United States
Environmental Protection
Agency
Office of Water
(WH-555F)
EPA 842-S-i2-012
June 1992
Final Report on Current
Meter Measurements at
the 106-Mile Site in
Support of Municipal
Waste Disposal
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FINAL REPORT
CURRENT METER MEASUREMENTS
AT THE 106-MILE SITE
IN SUPPORT OF
MUNICIPAL WASTE DISPOSAL
April 25, 1988
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Marine and Estuarine Protection
Washington, DC
Prepared Under Contract No. 68-03-3319
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TABLE OF CONTENTS
EXECUTIVE SUMMARY }
1. INTRODUCTION 1
1.1 Background . 1
1.2 Physical Oceanography of the 106-Mile Site 1
1.2*1 Overview.**»». .*..»..»*,».» .... »*.*...». 1
1.2.2 Field Programs Applicable to 106-Mile Site 6
1.2.3 Summary of Findings.* * 8
1.3 Obj ecti ves 23
2. FIELD PROGRAM 24
2.1 Moor i ngs 24
2.2 Data Processing . 28
3. DATA INTERPRETATION AND ANALYSIS * ** * 29
3.1 Introduction 29
3.2 Time Series 30
3.3 Basic Statistics 43
3.4 Inertial and Tidal Motions 52
3.5 Spectra and Coherence 63
4. SUMMARY 67
5. REFERENCES 71
LIST OF TABLES
Table 2-1 EPA 106-Mile Site Moorings 26
Table 3-1 Events Occurring at the 106-Mile Site,
September 1986 to April 1987 35
Table 3-2 Basic Statistics for 3-HLP and 40-HLP
records from moorings X and Y 49
Table 3-3 Basic statistics for 3-HLP and 40-HLP
records from MASAR moorings C and 0 for
two six-months periods in 1984 and 1985 51
Table 3-4 Frequency distribution of speeds and
directions and summary statistics for the
3-HLP current records from (a) X1Q61,
(b) X1063, (c) X1067, (d) X1068, and
(e) Y1061 53
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LIST OF FIGURES
Figure 1-1 Locations of slopewater between the edge of
the continental shelf and the Gulf Stream,
from Cape Hatteras to the Grand Banks 3
Figure 1-2 Mid-AtUntie Slope and Continental Rise
showing moorings in place during 1984-1985 7
Figure 1-3 Empirical scheme of slopewater circulation
containing Coastal Labrador Sea Hater (CLSW)
inflow from the Grand Banks partly retro-
fleeting, partly flowing southwestward along
the continental margin; a western Slope Sea
gyre; and inflow from the Gulf Stream
th ermoc line 9
Figure 1-4 Upper-level currents averaged over the period of
displaced Gulf Stream position, 10 October 1984-
January 1986 11
Figure l-5a MASAR hydrographic section for the northern transect
on February 21, 1985 14
Figure l-5b MASAR hydrographic section for the northern transect
on May 14, 1985 15
Figure l-5c MASAR hydrographic section for the northern transect
for September 9, 1985 16
Figure 1-6 The mean position, standard deviation and extreme
positions of the GS front taken from all
available digitized, satellite derived sea-
surface temperature maps for the month of
October 1985 19
Figure l-7a Temperature, salinity, and density distributions
in a vertical section across the continental shelf
on 19 July 1975 for a section off the eastern
shores of Virgina 21
Figure l-7b Temperature, salinity, and density distributions
in a vertical section across the continental
shelf on 21 July 1975 for the same transect as
Figure l-7a 22
Figure 2-1 A chart of the 106-Mile Site with EPA mooring
positions X and Y, and MASAR mooring positions
C and D, indicated 25
Figure 2-2 The configurations of EPA moorings X and Y deployed
southwest and northeast of the 106-Mile Site,
respectively* 27
Figure 3-1 40-HLP stick plot of the first three months of
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LIST OF FIGURES Cont'd
Figure 3-5a Oceanographic Analyses of AVHRR Satellite Imagery,
September 15, 1986 37
Figure 3-5b Oceanographic Analyses of AVHRR Satellite Imagery,
Octobers, 1986 38
Figure 3-5c Oceanographic Analyses of AVHRR Satellite Imagery,
November 19, 1986 39
Figure 3-5d Oceanographic Analyses of AVHRR Satellite Imagery,
November 24, 1986.... 40
Figure 3-5e Oceanographic Analyses of AVHRR Satellite Imagery,
December 29, 1986. « 41
Figure 3-5f Oceanographic Analyses of AVHRR Satellite Imagery,
February 18, 1987.. 42
Figure 3-5g Oceanographic Analyses of AVHRR Satellite Imagery,
March 23, 1987 43
Figure 3-5h Oceanographic Analyses of AVHRR Satellite Imagery,
April 13, 1987..... 44
Figure 3-6 Depth-time isotherm contours from mooring X.... 46
Figure 3-7a 3-HLP velocity records of the first three months
of deployment * 59
Figure 3-7b 3-HLP velocity records of the first three months
of deployment 60
Figure 3-8a 3-HLP velocity records of the second three months
of deployment. 61
Figure 3-8b 3-HLP velocity records of the second three months
depl oyment » 62
Figure 3-9a Variance preserving rotary spectra of currents
at 48m on mooring X 64
Figure 3-9b Variance preserving rotary spectra of currents
at 248m on mooring X.......... "... 65
Figure 3-9c Variance preserving rotary spectra of currents
at 249m on mooring Y 66
Figure 3-10a Coherence squared and phase differences between
current records from depths of 48m and 248m
on mooring X............. 68
Figure 3-10b Coherence squared and phase differences between
current records from depths of 248m and lOOOra
on mooring X 69
Figure 3-10c Coherence squared and phase differences between
current records from 248m on mooring X and 249m
on mooring Y .......I 70
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EXECUTIVE SUMMARY
In September 1986, two current meter moorings were deployed for a period
of seven months on the 2500-m isobath, respectively, northeast and southwest
of the 106-Mile Deepwater Municipal Sludge Disposal Site. The moorings were
designed to monitor the current and temperature structure of the upper layers
of the ocean in the vicinity of the 106-Mile Site in order to assess the
effects of various current regimes on the disposal of sludge. The southwest
mooring was instrumented with current meters at 50-, 100-, 250-, and 1000-in
nominal water depths, and with temperature recorders at approximately 25-m
spacing from the current meters from 50m to 250m. The northeast mooring had
data from the 250- and 1000-m levels.
The 106-Mile Site is situated, with water depths ranging from
approximately 1000m to 2700m, in a distinct part of the coastal ocean between
the Gulf Stream and the shelf break known as the Slope Sea. Mem flow, for
the deployment period, was 6-7 cm s-1 southwest along the isobaths and was
uniform with depth over the upper 250m of the water column. This
characteristic southwest flow at the 106-Mile Site is considered to be the
northern part of the anticlockwise western Slope Sea gyre. During the
seven-month deployment, an energetic warm-core ring (86-E) moved through the
site in November followed by a smaller, clockwise rotating, warm eddy that
was apparently a remnant of warm-core ring (86-F). Another small warm eddy,
mostly confined to the continental slope, was observed in October trailing a
large warm-core ring (86-A) that had passed through the 106-Mile Site in
early September. This small, energetic, October warm eddy drew out a 75- to
100-m thick filament of cool shelf water into the Slope Sea.
The January to March period was not affected by eddies and was
characterized by strong surface cooling that produced a homogeneous 250-m
deep layer of 12°C water by convective overturning in early February. After
the formation of the 12°C water, known as the slopewater pycnostad, the
upper 50 to 100m of the water column experienced a number of warm outbreaks.
One was due to a large Gulf Stream filament trailing a meander crest, and
one was due to the interaction of the Gylf Stream and a ring (86-1) that was
stalled near the Hudson Canyon in March and April.
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In February, at the 1000-m level, a cool, small ("12 km long)
Labrador Sea Water (LSW) anomaly was observed advecting along the 2500-ra
isobath with the mean flow. Many of the observed phenomena in these data,
including shelfwater intrusions, small warm eddies, warm outbreaks, and LSW
anomalies, have rarely or never been observed in time series records. Major
features such as warm-core rings, the mean southwest flow, and the formation
of the 12°C slope water have been previously studied in recent Slope Sea
observational programs; however, the databases from these studies are
limited.
11
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1. INTRODUCTION
1.1 BACKGROUND
The 106-Mile Deepwater Municipal Sludge Site was designated in 1986
for the disposal of municipal sludges. As part of a monitoring program for
the disposal of sludge at the site, two current meter moorings were deployed
to monitor the temperature and current structure of the upper ocean over a
seven-month period from September 1986 to April 1987, Background material
on the dumping of sludge at the 106-Mile Site is given in EPA (1987).
Both the near- and farfield fate of sludge dumped In the deep ocean
should be evaluated, including the analysis of the transport and mixing
processes at the site and 1n adjacent waters, for a wide range of space and
time scales. The physical disposal processes involved in the dispersal of
sludge are advection by the current field, mixing by turbulence and
large-scale shear flows, and sinking of sludge particles through the water
column. Transport and mixing are profoundly affected by the types of
circulations encountertd by the sludge particles on their way to the
seafloor. The Slope Sea, the region between the Gulf Stream and the shelf
break in the Mid-Atlantic Bight where the 106-Mile Site is situated, is a
very complex region of the coastal ocean. Circulation features affecting the
106-Mile Site include the Gulf Stream, warm-core anticyelonic rings, the
Slope Sea gyre, and shelf-slope front exchange processes. These processes
are briefly discussed in terms of the transport and mixing of sludge waste in
the Monitoring Plan ( EPA , 1987). The physical oceanography of the
southern part of the Mid-Atlantic Bight slope is summarized in the following
section.
1.2 PHYSICAL OCEANOGRAPHY OF THE 106-MILE SITE
1.2.1 Overview
The physical environment of the 106-Mile Site determines how dumped
waste will be transported away from the site and mixed with the surrounding
waters, and eventually determines its fate. A thorough knowledge of the
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physical environment is needed in order to assess where waste goes, what the
rates and mechanisms of mixing with the receiving waters are, and where on
the seabed it ends up. Because sewage sludge has essentially the same
density as sea water and the majority of particles are silt sized, the sludge
constituents will take several weeks to several months to reach the seabed in
water depths, typical of the site, of 200 to 2700m. Because the current
flows are fairly strong (~0-50 cm s-1), the particles may be transported a
considerable distance in that time. Therefore, a large scale view of the
circulation (i.e., patterns of water movement) is required to assess the fate
of the sludge due to currents and mixing. The physical environment needs to
be characterized on time scales ranging from days to years and length scales
of a few km to ~1000 km. The circulation at the 106-Mile Site should be
viewed in the context of the circulation of the Mid-Atlantic Bight
encompassing the continental shelf, slope, and rise, and the Gulf Stream (GS)
from New England to Cape Hatteras. The oceanographic regime occurring at the
time and place of a release is, of course, important for Initial mixing and
dispersion,* however long-term farfield transport and fate are a more complex
issue because of the variability of the receiving waters over a wide range of
length and time scales, and the close proximity of radically different
current systems in the Mid-Atlantic Bight.
The 106-Mile Site is situated within a distinct part of the ocean
known as the Slope Sea (Csanady and Hamilton, 1987). It Is probably one of
the most complex bodies of water in the world, and though the Slope Sea and
the Gulf Stream have been studied since the 1930s (Rossby, 1936), it is only
recently that a modern synthesis of the circulation has been put together
(Csanady and Hamilton, 1987; SAIC, 1987) and some of its features confirmed
by the recent Mid-Atlantic Slope and Rise (MASAR) and Shelf Edge Exchange
Program (SEEP-I) experiments. Figure 1-1 shows a sketch of the Slope Sea
bounded by the Gulf Stream in the south and shelf water in the north. The
upper 50 to 75m of the water column in the Slope Sea is often a mixture of
shelf water, slopewater and Gulf-Stream-derived watif, producing complex
salinity and temperature structures. The shelf break region between Cape
Hatteras and Georges Bank has a complex salinity and temperature front
separating the cooler, fresher shelf waters from the warmer, more saline
slopewaters. The mixing processes along this shelf-slope front, producing
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50°N-
40° -
SMLF
•S£A WATEK
SLOPtWAJtK
80°W
70C
60*
50'
Figure 1-1.
Location of alopewater between the edge of the continental shelf
and the Gulf Stream, from Cape Hatteras to the Grand Banks,
Most of the area is occupied by slopewater except In the
northeast corner where coastal Labrador Sea water intrudes.
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cross-front exchanges possibly important of introducing sludge onto the
shelf, are extremely complex and not well understood or comprehensively
measured.
The recent MASAR experiment has shown that the Gulf Stream has
profound influences on the circulation over the slope. The Gulf Stream, part
of the western boundary current system of the North Atlantic Ocean, leaves
the continental margin at Cape Hatteras and behaves like a free-meandering
jet, flowing into progressively deeper water. It is warmer than surrounding
waters and separates the Sargasso Sea on the south side from the much cooler
slopewater on the north. There is a sharp temperature front, easily observed
from satellites, between the shoreward side of the jet and the slopewater,
. that is often referred to as the "north wall" of the Gulf Stream (GS). The
high speed currents of the GS are generally restricted to the upper 1000m of
the water column. Maximum surface speeds may exceed 200 on/s. Gulf Stream
meanders are large shoreward (north) and seaward (south) sinuous
displacements of thi GS front that move east along the path of the jet. The
meanders have wavelengths ranging from 100 km to 1000 km, periods of 4 to 100
days, and downstream propagation speeds dependent upon wave periods such that
4- to S-day period waves, with wavelengths of about 180 km, propagate at
about 40 km/day and 40-day period waves, with wavelengths of 600 km,
propagate at about 20 km/day (Tracey and Watts, 1986). The meander
displacements are constrained near Cape Hatteras, but show an almost linear
increase in amplitude as they move east. Near the New England Seamounts, the
path of the GS has an envelope that is about 500 km wide (Cornillon, 1986).
