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1 Not to be cited without prior reference to the authors ICES CM 2002/M:37 The Upper Ocean Circulation at Great Meteor Seamount. Part II: Retention Potential of the Seamount Induced Circulation Aike Beckmann and Christian Mohn Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Max Planck Institute for Meteorology, Hamburg, Germany Abstract The circulation patterns at Meteor Seamount are investigated for implications for the marine ecosystem, using a numerical ocean circulation model. The importance of tidal rectification and internal tide generation has been documented in Part I of this study. Passive tracers confirm the idea that there is an area above the seamount which is largely isolated from the surroundings. Lagrangian particle trajectories are used to test and quantify the potential for retention. We find that passively advected organisms are more likely to remain in the near surface layers above Meteor Seamount than actively migrating organisms, who might escape from the area. Finally, the importance of strong wind events on the distribution of particles is illustrated. 1 Introduction 1.1 Aspects of Seamount Biology Marine biologists have been attracted to seamounts since the late fifties after the abundance of life above seamounts was discovered (Hubbs, 1959). Increased biomass and biodiversity are the characteristics of many seamounts, and the local conditions as well as their effects on larger scales have been investigated repeatedly. Comprehensive reviews were given by Boehlert and Genin (1987) and Rogers (1994). Our understanding of many of the above issues is still incomplete due to the large variety of physical and biological circumstances and the large effort that is necessary to investigate a seamount ecosystem as a whole. The effects of the oceanic circulation above seamounts and submarine banks on the distribution of biogeochemical parameters have also been discussed frequently (e.g., Loder et al., 1988; Perry et al., 1993). Comeau et al. (1995) and Dower et al. (1992) discuss the relationship between enhanced primary production and nutrient supply through locally upwelled water masses at Cobb Seamount in the northeast Pacific. Mullineaux and Mills (1997) found strong evidence that larvae of a wide range of benthic species are retained and aggregated by the tidally rectified three-dimensional flow system above Fieberling Guyot in the North Pacific. White et al. (1998) observed strong enrichment of nutrients over Porcupine Bank, a near-shore bank in the northeast Atlantic, located at the shelf edge west of Ireland. Summarizing our present knowledge of seamount biology, there are strong indications that the increased biomass and the level of endemism above many seamounts is profoundly influenced by hydrodynamical processes and phenomena (upwelling, turbulent mixing, closed circulation cells). 1

2 Two aspects of the physical situation at the seamount seem particularly important for the biological system: the strength and persistence of closed circulation cells and the effect of the time-dependent flow field on individual particles. In this second part of our Meteor Seamount study we present the results from numerical process studies on the stability of the circulation. We simulate the advection and dispersion of passive particles to examine possible biological implications. Finally, we investigate the sensitivity of the flow system at the seamount to atmospheric forcing in terms of stability and stationarity. 1.2 Physical Elements of Seamount Regimes Strong currents and a generally enhanced variability have been observed frequently at seamounts. During a first systematic survey, the Atlantic Seamount Cruises 1967, it was noted that the horizontal homogeneity was strongly disturbed near the top of the seamount (Horn et al., 1971). A large number of studies have since confirmed the existence of strong long-period, sub-inertial and tidal currents at steep isolated topographic features at many locations in the world s oceans (Huthnance, 1974; Hunkins, 1986; Genin et al., 1989; Codiga and Eriksen, 1997). Milestones in our theoretical understanding of flow around seamounts were works by Taylor (1917, 1923) on steady flow over topography, and the theory of freely propagating trapped waves at isolated topography by Chapman (1989) and Brink (1989, 1990). Numerical models have recently been used successfully to investigate the seamount regimes in the limit of steep slopes, tall topography and strong nonlinearity (Chapman and Haidvogel, 1992; Haidvogel et al., 1993; Beckmann and Haidvogel, 1997). Most investigations have focussed on circular seamounts, and many phenomena can be exemplified in this prototype Taylor columns/caps Steady flow encountering a seamount leads to the formation of a closed anticyclonic (clockwise on the northern hemisphere) circulation cell atop the topographic obstacle. This phenomenon is called a Taylor column, or, in stratified fluid, a Taylor cap and the corresponding doming of isopycnals has been observed frequently above seamounts. A region of weak or vanishing currents is found in the center of such a circulation cell. This stagnation area would be an exceptional place for biological (in-)activity and sedimentation. However, stagnant areas can be expected only in areas where mean flows are strong, and mesoscale variability and tidal currents are weak. This is the case only in a few places over the world ocean Trapped waves Periodically alternating flow (as from eddying or tidal motion) at sub-inertial ( ) frequencies can generate trapped waves (so-called seamount trapped waves) at a seamount. The gravest mode of these seamount trapped waves is again a dipole of vertical velocities and density anomalies rotating anticyclonically about the seamount. The corresponding horizontal flow features counterrotating cells. Seamount trapped waves and are bottom intensified with a typical vertical scale determined by the strength of the stratification. A continuous excitation of 2

