Not to be cited without prior reference to the authors
ICES CM 2002/M:20
The Upper Ocean Circulation at Great Meteor Seamount.
Part I: Structure of Density and Flow Fields
Christian Mohn and Aike Beckmann
Max Planck Institute for Meteorology, Hamburg, Germany
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Abstract
Observations of the hydrography and currents at the Great Meteor Seamount are combined
with a numerical model to investigate the three-dimensional structure of the flow regime at this
seamount. Signatures of periodic and mean flow are separated and interpreted. Tidal forcing
is the dominant process in this area, leading to internal wave generation, trapped waves, flow
rectification, and a system of closed circulation cells (horizontal and vertical). Steep slopes and
a flat summit plain lead to a previously unreported mixed layer thickness anomaly along the
edge of the seamount. Observations alone are found insufficient to derive a complete picture
of the circulation and water mass distribution. The model results will be used in Part II of this
study to further investigate biologically relevant questions.
1 Introduction
During the past decades, a large number of multidisciplinary studies have focussed on isolated
seamounts and submarine banks. As a result, our knowledge about the effects of seamounts
on marine ecosystems (Boehlert and Genin, 1987; Rogers, 1994) and on the ocean circulation
(Hogg, 1980; Beckmann, 1999) has grown steadily.
Most of the physical elements of isolated seamount regimes have been explored. These
include Taylor column/Taylor cap generation through a steady impinging flow (Chapman and
Haidvogel, 1992, e.g.), flow amplification (Hunkins, 1986; Eriksen, 1991), trapped wave generation (Brink, 1989, 1990), rectification related to tidal forcing (Haidvogel et al., 1993; Kunze
and Toole, 1997) as well as locally enhanced turbulent vertical mixing (Kunze and Toole, 1997;
Eriksen, 1998). In addition to observational programmes and theoretical considerations, numerical modeling has played a major role in these investigations. For example, the physical
processes at steep and tall seamounts have been explored in idealized settings (Chapman and
Haidvogel, 1992; Haidvogel et al., 1993; Goldner and Chapman, 1997). Realistic topography was used by Beckmann and Haidvogel (1997) for a quantitative study of flow rectification.
Recently, a similar concept was used for Maud Rise in the Southern Ocean (Beckmann et al.,
2001), to study the effects of a seamount on sea ice formation. Yet, some aspects have not
been addressed so far. Among these are (a) combined sub- and superinertial forcing, (b) critical
latitude effects and (c) strongly asymmetric topographies.
This study focuses on the Great Meteor Seamount in the central North Atlantic and combines observations and numerical modeling to investigate the physical situation and some of the
1
consequences for marine ecosystems. Three aspects of the physical situation at this seamount
make it particularly worth studying: the shape of the Great Meteor Seamount is north-south
elongated with three relatively sharp corners in the south, northwest and northeast and there are
two smaller seamounts nearby; the critical latitude for diurnal K tides separates the northern
from the southern half; and finally both diurnal and semidiurnal tides are important.
This paper is organized as follows: Chapter 2 introduces the observational and theoretical
background and a description of the most recent set of measurements as well as the ocean
circulation model and its configuration. In Chapter 3 we present the results of the observations
and selected numerical simulations on mean flow conditions as well as the variability. The
results are summarized and discussed in Chapter 4.
Great
Meteor Seamount
M423-A
30’
➤
Small
Meteor Seamount
20’
Closs Bank
10’
M423-B
➤
❶
o
30 N
➤
50’
M423-C
N
W
30 °
28 °
30’
❷
40’
30’
10’
29oW
50’
40’
30’
20’
10’
28oW
Figure 1: Great Meteor Seamount topography (Smith and Sandwell, 1997): three-dimensional
view (left), plain view (right), with CTD station grid, CTD transects and location of acoustic
current meter moorings during the the RV Meteor cruise 42/3 (29 August - 21 September 1998).
The depth contour interval is 250 m.