The larger meanders can pinch off to the north or south of the GS
to produce detached clockwise rotating warm-core rings or anticlockwise
rotating cold-core rings, respectively. A warm-core ring is essentially a
rotating, bowl-shaped parcel of Gulf Stream and Sargasso Sea water about 100
to 150 km in diameter, 1000-m deep in the center, with maximum speeds of
about ISO cm/s. Once it has formed in the Slope Sea, it moves west at
approximately 6 km/day (Joyce, 1984). These rings have complex life
histories before they coalesce with the Gulf Stream, often near Cape
Hatteras, Warm-core rings can interact with the GS and shelf waters,
extruding warm and cold streamers from these waters that can wrap around the
outer edges of the ring. The passage of a ring along the slope can
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apparently trigger instabilities and waves on the shelf-slope front that may
also form smaller warm- and cold-core eddies, about 20 to 50 km in diameter,
In the upper slope aiding the exchange of shelf and slopewaters. About five
to eight rings per year move into the western Slope Sea (Fitzgerald and
Chamberlin, 1983; Brown et al«, 1986), Thus, sludge dumped into the strongly
sheared currents of a ring will be transported and mixed in a different
physical and biological environment than when a warm-core ring is not present
over the dumpsite.
The previous discussion has concentrated on the upper-layer
circulation processes of the Slope Sea. Because sludge particles settle
through the water column, the deep circulation over the slope and rise also
needs to be considered. Strong fluctuating currents (20 to 60 cm/s) are
found near the bottom on the continental rise of the Mid-Atlantic Bight.
They are due to planetary wave motions, having periods of 10 to 100 days.
thought to be generated by the meandering Gulf Stream. These currents are
known as topographic Rossby waves (TRWs) (Rhines, 1971; Luyten, 1977;
Hamilton, 1984) and are characterized by fluctuating currents that increase
in magnitude from about 1000-m depth towards the seabed. These TRWs
propagate away from the deep 6S region towards the slope in a generally west
or southwest direction. The near-bottom TRW currents may be strong enough to
move fine sludge particles that reach the bottom, thus helping to disperse
them.
The near-bottom mean flow in water depths shallower than about
4000m on the continental rise and lower part of the slope is part of the
Western Boundary Undercurrent (WBUC). The WBUC is formed by the sinking of
cold arctic water in the region of the Denmark Strait and has been traced as
far south as the Grand Bahama Bank (27 R), On the Mid-Atlantic Bight slope
and rise, this mean current (~2 to 3 cm/s) follows the general trend of the
isobaths and passes under the Gulf Stream at Cape Hatteras. Thus, sludge
particles in the lower water column tend to be transported southwest by this
deep current system. Over the deeper part of the steep slope (1000- to
2000-m depth), fluctuating and mean currents seem to be weaker with a
different vertical structure than on the rise. This lower slope region way
be a sink for suspended sediment derived from land and the continental shelf.
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The few deep measurements, made by the SEEP-I and MASAR programs, over the
lower slope tend to confirm the existence of this low energy region.
This brief review of the various regimes and phenomena that affect
the farfield fate of sludge material dumped at the 106-Mile Site is followed
by a discussion of recent measurement programs (primarily MASAR and SEEP-I)
and their implications for transport and mixing. The discussion will
concentrate on the upper-layer circulation of the Slope Sea, including the
effects of warm-core rings and Gulf Stream incursions, and the possibilities
of exchanges with the outer shelf across the shelf-slope front.
1.2.2 Field Programs Applicable to 106-Mile Site
The most useful data on currents and hydrography of the western
Slope Sea and the Gulf Stream were taken during two extensive moored array
studies funded by the Department of Energy (DoE): the Shelf Edge Exchange
Program (SEEP-I, Aikman et al.f 1987); and the Minerals Management Service
(MMS): the Mid-Atlantic Slope and Rise (MASAR) study (SAIC, 1987); and
associated studies. A sketch map of the arrays is given in Figure 1-2. The
complete array (SEEP, MASAR, the Gulf Stream current meter moorings off Cape
Hatteras, and the Inverse Echo Sounder Array under the GS (funded by the
Office of Naval Research (ONR) and the National Science Foundation (NSF)) was
in place during 1984. The MASAR arrays were maintained from March 1984 to
March 1986. Both the MASAR and SEEP programs had extensive hydrographic
components. The MASAR study conducted eight seasonal hydrographic surveys
over the two years, using closely spaced stations along the two mooring
transects. This study produced the first documentation of the seasonal cycle
of formation and erosion of the slopewater pycnostad, a well-mixed layer of
water found between about 50m and 200m throughout the Slope Sea with a
temperature of 12°C, salinity 35.50/00, and hence a density of 27.0 sigma-t.
Because the MASAR northern mooring transect off New Jersey is just to the
south of the 106-Mile Site, it provides the best long-term set of current
meter measurements appropriate to the site. The U.S. Environmental
Protection Agency (EPA) measurement program, discussed in this report, was
designed to extend and complement to the MASAR study.
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76 M 75 W 74 H 73 H 72 H 71 M
N
70
41 N
40 N
39 N
SB H
37 N
•Inverted Echo Sounders
©Inverted Eeho Sounders
with Current Meters
ml tear MMS MOOT ings
if 6 Month MMS Moorings
A SUP
EPA Nuclear Dump Site
Mind Measurement Buoys
SEtP-J
36 N
35 N
42 N
N
N
39 N
38 N
37 N
36 N
78 W 75 M 74 W 73 M 72 N 71 W
70
3S N
Figure 1-2. Mid-Atlantic Slope and Continental Rise showing moorings in
place during 1984-1985. The 106-Mile Site is marked.
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Site-specific studies at the 106-Mile Site were conducted by the
National Oceanic and Atmospheric Administration (NOAA) between 1974 and 1978
(Ingham et al., 1977) concentrating on warm-core ring tracking (Bisagni,
1976) and hydrography. There is also extensive historical hydrography,
mainly in the eastern part of the Slope Sea, including Gulf Stream '60
(Fuglister, 1963) and McLellan's extensive surveys (McLellan et al., 1953;
McLellan, 1956, 1957). Prior to MASAR and SEEP, Woods Hole maintained a
mooring at "Site D" in 2600m of water south of Nantucktt Island (Thompson,
1977). However, because of the use of surface floatation on the moorings,
the upper-layer measurements were contaminated by mooring motions due to
surface waves and the later (1972 to 1973) subsurface moorings concentrated
on deep currents. Near bottom currents have been measured at a number of
sites on the continental rise, including the 2800-m low-level radioactive
waste site which is adjacent and southeast of the 106-Mile Site (Hamilton,
1982; 1984).
1.2.3 Summary of Findings
1.2.3.1 Slope Sea Circulation
The underlying circulation pattern of the Slope Sea has been
derived from historical hydrography (Csanady and Hamilton, 1987) and is shown
in Figure 1-3. The prominent western gyre is probably variable in size and
strength depending on the configuration of the Gulf Stream, the strength of
the inflow from the Labrador Sea, and the strength of the large scale wind
forcing. The detailed dynamics are not established, and the evidence for the
gyre and its variability is largely empirical. Large perturbations such as
warm-core rings and upper-slope eddies are essentially superimposed on this
basic circulation. The 106-Mile Site and the northern MASAR mooring transect
are approximately situated in the strongest part of the southwest flowing arm
of the anticlockwise gyre. This fact provides an explanation for the
predominantly southwest mean currents ("10 cffl/s) that have been measured at
the 106-Mile Site (Bisagni, 1983) and the MASAR northern transect (SAIC,
1987), Evidence for a distinct northeast return flow along the north wall of
the GS comes primarily from sea surface temperature maps derived from the
8
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50°-
45°-
Shelf Edge
70'
60'
50"
FIGURE 1-3. CONCEPTUAL MODEL OF THE CIRCULATION IN THE UPPER LAYERS OF
THE SLOPE WATER FROM CSANADY AND HAMILTON (1987). THE
106-MILE SITE IS SHOWN IN THE INSHORE ARM OF THE SLOPE SEA
GYRE. THE DASHED LINES INDICATE THE EDGE OF THE
CONTINENTAL SHELF AND THE HISTORIC MEAN POSITION OF THE
NORTHERN EDGE OF THE GULF STREAM.
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Advanced Very High Resolution Radiometer (AVHRR) on polar orbiting
satellites. Cooler, fresher water is often observed stretching from the
outer shelf and upper slope regions off Chesapeake Bay and extending in a
narrow band for several hundred kilometers along the north wall of the SS.
The position of this shelf and slope outflow can vary from the eastern shores
of Virginia to just north of Cape Hatteras, depending on the configuration of
the GS. The MASAR study measured coherent, upper-slope, along-isobath
currents between the north and south transects for a period of about 150 days
beginning in September 1984. There were comparable periods when there was no
observable correlation between the along-isobath current fluctuations of the
two transects, indicating that thera were substantial periods when the
western Slope Sea gyre did not extend to the Chesapeake Bay transect (SAIC,
-1987).
The position of the GS also has an indirect effect on the strength
of the upper-layer flow at 106-Mile Site. During most of the MASAR study
(March 1984 to March 1986), the GS was, on average, about 100 km closer to
the New Jersey shelf break than in a normal year. Only in the period between
May and September 1984 did the GS follow a path approximating its historical
or normal position. The mean upper-layer currents (at approximately 100-m to
200-m depths) for May to September 1984 (GS normal) and October 1984 to
January 1986 (GS displaced north of its historical mean position) are shown
in Figure 1-4. In the latter case, mean southwesterly flows are seen over
the New Jersey Slope (moorings B and C), but mean easterly flow is at the
3000-m isobath (mooring E). Thus, flow at mooring E appears to be in the
southern limb of the anticlockwise rotating gyre. In the normal GS period,
flows over the slope (200- to 2000-m isobaths) are weaker at both transects
than in the displaced GS period. The most substantial southwest flow is
observed at El; mooring H is in the GS most of the time. The implication is
that the gyre has more room when the GS follows its normal path and the
upper-layer of circulation is more diffused, with maximum southwesterly flow
occurring seaward of the 2000-m isobath. With the GS displaced, the surface
area of the western Slope Sea is reduced and the gyre appears stronger with
maximum speeds occurring closer to or over the slope. Monthly mean
along-isobath currents at 85 were found to be highly correlated (correlation
coefficient, R,~Q.79) with a monthly mean distance of the GS front from the
10
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39 N
76
36 N
37 N
36 N
76 W
75 W
74 U
I
75 W
74 U
73 W
72 U
71 W
70 u
39 N
I 00:
Um» MiTlblUtNT
73 U
72 W
1C li
«• 1
(a)
71 U
37 ti
70 U
36 r;
76 W
76 U
75
74 W
73 W
72 W
71 W
70 U
38 N
75 W
74 W
73 W
72 W
— 3fi N
— 37 N
71 W
70 U
36 N
Figure 1-4.
(a) Upper-level currents averaged over the period of displaced
Gulf Stream position, 10 October 1984 - 31 January 1986.
(b) Upper-level currents averaged over the period of normal
Gulf Stream path, 1 May - 21 September, 1984. The mean
position of the GS front for the indicated periods, taken
from daily AVHRR imagery, is shown by the solid line. The
position of the 106-Mile Site is shown.
11
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shelf break measured along the northern MASAR transect. This direct
relationship between the strength of the southwest flow over the New Jersey
slope and the position of the GS front is an important result of the MASAR
field study (SAIC, 1987). The dynamics of such a "teleconnection" are not
yet understood.
The transition of the GS from normal to displaced state and vice
versa was documented, with satellite imagery and current meters, three times
in the MASAR study. The transitions took about a week and seem to be caused
by the coalescence of cold-core rings (twice) or warm-core rings (once) with
the GS in the vicinity of Cape Hatteras (SAIC, 1987). The long time scales
of GS variability indicate that dispersion studies for farfield fate at the
106-Mile Site should obtain time series of several years duration in order to
obtain reasonable statistics on the variability of the circulation in the
western Slope Sea.
1.2.3.2 Slope Sea Hydrography
The depth of mixing and the salinity, temperature, and density
structure of the upper layers are important environmental constraints on the
mixing and dispersion of sludge. The surface mixed layer depth is an
important parameter for model calculations of farfield dispersion (Walker et
al., 1987). Typically, the upper layers of the Slope Sea in spring and
summer consist of a characteristic well-mixed layer above the main
thermocline between about 50- and 150-m depth. This layer is known as the
slopewater pycnostad and has a characteristic temperature and salinity of
about 12°C and 35.5°/00, respectively. Above the pycnostad, 0 to 50m in
depth, there are complex mixtures of shelf, GS, and slopewater. GS water can
overrun the surface waters of Slope Sea through filaments, warm outbreaks and
extrusions. The latter are usually due to the interaction of the GS with
warm-core rings. Filaments are shallow (20 to 50m deep) elongated fingers of
GS water that trail from the crests of some meanders. They are generated by
the circulation in the trough of a GS meander and are thought to be fairly
passive (Bane et al., 1981). Warm outbreaks are similar to but larger than
filaments and persist for several weeks (Cornillon, 1986). In the fall and
winter, more frequent storms and atmospheric cooling tend to mix the surface
12
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layers and reduce the vertical gradients of salinity and temperature for both
shelf and slopewater.
