The Upper Ocean Circulation at Great Meteor Seamount. Part II: Persistence and Retention Potential of Closed Circulation Cells, Ocean Dynamics, 2002, Submitted

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 com- bines 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 Small Meteor Seamount 30' M423-A Meteor Seamount ➤

20'

Closs Bank 10' M423-B ❶

➤ 30oN ➤

50' M423-C

W N ° 40' ❷ 30 28 ° 30' 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 40oN Azores

35oN 11 3

Madeira

30oN Canary Islands GMS 5 3

25oN 4

o 20 N 12

15oN Cape Verdean Islands

10oN 50oW 45oW 40oW 35oW 30oW 25oW 20oW 15oW 10oW 5oW

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; Kase¨ 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 semidiur-

nal 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 re-

alistic 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 rea- sonable 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 com-

posed 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 re- flect 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”

1It 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 M42/3-C

0 0 0 25 25 z [m] z [m] 25 z [m] 26 26 26 26.2 -100 26.2 -100 26.2 -100 26.4 26.4 26.4

-200 -200 -200 26.6 26.6 26.6 26.7 26.7 26.7 26.75 -300 -300 -300 26.8 26.75 26.75 26.8 26.8 26.9 -400 -400 26.9 -400 26.9 27 27 -500 27 -500 -500

27.1 27.1 27.1 -600 -600 -600 0 50 100 0 50 0 50

x [km] x [km] 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 prop- erties 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 near- surface 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.

SC-ADCP mooring north SC-ADCP mooring south

"! # $ % % &! # $

a a F a a F

')( * ')( ')( * cm s ')( cm s g GMT cm s cm s g GMT

M ¡ 11.9 -9.1 152.1 0.76 3.4 14.6 -6.4 325.6 0.44 4.2

S ¡ 4.5 -2.5 177.0 0.56 4.1 3.9 -0.8 313.0 0.21 3.5

K / + 2.0 -1.4 76.4 0.7 7.1 3.7 -3.4 49.2 0.92 13.2

O 1.5 -1.2 122.3 0.8 15.0 2.3 -1.1 197.8 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

$ -/.% "!102.1%,3- rotation. The phase relative to Greenwich is # , is the ellipticity ( ), 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 com-

pared 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

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 ab- sence 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

')( * ')( ')( * cm s ')( cm s g GMT cm s cm s g GMT

M ¡ 7.5 -1.7 227.5 0.23 2.2 8.1 -3.3 296.9 0.41 2.3

S ¡ 3.8 -0.9 179.4 0.24 3.4 4.9 -2.1 24.6 0.43 4.4

K / + 3.5 -3.0 70.2 0.86 12.5 4.0 -3.1 117.1 0.78 14.3

O 0.5 -0.3 40.2 0.6 5 0.6 -0.5 93.9 0.8 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

$ - .3% &!302.1%,4- to Greenwich is # , is the ellipticity ( ), and F is the ampliﬁcation factor relative to the far–ﬁeld values from the TPXO.5.1 tidal model.

SC-ADCP Model

northern northern 100 flank 100 flank

200 200 z(m) 300 300

East East 400 400 10 cm/s 10 cm/s

southern southern 100 flank 100 flank

200 200 z(m) 300 300

East East 400 400 10 cm/s 10 cm/s

−28.6 −28.4 −28.2 −28.6 −28.4 −28.2 longitude longitude

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

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 re- produce 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 2000 1000 1000

300 300

2000 2000 3000 3000

0 km 30 0 km 30

-0.05 -0.04 -0.03 -0.02 -0.01 0.0 0.01 0.02 0.03 0.04 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 condi-

tions 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 den-

sity 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 isopy-

cnal 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 km 30 4000 0 km 30 4000

3000 3000 2000 2000 1000 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 gen-

11 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)

4000 0 km 30 4000 0 km 30

3000 3000 2000 2000 1000 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 threed- imensional 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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