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Tidal influence on the hydrodynamics of the French Guiana continental shelf
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Continental Shelf Research 28 (2008) 951–961 www.elsevier.com/locate/csr
Tidal influence on the hydrodynamics of the French Guiana continental shelf A. Bourreta,, J.-L. Devenonb, C. Chevalierc IXSURVEY, 46 quai Franc- ois Mitterrand, 13600 La Ciotat, France Laboratoire d’Oce´anographie Physique et Bioge´ochimique, OSU/Centre d’Oce´anologie de Marseille, UMR 6535 CNRS/Universite´ de la Me´diterrane´e, Campus de Luminy—Case 901, 13288 Marseille Cedex 09, France c IRD—UR CYROCO, Campus de Luminy—Case 901, 13288 Marseille Cedex 09, France a
b
Received 2 November 2006; received in revised form 7 January 2008; accepted 11 January 2008 Available online 26 January 2008
Abstract This study investigates the circulation on the French Guiana continental shelf under tidal influence. Indeed, hydrodynamics are characterised by a weak salinity tongue located in the middle of the shelf and induced by the Amazon River, a coastal current flowing from the southeast, and a tidal standing wave whose co-range lines are parallel to the coast. In addition to field observations, a numerical model also is used to evaluate the tidal influence on coastal circulation. The model makes use of the MOBEEHDYCS code, a three-dimensional free surface time-splitting model whose domain is bounded with a closed coastal boundary, two active boundaries (offshore and lateral) and a passive boundary. The boundary configuration and hydrodynamics require a careful choice of passive open boundary conditions. The initial and boundary conditions come from field data. The tidal currents are essentially cross-shore and do not have a great influence on the main current direction on the offshore part of the shelf. The offshore currents remain parallel to the coast. In the inner shelf, the tidal influence is found to be much more important and the tidal currents can reach 0.45 m/s. Vertically, the tidal currents are barotropic, in spite of the high stratification and they induce a horizontal cross-shore migration (about 3 km) of the weak salinity tongue and vertical oscillations of the halocline without complete mixing. r 2008 Elsevier Ltd. All rights reserved. Keywords: French Guiana continental shelf; Coast circulation modelling; Tidal influence
1. Introduction The French Guiana continental shelf constitutes an interesting and important coastal zone from a hydrodynamical point of view. The presence of gold-bearing deposits on the seabed along with a high level of heavy metals and the proximity of rivers such as the Amazon River, rich in sediments and nutrients, justifies the interest and the study of this area. The French Guiana continental shelf is subject to a combination of hydrodynamic processes and atmospheric forcings. Hydrodynamics are characterised by a strong coastal current flowing from the southeast (Flagg et al., 1986; Richardson and Mckee, 1984), high Corresponding author. Tel.: +33 491 82 92 05.
E-mail address: [email protected] (A. Bourret). 0278-4343/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2008.01.008
stratification due to the plume of the Amazon River (Hu et al., 2004; Hellweger and Gordon, 2002), and by strong tidal currents and trade winds. Indeed, the vicinity of the InterTropical Convergence Zone, where the Southeast and Northeast Trade Winds converge, induces a seasonal alternation of winds. In addition, a strong coastal current flows along the coast to the northwest and is partly induced by the North Brazil Current (Richardson and Reverdin, 1987; Johns et al., 1998). Based on oceanographic observations during spring and numerical experiments, a weak salinity tongue located about 40 km off the coast is due to the proximity of the Amazon River. The outflows from local rivers have more importance at lesser distance only (Chevalier et al., 2004).
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The tide is mixed with two daily low and high tides of different amplitudes per day (Service Hydrographique et Oce´anographique de la Marine, 1975) and the co-range lines are almost parallel to the coast. The tidal standing wave presents five main tidal components, which are the M2, S2 and N2 semi-diurnal waves and the K1 and O1 diurnal waves. The tidal amplitude can reach 1.5 m at Cayenne (Fig. 1) (Service Hydrographique et Oce´anographique de la Marine, 1975) and the tidal currents have amplitude of 0.5 m/s (Pujos and Froidefond, 1995). The Amazon shelf has been the subject of a multidisciplinary survey (AMASSEDS: A Multidisciplinary Amazon Shelf Sediment Study) between 1989 and 1993. Observations indicate that, the currents on the Amazon shelf are the sum of semi-diurnal nearly barotropic tidal currents perpendicular to the coast and a strong long-shore current (Lentz and Limeburner, 1995). Additionally, the Amazon plume is constituted by a weak salinity tongue which spreads to the middle of the shelf (Lentz and Limeburner, 1995; Curtin, 1986; Gibbs, 1970). The tidal wave has been studied on the Amazon shelf and more precisely the M2 tide from mooring data and a simple, local homogeneous tidal model (Beardsley et al., 1995). Geyer et al. (1996) have also underscored the modulation of the stratification by the tide. From field data, Nikiema et al. (2007) have studied the influence of specific physical processes on the morphology of the Amazon plume by using a three-dimensional (3-D) hydrodynamic numerical model and realistic bathymetry and coastline of the northern Brazilian shelf. On the French Guiana shelf, the data are scattered and mainly hydrologic. Near the coastal zone, the pycnocline is located at a water depth between 8 and 10 m. The currents seem always directed to the northwest with a slight influence of the tide (Castaing and Pujos, 1976; Pujos
and Froidefond, 1995). The water masses off the French Guiana coast are characterised by a weak salinity plume at different periods of year (Colin and Bourle´s, 1994). The plume extends out to 80 km from the coastline. According to Pujos and Froidefond (1995), the currents are maximum in the middle of the shelf and seem to be the prolongation of the current flowing along the coast near the Amazon mouth. A field survey SABORD 0 was carried out during May 1996 from the Approuague to the west of Sinnamary (Gouriou et al., 1997). Five transects perpendicular to the coast were carried out. These transects show that the halocline is located between 6 and 12 m of depth. The surface salinity field presents a weak salinity area at the eastern end of the shelf. The sea surface temperature is quite homogeneous over the study area but with a colder area occurring near the Maroni River mouth (Fig. 1). Another field study, CHICO 0 was carried out during the rainy season between the Oyapock River and the Approuague River in April 1999 (Froidefond et al., 2002). Current measurements were carried out during this study, especially along the eastern boundary, between 30 and 140 km from the coast. Current velocities increased regularly from 0.3 m/s at 30 km off the coast and reached a maximum value of 1.1 m/s on the offshore part of the shelf. Hydrodynamic modelling on the French Guiana shelf has been used to study the influence of trade winds on hydrology and currents (Chevalier et al., 2004). Indeed, a northeastern wind induces a downwelling near the coast and this wind does not affect the vertical profile of the long-shore velocity. Wind effects on hydrology are limited in depth to 5 m. The influence of tide on shelf currents and stratification has not been studied. On the French Guiana continental shelf, as in many other estuarine and coastal settings, we would expect that the strong tidal currents would have an influence on the structure and dynamics of the salinity, temperature and current fields both horizontally and vertically. This study addresses the influence of the tidal phenomenon on the currents, and tracer (i.e. the temperature and the salinity) fields for the particular zone of the French Guiana continental shelf. We have studied more particularly the salinity field due to the influence of the Amazon freshwater. To our knowledge, this modelling constitutes the first simulation of this shelf area, which takes into account the tidal influence coupled with the long-shore current and stratification. The present article is composed of three parts: the first one concerns the numerical implementation. The second part presents the validation of the tidal signal and the last part constitutes an analysis of both vertically integrated and 3-D simulations. 2. Numerical implementation
Fig. 1. Bathymetry of the French Guiana continental shelf and locations of the SHOM tidal gauges.
