VHF radar observations of surface currents off the northern Opal coast in the eastern English Channel

ARTICLE IN PRESS

Continental Shelf Research 27 (2007) 2449–2464 www.elsevier.com/locate/csr

VHF radar observations of surface currents off the northern Opal coast in the eastern English Channel Alexei Sentcheva,, Max Yaremchukb a

Ecosyste`mes Littoraux et Coˆtiers (FRE-2816), Universite´ du Littoral-Coˆte d’Opale, 32 Av. Foch, 62930 Wimereux, France International Pacific Research Center, University of Hawaii, East– West Road 1680, POST Building 401, Honolulu, HI 96822, USA

b

Received 6 February 2007; received in revised form 7 June 2007; accepted 18 June 2007 Available online 27 June 2007

Abstract Two very high-frequency radars (VHFR) operating on the Opal coast of eastern English Channel provided a nearly continuous 35-day long dataset of surface currents over a 500 km2 area at 0.6–1.8 km resolution. Argo drifter tracking and CTD soundings complemented the VHFR observations, which extended approximately 25 km offshore. The radar data resolve three basic modes of the surface velocity variation in the area, that are driven by tides, winds and freshwater fluxes associated with seasonal river discharge. The first mode, accounting for 90% of variability, is characterized by an alongshore flow pattern, whereas the second and third modes exhibit cross-shore, and eddy-like structures in the current velocity field. All the three modes show the dominant semi-diurnal variability and low-frequency modulation by the neap-spring tidal cycle. Although tidal forcing provides the major contribution to variability of local currents, baroclinicity plays an important role in shaping the 3D velocity field averaged over the tidal cycle and may strongly affect tracer dynamics on larger time scales. An empirical orthogonal function (EOF) decomposition and a spectral rotary analysis of the VHFR data reveal a discontinuity in the velocity field occurring approximately 10 km offshore which was caused by the reversal in the sign of rotation of the current vector. This feature of local circulation is responsible for surface current convergence on ebb, divergence on flood and strong oscillatory vertical motion. Spectral analysis of the observed currents and the results of the Agro drifter tracking indicate that the line of convergence approximately follows the 30-m isobath. The most pronounced feature of the radar-derived residual circulation is the along-coast intensification of surface currents with velocity magnitude of 0.25 m/s typical for the Regions of Freshwater Influence (ROFI). The analysis has provided a useful, exploratory examination of surface currents, suggesting that the circulation off the Opal coast is governed by ROFI dynamics on the hypertidal background. r 2007 Elsevier Ltd. All rights reserved. Keywords: Tidal current; Residual current; Surface current convergence; VHF radar; ROFI dynamics; English Channel

1. Introduction

Corresponding author. Tel.: +33 3 2199 6417;

fax: +33 3 2199 6401. E-mail addresses: [email protected] (A. Sentchev), [email protected] (M. Yaremchuk). 0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.06.010

In recent years shore-based high frequency (HF) Doppler radars have been extensively used as an efficient tool for remote monitoring of surface currents in many coastal regions (Prandle, 1993; Graber et al., 1996; Shay et al., 1998; Marmorino

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and transport along the Opal coast. Despite numerous previous investigations, including modelling efforts (Bailly du Bois and Dumas, 2005; Sentchev and Korotenko, 2005; Ouahsine et al. (2006) and field measurements (Prandle et al., 1996; Lafite et al., 2000), the flow pattern, its magnitude and especially its dependence on various forcing factors remains relatively uncertain. This shortcoming is primarily due to the difficulties in acquisition of long-term in situ data because of the extremely intense traffic in the Channel. In that respect, remote sensing of surface currents by VHFRs provides a unique opportunity to establish a monitoring system of the eastern Channel on a regular basis. VHFR observations in the English Channel started more than two decades ago. Broche et al. (1986) were the first to report the results of 3-day long VHFR measurements carried out in 1982 in the Bay of Seine. In 1990–1991, Prandle et al. (1993)

et al., 1999; Haus et al., 2000; Breivik and Sætra, 2001; Kovacevic et al., 2004; Kaplan et al., 2005). In spring 2003, a very high-frequency radar (VHFR) array was deployed on the Opal coast (Coˆte d’Opale) of north-eastern France. The region occupies a 100 km segment of the French coastline in the eastern English Channel (EEC) between the Canche estuary and port of Calais (Fig. 1), where a unique combination of densely populated environment, tourism, industry and historical heritage requires new concepts of ecologically sustainable development. One of the latest interdisciplinary research activities in the region is the ‘‘eastern English Channel–southern North Sea’’ (EEC–SNS) project supported by the National Program for Coastal Environmental Research (PNEC). The radar experiment analysed below was conducted in the framework of this project in May–June 2003. In the present study we focus on investigation of the physical processes which govern the circulation

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Fig. 1. Experimental domain off the Opal coast in the eastern English Channel. Radar sites are shown by black circles, grey circles denote locations of the CTD soundings. Major rivers, contributing to the freshwater input and bottom topography are also shown. Contour interval is 10 m.