2 Material and Methods
2.1 Observational and Theoretical Background
Great Meteor Seamount is one of the largest isolated submarine features in the Atlantic Ocean,
located at 30 N and 28.5 W in the subtropical North Atlantic (Fig. 1). It is located approximately 1500 km west of the Canary Islands and 1000 km south of the Azores, far off coastal
boundaries (Fig. 2). It rises steeply from depths greater than 4500 m to depths of less than 300 m
and is characterized by an elliptically shaped flat plateau
with a maximum length of 54 km and
a maximum
width of 31 km. The average slope is 29 at depths 3000 m, locally exceeding
40 .
The flow system in the warmwatersphere of the subtropical North Atlantic is dominated
by the wind-driven subtropical gyre, which forms an anticyclonic recirculation from the Gulf
Stream system between 20 N and 35 N . While the major part of the gyre recirculation occurs
2
o
40 N
Azores
o
3
11
35 N
Madeira
o
Canary Islands
30 N
5
GMS
3
o
25 N
4
20oN
12
o
15 N
Cape Verdean
Islands
o
10 N o
50 W
o
45 W
o
40 W
o
35 W
o
30 W
o
25 W
o
20 W
o
15 W
o
10 W
o
5 W
Figure 2: Schematic picture of the upper ocean circulation in the eastern subtropical gyre of the
North Atlantic after Siedler and Onken (1996). The Great Meteor Seamount (GMS) complex is
located at 30 N and 28.5 W .
west of the Midatlantic Ridge, a substantial transport of water masses is also evident in the eastern basin (Schmitz and McCartney, 1993). The Great Meteor Seamount is located between the
eastward Azores Current and the southwestward recirculation, with relatively weak southwestward mean currents. Although the Azores Current system is a source of mesoscale variability
(LeTraon and DeMay, 1994; Käse and Krauss, 1996), there is no observational evidence of particularly strong eddy activity at Great Meteor Seamount. For example, Meincke (1971a) found
the residual (daily-mean) upper ocean flow in the vicinity of the seamount to be stationary over
several weeks.
The tidal characteristics of the Iberian and Canary basins were investigated by Siedler and
Paul (1991) based on a large-scale moored current meter array. They found typical semidiurnal
tidal currents amplitudes of 1 cm s and 3.5 cm s for the main semidiurnal constituents M (12.421 hours) and S (12.000 hours). In order to obtain the main diurnal tidal currents K
(23.943 hours) and O (25.819 hours), the inverse global tidal model TPXO.5.1 (Egbert et al.,
1994) was applied to the Great Meteor Seamount area and its surroundings. The main semidiurnal tidal constituents in the model agree well with the observations of Siedler and Paul (1991),
we therefore feel confident to rely on the modeled diurnal tidal amplitudes of 0.3 cm s (K )
and 0.1 cm s (O ), respectively, i.e., the main diurnal currents are one order of magnitude
smaller than the semidiurnal currents. In general, the barotropic tidal flow in the deep ocean
3
regions off the seamount is oriented mainly northeast-southwest. Relatively weak enhancement
of barotropic tidal currents above the seamount is predicted by this coarse resolution model.
The amplification of the main semidiurnal constituents is 2.4 (M ) and 2.3 (S ), respectively.
The strongest amplification factor of 10.3 is indicated for the diurnal O tide, while there is no
significant enhancement of K .
Isopycnal doming can be expected due to either long-period (steady) impinging flow or the
rectification by trapped waves for subinertial frequencies. This rectification occurs if low-mode
seamount-trapped waves are in near–resonance with the tidal forcing. Nonlinear interaction can
lead to an anticyclonic, along-isobath residual flow of substantial amplitude. This resonance
depends on the shape of the seamount, the ambient stratification and rotation (Brink, 1989;
Haidvogel et al., 1993). An overview of principal mechanisms and circulation patterns is given
in Part II of this study (Beckmann and Mohn, 2002). In case of the Great Meteor Seamount, the
large–scale flow field is relatively weak, but seamount trapped waves are possible for O and,
on the northern flanks only, K .
2.2 CTD and Current Measurements
The hydrographic sampling strategy was to obtain a snapshot of the local stratification and
flow conditions and to collect a representative validation data set for high-resolution numerical
experiments. A total of 52 CTD profiles were collected across the Great Meteor Seamount from
the surface to the seabed with a Seabird 911plus system in September 1998 (Fig. 1). The station
grid was less than 1 km along the seamount flanks to resolve phenomena above the steep slopes.