The simulation domain (Fig. 1) has a closed coastal boundary on southwest and three open boundaries: two
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cross-shore and one long-shore. Four rivers are taken into account for the modelling purpose of the shelf circulation. These are, from the east to the west, the Oyapock River, the Approuague River, the Mahury River and the Sinnamary River. The Maroni River located at the western part of the domain is not taken into account. The coastal, finite-difference, free-surface model MOBEEHDYCS (Hess, 1989; Chevalier et al., 2004) is used to reproduce the shelf circulation. The model is governed by the momentum equations, the continuity equation, heat and salinity conservation laws and the equation of state of sea water. The model follows the hydrostatic and the f-plane approximations and Boussinesq’s hypothesis. The numerical MOBEEDHYCS code makes use of a regular Cartesian horizontal grid associated with sigma vertical coordinates (Mellor and Blumberg, 1985) with closer mesh refinement near the free surface than near the bottom. The horizontal velocity is separated into a vertically integrated component (the barotropic mode) and a departure component (the baroclinic mode) according to a modesplitting numerical scheme. The vertical eddy viscosity coefficient follows the Prandtl’s model where the influence of stratification is taken into account thanks to the Richardson’s number (Ri) (Munk and Anderson, 1948). The grid spacing is regular (Dx ¼ Dy ¼ 2500 m) with 130 meshes in the along-shore direction and 77 in the crossshore one. The time step of the barotropic mode is 20 s and for the baroclinic mode is 100 s. The vertical discretisation of the model includes 19 sigma levels. A detailed description of the model is given in Chevalier et al. (2004). The particular topography characterising on the French Guiana continental shelf leads us to consider the studyarea as partly bounded by one southern coastal closed boundary (boundary 1), by two open cross-shore boundaries (boundaries 2 and 4) and one outer long-shore boundary (boundary 3). In this configuration (Fig. 1), tidal currents are essentially parallel to boundaries 2 and 4 whereas the coastal current flows perpendicularly into these boundaries. So, boundaries 2 and 3 are active, i.e. the value of the velocity and/or the sea surface elevation on these boundaries is imposed from a knowledge of conditions outside. Here, the incoming coastal current is imposed on the boundary and the tidal waves are specified on the boundary 3. The tidal forcing, which is essentially normal to the offshore boundary, is imposed on this boundary with a Flather type condition, adapted to a normal tidal forcing (Davies, 1983; Davies and Lawrence, 1994): 2pt Vt tþ1 ffiffiffiffiffiffiffi , Zi ¼ A0 sin þ pB1 and Ztþ1 (1) B ¼ Zi T gH where VB71 is the boundary-normal vertically integrated velocity just before boundary, Zi, the imposed SSE at boundary and ZB, the SSE at the boundary grid point. The modelling takes into account the main five tidal components (M2, S2, N2, K1 and O1). Their amplitudes and phases come from extrapolated data of tidal gauges
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belonging to the French Navy Hydrographic Service (Service Hydrographique et Oce´anographique de la Marine, 1975) (stations 15 and 23; see Fig. 1). The boundary 4 is passive, because the coastal current exits there, i.e. hydrodynamics inside the domain determine the values on the passive open boundary. The particular choice of the condition applied to the passive open boundary may be of primary importance for numerical results validity. The initial data of salinity and temperature fields are obtained from the sea campaign SABORD 0 study. It was conducted with the oceanographic vessel Antea in May 1996 on the French Guiana continental shelf from the Approuague River to the Sinnamary River (Gouriou et al., 1997). The coastal long-shore current is derived from CHICO 0 data. During this study, the currents were measured along a transect located on the eastern boundary of our modelling domain. The measurements used at the boundary 2 have a horizontal resolution of 1 km. They have been interpolated on the horizontal model grid and have been integrated on the vertical to force the vertically integrated model. 3. Calibration and validation of the numerical tidal representation For the northwestern boundary, the adapted choice of passive boundary conditions is of primary importance for the accuracy of modelling. A fundamental aspect in this choice is the adequacy between hydrodynamics and the passive OBC. Although, these last intervene locally, they may influence hydrodynamics and hydrology in the whole simulated domain. A complete description of the chosen open passive boundary condition is given in Bourret et al. (2005). Here, we have implemented a characteristic method (Hedstrom, 1979; Roed and Cooper, 1987) on the barotropic variables, a Neumann condition for the two baroclinic velocity components and a simplified advection condition on the tracers. The calibration operation of the model has been achieved by fitting the bottom friction coefficient: it has a great influence on tidal movement (Lefe`vre, 2000) and constitutes an imperfectly known model parameter depending on the bottom characteristics. The friction term is determined by the following formula (Hess, 1989): ! with r ¼ 9 104 þ 1 103 jju !jj, ~ t ¼ rru (2) b
b
b
where r corresponds to the bed friction coefficient and ub is the current velocity on the bottom. A comparison (Figs. 2 and 3) between the numerical results and seven tidal-gauge measurements located on the shelf (Fig. 1) has been conducted. The SHOM data do not allow a coherent statistical study because of the limited number of stations. These data illustrate, however, the quality of the tidal modelling with the MOBEEHDYCS code, the stations being located on the whole shelf (offshore and closer to the coast).
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Fig. 2. Comparison between the measured data from SHOM and numerical results for the amplitude and phase lag of the M2 wave (a), the S2 wave (b) and the N2 wave (c).
The amplitude and phase of the five main tidal constituents have been computed from sea surface elevation records using harmonic analysis (Foreman, 1978). Tidal amplitude and Greenwich phase lags are calculated via a least-squares fit method coupled with nodal modulation for only those constituents that can be resolved over the length of record. These sea surface elevations have been analysed using a Rayleigh criterion of 1 with no inference.
The Rayleigh criterion is used to determine whether a specific constituent can be identified and isolated from the other constituents. In order to quantify the errors in amplitude and in phase, the mean range between observed values and numerical values has been determined and is given in Table 1 according to the considered tidal wave. In terms of amplitude, the difference between observations and modelling are
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Fig. 3. Comparison between the measured data from SHOM and numerical results for the amplitude and phase lag of the O1 wave (a) and of the K1 wave (b). Table 1 Lag between the numerical results and the SHOM data for the amplitude and the phase of tidal elevation for each main component of spectrum Wave
Difference in amplitude (cm)
Mean wave amplitude (cm)
Difference in phase (1)
Mean wave phase (1)
M2 S2 N2 K1 O1
5.40 1.07 1.21 0.52 0.37
79.60 24.74 16.26 9.71 8.42
3.75 4.72 5.62 4.20 2.49
6.24 6.23 4.08 1.37 1.79
small for the N2, S2, K1 and O1 tidal waves. We observe a weak overestimation of the numerical M2 amplitudes (of the order of 5.4 cm) compared with the measured values. Other differences occur at specific SHOM stations (Fig. 1). For example, at station 25, there is a systematic lag (in phase and amplitude), probably linked to its location at the Oyapock mouth, an almost closed bay where the topography influences on the tidal wave propagation. In addition, the mesh size of 2500 m does not allow a fine representation of the coastline and of the bathymetry. Otherwise, the bathymetry is quite poorly known because of mud banks along the French Guiana coast (Froidefond et al, 1988). Station 05 also has a lag in amplitude or in phase, probably due to its location near the northwestern
boundary, where the mouth of the Maroni River is not represented. Also, the presence of a mud bank probably modifies the bathymetry. The adjustment quality for wave phase is less good than for wave amplitude. However, the range between data and modelling phases is quite small. The highest difference occurs at station 05. This close agreement between numerical results and tidal gauge measurements validates the modelling of the five main tidal waves. 4. Results and discussions We have focused our attention of the influence of tide on the general current pattern and the salinity field.