3 these waves at their characteristic frequency (e.g., by alternating tides) can lead to resonance effects, with substantial amplification (Haidvogel et al., 1993). In this case, nonlinearities may generate a residual current, which is also anticyclonic and has a similar structure as the that for steadily forced Taylor caps. Note however, that this residual flow is the result of and often masked by strong fluctuating currents Free waves At super-inertial frequencies (e.g., by tidal forcing), free waves are generated at the seamount flanks. These internal tides (with wavelengths of the order of the Rossby radius of deformation) can be excited on all flanks of a seamount, with some being reflected into the deeper ocean, some being transmitted into the shallower areas of the seamounts. The density perturbations caused by internal waves can be of substantial amplitude; in observations they may be found dominant. 2 The Meteor Seamount Regime 2.1 Biological Observations 0 Chl a (HPLC) [µg/l] Pressure [dbar] N Distance [km] S Ocean Data View Figure 1: Chlorophyll-a concentration on a North South transect crossing the summit of Meteor Seamount (M. Kaufmann, pers. comm.). At Meteor Seamount, a number of biological observations have been carried out (Nellen, 1973, 1999), in conjunction with hydrographic and current measurements. During the latter cruise, enhanced concentrations of phytoplankton (K. von Bröckel, pers. comm.) but no significant differences in zooplankton biomass (R. Diekmann, B. Martin, pers. comm.) between the seamount area and the surrounding ocean were found. Isopycnal doming was found on top of 3

4 the seamount (Nellen, 1999), as well as an isolated patch of chlorophyll in the upper thermocline (between 50 and 110 m depth) above the seamount s summit plain (M. Kaufmann, pers. comm., see Fig. 1). In general, all biological observations show a high variability (patchiness) on horizontal scales smaller than the seamount. Thus the biological situation at Meteor Seamount seems to be largely comparable to other seamounts with shallow summits in the World s Ocean (e.g., Cobb Seamount, Fieberling Seamount, Bowie Seamount; see also Genin et al. (1994)). However, it is unique with respect to its location at the critical latitude for diurnal tides and the high amplitudes of semidiurnal tides (see Mohn and Beckmann, 2002). Our goal here is to investigate the underlying mechanisms for biological tracer distribution and particle movement. The latter has been done for an idealized seamount by Goldner and Chapman (1997), assuming strong steady flow and relatively weak tides. At Meteor Seamount, the physical forcing features strong tides and a weak mean flow. 2.2 The Physical Situation km m Figure 2: Depth of the Upper Thermocline Layer (UTL) in the central model domain. Contour interval is 3 m. The wide variety of possible regimes (mean flow or tidally dominated, weakly or strongly stratified and rotating, abrupt or gently sloping, irregularly shaped) requires a detailed study for each seamount under investigation. Basically, the processes described in the Introduction section are generally valid, their relative strength, however, varies from seamount to seamount. The situation at Meteor Seamount was investigated by Mohn and Beckmann (2002), who found most of the above mechanisms and phenomena are at work here simultaneously. Basis for these experiments and analyses is the model configuration described in Part I of this study (Mohn and Beckmann, 2002). The primitive equation ocean general circulation model with terrain-following coordinates was configured as a periodic channel oriented northeast 4