At each CTD station up to 24 additional water samples were collected at different depths, using
a Seabird rosette system. The bottle samples were analyzed for salinity for later calibration
of the CTD conductivity. The rosette was equipped with mechanical reversing thermometers
(Gohla) at four depths for the calibration of CTD temperature and pressure. After the cruise,
the final processing and calibration of the CTD data was performed, achieving WOCE standard
accuracy.
Two Self-Contained Acoustic Doppler Current Profilers (SC-ADCP) with an operating frequency of 153 kHz were deployed at the northern and southern edge of the seamount. The
moorings were placed for a period of 3 weeks at water depths of 383 m (northern mooring) and
423 m (southern mooring), respectively (see Fig. 1). Current velocities were recorded in depth
intervals of 8 m and averaged to ensembles of 30 min. The resulting dataset was postprocessed
according to the requirements for backscatter calibration, sound absorption and beam geometry
(RDI-Primer, 1996). Poor quality velocity data with a percent good value of less than 25
were excluded from further analysis.
2.3 The Ocean Circulation Model
2.3.1 The model set-up
The main goal of the modeling effort is to support and extend the results of the observational
study, i.e., the seamount induced regime in the Great Meteor Seamount area under late summer
stratification conditions. A first step is to quantify the relative contributions of the time–mean
and the transient anomalies and then to identify the dynamical mechanisms at work.
4
We chose the fully three-dimensional, hydrostatic, nonlinear, terrain-following coordinate
primitive equation model SPEM (Haidvogel et al., 1991). SPEM has been successfully applied
in previous studies with idealized topography (Chapman and Haidvogel, 1992; Goldner and
Chapman, 1997) and realistic seamount configurations, such as Fieberling Guyot (Beckmann
and Haidvogel, 1997) and Maud Rise (Beckmann et al., 2001). By virtue of the nonlinearly
vertical coordinate transformation (Song and Haidvogel, 1994), shallow areas as well as the
surface and bottom layers are represented with increased vertical resolution.
Necessary ingredients for such a study are a realistic topography, background stratification, and time-mean and tidal forcing with realistic amplitudes and frequencies for the main
semidiurnal and diurnal constituents. Some idealizations have been made to simplify the configuration; the large-scale (steady and tidal) flow and the initial density field were assumed to
be horizontally uniform and thermodynamic effects are included in the model by a single-state
variable (potential density). Tidal ellipses are approximated by straight lines oriented northeast–
southwest.
30°N
0
km
100
28°30’W
Figure 3: Channel geometry and bottom topography of the numerical model. The contour
interval is 500 m. The prevailing direction of the far field oceanic flow is indicated by the
arrows. North of 30 N , the K tide is subinertial.
The bottom topography was taken from a satellite gravimetry based bottom topography data
set (Smith and Sandwell, 1997) with a horizontal resolution of 1/30 . It is placed in the center of
a periodic channel domain, oriented NE-SW according to the prevailing direction of the steady
and tidal oceanic far field flow and bounded by solid sidewalls (Fig. 3). A -plane is used
to include the effects associated with the critical latitude for the K frequency (at 30 N ). At
the boundaries of our periodic channel restoring zones were added to reduce the generation and
reflection of Rossby waves. We found that the boundaries are sufficiently far from the seamount
topography to not seriously influence the results at the Meteor Bank.
The model domain spans an area of 512 512 km. The horizontal grid contains 128 128
points with a variable grid spacing between 1.2 km in the center of the domain and 6.8 km at
5
the boundaries. The vertical grid consists of 20 levels and is stretched accordingly to properly
resolve small-scale processes especially at the bottom and above the seamount flanks. A weak
smoothing of the topography was introduced for numerical stability. A significant flattening
of the original Small Meteor Seamount summit depth of approximately 250 m was found after
smoothing. However, we consider this as an acceptable trade-off, since we concentrate on the
Great Meteor Seamount, where a validation of the model results with observations is possible.
2.3.2 Initialization and forcing
Detailed observations of far field stratification and currents immediately off the seamount are
sparse. The September 1998 measurements (Nellen, 1999) were used to determine the density
distribution for the initialization of the model.