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conducted an intense year-long HFR survey in the Strait of Dover, which allowed to obtain relatively accurate estimates of the tidal flow into the North Sea (Prandle and Player, 1993; Sentchev and Yaremchuk, 1999). This work can be considered as an extension of the above mentioned investigations. We report the results of 35-day long VHFR measurements off the northern Opal coast, that were supported by in situ observations. The domain (Fig. 1) is affected by river discharge from the northern French coast, which supplies the region with 0.05–0.20 km3 of freshwater per day depending on the season. Thus, the local dynamics involves baroclinic effects and, as we show, brings in certain features of circulation typical for the regions of freshwater influence (ROFI). With tidal range up to 8 m and tidal currents reaching 1.5–2 m/s, the background conditions can be classified as hypertidal environment (e.g., Dyer, 1997), making the studied area distinctly different from the ROFIs investigated by other authors (Visser et al., 1994; Souza and Simpson, 1996a, b; Marmorino et al., 1999, 2004; Estournel et al., 2001; Hyder et al., 2002). Our primary objectives are to separate tidal, wind- and buoyancy-driven flows in a way that provides an insight on the spatial structure and temporal evolution of the coastal currents and to analyse quantitatively the circulation and its forcing mechanisms. The paper is organized as follows. In the next section we present the available data on external forcing (winds, tides and river runoff) and describe VHFR observations of sea surface currents, drifter trajectories and ship-borne CTD soundings. In Section 3, the VHFR data are analysed using the principal component, rotary and EOF technique, the sea response to external forcing is decomposed into tidal, wind- and buoyancy-driven constituents and the surface convergence front is localized. Using the VHFR derived surface currents, we also estimate the mean residual flow and assess the Stokes velocity field across the region. Discussion of the results and conclusions are presented in Section 4. 2. Data 2.1. External forcing Water flow along the Opal coast of France is a combined result of the interaction between tidal motions, river discharge, meteorological forcing and

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non-tidal sea-level changes. Remotely forced tidal waves arriving from the Atlantic ocean and the North sea is the dominant factor which determines the variability of the sea surface height (SSH) and currents in the study area. The SSH variations range within 4–8 m, whereas the corresponding tidal currents have a typical magnitude of 1–1.5 m/s (e.g. Sentchev et al., 2006; Ouahsine et al., 2006). SSH variations in the port of Boulogne (Fig. 1) are shown in Fig. 2(a). The tidal signal has a predominant semi-diurnal period, with a pronounced fortnightly modulation due to the interference of the M2, S2 and N2 constituents. The local tidal current lags the sea level by approximately 2.5 h, resulting in the current reversal 2.5 h before the arriving of the high water (HW) or low water (LW) into the domain. Buoyancy forcing is dominated by the river runoff. Majority of the freshwater comes to the EEC from the Seine, Somme, Authie and other rivers on the northeastern coast of France and occurs in winter. In 2003, a peak total river discharge of 1900 m3 s1 was observed in January (Fig. 2(b)). The consequence of the freshwater input to the EEC is the existence of the haline front, which separates offshore saline waters of Atlantic origin from freshened near-shore waters. Annual cycle of the river runoff illustrates a strong time variability with secondary peaks occurring in late winter-early spring (Fig. 2(b)). Our study covers the period of a relatively low river runoff with a mean total discharge of approximately 500 m3 s1 (Fig. 2(b)) not typical for late spring. The Seine River (Fig. 1) provides majority (80%) of the freshwater input to the EEC. Fig. 2(c) shows winds measured during the experiment at the Boulogne light tower. Winds with speeds exceeding 10 m/s were generally coming from southwestern or, less often, from the northeastern quadrant with the dominant directions being about 201 and 2201. 2.2. Field measurements The radar experiment (ERMANO) was conducted in the framework of EEC–SNS project, which represents a multi-disciplinary study of physical and biological processes on the northeastern continental shelf of France. The field program involved several ship missions, Argo drifting buoy tracking and the VHFR measurements in the eastern part of the Channel.

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Fig. 2. External forcing data. (a) Tidal elevation observed in Boulogne during the experiment. (b) Total discharge rate R of the rivers on the northeastern French coast in 2003. Grey shading denotes the period of radar measurements. The data are available on http://www.eau-artois-picardie.fr and http://seine-aval.crihan.fr. (c) Wind measurements made at the Boulogne light tower (Met-office data). Positive velocities are from the South to North (S–N) and from the West to East (W–E).

In May–June 2003, two VFH radars were deployed to monitor surface currents off the French Opal coast for a 35-day period. One radar site was located on the Cape Gris Nez, 40 m above the sea

level. The other radar was 12 km farther south, at Wimereux (Fig. 1). This site had an altitude of a few metres about the sea level. The VHFR system ‘‘COSMER’’ (Broche et al., 1987), developed at the

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LSEET Laboratory (University of Toulon, France), uses a beam-forming emitter antenna (4 separate elements) oriented along the centre line of the radar beam coverage (Fig. 3(a)). The 25 m long receiving antenna is orthogonal to the emitter and consists of