5 southwest. A simulation of steady and tidally forced flow at Meteor Seamount was carried out for 90 days. The model results were validated against hydrographic and current measurements (see Part I, Mohn and Beckmann (2002)). The 30-day period between day 60 and day 90 was taken as a quasi-stationary regime after spin-up. The large scale flow is relatively weak, hence the corresponding Taylor cap is weak. We find doming due to the rectification of diurnal (K ) tides, as well as high levels of variability due to semidiurnal tides (including internal variability due to internal tides). We therefore expect substantial differences between Eulerian means (averaged pointwise in time) and Lagrangian means (averaged along individual trajectories), i.e., the spreading of tracers and the movement of particles may be significantly different as envisioned from the Eulerian time-mean flow. Finally, the relatively strong stratification leads to a tendency for decoupled surface from the summit layer flow, and also substantial internal tide generation. Our reference experiment (see Part I of this study, (Mohn and Beckmann, 2002)) simulates the development of the three-dimensional circulation at Great Meteor Seamount. Most notably, there is a ring-like structure of deepened mixed layer (upper thermocline layer, UTL) above the upper flanks of seamount, roughly indicating the area of seamount induced hydrography. Its lower boundary is determined by the depth of the 1.1 kg m potential density difference from the surface (in units) according to the observed density maximum of the seasonal thermocline. The UTL depth is presented in Fig Model Results 3.1 Passive Tracer Fields The goal of additional studies with the model is to further identify typical patterns, that might be relevant for marine biological distributions. This set of experiments uses passive tracers, thought to represent a biological variable (e.g., benthic larvae, an endemic species, of anomalies of phytoplankton, zooplankton or a nutrient) and look for typical distribution patterns above the seamount. From our numerous experiments, we present here two special cases; the first featuring a surface tracer, initially prescribed with values of 1 at the surface, and 0 below. Its advective/diffusive spreading during the 30-days integration from day 60 to day 90 illustrates the vertical penetration of surface properties into deep layers as well as the upwelling of deeper fluid into the mixed layer. In the second experiment we used a endemic species tracer which was initially set to 1 above the seamount in water shallower than 1500 m, uniform over depth, and 0 outside. This tracer is thought to represent the distribution of a tracer in the inner seamount regime, and we are interested in the lateral spreading of this quantity The surface tracer Upwelling above the seamount is evident from the tracer distribution in the surface layer (Fig. 3a). After 30 days, it features reduced tracer concentrations in the center of the seamount summit plain. This is the combined effect of a thinned mixed layer in this region and some vertical entrainment of tracer-free water into the mixed layer above the seamount. We find strong 5

6 a b 0 km km 1.0 Figure 3: 5-days-mean surface tracer distribution after 30 days of integration (a) in 50 m depth, and (b) on a southeast northwest transect, as indicated by the dashed line in (a). This tracer was initialized at the surface only and has been replaced by tracer-free fluid from below, illustrating the general upwelling above the seamount. indications of submesoscale variability in the tracer distribution, indicating again the highly turbulent regime above the seamount. Vertical motion and mixing is even more evident from the vertical section (Fig. 3b). Here the mixed layer above the seamount appears to be much shallower than in the surrounding ocean. The boundaries coincide with the aforementioned ring of increased mixed layer depths around the seamount (see Fig. 2). This area is a zone of increased upward transport of subsurface water masses and tracers. The distribution of this tracer can explain the existence of isolated patches of, e.g., subsurface chlorophyll above the seamount, as found in the recent observations The endemic species tracer The second tracer was released above the shallow areas of the seamount, uniform with depth, to show the degree of lateral isolation of the inner seamount regime, especially at depth. It was initialized throughout the water column in areas with less than 1500 m water depth. The resulting tracer distribution shows at least two interesting results. First, a maximum remains in the center of the flat seamount plain, while the concentration above the flanks is significantly reduced (Fig. 4a). The surface distribution shows large filaments emanating from the seamount into the open ocean (see also the distribution of variability in the seamount area in Mohn and Beckmann (2002)). By this process, most of the tracer initially prescribed in water m deep is lost to the surroundings. This is even more obvious from the transect through the seamount center (Fig. 4b). The tracer is confined to the shallow areas (depths 350 m) and two areas of increased concentration can be identified: the mixed layer and the central water column over the summit plain. The separation of the UTL signal from the interior confirms our earlier statements (see Part I, Mohn and Beckmann (2002)) on the decoupling of 6

