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The current dynamics inside the Strait of Bonifacio: Impact of the wind effect in a little coastal strait
Continental Shelf Research 31 (2011) 1–8
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The current dynamics inside the Strait of Bonifacio: Impact of the wind effect in a little coastal strait O. Gerigny , B. Di Martino, J.C. Romano UMR SPE CNRS 6134 , Universite´ de Corse, 20250 Corte, France
a r t i c l e in f o
abstract
Article history: Received 22 March 2010 Received in revised form 4 November 2010 Accepted 16 November 2010 Available online 25 November 2010
The dynamics of Bonifacio strait (south of Corsica) are investigated in a comprehensive study at long term from two set of data. First, by moored ADCP put in middle of the strait during six weeks (4 November 2004 to 7 January 2005), at a depth of 40 m and at few Kilometers from coast. Second, by velocity profiles obtained with a 314.4 kHz Acoustic Doppler Current Profiler (ADCP) during seven cruises (Cyrce) covering entirety strait. From moored ADCP data, a study is made on time correlation between wind stress and current. Shipboard ADCP data permits to obtain a general view in totality of strait and mostly to observe spatial variability. This study shows that the circulation is governed both by wind stress and by general circulation of Mediterranean Sea. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Wind Current data Coastal zone Moored ADCP Shipboard ADCP Strait of Bonifacio
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Data collection and processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Tidal current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Current data arising from profiler bottom-moored current meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. General scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Case study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Current data arise to boarded ADCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction The straits and passages of Mediterranean Sea are keys regions because they contribute to the understanding of Mediterranean basin-scale circulation. In fact, they act on the general circulation by controlling the water mass-transport (Beranger et al., 2005). The previously studies and literature showed that the inflow and outflow of water mass from basin to basin are characterised by
Corresponding author. Tel.: +33 4 95 45 00 41.
E-mail address: [email protected] (O. Gerigny). 0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.11.005
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low frequency and seasonal variability (Astraldi et al., 1999) with high variability of the current through these passages, particularly for little coastal area where there was many coastal meanderings (Plus et al., 2009). Circulation in strait could be governed by different process: atmospheric, tidal action, wind stress (the complex role of wind which mainly controls the surface circulation), topographic features and coastal geometry (which are obstacles to the mean circulation) and thermohaline boundary forcing. So the circulation in strait was complex and many studies were required to understand all the physical process playing a crucial role in water mass transport with a direct impact on ecological issues. This dynamics of circulation in coastal area and particularly inside
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Fig. 1. Strait of Bonifacio, south of Corsica.
pattern. Two types of current data were available: the first arise from profiler bottom-mounted ADCP and second arise from shipboarded ADCP. Profiler bottom-mounted ADCP, was deployed by the CETMEF-DDE (Direction Departemental de l’Equipement) during one and a half month on winter (to 24 November 2004 to 07 January 2005), and collected about 63 390 data scans. This bottom-mounted ADCP was installed at 40 m of depth, in the middle of the strait (near the Lavezzi Island—41.201N and 9.151E) and recorded, each 30 min, vertical profiles (30 levels) of current speed (in m s 1) and direction (in degree) along the water. The first level was 1.4 m above current meter and each level were separated by 1 m. ADCP system was equipped with pressure sensor that gave the height water in meter used for study the tide. In the major part of the figures and analysis, any filter was used for processing data. When statistical analysis was necessary, a specific tools concerning angular data. Indeed, the classical statistics developed for the analysis of linear data are not directly applicable to directional data. A standard average between several directions would not be right because these directions are taken between (an arbitrary) 0 and 3591. The same problem exists also when the standard deviation is calculated from a current direction distribution. To solve this problem, the averages and standard deviations of angular dataset was determined using a statistical analysis of circular data (Fisher and Lee, 1983; Fisher, 1993; Bowers et al., 2002) that are briefly described. Measured angles are noted yi . The value of the coefficient Cc, Cs and Rc was computed according to the following formulae: Cc ¼