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The simulation time for results is 60 days, in order to study current and tracers fields after the spin-up time and to analyse the effects of the main tidal waves. According to the initial and boundary conditions, the results of simulations presented here illustrate the circulation during the rainy season, i.e. during the boreal winter, the strong long-shore current is induced by the North Brazil Current flows (Richardson and Reverdin, 1987; Johns et al., 1998). 4.1. Current and sea surface elevation fields The sea surface elevation during 60 days for the station 13 is shown in Fig. 4. This station is close to the coast and so the tidal range reaches 2.5 m during spring tide. The spring-neap cycle is pronounced and one can observe the amplitude difference of sea surface elevation during two consecutive high waters due to the two diurnal tidal waves used in this simulation. The elevation amplitudes during high tide of spring tide and of neap tide are presented in Figs. 5a and b, respectively. During spring tide, the offshore co-range lines are about 1 m and increase when approaching to the coast to reach 1.5 m at the Oyapock and Maroni River mouths. The co-range lines are almost parallel to the coast, except at the Maroni and Oyapock River mouths. These distortions are linked to the coastline orientation change in the proximity of these mouths in comparison with the main orientation of the coastline between Kourou and Cayenne. During neap tide, the elevation amplitude is 0.35 m offshore and reaches 0.45 m at the Maroni River mouth and near Cayenne. At the Oyapock River mouth, the elevation amplitude is 0.50 m. The numerical results of sea surface elevation are comparable to elevations measured by the SHOM at Cayenne with spring-tide amplitude of 1.4 m and neap-tide amplitude of 0.45 m. The mean vertically integrated currents (Fig. 6) are essentially parallel to the coast offshore where the speed is greatest (1.2 m/s). In front of the Sinnamary mouth, the mean current is deflected to the coast due to the bathymetry, which presents a slightly pronounced submarine canyon. In the West, the current is concentrated on the offshore part of the shelf and it is oriented to the North. The main direction of vertically integrated current field is affected by the tide only during spring tide, especially near the coast. Indeed, the long-shore coastal current is weak close to the coast and the tidal currents, which are
Fig. 4. Time evolution of the sea surface elevation at the station 13.
Fig. 5. Elevation amplitude (m) at high tide during spring tide (left) and neap tide (right) on the French Guiana continental shelf.
essentially cross-shore, have more influence. At ebb of spring tide, the tidal currents are maximum and directed to the offshore. Near the coast, the total currents (Fig. 6a) are oriented to the offshore, particularly close to the Oyapock River mouth. At flood of spring tide, the tidal currents are maximum and directed to the coast, influencing the total currents (Fig. 6b) close to the coast. The intrusion in front of the Sinnamary mouth is also reinforced and at the west, the current is not more concentrated on the offshore part of the shelf. On the innershelf, the tidal currents are of the order of 0.15–0.45 m/s during the neap-spring cycle and reach 0.55 m/s in front of Sinnamary and off the Oyapoch River mouth due to a variation of the bathymetry. Offshore, the
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Fig. 7. Tidal residual transport (m2/s) for the M2 tidal wave.
number of the studied M2 wave period. In our case, a part of the stationary component is linked to the coastal current flowing from southeast and imposed on the southeastern boundary. It is not taken into account in the calculation of the residual transport. The tidal residual transport is presented in Fig. 7. It reaches 0.025 m2/s at the Oyapock River mouth and the Approuague and Sinnamary River mouths. On the shelf, it is weaker, from 0.005 to 0.15 m2/s with three different behaviours:
Fig. 6. Current fields during the maximum ebb of spring tide (a) and during the maximum flood of spring tide (b) after 60 days of simulation.
In the east, the tidal residual transport is weak and oriented coastward. Between Cayenne and the Sinnamary River mouth, it is weaker and oriented to the west, i.e. onshore. North of the Sinnamary River mouth, it is stronger and oriented to the northwest, i.e. almost parallel to the coast.
4.2. Surface salinity field
tidal currents are weaker, of the order of 0.05–0.10 m/s, during to the spring-neap cycle. The tide has an influence on the tracer fields (temperature, salinity, etc.) due to the diffusion processes linked to the oscillatory movement and by the mean transport induced by the residual circulation. The residual transport can be considered as the stationary total transport (Robinson, 1983). These residual currents constitute a persistent structure, linked to the bathymetry and the bottom friction. We have studied the residual transport induced by the main tidal constituent, i.e. the M2 wave, to model the French Guyana circulation. The residual transport is expressed as the temporal mean of transport by including an integer
The surface salinity field during high tide after 60 days of simulation occurs as a weak salinity tongue that is linked to two different influences (Fig. 8). The area located in the middle of the shelf, where the salinity is between 10 and 20, may be attributed to the Amazon influence. So, this salinity tongue is in agreement with the boundary conditions. The near coastal zone where the salinity is less than 10 is due to the French Guiana rivers influence. Their influence does not extend beyond the 30 m isobath. In order to evaluate the cross-shore migration of water masses induced by cross-shore tidal currents, the crossshore time displacement has been drawn from estimates of the position of the 15 surface isohalines in the model (Fig. 8). The time evolution of the surface cross-shore
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The observed phase lag of p/2 is due to the displacement that is maximum when the current direction changes. Indeed, under the influence of flood currents, the water masses are advected near the coast. After 6 h, the current changes direction from flood to ebb and the water masses located nearest to the coast are advected offshore. According to a study of the Amazon basin, Geyer (1995) also attributes the large salinity tidal fluctuations to the advection of salinity front by the cross-shelf tidal flow. An onshore–offshore displacement of 20 km is observed for tidal current amplitude of 1 m/s, which corresponds proportionally to our results. 4.3. Vertical structure of salinity fields and currents
Fig. 8. Sea surface salinity field at the high tide after 60 days of simulation. The point A represents the station where the vertical profile of current and salinity are drawn (see Fig. 9).
Fig. 9. Time evolution of the surface 15: isohaline movement (solid line) and the cross-shore surface velocity (dashed line) around the station A.
current is shown in Fig. 9. Note that negative values correspond to velocity directed offshore. The cross-shore tidal currents induce a migration of the salinity field perpendicular to the coast. This transversal displacement during a semi-diurnal tidal cycle is of the order of 3 km with tidal currents of 20 cm/s, and is out of phase by p/2 with the maximum cross-shore currents. The model crossshelf velocity has been integrated and compared to the cross-shelf displacement determined from the salinity field. The integral of the model cross-shelf velocity is similar to the model displacement of the salinity field. The displacement estimated from the salinity field is slightly decreased because of a diffusion effect.