The model is forced with a combination of barotropic steady and periodic inflow with realistic amplitudes and phases taken from the observations and tidal model results described in
Subsection 2.1. For our study we regard a barotropic steady far field flow of 1 cm s as a reasonable assumption (Meincke, 1971b). Due to the rigid lid condition, we can use a barotropic
mass transport streamfunction varying periodically in time to introduce the tidal currents.
This method was successfully used in previous periodic channel seamount studies (Haidvogel
et al., 1993; Beckmann and Haidvogel, 1997; Mohn and Beckmann, 2002).
Subgridscale mixing is represented by a constant biharmonic lateral viscosity along the
terrain-following model surfaces ( = 10 m s ) and a constant biharmonic lateral diffusivity
rotated to geopotential surfaces ( = 5 10 m s ). An adaptive scheme (Pacanowski and
Philander, 1981) computes vertical mixing as a function of stratification and vertical shear; a
velocity dependent quadratic bottom friction is used.
Each calculation begins from rest and is integrated for a period of 90 days with a time-step
of 67.5 s. The forcing is gradually increased to its full strength within the first 15 days of the
model integration to reduce the effect of inertial wave excitation. The response of the seamount
regime to the forcing is fully developed after 60 days of model integration. The time-mean
oceanic fields are then obtained by averaging another 30 days, thus including two complete
spring–neap cycles.
3 Results
3.1 The September 1998 Observations
3.1.1 Density field
The results of the CTD measurements are presented along three transects, which were composed from the available database (see Fig. 1). The vertical distribution of potential density
( kg m ) is used to describe the key characteristics of the local stratification.
The density fields shown in Fig. 4 exhibit systematic anomaly patterns which clearly reflect presence of the seamount (Fig. 4)1 . In general, the density field is marked by a strong
small–scale variability above the steep seamount flanks. The expected large-scale “dome-like”
1
It is, however, important to note that these fields represent neither time–mean nor instantaneous conditions, as
no tidal correction has been carried out.
6
M42/3-A
M42/3-B
0
25
z [m]
26
26.2
-100
0
0
25
z [m]
-100
26
26.2
-100
-200
26.6
-300
26.7
26.75
26.8
26.7
26.7
-300
26.75
26.8
26.8
-400
26.4
26.6
26.6
26.75
26
26.2
26.4
-200
-300
25
z [m]
26.4
-200
M42/3-C
26.9
-400
26.9
-400
26.9
-500
27
27
27
-500
-500
27.1
27.1
27.1
-600
-600
0
50
100
-600
0
x [km]
50
x [km]
0
50
x [km]
Figure 4: Vertical distribution of potential density ( kg m ) in the upper 600 m along transects M42/3-A - M42/3-C at the Great Meteor Seamount. The tick marks at the top of each
figure mark the locations of CTD measurements.
deformation of the density field above the seamount is well pronounced along the North–South
axis of the seamount (transect M423-A) where the sampling was extended to the deep oceanic
seamount surroundings. It constitutes a dense (cold) anomaly relative to the water mass properties off the seamount whose intensity and amplitude of uplifting strongly vary with depth.
Due to the limited extent of transects M42/3-B and M42/3-C along the East-West axis of the
seamount the dense anomaly is less distinctive but still evident.
A second prominent feature is the narrow but strong depression of isopycnals above the
steep flanks of the seamount, revealed by the reduced distance between the stations above the
flanks. It is visible at all transects in the immediate vicinity of the seamount summit area and
most pronounced at greater depths, but extends across the whole water column into the nearsurface layer.
3.1.2 Barotropic tides
To estimate the barotropic tidal activity at Great Meteor Seamount, each velocity component
of the 19 days time series from the SC-ADCP profiler records was spectrally analyzed. Due
to the data gaps we were not able to compute real barotropic velocities but using averages
over the available sampling depth ranges instead. These restricitions allow only a first-order
comparison of our observations with the results of the TPXO.5.1 tidal model. Table 1 presents
the the characteristics of the current ellipses for the main semidiurnal and diurnal constituents,
respectively.
7
a
M
S
K /+
O
cm s
SC-ADCP mooring north
#
$
a "!
F
')(
11.9
4.5
2.0
1.5
cm s
')(
-9.1
-2.5
-1.4
-1.2
*
g GMT
a%
cm s
152.1 0.76 3.4
177.0 0.56 4.1
76.4 0.7 7.1
122.3 0.8 15.0
SC-ADCP mooring south
#
$
a%&!