N X
cosðyi Þ,
Sc ¼
i¼1
N X
sinðyi Þ,
Rc ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðCc2 þ Cs2 Þ
i¼1
The circular mean direction mc , the circular variance Varc and circular standard deviation sc were then given by 8 1 if Sc 40, Cc 4 0 > < tan ðSc =Cc Þ, mc ¼ tan1 ðSc =Cc Þ þ p, if Cc o 0 > : tan1 ðS =C Þ þ 2p, if S o0, C 40 c c c c
2.2. Data collection and processing Varc ¼ 1 Wind data were obtained from Meteo-France (Pertusato’s station) between 1981–1990, and during the current record period, with frequency record every hour, in order to describe the wind
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The Strait of Bonifacio (SoB) is located in the North-western Mediterranean sea, between Corsica and Sardinia islands where the Liguro-provencal and Tyrrhenian basins communicate (between 8.81E and 9.51E and 41.31N and 41.61N, see Fig. 1). Narrowed of 14 km, the strait of Bonifacio dwell on 80.000 ha (Mouillot et al., 2008) and is dotted with many islands and islets. Along the Corsica west coast the continental shelf is relatively narrow with the 20 m isobath near the coastline and with the 100 m one which reached on average at 12 km of the coast. On the opposite, the Corsican east coast presents a wider continental shelf and the 100 m isobath is currently located at 25 km off the coast.
Corsica
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straits were complex but essential for many questions concerning environmental management (fisheries, tourism, conservation of marine fauna and flora, possible environmental accident, larva retention and transport Narvaez and Valle-Levinson, 2008). The main hydrodynamic features of currentology in the Northwestern Mediterranean sea were previously described and discussed in many studies (Millot, 1990, 1999) but at large scales of space and time. At the west of south Corsica coastline, a current comes from Atlantic water, enter by Gibraltar strait (with colder characteristic), and goes up to north, while along the east coast a current comes from the Tyrrhenian current goes up to south (water more cold and salt). At a smaller scale some current studies have included the dynamic and water balance between the two basins through the Strait of Bonifacio (SoB, south of Corsica) (Millot, 1987; Ovchinnikov, 1974) and the marks left by deep currents near the sea-floor on the sediments in the SoB by using sonar imagery (Pluquet, 2006). These informations confirm the presence of a north current along the west part of the strait and of a south current on the east. The main current circulation crosses the middle of the strait from west to east. But, there was not available data about water circulation and winds which are the main forcing factor for marine currents. Moreover in this study area, many critical environmental problems exist. On one hand, the presence of the biggest natural park in France which high level of protection, and on the other an intense commercial ship movements cruising by this international maritime way, and most of them carrying dangerous materials. For a policy of environmental management such as, it is very important to know the general current circulation and to have tool for management decision. In order to improve knowledge and data base, several investigations were developed. Two are reported in this paper. First: the UMR SPE CNRS no 6134 (University of Corsica) started in 2004 the Cyrce programme (collection of currentologic data with boarded ADCP (Acoustic Doppler Current Profiler) and hydrological data. The second collect of currentologic data was provided by bottommounted ADCP which was installed in middle of Strait by DDE (Direction Departemental de l’Equipement). In relation to an exhaustive collection of wind conditions, our aim was to describe the main features of water circulation in SoB by (1) determining what were the main influences over this circulation, (2) understanding the wind/current relationships, in surface and in the water column (3) by illustrating this circulation and interactions by some case studies.
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Rc , N
sc ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2logð1Varc Þ:
All the treatments proposed in this paper use these previous definitions for the statistical analysis circular data.