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8 equi-spaced elements. Signals from eleven 101 wide beams are separated by COSMER hardware into 600 m long ‘‘bins.’’ The radars operate at frequencies 45 Mhz (Cape Gris Nez) and 47.8 MHz (Wimereux), and can provide measurements over a distance up to 25 km offshore. A typical operation scheme involves simultaneous 8-min sampling from both sites (typically 6 incoherent summations 80 s each) followed by 12 min of data processing and stand-by. This sequence is repeated 3 times per hour and provides surface velocity vectors averaged over the bins shown in Fig. 3(a) at 20 min intervals. To increase the signal to noise ratio at far ranges, where vector returns dropped below 70% (typically for the Wimereux site), the power spectra were averaged over three adjacent cells for both radars. The resulting cell configuration is shown in Fig. 3(b). At the points of beam intersection, the two radial components of the current were combined to yield two-dimensional surface current vectors. Observations at the Wimereux site have a 20-h gap on May 21–22, caused by power supply failure. VHFR data were supplemented by in situ observations performed by the R/V ‘‘Coˆtes de La Manche.’’ On June 4, 2003, the ship deployed a surface moored ADCP current meter, which provided velocity profiles during that day. Three Argo drifters were released in the study area in the spring–summer of 2003, and one Argo drifting buoy was tracked for approximately one tidal cycle in the region covered by VHFR observations on June 4, 2003. Physical and biological samplings were made along the drifter trajectory by the R/V Coˆtes de La Manche. As a part of the EEC–SNS project, three shipboard surveys were also performed. Cross-shore sections were repeatedly occupied by the R/V along the French coast between the mouth of Seine and Belgium. Twelve stations were located in the region covered by VHFR observations (Fig. 1). 3. Currents from VHFR measurements

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Fig. 3. The VHF radar coverage of the area with eleven beams each 101 wide, separated into ‘‘bins’’ of 600 m spacing in the radial direction (a), and (b) with 1.8 km spacing adopted after the along-beam averaging. The regular grid is shown by grey circles in (b). Shading indicates the region where the vector returns are below the 75% level. Radar sites are shown by open circles. Thick solid lines indicate the orientation of emitter antenna array on each radar site: 1901 and 1501 on northern and southern sites, respectively.

3.1. Preprocessing and quality control The raw VHFR data were acquired in the form of backscattered power spectra along 11 beams (Fig. 3(a)) for each radar. Spectra along the beams were registered at 600 m radial resolution and then averaged with a 1.8 km window to improve the statistics. These 3-bin radially averaged spectra were used to identify radial velocities at 1.8 km resolution. A semi-automatic quality control has been

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Tidal flow dynamics can be effectively quantified by estimating the parameters of synthesized tidal current ellipses. To study the evolution of tidal currents in time and space, we applied the principal component analysis (PCA) technique (Emery and Thomson, 1997) to 12.3-h time series with 20-min resolution, at every grid point. The procedure allowed us to retrieve the horizontal structure of the tidal motion and to characterize the temporal evolution of tidal current magnitude. The results are summarized in Figs. 4 and 5. Synthesized ellipses provide two major properties of the tidal currents: the orientation and the magnitude of the dominant current. The shape of the tidal current ellipse, referred to as ellipticity, reveals the anisotropy of oscillatory tidal flow and can be quantified by the eigenvalue ratio of the velocity correlation tensor.

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applied to radial velocity data to remove the outliers and fill the gaps lasting less than 40 min (2-sample intervals). After that, the data have been interpolated on a regular grid with 2 km spacing (Fig. 3(b)). As a result, gridded surface velocity fields were obtained at 20-min intervals starting on May 1, 2003. Vector data were collected over the majority of the domain more than 95% of the time. Data return was affected by low signal-to-noise ratio at far ranges, were vector returns dropped to as low as 65%. Grid cells with less than 75% returns were excluded from consideration. They are located on the edge of the radar coverage zone highlighted by shading in Fig. 3(b). The estimated accuracy of measurements is of the order of 5 cm/s for both velocity components. In practice, this accuracy is multiplied by an additional amplification factor depending on the angle between the two intersecting beams, and on the current alignment relative to the beam direction (Prandle, 1991). In the northern part of the radar coverage zone at far ranges, a conventional minimum of 301 between the intersecting beams is not reached, providing additional argument for excluding distant cells from the analysis. The resultant configuration, which met all the data quality criteria, counts 50 grid cells. In these locations, the validated time series are 35 days long and cover the period from 00 h on May 1 (Julian Day (JD) 121) to 23.66 h on June 4 (JD 155). A total of 60 time steps (20 h) in the time series are missing on JDs 141–142 because of data acquisition problem on the Wimereux site.

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Fig. 4. (a) PCA-derived tidal ellipses on May 19, 2003. 12-hour averaged wind is shown in the upper left. (b) Time evolution of ellipses in the particular grid point indicated by asterisk in (a).

Fig. 4(a) reveals several features in the spatial distribution of tidal currents. Ellipse orientation shows that the current is strongly controlled by the topography, producing alignment of the major axes along the depth contours. One can distinguish anisotropy in current field with a relatively high ellipticity observed over the shallow sandbanks located in the middle of the Dover Strait whereas the current magnitude reaches its maximum (2.3 m/s) in the region over the ridge approximately 20 km offshore. Fig. 4(b) shows evolution of the tidal current ellipses over the neap-spring tidal cycle at one particular point located 6 km offshore in the vicinity of 30-m isobath (shown by asterisk in Fig. 4(a)). Pronounced fortnightly variability of the current is the result of superposition of the three semidiurnal constituents (M2, S2, N2), which dominate tidal spectrum in the region. The magnitude of tidal

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attains 0.4–0.45 m/s over the shallow sandbanks in the middle of the Strait of Dover. For calm periods (May 5–9, 15–16, 26–27), an eddy-like structure in the residual flow field could be identified west of the Cape Gris Nez (Fig. 6(a)). The existence of cyclonic eddy in the residual current field was documented in the experimental study of Prandle et al., 1993, and numerical study of Ouahsine et al. (2006).