F
')(
14.6
3.9
3.7
2.3
cm s
')(
-6.4
-0.8
-3.4
-1.1
*
g GMT
325.6
313.0
49.2
197.8
0.44 4.2
0.21 3.5
0.92 13.2
0.48 23.0
Table 1: Tidal analysis of mean velocity time series at Great Meteor Seamount. a %, and a "!
are the semimajor and semiminor axes of the tidal ellipse, negative a %&! indicate clockwise
rotation. The phase relative to Greenwich is # , $ is the ellipticity ( -/.%"!102.1%,3- ), and F is the
amplification factor relative to the far–field values from the TPXO.5.1 tidal model.
At both locations, enhanced tidal currents are found (compared to the oceanic far field tides
as computed with the TPXO.5.1 tidal model; see Section 2.1). The tidal current variance within
the sampling range is dominated by the semidiurnal frequency band at both mooring sites and
can be attributed to the main semidiurnal constituents M and S . Typical velocities within
the semidiurnal band range from 3 to 5 cm s for the S and 12 to 15 cm s for the M
constituents. The relative contributions of the main tidal constituents in the diurnal band are
much smaller. Typical current velocities are in the order of 2 - 3.5 cm s for K /f and 1.5 2 cm s for O . Due to the geographic latitude of the seamount ( 30 N ) and the short time
series the K and local inertial frequencies are not properly resolved from one another and cannot be treated separately. We find a three- to four-fold amplification for the main semidiurnal
tides relative to the far field values from the TPXO.5.1 tidal model, but more than a magnitude enhancement of the diurnal constituents with the strongest amplification occuring at the
southern slope. The observations generally overestimate the tidal model results with the K
constituent being surprisingly strong (see Section 2.1). There are several possible explanations
for this difference. Firstly, the accuracy of the tidal currents may be degraded by data gaps
due to the poor quality of the acoustic measurements within the top 60 m of the water column.
Secondly, the coarse resolution barotropic tidal model may not represent the topography and
consequently the resonance frequency of Meteor Seamount with sufficient accurracy.
3.2 Numerical Simulations
3.2.1 Barotropic tides and local residual flow
Although two single current meter moorings cannot give a representative picture of the prevailing flow system at the seamount, they are useful for model validation. To estimate the reliability
of the model results we extracted a 19 days time series of the modeled velocity profiles at each
of the SC-ADCP mooring locations. Barotropic tidal characteristics were calculated and compared with the observations and the TPXO.5.1 tidal model results. They are summarized in
Table 2. The barotropic tidal velocities in our model range from 7.5 to 8.1 cm s (M ) and 3.8
to 4.9 cm s ((M ) corresponding to a moderate amplification over the seamount from 2.2 to
2.3 (M ) and 3.4 to 4.4 (S ), respectively. This generally confirms the TPXO.5.1 tidal model
results (see Section 2.1). The diurnal band is marked by a strong amplification of the main
8
constituents K and O . While the barotropic O tide is somewhat underestimated in our model
(amplification factor 6) compared to Egbert’s tidal model, the amplification of the K tide even
exceeds the values of the SC-ADCP mooring observations. One explanation for the differences
between the observations and Egbert’s tidal model results was found to be vertical undersampling due to observational data gaps which allowed the calculation of depth-averaged, but not
fully barotropic velocity profiles. Since this is not the case in our model we suggest that the absence of any amplification of the K tide in Egbert’s tidal model is a consequence of its coarse
resolution which fails to properly represent the resonance frequency of Great Meteor Seamount.
SPEM (SC-ADCP mooring north) SPEM (SC-ADCP mooring south)
a%, a "!
#
$
F
a%, a "!