Exchange events can also occur with shelf water due to rings, upper-
slope eddies, and wind-driven flows. Hydrographic sections that transect
across the Slope Sea often show large subsurface elliptical shaped masses of
cooler, fresher water of shelf origin, apparently detached from shelf. The
recent SEEP studies have shown that these apparently isolated water masses in
the slopewater are probably attached to the shelf-slope front through complex
three-dimensional structures (R.W. Houghton, Lamont-Doherty Geological
Observatory, 1986, personal communication). The complexity and heterogeneity
of the upper 50- to 75-m of the water column at the 106-Mile Site indicate
that a single conductivity-temperature- depth (CTD) cast, particularly in
summer, at the dumpsite may not be representative of the hydrographic
structure over the length scales of the dispersion processes and may also be
difficult to interpret in terns of salinity and temperature of different
water masses. Examples of the variability of the hydrography are shown in
selected salinity, temperature, and sigma-t (density) sections from the
northern MASAR transect (Figures l-5a through l-5c).
An important result of the MASAR study was the documentation of the
formation and erosion of the slopewater pycnostad over the annual seasonal
cycle. Part of this cycle is illustrated by the sections in Figures l-5a
through l-5c. The slopewater pycnostad is formed by convective overturning
in the late winter (February) due to the cumulative effect of winter storms
and intense winter atmospheric cooling. The result is shown in Figure l-5a
where the upper 200m of the water column is perfectly mixed (temperature
12°C, salinity 35.40/00, 26.9 sigma-t). Time series of temperature from the
MASAR moorings and the southern transect hydrographic section for February
1985 showed that the formation of this deep mixed layer occurred throughout
the western Slope Sea at about the same time. Thus, the overturning of the
upper layers in late winter is not a local phenomenon. By the middle of May
(Figure l-5b), this mixed layer has been overrun by shelf and GS water, which
provide a stratified cap to the pycnostad (50 to 150-m), isolating it from
atmospheric exchanges. The fall section (Figure l-5c) shows no trace of the
pycnostad. There is now an intense seasonal thermocline at about 30-m depth
formed by atmospheric heating through the summer. Note in this figure the
13
-------
MMTHCIM LWC
owjise s
ITillOd 1 IS »
lTWC«»ni* 6C» C
MM • IS' MU < II. I*
JB ca aa
BISTJNCt (KM)
129 150
CRUISE S
ft V22/M
I 1J l|
KIN • 3«.)« IU< • 34.11
«a 89
DISTANCE (KIM
till(ON
i Si isa
ia
5 158
T
*•
a 200
efl 90
CI STANCE (KH)
MQATHCKM t,HC
cmjisc s
I/K/I* TO i/n/M
lUftIM I '0 II
IIOU-1
W« " UM NH - H.it
120 ISO
Figure l-5a.
MASAR hydrographic section for the northern transect on
February 21, 1985.
H
-------
I
X
, 1 I I H1 I I I I I I JL I' >
«ae -
sse -
3iB
MflS/SAIC/MASAH CRUISE 6
4/H'i! ia s/n/u
m TO «<
«c en c
HIM • *.i« «« * ti.ti
30 60 9B
DISTANCE (KM>
12B lit
mS/SAlC/HASAB OWISE 8
$/M/M f« 1/K/lf
IIM tax M 10 44
(«.I*lt> rfl
nw * U.li nu • M.U
urc
WS/SA1C/HASAR CRUISE 8
ly 14/44 19 9/iiiKi w ra 4<
KIN • 25 II MI •
38 60 SB
OrSTANCC
-------
CRUtSI
V V4i 10 «/ t/a
SJUI&fl M TO <«
T(i»c«i>ute oee e
nt« « 7.2) rut • ».M
20 60 50
DISTANCE mm
iU'ICM
ISO
i
nns/s*ic/.i*s**
«/ V«5 to 4/ 4'U
sunoxs is rui • 34.«
300 i
30 62 40
OiSFANCC (KH)
ItkttOH
UO ISO
LIC
CRu!S£
It «'« IS 1' 1'4)
$itri»<9 a ta <«
SI0<4*f
«!* « I*.34 »«J • J».M
300
JO 60 90
DISTANCE (Mil
Figure l-5c. MASAR hydrographic section for the northern transect for
September 9, 1985.
16
-------
intense salinity and temperature front in the lower part of the water column
at the shelf break and the occurrence of cool, fresh water of shelf origin
(Stations 30 and 26) at about 50-m depth at distances of more than 100 km
from the slope.
The erosion of the pycnostad between April and September is caused
by the upwelling of water from the GS thennocline (800m deep in the Sargasso
Sea), which is oxygen depleted and nitrate rich (Rossby, 1936; McLellan et
al., 1953). Convective overturning in the late winter increases the oxygen
content (~5 ml/L), and the high biological productivity In the spring
decreases the itrate levels (~7 nq at/L) of the pycnostad, from the values
found in the thennocline east of the GS.
Thus, sludge dumped in slopewater during the late winter convective
overturning could be readily mixed over the upper 200m of the water column.
In the late fall, however, an intense seasonal thennocline may limit vertical
mixing, due to winds, to the upper 20m of the water column. The frequent
occurrence of complex water mass structures and fronts in the upper 50 to 75m
of the Slope Sea may also mean that horizontal mixing is inhibited, with the
sludge possibly being confined to a distinct water mass, thus slowing the
processes that lead to dilution and dispersion.
1.2.3.3 Harm-Core Rings and the Gulf Stream
Warm-core rings are spawned from GS meanders, and are reabsorbed by
the GS after a lifetime of anywhere from 2 to 12 months. A ring in the Slope
Sea is a visitor, and although it can cause motions and exchanges of the
surrounding waters, it may be regarded as a temporarily detached part of the
GS which is, biologically, relatively unproductive compared to the
surrounding slopewater. Thus, sludge that is dumped into the center part of
ring may experience high currents ("150 cm s-1) in a variety of directions
depending on the position of the release relative to the ring center. Over
the slope, the characteristic signals from moored upper-layer instruments of
the passage of a ring are strong clockwise rotating northward currents
accompanied by a sharp increase in temperature. However, the mass transport
of the sludge as it is dispersed within the rotating mass of water is
southwest at about 5 cm/s-1 as the center of the ring moves towards Cape
17
-------
Hatteras. A ring in the region between the 106-Mile Site and Cape Hatteras
may be reabsorbed into the Gulf Stream or lose mass to the GS in ring-4$
interaction 3t any position. Therefore, sludge particles within a ring are
most likely to be introduced into the swiftly flowing jet of the GS proper.
However, rings on occasion show evidence of spiral-shaped filaments of shelf
or slopewater within the center part of the rotating mass. Thus, there is a
possibility of exchange with the shelf and slopewater even for sludge dumped
in the center of a ring, as further discussed in the next section.
The Gulf Stream itself may on occasion flow over the
106-Mile Site. Figure 1-6 shows the mean, standard deviation and extreme
positions of the GS north wall taken from all available (two per day) AVHRR
images for the month of October 1985. Thus, the shoreward extreme position
shows the GS flowing across the majority of the dumpsite. This situation is
probably fairly rare depending on the average path of the GS during the
year. In 1985, due to the displaced GS path as previously discussed, the
monthly statistical position diagrams (like Figure 1-6) show that the
106-Mile Site had direct GS current events three to four months of the year.
If a GS meander crest moves through the site, it may affect the flow regime
for several days at a time. Sludge released into the GS is probably the most
favorable situation for rapid dispersion of the waste because of high
velocities (""200 cm s-i) and strong vertical and horizontal velocity shears.
However, GS flows through the 106-Mile Site are likely to be east or
northeast and thus, in terms of LPCs at the eastern site boundary, it will
probably cause violations due to the speed at which the sludge waste would be
advected eastward. A similar situation of strong east currents would occur
if waste is dumped on the northern edge of a warm-core ring.
1.2.3.4 Shelf-Slope Exchanges
The most complex and difficult issues in determining the fate of
sludge waste are the transport and mixing processes across the shelf-slope
front and the possibility of introducing sludge particles onto the highly
productive, important fisheries of the outer continental shelf. A number of
mechanisms have been proposed for the exchange of shelf and slopewater across
the front, such as the generation of intrusions due to the passage of rings
18
-------
4SN
North-Woll Location Envelope for October. 1985
4CN • -
J5N -
33N
45N
40M
3SN
32N
5SM
Figure L-6. The mean position (haavy line)* standard deviation and extreme
positions (thin line) of the CS front (norch wall) taken from
all available digitized, satellite-derived (AVHRR) sea-surface
temperature maps for the month of October 1985. The position
of the 106-Mile Site is shown.
19
-------
along the slope, frontal instabilities, the effects of small clockwise and
anticlockwise eddies on the upper slope, wind-forced and density-driven
intrusions, and mixing mechanisms such as caballing and double diffusion
(Garrett and Home, 1978). Caballing is the process where two water masses
of similar density but different temperature and salinity (i.e., cooler and
fresher versus wanner and salty) mix to produce denser water (dye to the
non-linear nature of the equation of state for sea water), which then sinks.
Double diffusion Is the process where water masses separated by a temperature
and salinity front become interleaved along isopycnal surfaces. This usually
occurs at the shelf-slope front (see Figure l-5c). It is likely that these
mechanisms produce episodic events of limited spatial extent, though
intrusions seem to be relatively long lived (days to weeks), and the
onshore-offshore fluxes are estimated to be comparable to the average
onshore-directed cross-shelf flux of salt required for a salt balance on the
Mid-Atlantic Bight shelf (Ketchum and Keen, 1955). These mechanisms produce
movements of water from the slope to the shelf and vice versa which have not
been quantified or experimentally studied. Most of the evidence is from
satellite imagery and hydrographic sections across the shelf and slope.
However, intrusions of slopewater do occur episodically on the shelf, and
these provide pathways for sludge particles to be injected onto the shelf and
then transported by density- and wind-driven upwelling flows towards the
shore.
An example of wind, and possibly density-driven intrusion at
thermocline depth on the shelf, during summer, Is given in Figures l-7a and
l-7b. The transects were taken across the shelf off the eastern shore of
Virginia at about 38°N. The salinity intrusion at about 20-m depth extends
about halfway across the shelf on July 19, 1975, just above the cold pool of
8°C water- A sharp front separates the cold pool from slopewater
(temperature > 12C) in the lower half of the water column similar to the May
1985 MASAR transect (Figure l-5b). In two days, under strong upwelling
favorable southerly winds, the salinity intrusion has grown by about 40 km,
to reach the shallow inner shelf (Figure l-7b). This example of a rapidly
developing intrusion of slopewater provides a pathway for sludge particles
trapped in the seasonal thermocline to reach the inner shelf. In winter, the
evidence from hydrography and current meters is that onshore directed, wind-
20
-------
"I 4 * * * » « •» M S
»r ice IK
V-'
i: *: t: «:
9 '» - « -4 •' •( if Jf J
1C
to
f *c
i
o it
Figure l-7a. Temperature, salinity, and density distributions In a vertical
sceclon across the continence! shelf on 19 July 1975 for a
section off the eastern shores of Virginia
21
-------
is 40 e
40 *a m KB
Figure l-7b« Temperature, salinity, and density distributions in & vertical
section across the continental shelf on 21 July 1975 for the
same transect as Figure l-7a.
-------
driven flows are more likely to occur near the shelf bottom than at
thermocline depth (Boicourt and Hacker, 1976). The presence of canyons,
shelf valleys and other topographic features may locally enhance wind- and
density-driven shoreward flows.
The SEEP experiment and recent examination of historical
hydrography (Churchill et al., 1986) have shown that warm-core rings may be
responsible for substantial intrusions of slopewater onto the shelf. Thus,
waste dumped just south of a ring can be moved onto the shelf by shoreward
ring currents. Similarly, the occurrence of smaller clockwise and
anticlockwise rotating eddies in the wake of a ring or an anticlockwise cold-
core eddy preceding a ring (Churchill et al., 1986) can inject near surface
slopewater onto the shelf. There is some evidence that these events are more
likely in summer, when the shelf-slope front is confined to the lower water
column, than in winter, when the front extends from surface to bottom. Once
injected onto the shelf by an Intrusion or exchange process, sludge
particles would be transported and mixed by shelf circulations, including
wind-forced and density-driven flows associated with major river outflows.
1.3 OBJECTIVES
The main objective of the current meter program was to monitor the
current and temperature structure of the upper ocean near the 106-Mile Site
in order to assess the physical processes affecting the transport and mixing
of sludge. This study extends previous measurement programs by monitoring
currents and temperatures closer to the surface (50m) and by extending the
time series available for use with dispersion models (Walker et al., 1987)
for statistical characterization of the current and temperature fields.
The MASAR program, funded by MMS, maintained a transect across the
slope and rise of four to five moorings just to the south of the 106-Mile
Site between March 1984 and March 1986 (SAIC, 1987). The instruments nearest
the surface were deployed at 100m or deeper in MASAR. The deeper instruments
on the EPA moorings (100-m, 250-m and 1000-m nominal depths) correspond to
the depths of meters deployed on the MASAR deepwater moorings (SAIC, 1987).