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Fig. 5. Contour plot of the rotary coefficient, r, derived trough the rotary analysis of the velocity time series; r ranges from 1 for clockwise motion to +1 for anticlockwise motion (r ¼ 0 is unidirectional flow).

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currents increases by about 100% in 7 days, rising from 0.8 m/s on JD 132 to 1.5 m/s on JD 139. Similar evolution of current ellipses was observed at other grid points. To assess spatial variations of the sign of rotation of the tidal velocity vector, we also performed rotary spectral analysis (Emery and Thompson, 1997) of the time series at every grid point. Both clockwise S and anti-clockwise S+ power spectra have pronounced peaks at the semi-diurnal frequency, accounting for 51% and 47% of the total kinetic energy, respectively. Spatial distribution of the rotary coefficient r ¼ (S+S)/(S++S) shown in Fig. 5, reveals two distinct zones with opposite signs of rotation of the tidal current vector, suggesting that tidal motions produce divergent (convergent) surface currents during certain periods of the tidal cycle. Separation line between these two zones tends to follow the 30-m isobath. Fig. 5 also shows that the amplitude of the clockwise component increases in the regions deeper than 40 m, while the shallow regions (sandbanks, shoreline areas) tend to enforce the amplitude of the anticlockwise component. Subtraction of tidal currents from the velocity time series yields an estimate of the residual velocity field. Examination of the residual velocity maps, reveals periods of pronounced correlation with winds (Fig. 6(b)). The magnitude of residual current

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3.3. EOF decomposition Although the PCA technique is seemingly a straightforward and convenient method for separating the tidal signal from low-frequency motions, it may not be efficient in extracting weak, but spatially coherent patterns hidden on the background of strong tides. A common alternative technique is the empirical orthogonal function (EOF) analysis (e.g., Kaihatu et al., 1998), which is basically the velocity field expansion in the eigenmodes of its covariance matrix. The eigenvalue spectrum shown in Fig. 7 reveals three statistically significant (Sirovich et al., 1995) modes lying above the n3/2 power law in a log–log plot of eigenvalue vs mode number n. These three modes together account for 96% of the variance in the dataset. Spatial structure of these modes and the corresponding amplitude time series are shown in Figs. 8 and 9. An individual current speed for each of the three modes can be calculated by multiplying a vector in Fig. 8 by the corresponding a(t) value given in Fig. 9. Mode 1 clearly dominates the spectrum, accounting for 90% of the total variance of the surface velocity field. Evolution of its amplitude (Fig. 9(a)) strongly correlates with SSH recorded at the port of Boulogne (Fig. 1). The amplitude of the first mode is an order of magnitude higher than those of the other EOFs. Apparently, this mode of velocity field variation corresponds to tidal motions. The spatial structure of mode 2 is characterized by convergence of surface currents occurring

approximately 10 km offshore (Fig. 8(b)). The convergence zone has along-coast orientation and matches the deep part of the Dover Strait. This feature is caused by change in the sign of rotation of the surface velocity vector, indicating a transition from a vertically homogeneous to stratified structure of the water column typical for ROFI environments (Souza and Simpson, 1997). Rotation of the velocity vector depends on stratification of the water column and bottom proximity. In shallow wellmixed near-coastal regions with vertically homogeneous density distribution, the rotation is anticlockwise. Further offshore, where stratification comes into play, the rotation changes the sign and becomes clockwise due to baroclinic effects (Visser et al., 1994). The amplitude variation of mode 2 is also dominated by the semidiurnal tides (Fig. 9(b)), but has some pronounced features on longer time scales. Mode 3 is shaped as a cyclonic eddy located west of Cape Gris Nez (Fig. 8(c)). The analogous pattern was obtained first by Prandle et al. (1993) who analysed the year-long measurements of surface currents in the Strait of Dover. It is likely that the eddy is generated by interaction of the strong northeasterly tidal flow with the Cape Gris Nez (Signell and Geyer, 1990). The eastern side of the eddy and the flow in the nearshore sector of the Strait may be affected by the coastal freshwater plume. Time variation of the mode 3 amplitude (Fig. 9(c)) may seem to have some correlation with the freshwater input and wind. However, semidiurnal variations of the signal imply that contribution of tidal motions to this mode remains dominant.

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EOFs do not necessarily sort data into modes corresponding to distinct forcing mechanisms, unless the forcing patterns exhibit enough persistence in time to leave their inprint on EOF modes. In that respect, one may think that strong topographic control of the tidal currents and along-coast confinement of the river plumes may leave their signatures in certain EOFs. To investigate the relationships between the wind/ buoyancy forcing and the variability of the surface velocity field at low frequencies, we conducted correlation analysis between the EOF amplitudes ai(t), i ¼ 1,y3, the corresponding time series of the wind stress s available from the wind speed W(t) measured at the Boulogne light house, and the river

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runoff R. The EOF amplitudes and the wind stress time series were low-pass filtered by applying a 25-h running mean three times successively. River runoff data, R(t), represented by the daily-mean values, were not low-passed filtered. The correlation coefficients, c, were then computed between the series R(tdt), s (tdt) ¼ n W(tdt)|W (tdt)|, and ai(t), where dt is the time lag and n is the 2  2 matrix describing rotation of the wind stress vector with respect to the wind direction observed at the

lighthouse. Both dt and the rotation angle y were varied to maximize the correlation. The hourly data in the filtered time series, ai(t), and W(t) have a nonzero correlation value of 0.10 at a 95% confidence level. For the daily river runoff series the corresponding threshold is 0.32. Results of correlation calculations are assembled in Table 1. According to the table, wind forcing contribution to the variability of different modes appears to be rather different. Major correlation is found for

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Fig. 9. Amplitude time series (dimensionless, left axes) for modes 1 (a), 2 (b), and 3 (c). Tidal elevation in Boulogne is shown by thin line in (a). Bold curves in (b) and (c) are the low-pass filtered time series. River discharge is given by grey shading in (c).