#
$
F
M
S
K /+
O
cm s
')(
7.5
3.8
3.5
0.5
cm s
')(
-1.7
-0.9
-3.0
-0.3
*
g GMT
cm s
227.5 0.23
179.4 0.24
70.2 0.86
40.2 0.6
2.2
3.4
12.5
5
')(
cm s
8.1
4.9
4.0
0.6
')(
*
-3.3
-2.1
-3.1
-0.5
g GMT
296.9 0.41
24.6 0.43
117.1 0.78
93.9 0.8
2.3
4.4
14.3
6
Table 2: Tidal analysis of barotropic velocity time series at Great Meteor Seamount derived
from the numerical model at each SC-ADCP mooring. a % and a%"! are the semimajor and
semiminor axes of the tidal ellipse, negative a %&! indicate clockwise rotation. The phase relative
to Greenwich is # , $ is the ellipticity ( - .3%&!302.1%,4- ), and F is the amplification factor relative to
the far–field values from the TPXO.5.1 tidal model.
Model
SC-ADCP
northern
flank
100
northern
flank
100
200
200
300
300
z(m)
400
East
10 cm/s
400
East
10 cm/s
100
southern
flank
100
southern
flank
200
200
300
300
z(m)
400
East
10 cm/s
−28.6
−28.4
longitude
400
−28.2
East
10 cm/s
−28.6
−28.4
longitude
−28.2
Figure 5: Residual flow averaged over 19 days at the SC-ADCP mooring sites and the corresponding model locations at the northern (top) and southern seamount slope (bottom)
The observed time-mean flow at both SC-ADCP moorings show generally westward to
9
southwestward flow from 70 m down to about 250 m, with a magnitude of 5-7 cm s (Fig. 5).
Below that depth (which could be called a “level of slow motion”), the flow is indicative of an
along-isobath flow at the northern and southern (southward elongated) rim of the seamount.
A particularly close correspondence of the model results with the observations is found for
the northern flank mooring (Fig. 5). For the southern flank mooring, the model results fail to reproduce the level of slow motion and the strong southwestward near-bottom flow. We attribute
these differences in flow magnitude to the exposed location of the southern SC-ADCP mooring
at the southernmost tip of the seamount, where the bottom topography undergoes rapid lateral
changes compared to the straight northern seamount flank. A weak smoothing of the bottom
topography was introduced in the model for numerical stability. Details of the bottom topography might not be represented adequately in the model and the real currents are underestimated
in areas in extreme topographic changes over a few kilometers. Alternatively, the residual current profile at the southern flank could be the result of mesoscale variability not included in our
simulation. Nevertheless, the correspondence of the model results with the observations at the
northern flank mooring indicates, that the model is capable of a realistic representation of the
seamount flow regime. Some underestimations of the real flow may occur in areas of strong
topographic changes.
3.2.2 The seamount summit layer (SSL)
4000
4000
3000
3000
2000
1000
2000
1000
300
300
2000
2000
3000
3000
0
-0.05
-0.04
-0.03
-0.02
km
-0.01
0
30
0.0
0.01
0.02
0.03
0.04
km
30
0.05 kg/m 3
Figure 6: Horizontal density anomaly in 250 m based on CTD observations (left) and time–
mean model results (right).
To describe the time-mean hydrographic regimes at Meteor Seamount we concentrate on
two layers, which are relevant for the local ecosystem. To investigate near–bottom conditions we define a Seamount Summit Layer (SSL), which covers the density interval 26.7 26.8 kg m ( 250 - 350 m). This layer is important for trapped wave dynamics, as well
10
as the habitat of benthic organisms. The second layer spans the near-surface mixed layer and
the upper seasonal thermocline and is referred to as the Upper Thermocline Layer (UTL). Its
lower boundary was defined as the depth of the 1.1 kg m difference from the area-mean surface potential density and represents the mixed and upper thermocline layer.
We find that isopycnal doming, as known from many other seamounts (e.g. Roden, 1994;
Freeland, 1994) is also present here, although its magnitude is relatively small. Fig. 6 shows
a map of the horizontal summit layer density anomaly at the 250 m isobath produced from
the CTD sections and the model time-mean. This isobath corresponds approximately to the
upper SSL surface. In both fields a density maximum is centered above the summit plain.
Since the CTD data are biased by the interpolation through areas of missing data, the density maximum appears more distinct in the model. It generates a positive density anomaly of
0.06 kg m (CTD) and 0.04 kg m (model), respectively. It is well discernible from an isopycnal depression above the upper to middle seamount flanks with negative density anomalies of
-0.04 kg m in the observations and -0.02 kg m in the model. There are indications of a belt
of particularly strong negative anomalies around the seamount, which are also reproduced by
the model. This feature cannot fully be resolved by the observations due to the limited extent
of the observational grid. At least part of the density anomaly differences (the observed density
anomalies are generally higher) may result from the tidal activity still present in the data.