The statistical characterization of the EPA current measurements
includes evaluating the along-isobath coherence of the current fields at
23
-------
moderate (75 km) separation (i.e., approximately the north-south extent of
the site). This evaluation allows some estimation of the along-isobath
spatial scales over which current measurements at a single position have
some validity. This estimation is important for dispersion modeling using
current meter data. The MASAR data showed that current fluctuations in the
upper layer on the slope were intermittently coherent, depending on the
configuration of the Gulf Stream, between the New Jersey and Virginia Beach
transects--a distance of about 250 km (SAIC, 1987). This coherence is a
reflection of the large scales inherent in the gyre circulation of the
western Slope Sea, when large pertubations such as rings are not present.
2. FIELD PROGRAM
2.1 MOORINGS
Two moorings, X and Y, were deployed on the 2500-m isobath,
northeast and southwest of the 106-Mile Site on September 19, 1986, and
recovered on April 23, 1987. The positions are given in Figure 2-1, and
details of the instrumentation and mooring designs are given in Table 2-1 and
Figure 2-2, respectively.
Table 2-1 shows the data variables returned from each instrument.
Mooring Y, as deployed, had Aanderaa current meters at 48m and 97m (Figure
2-2) j however, these top two instruments were lost because the spindle failed
on the 97-m Aanderaa (see cruise reports for Work Assignment No. 31 for
details). All the instruments experienced a 95 to 100% tape transport, but
the majority ran out of tape before retrieval because of various delays in
scheduling the recovery cruise. The instrument problems are as follows: the
General Oceanics MK II Niskin Winged Current Meter (X1062) had a faulty
tiltmeter and thus, a good temperature, but no current record was obtained.
The Sea Data temperature recorder (X1066) at 199m had missing data at the
beginning of the tape; and the rotor of the deepest Aanderaa in Mooring Y
(Y1062) apparently fouled about 10 days after deployment, producing
unrealistically low speeds (<2 cm s-1).
24
-------
75 U
EPA I 06-MILE SITE
74 U
41 N
40 N
39 N
36 N
75 U
1986-1967 MOORINGS
73 U 72 U
Figure 2-1. A chart of the 106-Mile Site with EPA Coring positions X and Y,
and MASAR mooring positions C and D, indicated.
26
-------
Table 2-1. EPA 106-Mile Site M»rlngs.
tearing X:
Position!
Water Depth:
Southwest of Dunpsite
38»34.51N 72ft36.6'W
2500-m
losezuDent
ZD
Instrument
Depth fron Surface
On)
Variables
Data Return
X1061
X1062
X1063
XIC64
'X106S
X1066
X1067
X1068
Aanieraa RCM-4 48 m
General Oceanic* 72 a
Aandaraa RQt-4 95 m
Sea Data TR-2 124 m
Sea Data TR-2 150 m
Sea Daca IR-2 199 m
Aaoderaa ROM 243 m
Aandera R£M-5 1000 m
Spd, Dir, Tasp.
Teop.
Spd. Dir. Top.
leap.
leap.
leap.
Spd, Dir, Temp.
Spd, Dir, leap.
Good 100Z
Tile oecer failed
Good 1007.
Good 1002
Good 100Z
Short 88Z
Good 1002
Good 1002
tooring Y: Nbrtheasc of Donpsite
Poaicion: 38'54.4'M 71*51.
VbcerD«pcht Z500-m
Instnnsit
ID
Instrument
Depth from Surface
(m)
Variables
Data Ren an
Y1061
Y1062
Aanderaa ROM 249 m
Aandaraa R£H-5 1005 m
Spd, Dir, Taqp.
Tanp.
Good 1001
Rotor fouled soon
after deployment
26
-------
Mooring X
44
43
73
37
150
199
a.
LU
O
1004
o
2500 BOTTOM
44
46
50
97
248
a.
LU
a
1004
2500 BOTTOM
Figure 2-2. The configurations of EPA moorings X and Y deployed southwest and northeast of the 106-Mile Site,
respectively.
-------
2.2 DATA PROCESSING
The Aanderaa tapes and Sea Data Mfcrocassettes were transcribed to
computer files using Aanderaa and Sea Data tape readers by SAIC in Raleigh,
NC, whereas the General Oceanic cassettes were transcribed to 9-track
computer tape by General Oceanics. The digital data were converted to
engineering units using the manufacturer's calibration information and then
edited to remove out-of-range values and sharp spikes. The resulting small
gaps were filled by linear interpolation. After the start and end times of
each raw data time series were precisely established and the magnetic
variation applied to current direction data, the data were filtered with a
3-hour low pass (3-HLP) Lanczos kernel decimating to intervals of one hour.
This filter removes high frequency noise from the time series and
establishes a common time interval of one hour between data points. To
further display low frequency data, the 3-HLP data were filtered with a
40-HLP Lanczos kernel and deciiated to 6-hour intervals. This filter
effectively removes tidal and inertia! signals from the time series.
The current vector time series were resolved into U- and
V-components such that the positive V-component is directed approximately
along the trend of the local isobaths at 060°T, Thus, the positive
U-component is directed away from the slope at 150°T. (Such rotated vector
time series are denoted by "R60" after the meter identification (ID) in the
plot labels—refer ta Figure 3-1 in Section 3.2).
The five-character time series IDs are constructed from the mooring
designation X or Y, 106 denoting the 101-Mile Site, and the number of the
instrument from the top of the mooring. Thus X1067 represents the Sea Data
record from the seventh instrument down from the top of mooring X (see Table
2-1). This designation is often shortened to X7, for example, in the
following discussion.
28
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3. DATA INTERPRETATION AND ANALYSIS
3.1 INTRODUCTION
The current and temperature records are best described in a
phenomenalogical manner: the records illustrate the effects on the current
and temperature field of a sequence of events passing the moorings. These
major events are primarily the passage of warm-core rings, but could include
smaller eddies, extrusions or filaments of shelf water, and Sulf Stream
phenomena. These events are superimposed on the basic southwesterly flow of
upper slopewater through the site, which is part of the northern limb of the
western Slope Sea gyre, A brief description of the physical processes
affecting the circulation at the 106-Mile Site is given in Stction 1,2.
Refer to Csanady and Hamilton (1987) and SAIC (1987) for a review of
circulation in the region; Joyce (1984) for a description of the current and
hydrographic fields in a warm-core ring; to Brown et al, (1986) for a
statistical description using satellite imagery of the life history of rings
from birth by the pinching of a northward Gulf Stream meander, to death by
absorption, often near Cape Hatteras, with the Gulf Stream; and to Beardsley
and Boicourt (1982) for a review of shelf circulation in the Mid-Atlantic
Bight.
In the interpretation of mooring data events, it is necessary to
use AVHRR imagery where possible to obtain a spatial view of the features
passing the mooring and thus confirm the analyses. Because the satellite
imagery provides a detailed picture of the sea surface temperature field,
which is sometimes obscured by clouds, and the mooring provides subsurface
time series, the interpretation of the events is not always clear.
Harm-core rings are named by the Marine Climatology Investigation, National
Marine Fisheries Service (NMFS), Narragansett, Rhode Island, according to the
year of formation and a sequence letter. Thus, the prominent November
warm-core ring seen in the EPA current records and AVHRR imagery (see Figure
3-5c in Section 3.2) is named 86-E.
29
-------
3,2 TIME SERIES
The 40-HLP current meter records from both moorings are given in
Figures 3-1 and 3-2. The corresponding 3-HLP temperature records are shown
in Figures 3-3 and 3-4. A compilation of the features present in these
records is provided in Table 3-1 and the satellite images (NOAA/NESS
oceanographic analyses) are given in Figures 3-5a through 3-5N. The
strongest fluctuations result from warm-core rings and are characterized by
increases in temperature at depths up to IQQQ-m accompanied by strong (50 cm
s-l) upper-layer currents. If the mooring is on the offshore side of the
ring center as the eddy moves southwest along the slope, the current vectors
will have a southwest component and rotate cyclonically (anticlockwise) as
the ring passes the mooring. If the current meters are on the shoreward
side of the ring center, the current vectors have a northeast component and
rotate anticyclonically (clockwise).
The seven-month deployment measured one major ring in November
(86-E) followed by a smaller, weaker warm eddy in December which seems to be
situated inshore of the 2500-m isobath and is apparently a remnant of ring
6-F (Figures 3-5c, d and e). A similar sequence occurs in September and
October (Figures 3-5a, b). However, the current measurements in September
only caught the northern edge of the September warm-core ring (86-A), but the
trailing warm upper-slope eddy (not named by NMFS) showed substantial flows
to 1000m and was effective in extruding a tongue of shelf water into the
Slope Sea. The temperature signal Indicated that the shelf water was coming
from the total shelf depth of about 100-m. This kind of event was rarely
measured by the MASAR current meters because the uppermost slope and rise
meters were at 100-m depth. It is possible that smaller warm eddies are
remnants of large warm-core rings (i.e., 86-F), or generated by rings, or
that smaller upper-slope eddies are more ubiquitous than previously thought;
such eddies have very weak surface temperature signatures and are thus not
readily detected in satellite SST observations. However, such eddies may be
detected from satellite imagery by their effects on surrounding water masses
(i.e., cold filaments wrapping around the center - Figure 3-5d).
The November warm-core ring (86-E) translated from mooring Y to X
in about 10 days. This translation speed of 8.7 cm s-1 (7.5 km/day) is
30
-------
-------
I
i
2
Of
i
CO i
I
p»,
S
10 - =
0
• 10 .4
•20 ~i
•JO - =
Yl 249m
86-E
zoo
290
1—I—'
300
1—i—
310
X8 1000m
320
3(0
J50
360
370
360
JIAIAN DATS 1946
0»» 260 IS 9/1'/1986 [P* iOb-niU SI If 40-M.r CUADEHtS
Figure 3-1.
40-HLP stick plot of the first three months of
deployment. Stick vectors oriented upward represent
along-isobath currents directed toward 060°T.
Cross-isobath currents directed toward 150°T are
represented by horizontal sticks pointing to the
-------
3
i
f\»
I
K.
S
a
1
I
I
•so -^
Yl 249m
XI 48m
100 _3
ton ~3
so J
rfi " -**-
"-«ji'^" " i • P^^nr-i y^pi ""n/u^?"^" '"— • ii* "i^t *'i^pi|p«r '" —
X3 95m
X7 248m
X8 1000m
JUL1AM B*fS >M7
DM ( II I/ i/IM»
tf* tot-nui lite 40-nr eumCMii
figure 3-2. 40-HtP stick plot of th* second thfeo months of deployment.
-------
XI 48m
X2 72m
X3 95m
X4 124m
X5 150m
Weak X6 199m
X7 246m
Y1 249m
X8 tOOOm
Y2 1005m
Figure 3-3. 3-HLP teraperature records of the first three months of the
deployment.
33
-------
B —.
I S
1 S
i *
II
19 —
I —
It —,
f> —
it -
14
ia _
ft
it —
a J
A
* _
i —
6 —
« -I
t -
1.00
.M -1
,«
n -|
.*> J
-3
J» J
»
l.JS
1 *>
X1 4Bm
^JIU^U^A/^VJ^VL^*
XS 199m
X7 248m
t1 249m
X8 lOOOm
Y=^
Y3 I005«
T
•**»
M SO
I IS l/ I/1W> "» !«•"!( Jilt
iflO tlfi
Figure 3-4. 3-HLP temperature records of the second three months of the
deployment. ,,
-------
Table 3-1. Events Occuring at the 106-Hlle Site, September 1986 to April 1987.
Dates
Julian Day
Satellite
Imagery
Description
9/22-10/1
10/2-10/9
265-274
275-282
09/15
10/06
10/8-10/9
11/1-11/21
281-282
305-325
10/06
11/19
CO
en
11/10-12/10 314-344
11/24
12/2-12/10
336-344
11/24
12/24-1/4
358-369
12/29
flow of trailing edge of large ring [86-A] that was over the
site at the beginning of September.
The ring [86-AJ undergoes a strong interaction with GS off the
mouth of the Chesapeake. A small anticyclone, trailing this ring,
appears on the upper slope that pulls a filament of cooler shelf
water southward over mooring X. Eddy produces eyelonically
rotating currents at X at depths to 1000m.
Cool filament shows strong temperature signals at 48m and 72m at
mooring X.
An energetic warm-core ring [86-E] moves through mooring X.
Initial behavior of the current vectors similar to that of Y.
After day 326, strong NB flow indicates ring center has moved
further offshore and ring translation speed along the slope seems
to have decreased. Note that no clear ring signal is seen at
1000m in the currents.
The ring [86B1 moves through mooring X. Initial behavior of the
current vectors similar to that of Y. After day 326, strong NE
flow indicates ring center has moved further offshore and ting
translation speed along the slope seems to have decreased. Note
that no clear ring signal is seen at 1000m in the currents.
Moderate offshore flow ("20 cm/s). The filament of cooler shelf
water wrapped clockwise round the ring [86-E] is observed as
approx. 2°C step-like temperature drop at 48, 72, 95m at X on days
334-336.
Imagery shows a very weak eddy onshore of mooring X. This is
apparently a remnant of ring 86-F. A deep warm eddy temperature
signal is observed at 150, 199, and 248m, but not at the surface.
Only weak eyelonically rotating southerly flow is observed at XI,
X3, and X7 consistent with the mooring being on the outer edge of
the ring.