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All boarded ADCP measurements were carried out within the CYRCE framework (the Current and hYdrology programme of the Corsican Region) and seven cruises were performed onboard the OV-Thethys II (INSU). Measurements were taken using the boarded ADCP (Teledyne RD Instruments—broadband VMBB 314.4 kHz) at a cruise speed of eight knots as recommended by Petrenko et al. (2005) supported by differential-GPS positioning. In all cruises, the same configuration was adopted: 60 cells (4-m depth each), with an averaging process every minute (in realtime automatic control procedures) and bottom tracking when possible. Precision of the ADCP data is given as higher than 0.02 m s 1. For each CYRCE cruise, a first raw-data analysis was performed by the INSU’s Technical Division and all measurement errors were eliminated (Petrenko, 2003; Petrenko et al., 2005; Leredde et al., 2007). The current data was processed using Matlab 7.5.0 (R2007b), in association with the data-treatment software ‘‘V.4.lite’’ (created by INSU), and Microsoft Excel (with Analysis Tool kit for statistical analysis). In consequence of sensors precision, the wind and current speed values were rounded off. 3. Results and discussion 3.1. Winds Subject at Mediterranean climate (sub-humid at temperate winter), SoB is a particularly windy region where winds blow 328 days by year (171 days with a wind 4 16 m s1 , (Office de l’Environnement de la Corse, 2007, Mouillot et al., 2008).The wind regime in this region of South Corsica is first dominated (about 50 percent of wind regime) by westerly wind during all the year (called Libecciu, blowing between 2601 and 3001, with speeds generally superior at 8 m s1 ), and secondly dominated by easterly wind (about 30 percent of wind regime) blowing during winter (called Gregale, include between 601 and 1001, in majority between 5 and 8 m s 1). These violent winds (only 2 percent were smaller at 2 m s 1) are the main cause of strong currents in this area. During period record of bottom-mounted ADCP, mean velocity was 7 m s 1, standard deviation of 4.09, and the most-frequent direction is Libecciu. We supposed that Pertusato wind is represented of the wind on area to disregarding the local effect due to the topography same they could be tender (sensible) in area.
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3.2. Tidal current In Mediterranean Sea, the tidal flows could be considered almost rectilinear, with a clear predominance of the west–east component of current velocity over the south–north component (Mananes et al., 1998; Pillsbury et al., 1987) with non-linear phenomena affecting tidal. By virtue of his configuration (west– east), it was possible that currents in SoB are influenced by the tidal movement. In spite of low tide in Mediterranean Sea, the height of water was studied to understand the role of tide in current circulation. Fig. 2 presented (a) the Fast Fourier Transform filter (FFT) on water height, (b) the FFT on speed wind data and (c) the FFT on speed current data through water column. On first FFT (2a), two harmonics are observed corresponding at classical semidiurnal tidal (M2) and diurnal tidal (K1) (with for respective amplitude lower 0.1 and lower 0.05). On the current speed FFT (2c), the more important frequency are lower than 1 by day, and a significant signal can be observed around the 2 by day. At low frequency (lower to 1 cycles per day), a mass of energetic peaks were observed which were due to a low frequency forcing coming from circulation at big scale in western Mediterranean and Tyrrhenian basins. The influence of non-linear effects in their dispersions was accentuated by coastal phenomena (coastal morphology and bottom proximity, (Valle-Levinson et al., 2007). Even if the peak at 2 by day could be associated with the diurnal tide, doubt persist because there is any significant peak associated to a semidiurnal tide (which should be around 4 by day) though the more important than one. On the other hand, this peak could not be associated with barometric phenomenon (record during winter season) or direct wind effect since the FFT of wind data (2b) do not presents significant peak at this frequency. In every cases, the tide effect can be associated to a maximum current velocity from 3 to 5 cm s 1. Later in this study, the tidal impact will be neglected compared to other physical phenomena. 3.3. Current data arising from profiler bottom-moored current meter 3.3.1. General scope Surface speed average was of 50 cm s 1, resulting from strong currents but with high standard deviation 28 cm s 1: over all the measurement period, (the lowest speed of 2 cm s 1 and the
Fig. 2. (a) The Fast Fourier Transform filter (FFT) on water height, (b) the FFT on wind data and (c) the FFT on current data through water column.