0
4000
km
30
0
4000
3000
km
30
3000
2000
1000
2000
1000
300
300
2000
2000
3000
3000
➙
10 cm/s
290
287
284 281 278 275
272 269
266
263 260 m
Figure 7: Depth (left) of the top of the Seamount Summit Layer (SSL) at the
26.7 kg m isopycnal, horizontal time-mean circulation and vertical velocity within the SSL
(right).
A different view on this feature is given in Fig. 7, where the depth of the upper surface
of the SSL (the 26.7 kg m isopycnal) is shown. A 30 m upward displacement can be seen
above the relatively flat summit plain. Note also the small scale doming above the summit hills,
illustrating the effects of small scale topographic features.
Sensitivity experiments with single constituent forcing have shown that this doming is gen11
erated by the converging eddy buoyancy fluxes of the (weak) K constituent. This mechanism
is due to seamount trapped waves generated by a subinertial tidal constituent (Haidvogel et al.,
1993). This result is at odds with the results of the TPXO.5.1 model presented in Section 2.1,
which found stronger amplification for O . We believe that the coarse resolution of the tidal
model leads to a wrong resonance frequency of the seamount and hence an incorrect response
to diurnal tides. Finally, the fact the K is subinertial only on the northern flanks also reduces
the amplitude of the trapped waves and the corresponding effect on the mass field.
The time-mean horizontal flow in the SSL is composed of a primary anticyclonic circulation
cell at Great Meteor Seamount and some sub-mesoscale eddy-like variability at the seamount
periphery (Fig. 7). There is an along-isobath flow of significant magnitude in the near-bottom
layer with local areas of enhanced cross-isobath flow at the elongated southernmost tip of the
seamount. The strongest current velocities of up to 10 cm s occur along the southern seamount
flank but are generally weaker north of 30 N . These differences in the flow pattern between
the northern and southern areas of the seamount are not easily interpreted. They may be a
consequence of change of the 5
frequency from sub- to superinertial equatorward of 30 N .
Note that a similar flow pattern can be seen at the Small Meteor Bank.
3.2.3 The upper thermocline layer (UTL)
0
4000
km
30
0
4000
3000
km
30
3000
2000
1000
2000
1000
300
300
2000
2000
3000
3000
➙
10 cm/s
115
112
109 106 103 100
97
94
91
88
85 m
Figure 8: Depth (left) of the Upper Thermocline Layer (UTL), horizontal time-mean circulation
within and vertical velocity at the base of the UTL (right).
The thickness of the UTL is presented in Fig. 8. In the deep oceanic regions off the seamount
complex the UTL is approximately 120 m thick. Close to the outer rim of the seamount complex, a general uplift of the isopycnals is found, with minimum values of about 75 - 80 m.
The most prominent feature is the pronounced deepening of the UTL centered at the 2000 m
isobath, both at the Great Meteor Seamount and the smaller ancillary seamounts. This trench
12
exists on all flanks of the seamount; deeper depressions are located in three areas of increased
cross-isobath flows. The dynamical source of this feature is related to the net vertical motion
caused by the eddy–induced uplift of isopycnals in the center and the downwelling over the
flanks (see also Part II of this study). Model sensitivity studies indicated that it is mainly caused
by the semidiurnal tidal components.
In the near surface layers, the positive density anomaly associated with the doming at the
outer rim (Fig. 8) generates a large-scale anticyclonic recirculation with typical velocities of
6 cm s . There is a closed anticyclonic cell in the center of the summit plain, with an extension
above the southern hill and accompanied by a number of counterrotating cells above the rim.
Due to advection by the steady background inflow from the northeast the dominant closed
circulation cell is not centered on the summit plain but shifted downstream to the southwest.
The diameter of these cells is about 30 km, close to deformation radius on the summit plain.
Such circulation patterns are typical for weak impinging flows in strongly stratified fluid (Fennel
and Schmidt, 1991).