Table 1 Correlation coefficients between the low-frequency variability of the first three EOFs and the wind and freshwater forcing

tx ty R (dt ¼ 170 h)

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0.60 (y ¼ 01, dt ¼ 0 h) 0.85 (y ¼ 01, dt ¼ 0 h) 0.70

0.30 (y ¼ 01, dt ¼ 30 h) 0.25 (y ¼ 51, dt ¼ 30 h) 0.25

0.50 (y ¼ 351, dt ¼ 0 h) 0.60 (y ¼ 351, dt ¼ 0 h) 0.40

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modes 1 and 3. Specifically, the south–north component of the wind stress, which is parallel to the vector field of mode 1, tends to modulate the magnitude of surface current at low frequency (c ¼ 0.85). The cross-shore variability of the surface current field, associated with mode 1, seems to be influenced by the corresponding component of the wind stress. The variability of mode 2 does not seem to be related to wind forcing, with the values of c below 0.30. This is due to a rather variable spatial structure of the vector field of mode 2. Though the current structure described by mode 3 shows also a variation of vector orientation in space, the contribution of wind to the time variability of this mode seems to be statistically significant (anti-correlation of 0.50–0.60 for both components), being in qualitative agreement with the Ekman theory and with the value of y obtained by Prandle (1993) for the same region. In contrast to the wind, correlation with freshwater forcing fails to pass the 95% criterion (0.32) at time lags ranging from 0 to 6 days. Only at the time lag of 7 days, the freshwater input appears to play a role in the variability of modes 1 and 3. Statistically significant anti-correlation (c ¼ 0.70) is found for mode 1, and is less significant (c ¼ 0.40) for mode 3. The value of dt7 days corresponds well with the travel time needed for a freshwater parcel moving with the residual current (0.3 m/s) to reach the region of observations from the mouth of Somme.

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The field most relevant for the transport of material in a water environment is the residual circulation. It is of primary importance in tidally energetic basins such as the English Channel. The knowledge of the residual currents can be used for estimating a long-term transport and dispersion of material in oscillatory tidal flows (Salomon et al., 1995; Bailly du Bois and Dumas, 2005). We estimated the residual velocities by averaging the monitored surface currents over a complete period of radar measurements. The resultant field, shown in Fig. 10(a), includes tidally induced component and also wind and buoyancy forced residual currents. The control of the residual flow by tidal motions is not so evident: although the residual current pattern still keeps orientation along the depth contours, its spatial structure appears to be more rectilinear and less correlated with that of

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mode 1. The most pronounced feature of the residual circulation pattern is the along-coast current intensification characterized by the substantial decrease in current magnitude (from 0.25 to 0.05 m/s) in the offshore direction. Being the consequence of interaction between the buoyancy input and tidal dynamics, this intensification was documented by many authors (Prandle et al., 1993; Sentchev and Korotenko, 2005). It is important to emphasize that the Eulerian residual velocities, shown in Fig. 10(a), are not

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required to be the same as the comparable velocities that passive tracers might be expected to follow. Formally, the Lagrangian description of the velocity field is the most relevant for the purpose of transport diagnostics. This velocity field can be estimated in the framework of Lagrangian tracking (e.g. Sentchev and Korotenko, 2005). In our study such a technique is not applicable because the size of the domain is less than the tidal excursion length. However, the difference between the Eulerian and Lagrangian velocity fields, known as the Stokes velocity vs, can be estimated as vs ¼ /(v . r)vS, where brackets denote time integration over the period of observations. Using a sequence of 2460 surface velocity maps we have estimated the mean vs (Fig. 10(b)). The resultant field reveals a noticeable (up to 0.2 m/s) shoreward Stokes velocity in the French sector of the Dover Strait, suggesting that the two quantities of the flow field, Eulerian and Lagrangian velocities, are not equal here. Our result seems to support the findings of Bailly du Bois and Dumas (2005) who used numerical experimentation to show that tracer pathways in the Lagrangian residual velocity field tend to follow the northeastward direction in the vicinity of Cape Gris Nez. Numerical modelling study of Sentchev and Korotenko (2005) of the transport of material in the EEC also supports this result. Since the Stokes velocity is linearly related to the net energy flux (Prandle et al., 1993), the shoreward veering of vs (Fig. 10) may also be interpreted as a consequence of stronger interaction of tidal currents with bottom topography near Cape Gris Nez. This interaction may cause enhanced dissipation of tidal energy and, as a consequence, convergence of the net energy flux. 4. Discussion and conclusions In this study, we analyse the new measurements of the surface velocity field in the eastern English Channel. The surface currents were investigated by means of various numerical and statistical techniques (principal component, rotary, EOF and correlation analyses) which provided insight into the spatial structure of the local flow driven by a combination of tidal, freshwater buoyancy and wind forcing. The results are complemented by examination of drifting buoy trajectories and CTD measurements. River runoff from the French coast has a significant influence on the near-shore water dynamics in