The vertical profile measured at the station A (see Fig. 8) of the salinity (Fig. 10a) and the corresponding cross-shore currents (Fig. 10b) during a tidal cycle of spring tide show similar 12 h oscillations and the stratification is persistent in spite of currents greater than 0.2 m/s. The time of the strongest currents at depth corresponds to the end of ebb where the salinity field close to the coast is advected offshore under the influence of the tidal current. The tidal straining induces a deepening of the halocline at this station. A similar behaviour of the salinity profile has been observed during the sea campaign CHICO 0 at a temporal station (25 h), located at 41540 N and 511390 W, and identified by the point A in Fig. 8. We observed vertical oscillation of the halocline (Fig. 11) with a 12 h period without complete mixing of the stratification. In the analysis of the tide-induced mixing in the Amazon frontal zone, Geyer (1995) observed that the salinity stratification varies considerably in its vertical position but it never vanishes, even during maximum spring tides. These characteristics are even more pronounced on the French Guiana continental shelf, since the tidal currents are slightly weaker than the ones observed on the Amazon basin. There is also a slight phase lag (40 min) between surface and bottom layers, indicating that maximum tidal currents occur almost at the same moment over the entire water depth. In addition, the vertical profile of tidal cross-shore currents (Fig. 10b) presents barotropic structure in spite of high stratification. There is a little tidal shear at the level of the halocline. This is confirmed by the results of AMASSEDS (Geyer et al., 1996) where the tidal variability appears to be essentially barotropic although the subtidal flow exhibits vertical shear (Candela et al., 1992). The weakness of the tidal vertical shear at the level of the halocline could explain the persistence of the stratification on the French Guiana continental shelf, as observed in numerical results. The salinity stratification determined from the model is quite similar to that observed at station A. However, in the model, the gradient of salinity is weaker. Indeed, the salinity of the order of 20 is located between 0 and 5 m of
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Fig. 10. Time evolution of the vertical profile salinity of the station A (left) and of the cross-shore current (right) obtained with the numerical model on a typical tidal cycle.
depth in the in-situ measurements. According to the model, this layer is located between 0 and 1 m of depth. Otherwise, the layer of salinity greater than 36 is located at 15 m of depth in the model whereas it is located at 10 m of depth according to measurements. This difference may be due to the salinity values imposed on the southeastern active boundary. Indeed, the values imposed on this boundary come from the CHICO 0 study. The currents on the shelf are high enough and the time lag between measurements on the boundary 2 and at the station A might be very short. Thus, they do not correspond perfectly to the hydrological conditions having induced such a stratification at the station A. Otherwise, the station A could be located in the plume of the Oyapock River and this river could induce a modification of hydrological condition at the station A. This river is taken into account in the model but the imposed flow corresponds to annual averages since no real time data are available. So, it would be interesting to study the impact of the flow of the Oyapock River on the vertical hydrological conditions near the coast. 5. Conclusion This modelling study constitutes, to our knowledge, the first study regarding the tidal influence on the currents and tracer fields (salinity) on the French Guiana continental shelf. The numerical results correspond to a typical circulation of the rainy season when the North Brazil Current flows northwestward along the French Guiana
shelf. The particular configuration of the simulation domain leads us to choose with care the open boundary conditions and in particular the passive ones. A comparison between numerical results and SHOM sea surface elevation data from seven tidal gauges has validated the tidal representation used by the model. The simulation has shown that the tidal currents are essentially oriented cross-shore. In the innershelf zone, the tidal currents are preponderant compared with the longshore current. In the outershelf region, the total currents direction is essentially parallel to the coast under the influence of large-scale currents. However, in front of the Sinnamary River, the currents flow to the coast due to a slight bathymetry variation. This phenomenon is reinforced by the tidal influence. Under the influence of large-scale currents and the Amazon River discharge, a freshening of the surface waters is induced on the middle of the French Guiana continental shelf. The tidal currents induce a vertical oscillation and a horizontal migration of this weak salinity tongue. Moreover, the normally expected decoupling of tidal currents, above and beneath the halocline, is less pronounced than at higher latitude. On the French Guiana continental shelf, the vertical structure of tidal currents is weakly influenced by the stratification according to the numerical studies and the tidal currents are barotropic. The time evolution of the stratification is linked to tidal variations and there is no mixing induced by the tide. The lack of this decoupling effect between bottom and surface layers could explain the
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Fig. 11. Time evolution of the vertical profile salinity observed to the station A during the sea campaign CHICO 0.
permanent character of the stratification on the French Guiana shelf. References Beardsley, R.C., Candela, J., Limeburner, R., Geyer, W.R., Lentz, S.J., Castro, B.M., Cacchione, D., Carneiro, N., 1995. The M2 tide on the Amazon Shelf. Journal of Geophysical Research 100 (C2), 2283–2320. Bourret, A., Devenon, J.-L., Chevalier, C., 2005. Investigation on passive open boundary conditions adapted to the conjunction of strong currents, standing tidal wave and high stratification: application to the French Guiana Continental Shelf. Continental Shelf Research 25, 1353–1373. Candela, J., Beardsley, R.C., Limeburner, R., 1992. Separation of tidal and subtidal currents in ship-mounted acoustic dopller current profiler observations. Journal of Geophysical Research 97 (C1), 769–788. Castaing, P., Pujos, M., 1976. Interpre´tation de mesures hydrologiques effectue´es sur le plateau continental de la Guyane franc- aise. Bulletin de l’Institut Ge´ologique-Bassin Aquitaine 20, 99–106.
Chevalier, C., Baklouti, M., Ramamonjiarisoa, A., 2004. Modelling the influence of wind and rivers on current, salinity and temperature over the French Guiana continental shelf during the rain season. Journal of Coastal Research 20 (4), 1183–1197. Colin, C., Bourle´s, B., 1994. Western boundary currents and transports off French Guiana as inferred from Pegasus observations. Oceanologica Acta 17 (2), 143–157. Curtin, T.B., 1986. Physical observations of the plume region of the Amazon River during peak discharge. II. Water masses. Continental Shelf Research 6, 53–71. Davies, A.M., 1983. Application of a three-dimensional shelf model to the calculation of North Sea currents. In: North Sea Dynamics. Springer, Berlin, pp. 44–62. Davies, A.M., Lawrence, J., 1994. A three-dimensional model of the M4 tide in the Irish Sea: the importance of open boundary conditions and influence of wind. Journal of Geophysical Research 99, 16,197–16,227. Flagg, C.N., Gordon, R.L., Mc Dowell, S., 1986. Hydrographic and current observations on the continental slope and shelf in the Western Equatorial Atlantic. Journal of Physical Oceanography 16, 1412–1429. Foreman, M.G., 1978. Manual for tidal currents analysis and prediction. Pacific Marine Science Report 77-10, Institute of Ocean Science, Victoria, BC, 97pp. Froidefond, J.M., Pujos, M., Andre´, X., 1988. Migration of mud banks and changing shoreline in French Guiana. Marine Geology 84, 19–30. Froidefond, J.M., Gardel, L., Guiral, D., Parra, M., Ternon, J.F., 2002. Spectral remote sensing reflectances of coastal waters in French Guiana under the Amazon influence. Remote Sensing of Environment 80, 225–232. Geyer, W.R., 1995. Tide-induced mixing in the Amazon frontal zone. Journal of Geophysical Research 100 (C2), 2341–2353. Geyer, W.R., Beardsley, R.C., Lentz, S.J., Candela, J., Limeburner, R., Johns, W.E., Castro, B.M., Soares, I.D., 1996. Physical oceanography of the Amazon shelf. Continental Shelf Research 16 (5/6), 575–616. Gibbs, R.J., 1970. Circulation in the Amazon River estuary and adjacent Atlantic Ocean. Jounal of Marine Research 28, 113–123. Gouriou, Y., Oudot, C., Chuchla, R., Bourles, B., Baurand, F., Diverres, D., Fournier, P., Marin, F., Ternon, J.-F., 1997. Ope´ration Sabord 0, N.O. Ante´a (25–29 mai 1996): recueil de donne´es: introduction, mesures ‘en route’, mesures CTD et traceurs ge´ochimiques, coupes de distributions verticales, cartes de distributions horizontales. IRD, Cayenne (GUF), 1997, 139pp. Hedstrom, G.W., 1979. Non-reflecting boundary conditions for nonlinear hyperbolic systems. Journal of Computational Physics 30, 222–237. Hellweger, F.L., Gordon, A.L., 2002. Tracing Amazon River water into the Caribbean Sea. Journal of Marine Research 60 (4), 537–549. Hess, K.W., 1989. MECCA program documentation, Part 1. NOAA Technical Report NESDIS 46, Washington, DC, 97pp. Hu, C., Montgomery, E.T., Schmitt, R.W., Muller-Karger, F.E., 2004. The dispersal of the Amazon and Orinoco River water in the tropical Atlantic and Caribbean Sea: observation from space and S-PALACE floats. Deep Sea Research II (51), 1151–1171. Johns, W.E., Lee, T.N., Beardsley, R.C., Candela, J., Limeburner, R., Castro, B.M., 1998. Annual cycle and variability of the North Brazil Current. Journal of Physical Oceanography 28, 103–128. Lefe`vre, F., 2000. Mode´lisation des mare´es oce´aniques a` l’e´chelle globale: assimilation de donne´es in situ et altime´triques. The`se, Universite´ de Toulouse III—Paul Sabatier Lentz, S.J., Limeburner, R., 1995. The Amazon River plume during AmasSeds: spatial characteristics and salinity variables. Journal of Geophysical Research 100, 2355–2375. Mellor, G.L., Blumberg, A.F., 1985. Modelling vertical and horizontal diffusivities with the sigma coordinate system. Monthly Weather Review 113, 1379–1383. Munk, W.H., Anderson, E.R., 1948. Notes on the theory of the thermocline. Journal of Marine Research 7, 276–295.