7 the different layers above the seamount summit a b 0 km km 1.0 Figure 4: 5-days-mean surface tracer distribution after 30 days of integration (a) in 150 m depth, and (b) on a northeast southwest transect, as indicated by the dashed line in (a). This tracer was initialized in water shallower than 1500 m throughout the water column and a large portion still remains above the seamount. These tracer simulations lead us to conclude that there is a strong retention potential above Meteor Seamount. The results indicate the areas of retention, as well as the existence of an inner regime in areas with a water depth of less than 350 m as well as vertically separated regimes above the summit plain. 3.2 Particle Trajectories In this section, we address the retention aspect of flow at Meteor Seamount by looking at the individual based dynamics. A growing number of modeling activities use so-called individual based models (e.g., Bartsch and Coombs, 2001), where a group of organisms is not simulated as a continuum but treated as discrete individuals. This Lagrangian approach computes the threedimensional trajectories of particles, and even includes active behavior (diving, swimming) of the marine life forms. Occasionally, they are combined with NPZ (nutrient phytoplankton zooplankton) models, which then predict the three-dimensional distribution of nutrients and plankton. In a much simpler approach, passive particles can be advected with the flow field, and the results can be interpreted in terms of their implications for marine biology. It should be noted that the integration of trajectories does not explicitly include the effects of turbulent mixing; subgridscale variability, which leads to a dispersion of particles is not accounted for. This is particularly relevant in areas with high levels of turbulence such as the surface mixed layer and convective overturning and internal wave breaking events. Therefore, the interpretation of Lagrangian results has to take into account the statistical aspect of the method. 7

8 3.2.1 Passively advected floats 0 m km Figure 5: Modeled three-dimensional trajectories of passive particles released at 50 m depth. Plotted are daily positions for 30 days, thus excluding the tidal cycle from the picture. A total number of 1024 numerical floats were released at 50 m depth in a regular pattern across the Meteor Seamount area and their positions were recorded for 30 days. The threedimensional trajectories are shown in Fig. 5, with color-coded depth. A first visual inspection shows again two regimes; an outer regime, with southwestward translation of the floats, at about 1 cm s, with the steady flow, slightly increased due to the divergence of the streamlines around the topographic obstacle. In the area of the Great Meteor Complex (roughly coinciding with the 4400 m depth contour), however, the time-mean translation is much reduced. We find increased velocities just above the summit plain, where many of the floats recirculate. Further to the west there is a region of weak flow (apparently a shadow zone in the lee of the main obstacle), with irregular motion of the floats. The first impression is that there are three regimes: the first within the 1500 m depth contour, where individual floats are retained for an extended time, a second shadow zone to the west and finally the unobstructed flow regime outside the 4400 m depth contour. To illustrate the tidal contribution to the advection, Fig. 6 shows the diurnal cycle of some of 8