the EEC. As was emphasized by Simpson (1997), Souza and Simpson (1996b), within such regions, named the regions of freshwater influence (ROFI), buoyant spreading of low-salinity water from a river mouth tends to generate a coastal current and produces a physical regime which is different from those in the offshore regions. The baroclinic flow acts to induce stratification of the water column of the ROFI in competition with the tidal mixing and stirring processes. Winds may also alter the structure of the fresh-water plume due to upwelling and downwelling phenomena (Blanton, 1996; Joordens et al., 2001). We applied a rotary analysis, in combination with a PCA, to the surface velocity time series and found that the tidal currents exhibit opposite rotations across the line roughly following the 30-m isobath. This suggests that tidally induced surface currents are convergent (divergent) approximately 10 km off the Opal coast in certain periods of the tidal cycle. Convergence occurs on ebb, during the falling tide (it starts 3 h before the LW in Boulogne), and divergence on flood, during the rising tide (2 h before the HW). The detected change in sign of rotation of the surface velocity vector indicates transition from a vertically homogeneous to stratified structure of the water column, usually found in the ROFI environments (Souza and Simpson, 1996a, b; Visser et al., 1994). This is confirmed by CTD mesurements at the cross-shore sections occupied by the R/V ‘‘Coˆtes de La Manche’’ in April 2004. The density profiles show a pronounced stratification of the surface 15 m thick layer extending from 8 to 18 km offshore (Sentchev, 2007), whereas spatial distribution of the higher surface salinity gradients (Fig. 12) approximately corresponds to the location of the convergence zone (Fig. 8(b)) thus giving an additional evidence of the importance of freshwater forcing. Baroclinicity decouples the dynamics of surface low-salinity waters from the bottom frictional effects allowing clockwise rotation of the current vector, whereas the rotation is anti-clockwise in the homogeneous waters closest to the shore. This difference in velocity vector rotation in the nearshore and offshore regions are also confirmed by drifter trajectories (Figs. 11 and 12) and examination of instantaneous VHFR surface flow patterns. Drifter B in Fig. 11, travelling within the near-shore region follows the anti-clockwise path, whereas trajectories of drifters A, C and B, travelling farther offshore, show the opposite sign of rotation. The

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Fig. 11. The Argo drifter trajectories. Position of the convergent frontal zone is shown by dotted line. Initial drifter positions are shown by open circles.

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Fig. 12. Salinity (psu) averaged over the 4-m thick upper layer in May 2003. Black circles indicate locations of CTD stations.

significant difference in tidal excursion length inferred from drifter trajectories is related to the fortnightly modulation of tidal currents, analysed in Section 3.1 (Fig. 4(b)). Modification of the current ellipse parameters might also be related to the effect of drying beaches.

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Modelling and HFR observations performed in the Irish Sea by Prandle (1991) revealed shoreward veering of the major axes of current ellipses, accompanied by their enhanced eccentricities and anti-clockwise rotation near the coastline. However, peculiarities of the bottom morphology and water circulation along the Opal coast make it difficult to quantify the effect of drying beaches in generating the offshore current convergence. Better support of the remotely sensed VHFR observations by in situ measurements is required for more complex further exploration of a ROFI impact on coastal water dynamics in this region . The oscillatory character of the surface circulation, in which motion of the cross-shore currents alternates between a convergence and a divergence, tends to generate intensified vertical motions, and a tendency for passive tracers and biological material to concentrate along the observed convergence (frontal) zone. Auxiliary in situ oceanographic observations, conducted in the study area, documented strong semidiurnal variations of salinity, density and chlorophyll-a distribution, supporting a hypothesis of the dynamic structure of the frontal (convergence) zone. Salinity anomalies tend to be advected shoreward and upward during flood, enforcing stratification, whereas during the ebb, they are advected offshore, causing vertical homogenization of the water column and reinforcing horizontal gradients in the density field. The maximum horizontal density gradients are observed during current reversal at low water. The temporal mean field inferred from the VHFR data, contains a signature of the along-coast flow with a maximum velocity of the order of 0.25 m/s and a width of 10–12 km (Fig. 10(a)). EOF analysis of the same data has shown that the first three EOF modes account for 96% of the variance, with the major contribution of the first mode. This mode accounts for tidal currents. Its spatial structure shows clear correlation with topographic features, allowing to conclude that the surface tidal currents are strongly controlled by the bottom topography. A fortnightly variability in the time-depending amplitude of the mode 1 is found as well. Additional statistical analysis has also shown that along-coast winds may significantly affect the magnitude of the oscillatory currents represented by mode 1. Buoyancy forcing affects motions associated with the second EOF. Its spatial structure is characterized by a convergence zone aligned along the coast and located at the distance of 8–10 km offshore. On