ARTICLE IN PRESS A. Bourret et al. / Continental Shelf Research 28 (2008) 951–961 Nikiema, O., Devenon, J.-L., Baklouti, M., 2007. Numerical modelling of the Amazon River plume. Continental Shelf Research 27, 873–899. Pujos, M., Froidefond, J.M., 1995. Water masses and suspended matter circulation on the French Guiana continental shelf. Continental Shelf Research 15 (9), 1159–1171. Richardson, P.L., Mckee, T.K., 1984. Average seasonal variation of the Atlantic equatorial currents from historical ship drifts. Journal of Physical Oceanography 14, 1226–1238. Richardson, P.L., Reverdin, G., 1987. Seasonal cycle of velocity in the Atlantic North Equatorial Undercurrent as measured by surface
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drifters, current meters and ship drifts. Journal of Geophysical Research 92, 3691–3708. Robinson, I.S., 1983. Tidally induced residual flows. In: Physical Oceanography of Costal and Shelf Seas, pp. 321–356. Roed, L.P., Cooper, C.K., 1987. A study of various open boundary conditions for wind-forced barotropic numerical models. In: Nihoul, J.C.J. (Ed.), Three-Dimensional Models of Marine and Estuarine Dynamics. Elsevier Oceanography, pp. 305–335. Service Hydrographique et Oce´anographique de la Marine, 1975. Instructions nautiques. Ame´riques du Sud (Coˆte Est), Se´r. H 4, 325.
Continental Shelf Research 28 (2008) 951–961 www.elsevier.com/locate/csr
Tidal influence on the hydrodynamics of the French Guiana continental shelf A. Bourreta,, J.-L. Devenonb, C. Chevalierc IXSURVEY, 46 quai Franc- ois Mitterrand, 13600 La Ciotat, France Laboratoire d’Oce´anographie Physique et Bioge´ochimique, OSU/Centre d’Oce´anologie de Marseille, UMR 6535 CNRS/Universite´ de la Me´diterrane´e, Campus de Luminy—Case 901, 13288 Marseille Cedex 09, France c IRD—UR CYROCO, Campus de Luminy—Case 901, 13288 Marseille Cedex 09, France a
b
Received 2 November 2006; received in revised form 7 January 2008; accepted 11 January 2008 Available online 26 January 2008
Abstract This study investigates the circulation on the French Guiana continental shelf under tidal influence. Indeed, hydrodynamics are characterised by a weak salinity tongue located in the middle of the shelf and induced by the Amazon River, a coastal current flowing from the southeast, and a tidal standing wave whose co-range lines are parallel to the coast. In addition to field observations, a numerical model also is used to evaluate the tidal influence on coastal circulation. The model makes use of the MOBEEHDYCS code, a three-dimensional free surface time-splitting model whose domain is bounded with a closed coastal boundary, two active boundaries (offshore and lateral) and a passive boundary. The boundary configuration and hydrodynamics require a careful choice of passive open boundary conditions. The initial and boundary conditions come from field data. The tidal currents are essentially cross-shore and do not have a great influence on the main current direction on the offshore part of the shelf. The offshore currents remain parallel to the coast. In the inner shelf, the tidal influence is found to be much more important and the tidal currents can reach 0.45 m/s. Vertically, the tidal currents are barotropic, in spite of the high stratification and they induce a horizontal cross-shore migration (about 3 km) of the weak salinity tongue and vertical oscillations of the halocline without complete mixing. r 2008 Elsevier Ltd. All rights reserved. Keywords: French Guiana continental shelf; Coast circulation modelling; Tidal influence
1. Introduction The French Guiana continental shelf constitutes an interesting and important coastal zone from a hydrodynamical point of view. The presence of gold-bearing deposits on the seabed along with a high level of heavy metals and the proximity of rivers such as the Amazon River, rich in sediments and nutrients, justifies the interest and the study of this area. The French Guiana continental shelf is subject to a combination of hydrodynamic processes and atmospheric forcings. Hydrodynamics are characterised by a strong coastal current flowing from the southeast (Flagg et al., 1986; Richardson and Mckee, 1984), high Corresponding author. Tel.: +33 491 82 92 05.
E-mail address: [email protected] (A. Bourret). 0278-4343/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2008.01.008
stratification due to the plume of the Amazon River (Hu et al., 2004; Hellweger and Gordon, 2002), and by strong tidal currents and trade winds. Indeed, the vicinity of the InterTropical Convergence Zone, where the Southeast and Northeast Trade Winds converge, induces a seasonal alternation of winds. In addition, a strong coastal current flows along the coast to the northwest and is partly induced by the North Brazil Current (Richardson and Reverdin, 1987; Johns et al., 1998). Based on oceanographic observations during spring and numerical experiments, a weak salinity tongue located about 40 km off the coast is due to the proximity of the Amazon River. The outflows from local rivers have more importance at lesser distance only (Chevalier et al., 2004).