9 m km Figure 6: 30-days-trajectories for a selected number of floats to show the diurnal cycle and the vertical displacements above the seamount. the floats above the seamount. The tidal excursions are relatively small, a few km at most, higher in shallower water. Downward motion mainly occurs over the flanks, some of the particles are spiraling down to more than 100 m within a few days. However, up- and downwelling centers are not clearly separated in this highly variable flow field. Instantaneous velocities at Meteor seamount exceed 30 cm s in the near bottom layers. Typical tidal displacements during one period are up to 3 km (for 50 cm s flows) in middepths and less near the bottom and the surface. Typical time-mean flows are 10 cm s, thus the time needed for one loop around the seamount is also about 60 days. The horizontal Lagrangian mean flow coincides with the Eulerian mean flow in most places. In the vertical, however, upwelling occurs above the central summit plain, while downwelling takes place over the upper flanks. A passive particle (phytoplankton, nutrients, benthic larvae) will therefore be advected upward above the seamount. Maximum vertical displacements are a few tens of meters per day. 9

10 3.2.2 Actively vertically migrating particles Some marine organisms (zooplankton) avoid daylight in the near-surface layers and migrate several tens to a few hundred meters downward at dawn. To mimic the diurnal vertical migration of individuals, we have prescribed two target depths ( for the day and! #"$&%'( for the night), which are approached with maximum speed (assumed to be 2 cm s ). For simplicity we assume that day and night are of equal length, which is representative for the months of March and September at the latitude of Meteor Seamount. PASSIVE 0 km 50 ACTIVE 0 km cm/s 1 cm/s Figure 7: Lagrangian mean velocities from floats without (left) and with (right) vertical migration. In Fig. 7 we show two representative sets of 30-days Lagrangian mean velocities. The ensemble of floats largely unaffected by the seamount moves southwestward with the steady flow, while above the seamount, both the direction and the magnitude of the mean flow is entirely different: currents are irregular and although some of the floats may escape the seamount summit area, an increased residence time is obvious. In the case with vertical migration, the Lagrangian mean-flow pattern atop the seamount is entirely different. There is a systematic west to southwest flow, of the same order of magnitude as the far field currents. Our explanation for this unexpected result is that the current regime below the main thermocline is largely decoupled from the near-surface layers. In the intermediate and bottom layers the horizontal currents related to seamount trapped waves lead to a systematic lateral displacement, which on longer time scales (e.g., months) is dominated by the far field steady flow. This significantly different behavior may explain the observed differences in distributions of some phyto- and zooplankton species above Meteor Seamount The influence of a storm Numerical seamount studies have so far focussed on the effects of a steady periodic (barotropic tidal) flow, thus neglecting other potentially important forcing mechanisms. One of these, which 10

11 may be particularly important for the retention aspect of seamount flows, is atmospheric forcing in form of strong winds. To further quantify the retention potential of seamount generated closed circulation cells we investigated the effect of a passing storm on the phytoplankton community above Meteor Seamount. The storm was simulated by adding a uniform northwestward surface wind stress to the model for the duration of about two days. The wind stress is directed northwestward, such that the resulting Ekman transport is opposite to the large scale flow. In addition to the applied wind stress, the vertical near surface mixing was also increased during the passage of the storm. PASSIVE STORM 0 km 50 0 km cm/s 1 cm/s Figure 8: Lagrangian mean velocities from floats without (left) and with (right) strong wind forcing. During the storm, the resulting near-surface flow pattern features a butterfly dipole, with the main axis oriented north-south and typical flows of 30 cm s. This has a pronounced effect on the near-surface particles, which are systematically moved northeastward during this period. Even on a 30-days mean (Fig. 8), the resulting displacement is significantly altered. This is not surprising, as the displacement due to the additional wind-induced currents is about 20 km, a third of the seamount diameter. Note also that the direction of the far field advection is also drastically changed. We conclude that during this event, a large number of particles escaped the shallow areas above the seamount and a large number of the near-surface ecosystem members have been replaced. Obviously, the biological system Meteor Seamount regime is quite sensitive to such isolated wind events. 4 Summary and Conclusions Passive tracer distributions and Lagrangian trajectories in a numerical model of the highly variable flow field near Meteor Seamount have been analyzed for typical distribution patterns and time-mean displacements. 11