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the semidiurnal time scale this mode is characterized by a discontinuity in the surface velocity field induced by the change in sign of rotation of the tidal ellipses across the convergence zone. The rotary analysis of velocity time series confirms this behaviour of tidal currents, although we did not find a statistically significant correlation of mode 2 with river runoff. We explain this by a relatively weak freshwater discharge (four times less than the maximum discharge value observed in 2003), and the absence of an apparent tendency in its variation during the period of radar experiment (Fig. 2(b)). The spatial structure of mode 3 resembles an eddy-like pattern located west of Cape Gris Nez (Fig. 8(c)). This feature is found to be consistent with previous studies of Prandle et al. (1993), who derived a similar pattern through the analysis of the year-long measurements of surface currents in the Strait of Dover, and Ouahsine et al. (2006), who studied the tidal circulation in the EEC numerically. According to Signell and Geyer (1990), such a structure of the current field could be generated by the interaction of the tidal flow with Cape Gris Nez. Temporal variation of the mode-3 amplitude demonstrates modulation by wind forcing, variable strength of the tide, and, to minor extent, freshwater input (Table 1). However, semidiurnal variations of the signal (Fig. 10(c)) confirm that contribution of tidal motions to this mode remains dominant. A combination of the principal component, EOF and correlation analyses of the VHFR data allowed us to assess the role different forcing terms in lowfrequency variability of near-shore velocity field. We demonstrated that the freshwater input plays an important role in generating the convergence of surface currents oscillating at tidal frequency. The documented features of the variability of currents off the Opal coast are similar to the baroclinically forced evolution of the flows previously observed in the ROFI systems on the other shelves: near the mouth of Rhine (Souza and Simpson, 1996b), in the Chesapeake Bay (Marmorino et al., 1999), and other rivers and estuaries (Hyder et al., 2002; Kasai et al., 2002). Therefore, hydrodynamic environment along the Opal coast could be classified as a ROFI system in the hypertidal regime, where tidally forced component exceeds by an order of magnitude the baroclinically forced flows. The existence of surface current convergence front 8–10 km off the Opal coast is the major finding of the present study. This feature is

documented by the VHFR observations and evidenced by the cross-frontal change in sign of the rotation of tidal currents. The convergence zone has a tendency to follow the 30-m isobath. The mean velocity field, derived from the radar measurements, contains a signature of the coastal flow induced by coupling between the buoyant spreading of lowsalinity water and tidal dynamics. A cyclonic eddy, generated by interaction of the strong northeasterly tidal flow with the Cape Gris Nez, is detected by the VHFR observations as well. In the present study we have documented and qualitatively described only the major features of tidal circulation off the Opal coast of France: tidal current convergence and the near-shore intensification of residual currents. To quantify physical mechanisms of the ROFI impact on the regional circulation further research is needed. These studies would require better support of the remotely sensed VHFR observations by in situ data. To provide the best efficiency for future monitoring, the data sets should be synthesized with dynamical information supplied by numerical coastal models in the framework of the data assimilation technique. Implementation of this approach is a subject of our further research. Acknowledgements The authors acknowledge funding support by the Universite´ du Littoral and the Japan Agency for Marine–Earth Science and Technology (JAMSTEC), through its sponsorship of the International Pacific Research Center. Part of the work was completed when one of the authors (MY) was visiting the Universite´ du Littoral with support from the French Ministry of Education and Research. This study was conducted as a part of the PNECManche project. We gratefully thank the scientific staff at the Universite´ de Toulon, Pierre Broche, Philippe Forget, Yves Barbin and Joel Gaggelli, for planning the radar experiment and their contribution to data processing. This manuscript is IPRC/ SOEST Contribution number 461/7135. References Bailly du Bois, P., Dumas, F., 2005. Fast hydrodynamic model for medium- and long-term dispersion in seawater in the English Channel and southern North Sea, qualitative and quantitative validation by radionuclide tracers. Ocean Modelling 9 (2), 169–210.

ARTICLE IN PRESS A. Sentchev, M. Yaremchuk / Continental Shelf Research 27 (2007) 2449–2464 Blanton, J., 1996. Reinforcement of gravitational circulation by wind. In: Aubrey, D.G., Friedrichs, C.T. (Eds.), Buoyancy effects on coastal and estuarine dynamics, In: Coastal and Estuarine Studies, vol. 53. AGU, Washington, DC, pp. 47–58. Breivik, O., Sætra, O., 2001. Real time assimilation of HF radar currents into a coastal ocean model. Journal of Marine Systems 28, 161–182. Broche, P., Salomon, J.C., Demaistre, J.S., Devenon, J.L., 1986. Tidal currents in the Baie de Seine: comparison of numerical modelling and HF radar measurements. Estuarine, Coastal and Shelf Science 23, 465–476. Broche, P., Forget, P., Devenon, J.L., Crochet, M., 1987. VHF radar for ocean surface current and sea state remote sensing. Radio Science 22, 69–75. Dyer, K.R., 1997. Estuaries, A Physical Introduction, second ed. Wiley, Chichester, 195pp. Emery, W.J., Thompson, R.E., 1997. Data Analysis Methods in Physical Ocenography. Pergamon, NY, 634pp. Estournel, C., Broche, P., Marsaleix, P., Devenon, J.-L., Auclair, F., Vehil, R., 2001. The Rhone river plume in unsteady conditions: numerical and experimental results. Estuarine, Coastal and Shelf Science 53, 25–38. Haus, B.K., Wang, J.D., Rivera, J., Martinez-Pedraja, J., Smith, N., 2000. Remote radar measurement of shelf currents off Key Largo, Florida, USA. Estuarine, Coastal and Shelf Science 51, 553–569. Hyder, P., Simpson, J.H., Christopoulos, S., Krestentis, Y., 2002. The seasonal cycles of stratification and circulation in the Thermaikos Gulf Region Of Freshwater Influence (ROFI), north-west Aegean. Continental Shelf Research 22, 2573–2597. Joordens, J.C., Souza, A.J., Visser, A., 2001. The influence of tidal straining and wind on suspended matter and phytoplankton distribution in the Rhine outflow region. Continental Shelf Research 21, 301–325. Kaihatu, J.M., Handler, R.A., Marmorino, G.O., Shay, L.K., 1998. Empirical orthogonal function analysis of ocean surface currents using complex and real vector methods. Journal of Atmospheric and Oceanic Technology 15 (8), 927–941. Kaplan, D.M., Largier, J., Botsford, L.W., 2005. HF radar observations of surface circulation in Bodega Bay (northern California, USA). Journal of Geophysical Research 110, C10020. Kasai, A., Fujiwara, T., Simpson, J.H., Kakehi, S., 2002. Circulation and cold dome in a gulf-type ROFI. Continental Shelf Research 22, 1579–1590. Kovacevic, V., Gacic, M., Mosquera, I.M., Mazzoldi, A., Marinetti, S., 2004. HF radar observations in the northern Adriatic: surface current field in front of the Venetian Lagoon. Journal of Marine Systems 51, 95–122. Lafite, R., Shimwell, S., Grochowski, N., Dupont, J.-P., Nash, L., Salomon, J.-C., Cabioch, L., Collins, M., Gao, S., 2000. Suspended particulate matter fluxes through the Straits of Dover, English Channel: observations and modelling. Oceanologica Acta 23 (6), 687–700. Marmorino, G.O., Shay, L.K., Haus, B.K., Handler, R.A., Graber, H.C., Horne, M.P., 1999. An EOF analysis of HF Doppler radar current measurements of the Chesapeake Bay buoyant outflow. Continental Shelf Research 19, 271–288.