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The tide is mixed with two daily low and high tides of different amplitudes per day (Service Hydrographique et Oce´anographique de la Marine, 1975) and the co-range lines are almost parallel to the coast. The tidal standing wave presents five main tidal components, which are the M2, S2 and N2 semi-diurnal waves and the K1 and O1 diurnal waves. The tidal amplitude can reach 1.5 m at Cayenne (Fig. 1) (Service Hydrographique et Oce´anographique de la Marine, 1975) and the tidal currents have amplitude of 0.5 m/s (Pujos and Froidefond, 1995). The Amazon shelf has been the subject of a multidisciplinary survey (AMASSEDS: A Multidisciplinary Amazon Shelf Sediment Study) between 1989 and 1993. Observations indicate that, the currents on the Amazon shelf are the sum of semi-diurnal nearly barotropic tidal currents perpendicular to the coast and a strong long-shore current (Lentz and Limeburner, 1995). Additionally, the Amazon plume is constituted by a weak salinity tongue which spreads to the middle of the shelf (Lentz and Limeburner, 1995; Curtin, 1986; Gibbs, 1970). The tidal wave has been studied on the Amazon shelf and more precisely the M2 tide from mooring data and a simple, local homogeneous tidal model (Beardsley et al., 1995). Geyer et al. (1996) have also underscored the modulation of the stratification by the tide. From field data, Nikiema et al. (2007) have studied the influence of specific physical processes on the morphology of the Amazon plume by using a three-dimensional (3-D) hydrodynamic numerical model and realistic bathymetry and coastline of the northern Brazilian shelf. On the French Guiana shelf, the data are scattered and mainly hydrologic. Near the coastal zone, the pycnocline is located at a water depth between 8 and 10 m. The currents seem always directed to the northwest with a slight influence of the tide (Castaing and Pujos, 1976; Pujos
and Froidefond, 1995). The water masses off the French Guiana coast are characterised by a weak salinity plume at different periods of year (Colin and Bourle´s, 1994). The plume extends out to 80 km from the coastline. According to Pujos and Froidefond (1995), the currents are maximum in the middle of the shelf and seem to be the prolongation of the current flowing along the coast near the Amazon mouth. A field survey SABORD 0 was carried out during May 1996 from the Approuague to the west of Sinnamary (Gouriou et al., 1997). Five transects perpendicular to the coast were carried out. These transects show that the halocline is located between 6 and 12 m of depth. The surface salinity field presents a weak salinity area at the eastern end of the shelf. The sea surface temperature is quite homogeneous over the study area but with a colder area occurring near the Maroni River mouth (Fig. 1). Another field study, CHICO 0 was carried out during the rainy season between the Oyapock River and the Approuague River in April 1999 (Froidefond et al., 2002). Current measurements were carried out during this study, especially along the eastern boundary, between 30 and 140 km from the coast. Current velocities increased regularly from 0.3 m/s at 30 km off the coast and reached a maximum value of 1.1 m/s on the offshore part of the shelf. Hydrodynamic modelling on the French Guiana shelf has been used to study the influence of trade winds on hydrology and currents (Chevalier et al., 2004). Indeed, a northeastern wind induces a downwelling near the coast and this wind does not affect the vertical profile of the long-shore velocity. Wind effects on hydrology are limited in depth to 5 m. The influence of tide on shelf currents and stratification has not been studied. On the French Guiana continental shelf, as in many other estuarine and coastal settings, we would expect that the strong tidal currents would have an influence on the structure and dynamics of the salinity, temperature and current fields both horizontally and vertically. This study addresses the influence of the tidal phenomenon on the currents, and tracer (i.e. the temperature and the salinity) fields for the particular zone of the French Guiana continental shelf. We have studied more particularly the salinity field due to the influence of the Amazon freshwater. To our knowledge, this modelling constitutes the first simulation of this shelf area, which takes into account the tidal influence coupled with the long-shore current and stratification. The present article is composed of three parts: the first one concerns the numerical implementation. The second part presents the validation of the tidal signal and the last part constitutes an analysis of both vertically integrated and 3-D simulations. 2. Numerical implementation
Fig. 1. Bathymetry of the French Guiana continental shelf and locations of the SHOM tidal gauges.
The simulation domain (Fig. 1) has a closed coastal boundary on southwest and three open boundaries: two
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cross-shore and one long-shore. Four rivers are taken into account for the modelling purpose of the shelf circulation. These are, from the east to the west, the Oyapock River, the Approuague River, the Mahury River and the Sinnamary River. The Maroni River located at the western part of the domain is not taken into account. The coastal, finite-difference, free-surface model MOBEEHDYCS (Hess, 1989; Chevalier et al., 2004) is used to reproduce the shelf circulation. The model is governed by the momentum equations, the continuity equation, heat and salinity conservation laws and the equation of state of sea water. The model follows the hydrostatic and the f-plane approximations and Boussinesq’s hypothesis. The numerical MOBEEDHYCS code makes use of a regular Cartesian horizontal grid associated with sigma vertical coordinates (Mellor and Blumberg, 1985) with closer mesh refinement near the free surface than near the bottom. The horizontal velocity is separated into a vertically integrated component (the barotropic mode) and a departure component (the baroclinic mode) according to a modesplitting numerical scheme. The vertical eddy viscosity coefficient follows the Prandtl’s model where the influence of stratification is taken into account thanks to the Richardson’s number (Ri) (Munk and Anderson, 1948). The grid spacing is regular (Dx ¼ Dy ¼ 2500 m) with 130 meshes in the along-shore direction and 77 in the crossshore one. The time step of the barotropic mode is 20 s and for the baroclinic mode is 100 s. The vertical discretisation of the model includes 19 sigma levels. A detailed description of the model is given in Chevalier et al. (2004). The particular topography characterising on the French Guiana continental shelf leads us to consider the studyarea as partly bounded by one southern coastal closed boundary (boundary 1), by two open cross-shore boundaries (boundaries 2 and 4) and one outer long-shore boundary (boundary 3). In this configuration (Fig. 1), tidal currents are essentially parallel to boundaries 2 and 4 whereas the coastal current flows perpendicularly into these boundaries. So, boundaries 2 and 3 are active, i.e. the value of the velocity and/or the sea surface elevation on these boundaries is imposed from a knowledge of conditions outside. Here, the incoming coastal current is imposed on the boundary and the tidal waves are specified on the boundary 3. The tidal forcing, which is essentially normal to the offshore boundary, is imposed on this boundary with a Flather type condition, adapted to a normal tidal forcing (Davies, 1983; Davies and Lawrence, 1994): 2pt Vt tþ1 ffiffiffiffiffiffiffi , Zi ¼ A0 sin þ pB1 and Ztþ1 (1) B ¼ Zi T gH where VB71 is the boundary-normal vertically integrated velocity just before boundary, Zi, the imposed SSE at boundary and ZB, the SSE at the boundary grid point. The modelling takes into account the main five tidal components (M2, S2, N2, K1 and O1). Their amplitudes and phases come from extrapolated data of tidal gauges
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belonging to the French Navy Hydrographic Service (Service Hydrographique et Oce´anographique de la Marine, 1975) (stations 15 and 23; see Fig. 1). The boundary 4 is passive, because the coastal current exits there, i.e. hydrodynamics inside the domain determine the values on the passive open boundary. The particular choice of the condition applied to the passive open boundary may be of primary importance for numerical results validity. The initial data of salinity and temperature fields are obtained from the sea campaign SABORD 0 study. It was conducted with the oceanographic vessel Antea in May 1996 on the French Guiana continental shelf from the Approuague River to the Sinnamary River (Gouriou et al., 1997). The coastal long-shore current is derived from CHICO 0 data. During this study, the currents were measured along a transect located on the eastern boundary of our modelling domain. The measurements used at the boundary 2 have a horizontal resolution of 1 km. They have been interpolated on the horizontal model grid and have been integrated on the vertical to force the vertically integrated model. 3. Calibration and validation of the numerical tidal representation For the northwestern boundary, the adapted choice of passive boundary conditions is of primary importance for the accuracy of modelling. A fundamental aspect in this choice is the adequacy between hydrodynamics and the passive OBC. Although, these last intervene locally, they may influence hydrodynamics and hydrology in the whole simulated domain. A complete description of the chosen open passive boundary condition is given in Bourret et al. (2005). Here, we have implemented a characteristic method (Hedstrom, 1979; Roed and Cooper, 1987) on the barotropic variables, a Neumann condition for the two baroclinic velocity components and a simplified advection condition on the tracers. The calibration operation of the model has been achieved by fitting the bottom friction coefficient: it has a great influence on tidal movement (Lefe`vre, 2000) and constitutes an imperfectly known model parameter depending on the bottom characteristics. The friction term is determined by the following formula (Hess, 1989): ! with r ¼ 9 104 þ 1 103 jju !jj, ~ t ¼ rru (2) b
b
b
where r corresponds to the bed friction coefficient and ub is the current velocity on the bottom. A comparison (Figs. 2 and 3) between the numerical results and seven tidal-gauge measurements located on the shelf (Fig. 1) has been conducted. The SHOM data do not allow a coherent statistical study because of the limited number of stations. These data illustrate, however, the quality of the tidal modelling with the MOBEEHDYCS code, the stations being located on the whole shelf (offshore and closer to the coast).
