure 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/"tow-yo" CTD measurements taken concurrently
during a two-ship 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.
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 13a). 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-shelf-free" 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 Warm-Core 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.
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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 |