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Marmorino, G.O., Cooper, A.L., Mied, R.P., Lindemann, G.J., Trizna, D.B., Porter, D.L., 2004. Onshore propagation of a buoyant ocean front observed using a shore-based marine radar. Continental Shelf Research 24, 951–964. Ouahsine, A., Smaoui, H., Sentchev, A., 2006. Modelling of tide and tidally induced hydro-sedimentary processes in the eastern part of the English Channel. Journal of Marine and Environmental Engineering 8, 263–297. Prandle, D., 1991. A new view of near-shore dynamics based on observations from HF Radar. Progress in Oceanography 27 (5–6), 403–438. Prandle, D., 1993. Year-long measurements of flow through the Dover Strait by HF radar and acoustic Doppler current profilers (ADCP). Oceanologica Acta 16 (5–6), 457–468. Prandle, D., Player, R., 1993. Residual currents through the Dover Strait measured by HF radar. Estuarine, Coastal and Shelf Science 37, 635–653. Prandle, D., Losch, S.G., Player, R., 1993. Tidal flow through the Straits of Dover. Journal of Physical Oceanography 23 (1), 23–37. Prandle, D., Ballard, G., Flatt, D., Harrison, A.J., Johnes, S.E., Knight, P.J., Losch, S., McManus, J., Player, R., Tappin, A., 1996. Combining modelling and monitoring to determine fluxes of water, dissolved and particulate metals through the Dover Strait. Continental Shelf Research 16 (2), 237–257. Salomon, J.C., Breton, M., Guegueniat, P., 1995. A 2D long term advection–dispersion model for the Channel and southern North Sea. Part B: Transit time and transfer function from Cap de La Hague. Journal of Marine Systems 6 (5–6), 515–527. Sentchev, A., 2007. The influence of freshwater input on current dynamics and tracer concentration variability in the eastern English Channel. Marine Pollution Bulletin submitted for publication. Sentchev, A., Korotenko, K., 2005. Dispersion processes and transport pattern in the ROFI system of the eastern English Channel derived from a particle-tracking model. Continental Shelf Research 25, 2294–2308. Sentchev, A., Yaremchuk, M., 1999. Tidal motions in the Dover Straits as a variational inverse of sea level and surface velocity data. Continental Shelf Research 19, 1905–1932. Sentchev, A., Yaremchuk, M., Lyard, F., 2006. Residual circulation in the English Channel as a dynamically consistent synhtesis of shore-based observations and currents. Continental Shelf Research 26, 1884–2004. Shay, L.K., Lentz, S.J., Graber, H.C., Haus, B.K., 1998. Current structure variations detected by High-Frequency radar and vector-measuring current meters. Journal of Atmospheric and Oceanic Technology 15, 237–256. Signell, R.P., Geyer, W.R., 1990. Numerical simulation of tidal dispersion around a coastal headland. Coastal and Estuarian Studies 38, 210–222. Simpson, J.H., 1997. Physical processes in the ROFI regime. Journal of Marine Systems 12, 3–15. Sirovich, L., Everson, R., Manin, D., 1995. Turbulence spectrum of the Earth’s ozone field. Physical Review Letters 74, 2611–2614. Souza, A.J., Simpson, J.H., 1996a. Interaction between mean water column stability and tidal shear in the production of semi-diurnal switching of stratification in the Rhine ROFI, 1996. In: Buoyancy Effects on Coastal and Estuarine

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Dynamics. In: Coastal and Estuarine Studies, vol. 53, pp. 83–96. Souza, A.J., Simpson, J.H., 1996b. The modification of the tidal ellipses by stratification in the Rhine ROFI. Continental Shelf Research 16, 997–1007.

Souza, A.J., Simpson, J.H., 1997. Controls of stratification in the Rhine ROFI system. Journal of Marine Systems 12, 311–323. Visser, A.W., Souza, A.J., Hessner, K., Simpson, J.H., 1994. The effect of stratification on tidal profiles in a region of freshwater influence. Oceanologica Acta 17, 369–381.