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Fig. 2. Comparison between the measured data from SHOM and numerical results for the amplitude and phase lag of the M2 wave (a), the S2 wave (b) and the N2 wave (c).
The amplitude and phase of the five main tidal constituents have been computed from sea surface elevation records using harmonic analysis (Foreman, 1978). Tidal amplitude and Greenwich phase lags are calculated via a least-squares fit method coupled with nodal modulation for only those constituents that can be resolved over the length of record. These sea surface elevations have been analysed using a Rayleigh criterion of 1 with no inference.
The Rayleigh criterion is used to determine whether a specific constituent can be identified and isolated from the other constituents. In order to quantify the errors in amplitude and in phase, the mean range between observed values and numerical values has been determined and is given in Table 1 according to the considered tidal wave. In terms of amplitude, the difference between observations and modelling are
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Fig. 3. Comparison between the measured data from SHOM and numerical results for the amplitude and phase lag of the O1 wave (a) and of the K1 wave (b). Table 1 Lag between the numerical results and the SHOM data for the amplitude and the phase of tidal elevation for each main component of spectrum Wave
Difference in amplitude (cm)
Mean wave amplitude (cm)
Difference in phase (1)
Mean wave phase (1)
M2 S2 N2 K1 O1
5.40 1.07 1.21 0.52 0.37
79.60 24.74 16.26 9.71 8.42
3.75 4.72 5.62 4.20 2.49
6.24 6.23 4.08 1.37 1.79
small for the N2, S2, K1 and O1 tidal waves. We observe a weak overestimation of the numerical M2 amplitudes (of the order of 5.4 cm) compared with the measured values. Other differences occur at specific SHOM stations (Fig. 1). For example, at station 25, there is a systematic lag (in phase and amplitude), probably linked to its location at the Oyapock mouth, an almost closed bay where the topography influences on the tidal wave propagation. In addition, the mesh size of 2500 m does not allow a fine representation of the coastline and of the bathymetry. Otherwise, the bathymetry is quite poorly known because of mud banks along the French Guiana coast (Froidefond et al, 1988). Station 05 also has a lag in amplitude or in phase, probably due to its location near the northwestern
boundary, where the mouth of the Maroni River is not represented. Also, the presence of a mud bank probably modifies the bathymetry. The adjustment quality for wave phase is less good than for wave amplitude. However, the range between data and modelling phases is quite small. The highest difference occurs at station 05. This close agreement between numerical results and tidal gauge measurements validates the modelling of the five main tidal waves. 4. Results and discussions We have focused our attention of the influence of tide on the general current pattern and the salinity field.
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The simulation time for results is 60 days, in order to study current and tracers fields after the spin-up time and to analyse the effects of the main tidal waves. According to the initial and boundary conditions, the results of simulations presented here illustrate the circulation during the rainy season, i.e. during the boreal winter, the strong long-shore current is induced by the North Brazil Current flows (Richardson and Reverdin, 1987; Johns et al., 1998). 4.1. Current and sea surface elevation fields The sea surface elevation during 60 days for the station 13 is shown in Fig. 4. This station is close to the coast and so the tidal range reaches 2.5 m during spring tide. The spring-neap cycle is pronounced and one can observe the amplitude difference of sea surface elevation during two consecutive high waters due to the two diurnal tidal waves used in this simulation. The elevation amplitudes during high tide of spring tide and of neap tide are presented in Figs. 5a and b, respectively. During spring tide, the offshore co-range lines are about 1 m and increase when approaching to the coast to reach 1.5 m at the Oyapock and Maroni River mouths. The co-range lines are almost parallel to the coast, except at the Maroni and Oyapock River mouths. These distortions are linked to the coastline orientation change in the proximity of these mouths in comparison with the main orientation of the coastline between Kourou and Cayenne. During neap tide, the elevation amplitude is 0.35 m offshore and reaches 0.45 m at the Maroni River mouth and near Cayenne. At the Oyapock River mouth, the elevation amplitude is 0.50 m. The numerical results of sea surface elevation are comparable to elevations measured by the SHOM at Cayenne with spring-tide amplitude of 1.4 m and neap-tide amplitude of 0.45 m. The mean vertically integrated currents (Fig. 6) are essentially parallel to the coast offshore where the speed is greatest (1.2 m/s). In front of the Sinnamary mouth, the mean current is deflected to the coast due to the bathymetry, which presents a slightly pronounced submarine canyon. In the West, the current is concentrated on the offshore part of the shelf and it is oriented to the North. The main direction of vertically integrated current field is affected by the tide only during spring tide, especially near the coast. Indeed, the long-shore coastal current is weak close to the coast and the tidal currents, which are
Fig. 4. Time evolution of the sea surface elevation at the station 13.
Fig. 5. Elevation amplitude (m) at high tide during spring tide (left) and neap tide (right) on the French Guiana continental shelf.
essentially cross-shore, have more influence. At ebb of spring tide, the tidal currents are maximum and directed to the offshore. Near the coast, the total currents (Fig. 6a) are oriented to the offshore, particularly close to the Oyapock River mouth. At flood of spring tide, the tidal currents are maximum and directed to the coast, influencing the total currents (Fig. 6b) close to the coast. The intrusion in front of the Sinnamary mouth is also reinforced and at the west, the current is not more concentrated on the offshore part of the shelf. On the innershelf, the tidal currents are of the order of 0.15–0.45 m/s during the neap-spring cycle and reach 0.55 m/s in front of Sinnamary and off the Oyapoch River mouth due to a variation of the bathymetry. Offshore, the
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Fig. 7. Tidal residual transport (m2/s) for the M2 tidal wave.
number of the studied M2 wave period. In our case, a part of the stationary component is linked to the coastal current flowing from southeast and imposed on the southeastern boundary. It is not taken into account in the calculation of the residual transport. The tidal residual transport is presented in Fig. 7. It reaches 0.025 m2/s at the Oyapock River mouth and the Approuague and Sinnamary River mouths. On the shelf, it is weaker, from 0.005 to 0.15 m2/s with three different behaviours:
Fig. 6. Current fields during the maximum ebb of spring tide (a) and during the maximum flood of spring tide (b) after 60 days of simulation.