12 Passive tracer distributions indicate a substantial degree of isolation of the layer above the Great Meteor Seamount, both laterally and vertically. Most notably is a belt of increased mixed layer thicknesses found around the seamount. Which separates the inner from the outer seamount regime. This result is supported and refined by the analysis of particle trajectories, which also show different regimes above and outside the seamount. The main goal was to quantify the retention potential at Meteor Seamount. Retention in the specific circumstances at Meteor Seamount is defined to the ambient far-field current of 1 cm s. Translated into the time for a particle to be moved across the diameter of Meteor Seamount, we find 60 days for undisturbed motion. Any increase of this time could be called a retention. The quantification of retention on the basis of either passive tracers or Lagrangian floats remains difficult, as it requires a subjective and to a certain degree arbitrary selection of threshold values for tracer concentrations or float trajectories. Based on the presented model results, we have chosen to proceed by defining two regimes, which are clearly separated: the inner seamount regime (water depths less than 1500 m) and the outer seamount regime (water depths larger than 4400 m). The numerical floats are thus subdivided into two ensembles, and we compute the translation of their respective centers of mass. wind passive mean flow active Figure 9: Schematic representation of the ensemble-mean drift of particles in the inner (solid arrows) and outer (broken arrows) seamount regime for the three cases: passive (blue), actively migrating (green) and passive with southeasterly wind stress (red). The far field flow vector (purple) is added as a reference. The results are summarized in Fig. 9, where we compare the two regimes for the three cases of passive, actively migrating and wind-influenced particles. In the standard case (passive advection), particles outside the 1500 m depth contour are mostly advected at the far-field speed, with a slight increase due to the flow around the seamount complex. Ensembles of particles within the 1500 m depth contour show a drastically different mean movement. The center of mass moves irregularly, with several tenths of cm s, but the 30 days residual is of a few 12

13 mm s. This translates into a tenfold increase in residence time. Note that these residence times are longer than the reproduction rate of most organisms. In case of actively moving individuals, no such retention can be found. Both ensembles move southwestward with similar speeds, the direction, however, is different. Finally, the addition of a storm causes the particles in the inner regime to move upstream relative to the oceanic gyre circulation. To summarize our results, we find that ) a ring-like quasi-permanent u -shaped deepening of the mixed layer exists above the upper flanks of Meteor Seamount, separating the inner from the outer seamount regime; ) the influence area of the Meteor seamount topography extends to about 4 times the area of the summit plain, thus including much of the deep ocean ( 4400 m) as well. This is in agreement with the area of enhanced eddy energy as shown in part I of this study Mohn and Beckmann (2002); ) interpretations of biological data based on time-mean Eulerian flow fields are likely to be inadequate, as the local tidal flows are much stronger than the Eulerian residual and the spatial variations are large; ) actively migrating particles (zooplankton species, see also Wilson and Firing (1992)) are much less retained, as they are advected by the larger currents in deeper layers. This is perhaps the most unexpected result of our studies, and may be used to explain differences in residence times between zooplankton and phytoplankton; and finally ) strong wind events can affect the near surface layers and move a large number of marine organisms out of the retention zone. A final conclusion is that the phase of the tide, the length of the period with daylight and the history of extreme weather events in the weeks before and during the observations need to be taken into account when observational (physical and biological) data sets are analyzed and interpreted. It is hard to imagine that a seamount field survey without supporting modeling study will be successful in explaining the measurements. Acknowledgements The authors gratefully acknowledge helpful discussions with Catriona Clemmesen, Rabea Diekmann, Frank Hartmann, Inga Hense, Manfred Kaufmann, and Bettina Martin. This work was funded by the DFG under contracts Me 487/38-2 and Be 1851/1-1 as part of the Great Meteor Seamount project. References Bartsch, J., and S. H. Coombs, An individual-based growth and transport model of the early life-history stages of mackerel (scomber scombrus) in the eastern north atlantic, Ecological Modelling, 138, ,

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