-------
Table 3-1 Cont'd
Dates
Satellite
Julian Day Imagery
Description
12/12-12/24 346-358
1/1-2/17
2/17-2/23
CO
en
2/25, 3-5
3/29-4/12
4/4-4/2
1-58
48-54
12/29
None
2/18
56,64
88-102
94-112
None
3/23
4/13
This weak eddy [86-F] passed Yl centered around day 352. Stronger
southerly currents are observed at this time indicating that Y was
closer to the center that vhen the eddy passed X.
January is characterized by slow cooling of the surface layers and
generally weak southwestward flows.
A massive warn event occurs at the surface with decreasing
amplitude with depth. It is not observed below 150m (X5).
Clockwise rotating currents northeasterly ("20 cm/s) are observed
at XI and very weakly at X3. Imagery shows that this surface warm
event is due to a large GS filament overlying the lower slope.
A cool temperature spike is observed first at Y2 and then at X8
(1000m depth) implying a small lens of cooler fresher water as
moving along the isobath consistent with the southwesterly
currents observed at X8.
A large ring [86-1] approaching the site shows strong onshore
northerly currents at Yl. The center is east of the mooring.
The eddy {86-1] remains fairly stationary near the Hudson Canyon
and appears to be advecting warm water, from a GS warm outbreak,
south of the ring into the region of mooring X* XI and X3 show
weak onshore flow with high temperature. No warming Is observed
below 150m.
-------
EMnauumc Mouszs/mnami. matBBi samc^MMrrani. mem sxrmtxm £nmCB BKR<
*par*d by IB5S MuOLyst Jfmif»r Clark
MXE:
-. <<••• • I'i ,<=
£b^—I—
I y. ^** . *-l
GS GULF STREAM
WC WARM RING (EDDY)
CE COLO RING (EDDY)
ShM SHEIF WATER
SIM SLOPE UATER
WX ICE fftGE FRONTAL LOCATION
19 SI* SURFACE TEMP ("Cl (0-3 days old)
—*• BIRECftON Of riOH FRONTAL LOCATION
MOOIftCAflOH Bt MCI {3-? days old)
OF HARM CORE RING -•- ESTIMATED
(H»l vrie < ««'»TIA>I
Figure 3-5a. Oceanographlc Analyses of AVHRR Satellite Imagery, September 15, 1986»
-------
AHMJfSlS/HMrtOBM. WGASHER SEfWlCK/HUIOBM. UgXB SKTSBUnC SBRICE
by BESS M>*lyat Jmmifar Clark
by lUruw ClLMtoloqy InvMtigatioa (not), WVS, brraguia.tt, 81 02882
03
CS GUtF SWAM
WE WARM RING (EDO?)
U COLD BIND {EDOV)
ShH SHELF WATER
S1W SLOPE WATER
SA« SARGASSO WATER
XXX ICE EOCC FRONTAL LOCATION
19 SEA SURFACE TEMP ("Cj (0-3 days old)
—* DIRECTION OF FLOW FftOHTAL LOCATION
MODIFICATION B* HCl (3-7 days 0\
-------
OCKANDGKAFHIC MttUreXS/NNrZOMU. IKU1HER SEKVICE/mTItMaUL (JUtTO SATELLITE SERVICE DKT&:
Vr«t»tMi toy MESS hn*lynl J*nuif«i Clkrk s~)/\ A I L
HodiCi«d by Nacin* Clljwtoloqy Umesei^tion (NCII, Mrs, IUrrft9>ns«tt, U 02*12 IWre: &fyQ *V$t&Jltx0^
C CWJAIt
L IIOONIA
H, NtMOCMMIM
iifiMiris
tl
H HUDSON
U UUMtNSISM
\ V,XXS GULF s rut AM
SVMUOL • .X«£ WARN flIHG (EDDY)
LtGEMO ~~^ Ct tOLB.RtnG (EDDY)
^ SiiM SlilF MATE«
SIM SLOPE HATER
SAR SAHGASSO
fRonrAi.
19 5E» SUBFACE TEMP f°C) (0-3 days old)
—» OIBECfiSN OF riOH fft(Mir«L tOCAriOM
MQOlflCWlOK B« HCI (J-? 4*ys old)
OF HAIIH CO«E DING -•- ESriMAIEO rROHTAL
AtMLVSiS LOCATION
Figure 3-5c. Oceanographic Analyses of AVHRR Satellite Imagery, November 19, 1986.
-------
i!
OCBWWGSAPHIC AWiIJTSIS/WkTIOKM. WEATHER SEHVICZ/BWI«1M, EAKIB SKTRLUIC SEKVHX
Prepared by HESS Muilyat Jmtmifta Clark
Hodifiw) lay Matin* CliiMttoloqy Iz>v«Htiq»tit» (j*3i, HVS. Ttafit^utMtt, HI O2ft«2
DKXC:
CA.TE:
ICE EDGE
SEA SURFACE TEW {'C)
DIRECTION or FLOW
MODIFICATION BIT MCI
OF HAM CORE RING
ANALYSIS
GULF STREAM
WARM RING (EDO^)
COtD ft)TO jlDDr)
SIIELF MATER
SLOPE MATIR
SARGASSO ««,T£ft
fRO«TAL LOCATION '
(0-3 days old)
fftOMTAL tOCAHOIJ
(3-7 days old)
ESTIMATED FRONTAL
LOCATION
Figure 3-5d. Oceonographic Analyses of AVHRR Satellite Imagery, November 24, 1986.
-------
OCEANOGRAPHIC ANALYSIS/HATIONM, WEATHER SEBVJOE/HATIOMM. KARTH SATTKLtlfK SEBWTCE
Pr«p«red by HESS Analyst Jmuufor Clark
Modified by Maria* Climatology Investigation IMCI), MMK, Harragansett, Rl 02*82
DMPK:
7 I G&S
t / /&&
t inonii
HI
A
ai BLOCK
H HIOSOII
H UIlHtUCKM
IMSMINCtOH
It 6& 8
li 59.0
N if.
13
tt '.J 6
^J&sfA
'K&r-'*
'*£/ L*%
rfer • - *{\
L-Ati i i_l
Wj'r f^1 '
>^
32.
/
T/. *
*N
-gfij
h'^
1|..
/" t
k>J -
' /'/
^-+
V5
SVHOOL
LEGEND
MK ICE EDGE
19 SEA SURFACE TEMP (°C)
— *• OlffECTION OF FLOM
A*.«-HODlFtCATION BY mi
OF WARM CORE RING
ANALYSIS
GS GULF SI REAM
ME WARM RIHC (EDDY
C£ COLO RING (EDOlf
ShH SHELF WATER
SIM SLOPE HATER
SAR SARGASSO MATER
- FRONTAL IOCAT10I
(0-3 days old)
--- ffiONfAL LOCATION
(3-? days old)
— •— ESTIMATED FRONTA
tOC'.TIOfl
Figure 3-5e. Oceanographic Analyses of AVHRR Satellite Imagery, December 29, 1986,
-------
OCEASOGRAPBJC ANALTSIS/lttTIOWO. NEJOKER SERVICE/WJ-IORWL EAKTB SATXELLRC SEKWTCE
Prepared by HESS Muilyfit Jennifer Clfttk
by fUtina cliMtology Znmstigation (HCI), IMPS. ltotr«g«n&«tt, BI 028*2
OWE:
001*4 «r*a ve* nodld.d by UCI (..Hftivn*! (roa iifllal
••i*i 11 u dai* ("C|
DIRECTION Of rtOM
HOOIFICArtON 0V HCt
OF HARM CORE KING
ANALYSIS.
I
CULT STREAM
MAM) Him; (CODY)
COtl) RI'W (EDDVJ
SHELF WAfCn
SLOCC UAfCR
SARGASSO MAfEl>
rftOtUM. LOCAriOH
(0-3 days old)
FnOIITAl LOCAflON
(3-/ days old)
Figure 3-Sf. Oceanographic Analyses of AVHRR Satellite Imagery, February 18» 1987.
-------
OCEAnOGRATHIC ARM-VSIS/WJIONM, WEATHER SEHVIOE/MATIOflM, EAKTH SATTELIJTE SEtmOE DMT:
Pcepaiwl by IfSSS Aiulyst Jennifer Clark
Modified by HarinH Climatology Investigation (MCI), MIPS, tlarragansett, U 02SS2 BMTE
. M March
, <$**/March
t, CORSAIS
i i room*
H?
*
ei SLOCK
H HUBsou
WIlHIMCiOU
GS GULI STliCAH
m HARM RING
CE COID RIMG (£00r
SliM SHED* MAftn
SIM SLOPt MAIE«
SAfl SARGASSO HATER
XXX ICE EDGE FRONTAL LOCATIOI
19 SEA SURFACE TEHP ("C) (0-3 days ol
-------
MUiYSIS/RftCTOHM. WEATKEB SEKVICE/RftllttUiX. EMtfR fiMnXLXTE SOWICE DUE:
Prepared by HESS Analyst Jennifer Clark
rtodified by Marine cliMtolo^y Inv«ctiq»tion
-------
higher than average speeds of 6.5 cm s-1 reported by Brown et al., 1986.
However, the ring seemed to slow down and move off the slope as the northern
part of it passed looring X. It took 8-10 days for the center of the
trailing smaller eddy (remnant of 86-F) (JO 352-368) to pass the locations of
Y and X and thus catches up with ring 86-E by the end of December. Compare
the positions of 6-E and 86-F in Figures 3-5d and 3-5e.
January 1987 showed no ring activity, but February and March showed
some significant warm surface-layer events generally restricted to the upper
150m of the water column. Because these events are characterized by weak
currents, they are not directly ring-related. The warm outbreak occurring on
February 17 to 23 1s due to a large filament trailing a GS meander crest
(Figure 3-5f). Tht large warm outbreak, affecting much of the western Slope
Sea, shown in the March 23 analysis (Figure 3-5g) does not seem to have an
observable cause (i.e., a GS filament or ring - GS interaction) and may be
attributed to GS overrunning. The warm outbrtak seen at mooring X in April
seems to have been spavined by the' interaction of two large rings (86-1 and
87-B) off the New England and Georges Bank slopes with the Gulf Stream
(Figure 3-5h). These warm outbreaks seem to affect the upper 100m of the
water column and are slow moving and fairly passive. At the beginning of
April, the warm-core ring (86-1) near the Hudson Canyon begins to influence
the flow at looring Y.
Another view of the temperature time series from mooring X is given
by contouring isotherms with depth and time (Figure 3-6), The warm eddies
and ring 86-E (JO 277, 324, and 362) are clearly shown by the dipping of the
isotherms at all levels. The seasonal cooling of the surface waters through
November, December and January is clear. The water column becomes
homogeneous at the beginning of February when the upper and lower 12°C
isotherms meet (JD40). This convergence of the 12°C isotherms illustrates
the formation of the 12°C slope- water pycnostad by convective overturning in
late winter, which has been documented previously in MASAR and has been
surmised from earlier hydrographic work (Csanady and Hamilton, 1987).
Apart from the warm outbreaks, the water column remains homogeneous
between 50m and 250m until the beginning of April, usually the time when the
bottom of the pycnostad reaches its maximum depth (250m). This mooring X
temperature data is the third observation of a late winter overturning made
45
-------
EPA 106-MILE SITE MOORING X
72
c/3
a:
>_
UJ
JT
n,
UJ
124
150
• 199
-4V
K
a
^
1 J £W?
-------
by thermistor chains (SAIC, 1987), which helps to confirm the timing of the
formation and renewal of the slope water mass as well as its characteristic
temperature of about 12°C.
Perhaps the most intriguing event evident in the records is the
occurrence of the sharp temperature decreases at 1000m at both Y2 and X8
between JD 56 and 65, 1987 (Figure 3-4). The temperature drop of 0.5 to
0.6°C is enormous at this depth and the narrowness of the spike implies a
very small water mass being advected past the mooring. The 0.5 to 0.6°C
temperature change is of order 5 times the standard deviation of the X8
temperature record. The mean long-isobath speed between Days 56 and 65| v,
at X8 is -13.5 cm s-i. If the distance between moorings is 75 km, the time
for a water particle to move from Yl to X8 is 154 hours; the measured time
difference, using the time of the occurrence of the lowest temperatures, is
159 hours. Thus the supposition that the anomalous, cold water mass advects
along the isobath with the current is confirmed. Using the mean speed and
temporal width of the spikes, the length of the anomaly is between 11 and 13
km.
The mean temperatures of the Y2 and X8 records are about 4.2°C,
very close to the temperatures measured at 1000-m depth in the Slope Sea in
the MASAR hydrographic sections (SAIC, 1987). Salinities for this depth and
water mass range from about 34.95 to 34.98. If the anomaly has the same
density as T=4.2°C( 5=34.98 °/eo, thus sigma-t = 27.75, water, the salinity
corresponding to T=3.6°C, sigma-t = 27.75, is 34.90 °/00. This is
considerably fresher than is usually observed in hydrocasts in the western
slope sea. The water mass between 1000 and 2000m, with temperatures ranging
from 3.4 to 4.6°C, is characteristic of Labrador Sea Water (LSW) anomalies.
Historical data suggest that LSH flows southward along the continental
margin around the Ground Banks of New Foundland, and that the anomalies
weaken rapidly farther southwest along the slope. The small spatial scale of
low temperature spike and its inferred low salinity imply that mixing of LSW
may be intermittent and patchy. The true size and depth range of this nomaly
are, of course, impossible to estimate from two time series at a single
depth. However, such a strong anomaly does not appear to have ever been
detected by CTD casts in this region, although fresh LSW anomalies have been
detected further east (McCartney et al., 1980). This implies either its
47
-------
rarity or small scale patchiness or both. Normally, temperatures of 3.6°C
are measured at 1600m or deeper.