In the east, the tidal residual transport is weak and oriented coastward. Between Cayenne and the Sinnamary River mouth, it is weaker and oriented to the west, i.e. onshore. North of the Sinnamary River mouth, it is stronger and oriented to the northwest, i.e. almost parallel to the coast.
4.2. Surface salinity field
tidal currents are weaker, of the order of 0.05–0.10 m/s, during to the spring-neap cycle. The tide has an influence on the tracer fields (temperature, salinity, etc.) due to the diffusion processes linked to the oscillatory movement and by the mean transport induced by the residual circulation. The residual transport can be considered as the stationary total transport (Robinson, 1983). These residual currents constitute a persistent structure, linked to the bathymetry and the bottom friction. We have studied the residual transport induced by the main tidal constituent, i.e. the M2 wave, to model the French Guyana circulation. The residual transport is expressed as the temporal mean of transport by including an integer
The surface salinity field during high tide after 60 days of simulation occurs as a weak salinity tongue that is linked to two different influences (Fig. 8). The area located in the middle of the shelf, where the salinity is between 10 and 20, may be attributed to the Amazon influence. So, this salinity tongue is in agreement with the boundary conditions. The near coastal zone where the salinity is less than 10 is due to the French Guiana rivers influence. Their influence does not extend beyond the 30 m isobath. In order to evaluate the cross-shore migration of water masses induced by cross-shore tidal currents, the crossshore time displacement has been drawn from estimates of the position of the 15 surface isohalines in the model (Fig. 8). The time evolution of the surface cross-shore
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The observed phase lag of p/2 is due to the displacement that is maximum when the current direction changes. Indeed, under the influence of flood currents, the water masses are advected near the coast. After 6 h, the current changes direction from flood to ebb and the water masses located nearest to the coast are advected offshore. According to a study of the Amazon basin, Geyer (1995) also attributes the large salinity tidal fluctuations to the advection of salinity front by the cross-shelf tidal flow. An onshore–offshore displacement of 20 km is observed for tidal current amplitude of 1 m/s, which corresponds proportionally to our results. 4.3. Vertical structure of salinity fields and currents
Fig. 8. Sea surface salinity field at the high tide after 60 days of simulation. The point A represents the station where the vertical profile of current and salinity are drawn (see Fig. 9).
Fig. 9. Time evolution of the surface 15: isohaline movement (solid line) and the cross-shore surface velocity (dashed line) around the station A.
current is shown in Fig. 9. Note that negative values correspond to velocity directed offshore. The cross-shore tidal currents induce a migration of the salinity field perpendicular to the coast. This transversal displacement during a semi-diurnal tidal cycle is of the order of 3 km with tidal currents of 20 cm/s, and is out of phase by p/2 with the maximum cross-shore currents. The model crossshelf velocity has been integrated and compared to the cross-shelf displacement determined from the salinity field. The integral of the model cross-shelf velocity is similar to the model displacement of the salinity field. The displacement estimated from the salinity field is slightly decreased because of a diffusion effect.
The vertical profile measured at the station A (see Fig. 8) of the salinity (Fig. 10a) and the corresponding cross-shore currents (Fig. 10b) during a tidal cycle of spring tide show similar 12 h oscillations and the stratification is persistent in spite of currents greater than 0.2 m/s. The time of the strongest currents at depth corresponds to the end of ebb where the salinity field close to the coast is advected offshore under the influence of the tidal current. The tidal straining induces a deepening of the halocline at this station. A similar behaviour of the salinity profile has been observed during the sea campaign CHICO 0 at a temporal station (25 h), located at 41540 N and 511390 W, and identified by the point A in Fig. 8. We observed vertical oscillation of the halocline (Fig. 11) with a 12 h period without complete mixing of the stratification. In the analysis of the tide-induced mixing in the Amazon frontal zone, Geyer (1995) observed that the salinity stratification varies considerably in its vertical position but it never vanishes, even during maximum spring tides. These characteristics are even more pronounced on the French Guiana continental shelf, since the tidal currents are slightly weaker than the ones observed on the Amazon basin. There is also a slight phase lag (40 min) between surface and bottom layers, indicating that maximum tidal currents occur almost at the same moment over the entire water depth. In addition, the vertical profile of tidal cross-shore currents (Fig. 10b) presents barotropic structure in spite of high stratification. There is a little tidal shear at the level of the halocline. This is confirmed by the results of AMASSEDS (Geyer et al., 1996) where the tidal variability appears to be essentially barotropic although the subtidal flow exhibits vertical shear (Candela et al., 1992). The weakness of the tidal vertical shear at the level of the halocline could explain the persistence of the stratification on the French Guiana continental shelf, as observed in numerical results. The salinity stratification determined from the model is quite similar to that observed at station A. However, in the model, the gradient of salinity is weaker. Indeed, the salinity of the order of 20 is located between 0 and 5 m of
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Fig. 10. Time evolution of the vertical profile salinity of the station A (left) and of the cross-shore current (right) obtained with the numerical model on a typical tidal cycle.
depth in the in-situ measurements. According to the model, this layer is located between 0 and 1 m of depth. Otherwise, the layer of salinity greater than 36 is located at 15 m of depth in the model whereas it is located at 10 m of depth according to measurements. This difference may be due to the salinity values imposed on the southeastern active boundary. Indeed, the values imposed on this boundary come from the CHICO 0 study. The currents on the shelf are high enough and the time lag between measurements on the boundary 2 and at the station A might be very short. Thus, they do not correspond perfectly to the hydrological conditions having induced such a stratification at the station A. Otherwise, the station A could be located in the plume of the Oyapock River and this river could induce a modification of hydrological condition at the station A. This river is taken into account in the model but the imposed flow corresponds to annual averages since no real time data are available. So, it would be interesting to study the impact of the flow of the Oyapock River on the vertical hydrological conditions near the coast. 5. Conclusion This modelling study constitutes, to our knowledge, the first study regarding the tidal influence on the currents and tracer fields (salinity) on the French Guiana continental shelf. The numerical results correspond to a typical circulation of the rainy season when the North Brazil Current flows northwestward along the French Guiana
shelf. The particular configuration of the simulation domain leads us to choose with care the open boundary conditions and in particular the passive ones. A comparison between numerical results and SHOM sea surface elevation data from seven tidal gauges has validated the tidal representation used by the model. The simulation has shown that the tidal currents are essentially oriented cross-shore. In the innershelf zone, the tidal currents are preponderant compared with the longshore current. In the outershelf region, the total currents direction is essentially parallel to the coast under the influence of large-scale currents. However, in front of the Sinnamary River, the currents flow to the coast due to a slight bathymetry variation. This phenomenon is reinforced by the tidal influence. Under the influence of large-scale currents and the Amazon River discharge, a freshening of the surface waters is induced on the middle of the French Guiana continental shelf. The tidal currents induce a vertical oscillation and a horizontal migration of this weak salinity tongue. Moreover, the normally expected decoupling of tidal currents, above and beneath the halocline, is less pronounced than at higher latitude. On the French Guiana continental shelf, the vertical structure of tidal currents is weakly influenced by the stratification according to the numerical studies and the tidal currents are barotropic. The time evolution of the stratification is linked to tidal variations and there is no mixing induced by the tide. The lack of this decoupling effect between bottom and surface layers could explain the
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Fig. 11. Time evolution of the vertical profile salinity observed to the station A during the sea campaign CHICO 0.
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