3.2.4 Synopsis of the time-mean results
The combined observational and model results give rise to the following conceptual picture
(Fig. 9): The likely persistent deformations of the mass field can be described by the thickness
anomalies of two layers: the near-surface UTL and the subsurface SSL. Note that the relatively
flat summit plain is capable of supporting several closed time-mean horizontal circulation cells.
Both layers seem to be connected by a vertical mean motion, featuring downwelling in the
center, upwelling above the steep flanks, with high spatial variability due to the irregularities of
the topography (smaller hills on the seamount plain, as well as the southern, the northwestern
and northeastern corners). Note, however, that the eddy buoyancy fluxes are responsible for
setting up the density anomalies and the fluctuating currents are always the dominant signal.
The agreement between observations and model solutions is good in the SSL. Close to the
surface, the agreement is much less because the observations are not at all representative of a
time–mean.
UTL
SSL
UTL
✕
●
SSL
METEOR
SEAMOUNT
Figure 9: Schematic view of the time-mean circulation in the UTL (left), SSL (middle), as well
as the vertical overturning motion.
13
4 Summary and Conclusions
The dynamical regime at Meteor Seamount, a steep and tall topographic feature in the central
North Atlantic, has been investigated with a combined observational-modeling approach.
The Meteor Seamount is located in a region where both diurnal and semi-diurnal tidal constituents are of importance. In addition, a weak southwestward mean flow is present. As a
further complication, the critical latitude for the K tidal frequency cuts across the seamount.
This gives rise to a number of processes at the bank: “vortex cap” formation due to rectification
of amplified K tidal currents and a downstream shift of seamount induced anomalies.
The rectification process lead to a modest (30 m) time-mean doming of the isopycnals above
the seamount. This doming is mainly generated by the K currents, which must be closer to the
resonance frequency of Meteor Seamount than O . However, the leakage of K energy into
freely propagating waves on the southern slopes limits the magnitude of the doming and the
corresponding anticyclonic flow around the seamount.
Substantial amplification of tidal currents leads to a high level of submesoscale (20-40 km
eddy diameter) variability in the area. This complicates the interpretation of observations and
the validation of the model. At the northern flank we do find good agreement between model
and observations for those quantities that are representative for a longer period. At the southern flank mooring site the model strongly underestimates the observed current amplitudes. At
this location the topography is marked by abrupt lateral changes which may not be resolved
adequately in the model.
A striking phenomenon that has not been reported at other seamounts so far is the upper
thermocline layer thickness anomaly along the edge of the seamount. It is caused by the surface convergence of eddy fluxes due to the semidiurnal tidal components. This “ring” of deeper
mixed layers may be masked by mesoscale variability, and even remain undetected in observational studies due to large station distances. In any case it marks the center of the seamount
regime and we expect isolation to some degree within this ring.
Atop the seamount, a complex pattern time-mean recirculation cells have been extracted
from the model solution. However, the existence of closed circulation cells in the Eulerian
time-mean should not lead to the conclusion that the area above the seamount is largely isolated
from its surroundings. The high level of variability suggests that Lagrangian trajectories might
be substantially different from the Eulerian mean flow. This is the focus of Part II of this study
(Beckmann and Mohn, 2002).
The role of the ancillary seamounts seems to be small. They both exhibit their own doming,
as well as some internal wave generation (not shown). But there are no obvious interactions
with the Great Meteor Seamount. The reasons may be physical (relative to the gyre-scale flow,
the smaller seamounts lie downstream of the Great Meteor Seamount) or numerical (the model
resolution is not high enough to capture their shape in full detail).
Discrepancies between model results and observations cannot easily be attributed to either
the observational data set or model deficiencies (spatial undersampling and non-synopticity,
model resolution and forcing). Clearly, different observational strategies are necessary to obtain a set of measurements where strong tidal effects can be identified unambiguously. In any
case, the model helps to identify robust features by offering a complete coverage of the threedimensional flow and mass fields.
14
Acknowledgements
Helpful discussions with Adriene Pereira are gratefully acknowledged. We also like to thank
the captain, crew and scientifc staff on board RV Meteor 42/3. This work is a contribution to
the Great Meteor Seamount project and was funded under DFG contracts Me 487/38-2 and Be
1851/1-1.
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