The most likely explanation for the isolated LSW anomaly is that it
is an example of a Submesoscale Coherent Vortex (SCV). SCVs are described
and theoretically discussed in a review paper by McWiniams (1985). A
subthemocline SCV was measured during the POLYMODE Local Dynamics Experiment
with a radius of about 12 km, a depth scale of under 500m, and a fairly
vigorous anticlonic circulation (speeds ~20 cm s-1). There is a suggestion
of cyclonic rotation of the current vectors as the anomaly advects past X8,
but the measured speeds do not markedly differ from the mean advection
velocity. SCVs containing LSW anomalies are thought to be formed from the
mixing of near-surface water in the Labrador Sea to its depth of neutral
buoyancy (1000 to 2000m), whereby a geostrophic adjustment process, an
isolated vortex, is generated preserving the density and potential vorticity
of the patch. McWilliams (1985) has estimated that SCVs have lifetimes of
several years.
3.3 BASIC STATISTICS
The basic statistics of all the time series from both moorings are
given in Table 3-2. The mean along-isobath flow is a substantial 6.5 cm s-1
to the southwest and is remarkably uniform with depth at Mooring X. The
corresponding mean flow at Yl is about 4.9 cm s-1 and is directed eastward,
corresponding to the 2500-m isobath direction at mooring Y (see Figure 2-1).
Mean and maximum velocities were also calculated for the November warm-core
ring [86-E] and the quiescent period from January 1 to March 26, 1987, when
no eddies moved through the site. For the November warm-core ring, maximum
speeds were the same as those given in Table 3-2, at and above 250m. Mean
speeds, however, were approximately twice the record means given in Table 3-2
and mean temperatures were up to 3°C higher at 50- and 100-m levels. The
mean along-isobath velocity cdmponent, v, for the November period was
directed northeast at between 12 and 15 cm s-1 in the upper layers (XI, X3
and X7), but the mean flow of 5 cm s-1 was still southwest at 1000m (X8).
The January to March period, which may be more representative of the basic
Slope Sea gyre currents because no large eddies were affecting the'region
48
-------
Table 3-2*
Basic statistics for 3-HLP and 40-HLP records
from noorings X and Y«
tP» 106-Nltl SITE HOOI1MCS
ft*100 I «4/ 9/21, « - ll/ 4/ f, O
VARIABLE
lit I Lit ID DEPTH(H) fUlfCflOM Or U,V.$.P(D«) (1-KLPI
XlbM 4a 60 U
2500 «
S
f
XIO&J 72 T
XI06J 95 . 60 «
V
S
t
X»IM>4 124 t
X10&V 150 T
•XlOik I»» T
IlOfc? 241 60 U
V
t
UOM 1OOO 40 II
*
S
T
I SOU V
t
t
nut loos t
4B.9
• i. a
91 .9
21.00
20.31
100. •
108.0
19.19
I«.I4
18.49
17.94
47.2
77.1
79.0
16.94
10.4
32.1
4. ft)
67. 1
** .1
69.1
11.01
4.1.
-91.2
-II. 0
.2
9.1*
-47. •
.9
9,56
10.22
10.24
IO.OS
-55.1
.§
7.59
-11.0
-29.5
-64.1
-5t .4
.2
«.t5
1.61
HE AN
-.12
-4.44
21 .64
14,714
.18
-6.58
22.81
14.006
13.785
13.487
12.912
-.42
-6.89
18.60
11 .201
l.ll
-7.62
• .74
4.209
-2.56
14.61
10. HI
4.. 211
1ATIO OtlCNTATlON OT
VARIANCE tfC. 40-NtP 1 PHlMCfPAL AX£S
O-MLP) (40-Hl.r) 3-HLP KE (DCADECS TB«C)
291.49
114.12
182,99
13.145
4.59t
381,52
959.41
241. Of
4.711
3.2S6
2.391
1 .665
233.46
168.97
2. 106
11,21
29.46
21.57
.000
240.81
141.54
151.18
l.94>
.Oli
219.15 .778
250.96
149.68
12.496
6.511
124.07 .fl«t
112.07
734.24
4.708
3.210
2,153
I .610
172,76 .761
179.58
141.1
2.041
4.65 .680
22.99
18.92
.007
195.14 .772
110.14
121. 87
t .8«&
.012
94,6
107.6
101, S
47.0
U7.8
Hote - Vatiabla S « Speed -
-------
south of the Hudson Canyon, has mean along-isobath, southwest flows of about
7 to 8 m s-1 at all levels on mooring X. These southwest currents are very
similar to the total record means given in Table 3-2. Maximum speeds
measured for this January to March period were about 50 cm s-1 above 250m and
22 cm s-1 at 1000m. During this period, the Gulf Stream east of Cape
Hatteras was near or south of its normal position (Figures 3-5f and 3-5g).
Variability is also reasonably uniform over the upper 250m of the
water column, with maximum speeds and variances of the 40-HLP fluctuations
occurring at the 100-ra level rather than closer to the surface. The lack of
vertical shear occurs for both ring and non-ring related low frequency
currents. The weak vertical shear and maximum subsurface velocity measured
during the major ring event imply that mooring X was in the center part of
the ring, which is usually in near solid-body rotation (Joyce, 1984). The
principal axes are directed offshore of the trend of the isobaths and the
near equality of the U and V 40-HLP variances above 250m may be interpreted
as ring currents dominating the fluctuations, again with the center of the
eddies passing close to the 2500-m isobith. The low frequency variances,
excluding tides and inertia, 1 currents, are 70-85% of the total variance;
thus most of the energy is contained in the low frequency fluctuations.
However, during the quiescent January to March period, high frequency motions
(periods > 40 hours) contained about half the total kinetic energy.
It is useful to compare the statistics in Table 3-2 with similar
six-month statistics from MASAR moorings C and D, positions shown in Figure
2-1. Moorings C and D were in 2000- and 2500-m water depth, respectively and
were the closest MASAft current measurements to mooring X. Moorings C and
statistics for two separate six-month periods are given in Table 3-3.
Discussion of the data from moorings C and D may be found in SAIC (1987).
The second period (April to September, 1985) contained a large warm-core ring
event (ring 84-G), which had higher maximum speeds and variances than the
November 1986 ring (128 cm s-1 versus 108 cm s-1 at 100-m). However, the
most noteworthy difference between the statistics for both of the MASAR
periods in Table 3-3 and the EPA data (Table 3-2) is the strength of the
southwest, along-isobath mean flows. The MASAR periods show mean southwest
velocities of more than twice those measured at mooring X at both the 100m
and 1000m levels. The October 1984 to October 1985 period was characterized
50
-------
Table 3-3. Basic statistics for 3-HLP and 40-HLP records from MASAR moorings C
and D for two six-months periods In 1984 and 1985.
Ha-J ft ft MOORINGS C AND D
PERIOD i B4/JO/ 1,0- n/ 4/ I,
VAHlAiU
MEIER 18 6t?TH
• OF B&fS UAUR OePIH W-CaMPQNEHt Tl C),f>(P8)
HC2B1 110 52 U
2000 V
S
T
P
HC203 1000 S3 U
V
S
t
n AII nun nlNiniiN
U'HtPJ
50.6 -34. i
14.5 -5S.B
Si. 4 .B
17.45 10,44
125. 45 109. 82
13.1 -H.I
10. B -29.7
29. B .3
4.41 3.98
HCAN
1.4?
J2.52
13,538
-10, 38
11.43
4.151
HASAR HOORIW6S £ AM ft
PERIOD i BS/ 4/ 1, B - 85/iO/ 1,
WftftlftBLE
MEIEfi ID OfPfHIBI DlflECIlOH OF U.V.SICfl/Sl
1 OF J)A*S i*J« BII>»H V-COBPOHINr I( CI,Pfti8)
NC263 1006 32 «
V
5
KD302 125 52 U
2SOO V
5
I
NDJ03 925 52 U
V
E
T
HAIIHUH niNinun
O-W.P>
It. 2 -*.t
19.9 -34.1
35.4 .3
«. -127.2
0S. 2 -49.4
I2B.4 .1
is.se a. 72
U.? -li.7
10.1 -32.6
33.3 ,6
4. S3 3. 89
MEAN
2.14
-10.04
.14
-14.57
31.20
12.413
-1.42
ie.it
4. 149
J3-W.Pt
164.85
1.S25
4. J48
11.0k
32. B4
.003
0
VARIANCE
IS -HIP)
12,37
47.08
M.34
447. M
902.30
421. »J
1.744
13. ?2
39.18
48.15
.008
R/tnn
(40-H1.PI 3-MLP KC
42.18 .416
71.53
44. fit
5.' 701
2, S3 .tbt
J4.44
25.44
.60?
ftAfiO
ETC. 40-HLP i
«46-MV.I>» 3-Hlf *£
7,50 .886
43.05
444,00 .940
479.31
S
-------
by. the Gulf Stream being displaced north of its normal path, which apparently
strengthened the gyre flow over the New Jersey slope. This phenomenon is
discussed in Section 1.2. The October 1984 to April 1985 period was
characterized by little eddy activity south of the Hudson Canyon in the Slope
Sea. The variances for mooring C (Table 3-3) are very similar to the
variances calculated for the January to March 1987 quiescent period at
mooring X. The 1000-m temperature records at C and D showed no cold LSW
anomalies such as the one captured by moorings X and Y 1n February 1987. The
minimum measured temperature at C and 0 was 3.89°C, compared with 3.51°C at
Mooring X on JD65, 1987.
Tables 3-4a through 3-4e show the statistics for the 3-HIP current
meter records broken into speed and direction classes. This method of
presenting current data allows estimation of the percentage of time that
current velocity vectors have speeds in a given speed range and the most
probable compass direction. This information, in the form of frequency
distributions, should be useful in determining likely plume tracks for sludge
disposed at the site for the six months of the current monitoring program
with its particular sequence of vents (warm-core rings, eddies, etc.) which
will differ from other six-month periods, as comparison with the limited
MASAR data in Table 3-3 shows.
3.4 INERTIAL AND TIDAL MOTIONS
Short-period motions (less than a day) in the deep ocean are
generally due to internal waves. The astronomical tide produces very weak
barotropic currents (~ 0.1 cm s-1) in 2500-m water depth. The principal
energy sources are (1) the wind action on the sea surface which generates
inertial oscillations at a frequency close to the Coriolis parameter, f, and
(2) the semi-diurnal M2 tide which generates internal waves at the shelf
break. A harmonic tidal analysis of the X3 (95m) record showed that the M2
tide had an amplitude of 2.5 cm s-1. Upper-level inertial current
oscillations can have amplitudes of 10 to 30 cm s-1 and may reach 50 cm s-1
after major storms (Mayer et al., 1981). Inertial currents are important
because they provide energy for mixing and entrainment and are the mechanism
by which storm energy is propagated into deep water. " .
52
-------
Table 3-4a. Frequency distribution of speeds and directions (expressed as a percentage of total
record length) and summary statistics for the 3-HLP current records from X1061.
tn
FREOUfNCY DISTRIBUTION
1.00 HOURLY OA1A
DIRECTION
DECRIII
0- 34
30- 40
60- 90
90-120
120-150
150-130
IBO-210
j JO-140
2IO"i?t>
270-300
SCO- 130
35B-3&0
SPE6B
CN/S
HEAN Dlft
STD DEV
MEAN SPEED
k 1.2
9 2.*
,1 3.2
,2 3. 1
.6 5.0
1 4.X
fl 3, J
fi 3.2
0 2,2
0 9
i t
9 IB
If 8 205
89 B?
. 7
.6
1,5
1.9
3.8
*.1
2.0
18
i
27
214
78
SlATIONi II04I
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1.0
1,0
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.7
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2.5
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1.3
1.0
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27
i
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208
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.5
.2
.8
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45
221
95
, i
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.6
.0
.3
. 4
.2
.3
.3
45
i
34
214
101
3HRLP
.S
,0
i
63
242
"
* 21 .61 CH/S KAIINUrt •
STANDARD
DEVIATION
« U.
.0
.4
.1
.6
,0
.0
.2
.1
.0
.3
.2
63
1
7Z
227
109
SUB
91.69
,6
.0
,1
.8
.6
.0
.6
.3
72
|
81
212
124
MftftV
cn/s
,5
.0
.0
-ft
.0
01
i
90
250
104
SPANKING 9/ZD/B4 TO «/ 9/67 4824 DATA POINTS
PERCENT HEAN HIM HAI STD. fi£V.
smB S*EST» sniti
.0
,0
,8
,0
.0
.0
.0
.0
.1
.0
90
i
99
321
741
statistics
niNinun * .if cn/a
30 CH/S
SKEWMESS - t.66
30 1951 3*t$ *0 5i tiiQ
tA I ^ il. 1^1 ^Q ft7 o to
13.0 10,98 .48 55.22 10.40
MS 2275 59 ?t^0 J203
$ 2 27 00 19 ?1 69 2& 96
AS 235^ ft^ 15 i t it. "><*.
fiflHBE • -91.70 CH/S
IN A COORDINATE SYSItfl WHOSE V AXIS IS POSITIONED .00 DE6ftEES CLOCKH1SE FAON TRUE MOfiTti
HEAN I COHPONiNt - -5.95 Cfl/5 STANDARD OfUIATlON - IB. *1 CH/S SKCHNISS - , 33
t COMPONENT * -3.03 CN/S STANDARD DEVIATION * 14.24 CH/S SKfHHIig • .82
-------
Table 3-4b. Frequency distribution of speeds and directions (expressed as a percentage of total
record length) and summary statistics for the 3-HLP current records from X1063.
1,00 HOURLY 0»IA BTATIONi 11043
DIRECUON
DEGfttfS
0- 3 180-210
210-240
J40-270
170-iaa
300-330
330-340
SPEED
CH/5
1.2:
.8
1, 1
1 1
.4
.2
. J
' 4
, 4
,5
, J
1.0
0
1
ta
l.i
1. 3
2,4
2,7
3. J
4,4
1 3
5.7
3,7
2,5
2,2
10
I
20
.5
. J
1,1
2.5
3,9
4.0
4.2
2.3
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20
i
30
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1.0
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1. 1
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i
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<
50
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.4
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i
40
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>*
i
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. 1
80
i
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^
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, 1
90
i
too
S? ANN 1MB 9/20/04 TO 4/
.0
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.0
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too
1
no
9/a; 4874 DATA fO{NTI
PERCtHt
5 0
t k
5.4
4ft
4.9
7 t
11.6
17.2
U|
t. 3
6|
3.4
FIE AH fl|H
SPEED SW»
22* IS
j£.£9
20.04
20,^1
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.n
,48
19. 9S 1.19
25. Ji 2,32
MAI
sm»
108,00
A _
i 04, jj
45. 12
51.21
48,08
89.05
104,59
94.41
SID. OEV.
26. n
14.99
1C. 59
9. 74
10.2?
11.32
10.48
20194
P£»C£Nt 15.9 34.9 28.7 8.7 3.4 1.9 1.5 1.2 .4 1.0 .2
KIAH Olft 164 210 2)5 2)3 H? 194 233 219 257 218 7tO
SID SEV «3 II 70 74 65 111 97 Hi 104 126 1011
100.00
fltdM SPEED •
22. 7J Cfl/S
STANDARD DEV1ATIDM
sunnAft»
• ioi.oo cn/s
U.33 Cn/S
KJHinUH
SVCHMISS
.91 CH/9
RflNGE * 107.09 CM/9
IN A COORDINATE SySl£H WHOSE V AIH IS POSITIONED .00 DECREES CLOCKWISE FftDM TRUE XOftlH
HEAM 1 COAfOHItil ' -5.75 CR/& SI AND Aft 0 DEVIATION * 20.44 CM/9 SKENNESS » .95
DEAN y COflPDN£Mf * -3.32 Cn/S SIOMOADO DEVIATION • 17.4f Cfl/S SKEKMESS * 1.42
-------
Fable 3-4e, Frequency distribution of speeds and directions (expressed as a percentage of total
record length) and summary statistics for the 3-HLP current records from X1067.
fftEBUCHCY DISTRIBUTION
1.00 HOURLY DATA STAIlONi
DIRECTION
DfERCCS
0- 30
30- 49
10- 10
90-120
120-150
150-180
186-210
££ Jt&-J4tt
240-270
270-300
300-330
jjo-no
SPE8D
CM'S
PERM*!
MAN Olft
STP Dfv
1.2 ,8
.? .6
.B .4
1.0 12
1.3 14
1.5 2.7
?.2 4.2
2.t e.i.
3.2 «.4
l.t 2.0
t.2 l.J
0 »
i i
4 18
20.5 3*.»
209 225
92 *4
,3
, 4
I 4
1.4
t.l
2.B
S.3
5.3
2 4
1.5
.4
IB
t
27
22.fr
218
48
.2
.1
.2
.4
.9
.5
.8
.9
1.8
, j
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27
>
24
?,4
207
77
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.1
.1
.8
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34
t
45
4.1
2IS
»*
110*7 3HHLP
.1
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J
.0
.1
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I
.4
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1
54
2.1
188
JOS
,0
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,$
,2
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.0
.0
.0
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;
43
i.S
1»9
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.1
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.0
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.4
43
i
72
1.3
272
104
SPANNING 9/20/B&
0
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0
.0
.0
.6
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0
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72
i
il
.3
210
155J
TO J»
n} *
< * <•>
14.79
15.42
17. U
17.44
tf jt J
I J . 6 *
25.09
24.67 i
fid PK
,«fB 58.
.03 74.
.H 78.
_ . _
.01 70.
t « * ^
• 13 42.
.00 51.
.03 St.
.57 30.
.00 55.
!fc7 H.
.64 18.
A L
Bo
44
64
2*
« ^
7^
6?
34
50
04
98
30
STD. OEV.
13. 17
18, 11
32.02
15. 43
10.24
7,45
9,04
*
20,44
52. M
SUMHOSY SJAT1S1ICS
«tft« Sfttft
16.52 C«rS
STANDARD DEVIATION
CN/S
» 12,»8 CH/S
HtHlHUH
SKCHNCSS
.4! CH/S
RftHSC
78.41 Cn/S
IN A COOROtNATt SvSTF.n UltOSf V AI1S IS POSIUUNEO .00 DEGRECS CLOCKWISE FflOM TRUE NORTH '
ME/sn I CO'.POHEH- . -6.24 CN/S SfAHOfiftt DEVIATION • 14.25 CH/S SKCUNE5S » 1.04
MEAN T COn?tmtM! • -2,96 CN/5 STANDARD DEVIATION • 14.11 CM/B SKENNESS • l.St
-------
Table 3-4d. Frequency distribution of speeds and directions (expressed as a percentage of total
record length) and summary statistics for the 3-HLP current records from X1068.
Cn
FREQUENCY DISlRlfaLllOH
1.00 HDUfiLV DfttA
DlAECMDtt
DtGREES
0- SO
30- 40
iO- 40
90~ S 20
120*150
* ftft -7 1 ft
210-240
2*0*270
2*0-300
360-330
3JO-J40
smo
cri/S
HEAH D1R
SID DEV
J
2,0
1.0
.7
.5
0
1
3
B 4
319
13
3 S
7. 0
2.0
,S
.5
3
1
4
21 S
224
52
4 0
13.0
1.4
.3
.2
&
i
9
324
44
SIA110HI
2.3
11.4
5.6
.8
,1
.0
1
1
12
229
32
.0
.0
.3
4.4
2.4
.J
.0
.0
12
i
13
9 1
233
23
1I04B 3HDLI>
,0
.6
.a
.2
3.3
1,0
.1
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.0
IS
1
ta
i.B
232
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,0
.0
.0
.0
J
.9
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IB
i
21
1.4
231
17
,0
. CP
.ft
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. I
.4
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21
1
24
.i
236
21
.0
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,0
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,0
.2
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.1
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.0
.0
24
i
27
i.i
227
40
&
.ft
.0
.0
.0
.0
.2
.5
.1
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.0
.0
2)
i
30
.8
321
34
SPANhtNS 9/20/64 10 4/ 9/B7 4SI& DATA PQIMIS
6
.0
.0
.0
.0
.6
.1
.0
.0
.0
.0
30
i
33
. I
211
£41
1
1
1
2
12
44
2S
S
1
i
100
. 1
.1
.1
.0
.8
.4
-J
.S
.7
.4
.2
.00
HEAN
SPEED
S.Oi
4.91
5.40
S.Oi
4.97
3. 10
8.14
10. OS
8.48
4.24
4.11
3.79
HIM
SPEED
1.31
1.20
1.31
1.13
.70
.4?
i.Ol
.a;
1.23
1.25
HA I
SPEED
II.
to.
II.
11.
10.
11.
30.
32.
?ft.
14.
10.
10.
,?
34
14
3*
58
19
SO
70
59
M
S9
22
SIB.
2.
2.
3.
2.
2.
2.
S.
S.
1.
3.
2.
2.
DEV.
if
IS
IS
47
41
SB
30
10
27
54
74
4B
SPEED
STAIISI1CS
e.tt cn/s
SfANOAflD DEVIATION
NAIlNUn * 32.71 CH/S
4.as CH/S
HIN1HUM
SKEHNESS
.44 CH/S
1.31
RANGE
32.12 CH/S
IN A COOREINAIE SVS1EH WHOSE V AXIS IS POSITIONED .00 DEGREES ClOCKHISE FROM TflUE NOftfH
REAN 1 CflflPCKENT • -S.94 CH/S StANDAfiO BEV1AIIDN * 4.62 CH/S SKENNCSS • -.34
HEAN V COnPONCN! • -4.12 CH/S SIANDAfiB OCV1AI10N • 4.44 CH/S SKENttfSS » -.77
-------
Table 3-4e. Frequency distribution of speeds and directions (expressed as a percentage of total
record length) and summary statistics for the 3-HLP current records from YI061.
- — ==.
>RE«UENCY DISTRIBUTION
1.00 HOURLY DATA
D I R{C II ON
DEGREES
0- 30
30- 60
60- 90
90-120
no-ISO
150-190
UO-210
210-240
240-270
2)0-300
300-330
330-360
SPEED
CM/5
1. 3
1.0
.6
1.2
1.0
1.0
1.1
1.9
2.3
2.B
J.O
1.9
0
7
.a
.0
.7
,T
.3
.7
2.4
4.4
6.3
S.4
4.2
2.9
7
14
I. I
.6
.7
1.0
.6
.0
1,1
2.3
4.8
3.4,
2,1
1.9
14
21
STATION) YI06I
.6
.3
.2
.2
.3
.6
.t
.3
.6
.7
. 1
-J
21
26
1.0
• 2
.2
.1
.2
.2
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1.7
.7
,7
,7
I.I
29
35
.6
.1
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.1
.3
.2
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.6
.3
.1
.1
.J
35
42
3HRLP
.7
.0
.0
.0
.4
.6
.0
.5
.2
.t
.1
.3
42
49
.4
.0
.0
.0
.3
.1
.0
.0
.0
.0
.0
.3
49
36
.1
.0
.0
.0
.3
.1
.0
.0
.0
.0
.0
.6
36
63
SPANNING 9/20/BA TO
.0
.0
.0
.0
.4
.0
.0
.0
.0
.0
.0
.1
63
70
4/ 9/B7 4924 DATA POINTS
PCftCEHt
1.
1.
3.
4.
3.
5.
5.
12.7
16.3
II.)
10.3
10. 7
MEAN
SPEED
22. 7J
12.92
12.31
12.17
26.3?
20.07
13. 9«
17.75
14.00
14. OJ
14.21
20.78
MIH
SPFFD
1,2%
.99
.66
.15
.16
1.46
I.D2
1.03
.20
.30
.51
1.59
SPFFD
60. BO
41. 10
J7.77
19.21
69. IS
J7.98
40.96
52.33
51. 3S
41.47
49.21
ii.33
Sit. t>£V.
15.60
9.77
7.74
7.4J
21.15
15.40
9. S3
It. 59
9.49
fl. 30
9.07
16.06
PERCENT IB.4 35.0 20.5 «.8 7,1 3.2 2.9 1.5 1.0 .3
HEAN DIR 211 223 ?J6 233 2M 174 163 161 243 199
£10 DEV 103 92 92 92 167 110 104 120 120 911
100.00
MEflN SPEED
SlinnAAY STATISTICS
i6.es cn/s
STANDARD DEVIATION
HAiinun •
12.31 CM/S
69.13 CH/S
niKlnun »
SKCUNESS •
.13 Ch/9
1.30
RAN6E • 68.9* CH/S
IN A COORDINATE S»51Ef1 UHOSE V AIIS IS POSITIONED .00 DEfiRtES CLOCKUISf fflON TRUE NOR1H
MEAN I COMPONENT • -4.B7 CM/S STANDARD DEVIATION • 12.67 CD/S SKEHNESS • .Si
Y COMPONENT • .64 CH/S STANDARD OtVIAMOU • 13.84 Crt/S SKEONESS • .36
-------
The 3-HLP U- and V-components of velocity are given in Figures 3-7a
and 3-7b» and 3-8a and 3-8b and correspond to the 3-HLP temperature time
series presented in Figures 3-3 and 3-4. The prominent oscillations
superimposed on the low frequency signal are primarily inertia! with a period
of 2 jr/f - 19 hours (1.26 cpd). Oscillations are ubiquitous throughout the
record at all depths and highly intermittent with considerable variability in
amplitude.
Two major inertia! events art of particular interest. The first
occurs during the passage of the major ring 86-E at mooring X on JD 322-328.
A burst of inertia! oscillations with 30 cm s-1 amplitudes occurs as the ring
currents turn from onshore to northeast. These current oscillations are
possibly representations of inertia! jet structures in the outer part of
rings (Joyce and Stalcup, 1984) or possibly inertial internal waves trapped
by the horizontal shear of the low frequency currents. The presence of
inertial currents indicates that small scale structures with large shears,
which could aid the dispersion of slydge, exist in rings. There is some
suggestion that a similar phenomenon occurs at both moorings Y and X in the
small eddy that trailed the major ring (OD 352-360).
The second period of intense inertial wave activity begins on
January 22 (Figure 3-8). These waves were generated by an intense winter
storm (a northeaster) that moved along the eastern seaboard on January 22 and
23, producing heavy snow in Delaware and Hew York. At the latitude of the
moorings, the storm center was at the coast giving the strongest winds with
clockwise rotating wind vectors over the ocean (i.e., at the moorings, the
winds backed from easterly to southerly to westerly as the storm passes to
the north). The fast moving storm generated an inertial wake with the
strongest current oscillations east of the center low pressure. The strong
inertia! currents lasted from 10 to 20 days with highest speeds (40 cm s-1)
occurring at the 250-m level (X7) about 10 days after the northeaster passed.
The delayed response is due to the relatively slow vertical propagation speed
of inertial internal waves. Because the water column was already well mixed
when the storm hit, no mixed layer deepening is seen in the temperature
records (Figure 3-4) at this time.
58
-------
Y| 249m
§
§
8
m
a
ui s
XI 48m
X3 95m
X7 248m
X8 1000m
^V/A^V-
Figure 3-7a. 3-HLP velocity records of the first three months of depJoymcnt.
U-component.
-------
1
g
I
YI 249m
Xt 48m
X3 95m
tor^^
X7 248m
X8 1000m
JULIAN MY9 1*56
OAt 2flO 18 l/l?/l«ft
CM 106- nlU Sin J-IO »-COIfQ(AHT
Figure 3-7b. 3-HLP velocity records of the first three months of deployment,
V-coraponent.
-------
•t to
s
m.
Cf,
3
too
50
0
•90
•100
100
50
0
•50
•100
too
50
0
-40
-too
100
50
0
-50
-100
to
0
-10
•20
-M
*V«v*Vvft^M»vpr«»*
f^MM^
JULIAN MVS I
Mk« I l£ U
WWVhjT^
i^V%^N»v*V*
fj^4^%A^M^s^V^
M
30
«
—I—
50
60
>0
M
100
249
m
XI 48m
X3 95m
X7 248m
X8 1 000m
MO
120
IQA-HttC SITE
Figure 3-8a. 3-HLP velocity records of the second three months of deployment.
U-component.
-------
100 —,
M —
irjmwc^v-vflj
AMiJ^Aj^iftfl/u^Mi^^^VAftihM ••!! t
^ ^S^^A*^
Yl 249m
-JO _
-100 -
100 —
50 -
48m
*K-
,,„ i/wAia**..- i Jyrk i
wvw" ' *r ^/*vir *v
^^
-50 ~
•100 —
100 —
50
J?^.
X3 95m
V
*UL . 1^%. A*"^
*^>vVA«i>/W~*j^^
ID
JULIAN DAIS 1967
0*r i is )/
X8 1000m
100 MO
120
io»-nii_£
Figure 3-8b. 3-HLP velocity records of che second three months of deployment,
V-component.
-------
3.5 SPECTRA AND COHERENCE
The distribution of energy in frequency bands is illustrated by the
spectra in Figures 3-9a through 3-9c. Rotary spectra are presented, where
the current vector time series are decomposed into anticlockwise (+) and
clockwise (-) components rotating at a given frequency (Gonella, 1971).
Thus, clockwise rotating inertial currents show a strong peak in the
clockwise rotary spectra in a frequency band around f. Purely rectilinear
current fluctuations have equal magnitude clockwise and anticlockwise rotary
spectra. The spectra are variance preserving in that equal amounts under the
curve represent equal current variance. The most energetic part of the
spectrum is restricted to periods longer than 10 days, which reflects the
warm eddy activity in these records. The mixture of clockwise and
anticlockwise components in this low frequency band varies with mooring and
depth because of the relative position of the centers of the clockwise
rotating warm eddies moving through the site. The predominance of clockwise
currents at Mooring X indicates that the moorings were generally inshore of
the large warm-core ring centers. Mooring Y shows more rectilinear
fluctuations and thus is generally closer to eddy centers. The records,
however, are dominated by the major ring that passed the site between JD 312
and 344. Typical of continental rise locations, there is little energy in
the records between 1- and 10-day periods.
The high frequency part of the spectra is dominated by the inertial
and M2 tidal peaks. The latter is weak, as indicated by the harmonic
analysis discussed previously. The inertial peak is quite broad, a
reflection of the intermittent nature of inertia! oscillations; the peak
height varies with depth. Near the surface, the inertial currents are
forced by local winds, but at greater depth, the inertia! signal can have
contributions from non-local, higher latitude areas because inertial waves
propagate energy both downward and horizontally (Hamilton, 1984; Phillips,
1977).
The stick plots (Figures 3-1 and 3-2) clearly show a high degree of
visual correlation between the fluctuations over the upper 250m of the water
column. This correlation is more quantitatively illustrated by the coherence
squared and phase differences between XI and X7, and X7 and X8, U and V
63
-------
iOO
ROTARY SPECTRA
Period (Days)
10 1
a:
0.1
•-2
CYCLES/DAY
1 X1061 R 60 48.CM) SOLID + COMPONENT
2 XI061 R 60 48.CM) DASHED - COMPONENT
"DATE « as/ 9/21: 0 TIME SERIES LENGTH : 20j DAYS
DEGREES OF FREEDOM i 18 BANDWIDTH : 0.04485051
CPD I
Figure 3-9a. Variance preserving rotary spectra of currents at
48m on mooring X.
64
-------
Ljj
O
Oi
<
ROTARY SPECTRA
100
Period (Days)
1000
a
CYCLES/DAY
1 XI067 R 60 248.
-------
LJ
o
^_.
***s,^.
100
500 —
0-
10
-2
ROTARY SPECTRA
Period (Days)
• inectial
i l r
1^
I0"1 i
CYCLES/DAY
-L-> * "
10
j
1 Y10BJ R 60 249.(M) SOLID * COMPONENT
2 Y108I R 80 249.(M) DASHED - COMPONENT
DATE s 88/ 3/21i 0 TIME SERIES LENGTH i 201 DAYS
DEGREES OF FREEDOM » 18 BANDWIDTH : 0.0448605! CPO
Figure 3-9c.
Variance preserving rotary spectra of currents at
249m on mooring V.
66
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components (Figures 3-10a and 3-10b). The upper water column has highly
coherent, in-phase, fluctuations at frequencies less than about 0.2
cycles/day. The offshore-onshore fluctuations (U) tend to be more coherent
than the along-isobath fluctuations (V). Between 250m and 1000m, the U
components are coherent between 0.05 and 0.1 cycles/day, which probably
reflects the penetration of warm-core eddies to 1000m.
Coherence between the moorings (Yl and X7) at the 250-m level
(Figure 3-10c) shows high coherence between U components with about a 180°
phase shift for the low frequency band, but virtually no coherence between
along-isobath components. Again this coherence reflects the influence of
the southwest propagating warn eddies end indicates that Stations X and Y are
less than half a wave length apart. The lack of coherence in the
along-isobath component probably reflects the different sizes of the eddies
and the relative positions of X and Y vnth respect to the trajectory of
their centers. The visual coherence of the Yl and X7 stick plots when -eddies
are not present (i.e., January 1987, Figure 3-2) is also not very evident.
At about day 30, the predominant flow at Yl changes from southwest to
northeast, but the flow at X7 remains southwest, implying a divergent flow
field that could be supplied from either off- or onshore.
4. SUMMARY
The two current meter moorings deployed on the 2500-m isobath
northeast and southwest of the 106-Mile Site provide further evidence of the
richness of variability of both currents and density structures in this
region of the Slope Sea. In the seven-month period (September 1986 to April
1987), sludge dumped at the site would have encountered several distinct
environments including a large warm-core ring (86-E), two smaller warm
eddies, cool filaments of shelf water extruded from the shelf by the eddies,
warm Gulf Stream outbreaks, moderate southwesterly flow of the Slope Sea
gyre, strong inertial currents due to a northeaster and vertical density
profiles ranging from strongly stratified in September to completely
homogeneous over the 250-m surface layer in late January. Some of these
phenomenon have not been apparent in previous moored instrument programs
because of the lack of sampling above 100m, Previous time series analyses
67
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(80
-taa
ta
XI 061
XI067
xr0st
XT 087
CYCLES/DAY
R 60
R 60
R 60
R 80
48.
24&.CM)
U COMPONENT
'U COMPONENT
DATE t 88/ 9/2 I
48%(M) V COMPONENT
248.(M) V COMPONENT
0 TIME SERIES LENGTH
201 DAYS
DEGREES OF FREEDOM • 18 BANDWIDTH
0.04485061 CPD
Figure 3-10a.
Coherence squared and phase differences between
current records from depths of 48m and 248m on
mooring X. £0
08
-------
UJ
o
Of
UJ
X
o
o
0.
180
» i r
CYCLES/DAY
XI087
XI268
XI087
X1088
R 60
R 60
R 60
R 60
DATE s 86/ 8/2 1
248.(M)
J000.(M)
248.(M)
i000.(M>
0
U COMPONENT
U COMPONENT
V COMPONENT
V COMPONENT
VRS
VRS
TIME SERIES LENGTH » 20 t DAYS
DEGREES OF FREEDOM •* 18 BANDWIDTH
0.0448506! CPD
Figure 3-10b.
Coherence squared and phase differences between
current records from depths o£ 248rn and 1000m on
mooring X.
-------
180
-180
CYCLES/DAY
R 60 248.(M)
1 XI067 R 60 248.(Ml
9 Y106I R 60 248.
-------
in this region have not detected cool shelf filaments, the small upper-slope
warm eddies (though thfeir presence has been suspected), and the passive warm
Gulf Stream outbreaks that occurred after the winter overturning event.
Similarly, the presence of a small body of anomalously cool and, by
implication, fresher water advecting along the isobaths at 1000-m depth has
important implications for the mixing of LSW with the surrounding deep North
Atlantic water mass. This anomaly, which may be a submesoscale coherent
vortex (McWilliams, 1935) being advected by the along-Isobath mean flow, may
not have direct impacts on the disposal of sludge at the 106-Mile Site, but
may have an effect upon the oceanography of the continental margin.
The seven-month EPA current monitoring program has demonstrated a
complex sequence of events, some of which are reasonably well understood
(i.e., warm-core rings), and others that have been observed in satellite
imagery, but have had few in situ measurements on their depth structures, and
are less well understood (i.e., smaller warm eddies, shelf filaments, and
warm outbreaks). The September 1986 to April 1987 period differed in type
and sequence of events, and thus overall statistics, from any previous
six-month period of current measurements taken by the MASAR program. The
two-year MASAR current meter observations were Tirolted in depth coverage and
thus, upper ocean complex events, such as shelf water extrusions, were not
captured by that program. Thus, information is particularly needed on the
smaller upper slope warm eddies and their role in forcing extrusions of shelf
water into the Slope Sea and intrusions of the slope water onto the shelf.
The origin and formation, relationship to warm-core rings, and decay of such
eddies is presently a raystery. The interannual time scales associated with
the Slope Sea gyre, Gulf Stream axis shifts, and the occurrence of warm-core
rings as well as the seasonal cycles associated with the shelf-slope front
and the formation and erosion of the slopewater pycnostad indicate that an
adequate description of the statistics of surface layer currents in the
region of the 106-Mile Site will require further monitoring.
5. REFERENCES
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Municipal Sludge Site. Environmental Protection Agency. EPA 842-
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Gonella, J. 1971. A local study of inertial oscillations in the upper
layers of the ocean. Deep-Sea Res. 18:775-778.
Hamilton, P. 1982. Analysis of current meter records at the northwest
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Hamilton, P. 1984. Topographic and inertia! waves on the continental rise of
the Mid-Atlantic Bight. J. Geophys. Res. 89:695-710.
Ingham, M.C., J.J. Bisagni and 0. Mizenko, 1977. The general physical
oceanography of deepwater dumpsite 106, in Baseline Report of
Environmental Conditions in Oeepwater Dumpsite 106 (Volume 1), NOAA
Dumpsite Evaluation Report 77-1, Rockville, MD, pp 29-86.
Joyce, T. 1984. Velocity and hydrographic structure of a Gulf Stream
warm-core ring. J. Phys. Oceanogr. 14(5):936-947.
Joyce, T.M., and M.C. Stalcup. 1984. An upper ocean current jet and internal
waves in a Gulf Stream warm core ring. J. Geophys. Res. 89il997-2994.
Ketchum, B.H., and D.J. Keen. 1955. The accumulation of river water over
the continental shelf between Cape Cod and Chesapeake Bay. Deep-Sea
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Luyten, J.R. 1977. Scales of motion in the deep Gulf Stream and across the
continental rise. J. Mar. Res. 35:49-64.
Mayer, D.A., H.O. Mofjeld, and K.D. Leaman. 1981. Near-inertial waves
observed on the outer shelf in the Middle Atlantic Bight in the wake of
Hurricane Belle. J. Phys. Oceanogr. 11:87-106.
McCartney, M.S., L.V. Worthington, and M.E. Raymer. 1980. Anomalous water
mass distributions at 55 W in the North Atlantic in 1977. J. Mar. Res.
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McLellan, H.J., L. Lauzier, and W.B. Bailey. 1953. The slopewater off the
Scotia Shelf. J. Fish. Res. Bd. Can. 10(4):155-176.
McLellan, H.J. 1956. On the sharpness of oceanographic boundaries south of
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McLellan, H.J. 1957. On the distinctness and origin of the slopewater off
the Scotian Shelf and its easterly flow south of the Grand Banks. 0.
Fish. Res, Bd. Can. 14:213-239.
McWilliams, J.C. 1985. Submesoscale, coherent vortices in the ocean. Rev.
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Rhines, P.B. 1971. A note on long-period motions at Site D. Deap-Sea Res.,
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Rossby, C.G. 1936. Dynamics of steady ocean currents in the light of
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