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Fig. 3. (1) To 1 at 500 h of recording: wind and current speed, (a) wind and current surface speed, (b) current speed in column water. (2) To 500 at 1056 h of recording: wind and current speed, (a) wind and current surface speed, (b) current speed in column water.
strongest current of 146 cm s 1). Figs. 3 and 4 show the speed and direction of currents and winds (with Fig. 3(1) and Fig. 4(1) to 1 at 500 h of recording and Fig. 3(2) and Fig. 4(2) to 500 at 1056 h of recording): (a) in the surface layer and (b) current data in the whole water column. The curve of Fig. 3(a) showed the strong variability of surface current dynamic. Deeper, with an average value of 16 cm s 1 in association with an high standard deviation value (12 with a minimum of 0 cm s 1 and a maximum of 81 cm s 1) the no-normal distribution of deep current speed exhibits also a high variability of depth current speed. As expected, the average current speed decreased with depth (Fig. 3). This decrease is of linear type and the difference between surface and deep-layer current speed is significant at p o 0:01 level when tested by a one-factor ANOVA analysis. Two major directions of current were identified: the first is centred on 701 and second on 2301 (see Fig. 4), in correspondence of wind direction dominance. The directions have only few light variations in water column and appeared to be strongly constrained by the bathymetry and the topography of the studied
marine area and did not show the classical pattern of Eckman spiral. Current direction variability decreased when current speed increased (Fig. 5) so the more the current were strong, the least their direction varied. 3.3.2. Case study To understand the general viewing of this hydrodynamic system, we analysed data by study of some particular cases. Links can be identified (Fig. 3a) between surface current and wind speed, but not always by the same way: strong winds are associated to high current intensities, but also with low currents, and, on the contrary, strong currents may appear with low winds. We identified two categories: the ones may be considered as basic cases because they concern all the studied area and the second no basic cases, can be detected at a local space scale. Of course, inside the basic cases, the correlation between wind and current speed was obvious, but a finer analyse of these temporal datasets showed well the complexity of hydrodynamic area.
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Fig. 4. (1) To 1 at 500 h of recording: wind and current direction, (a) wind and current surface direction, (b) current direction in column water; (2) To 500 at 1056 h of recording: wind and current direction, (a) wind and current surface direction, (b) current direction in column water.
3.3.2.1. Basic cases. The situation may be considered as basic when, first the current speed follows the wind speed, and second, when low winds are associated with erratic currents. The most-classically cases are when the changes in current speed follow those of wind, as presented in Fig. 3: current speed increase classically follows a wind speed (as example between 200 and 310 h or from to 1 to 3 December 2004), isolated by dotted line in Fig. 3(1)(a), within a slight lag time. After statistical tests performed on current speed and wind (in cases of wind speeds 4 7 m s1 ), the Ho hypothesis can be rejected exhibiting a positive correlation between current speed and wind speed in the whole water column (r ¼0.23 for surface current and r ¼0.35 for depth current, where r was the correlation coefficient, n¼543). In this set of data, surface current and wind directions are strongly linked (Fig. 4, with r ¼ 0.46 for the surface and r ¼ 0.35 for n ¼543, the bottom (the negative sign is only the consequence of the conventional inverse formulation for the directions: ahead for the currents, origin for the winds)). Another case was identified between times 300 and 450 (from 5 to 11 December 2004), where the current speed was directly
correlated with wind speed, the two curves being roughly parallel. The second kind (or type) of situation which may be considered as basic are the cases where low winds are associated with erratic currents (they are named here as erratic since they exhibit strong direction variability at a small scale of time and space, Fig. 6). In these cases, there is none correlation between respective wind and current speed. But, as presented in Fig. 5, lower is the wind speed and higher is the dispersion of current direction. In this figure each point is the average of respective twenty adjacent data for, respectively, mean of wind intensities and standard deviation of current directions; and the exponential fit of relationship is of exponential form and significant at p 0:05 level (n¼ 52, R2 ¼0.23, unilateral test). When strong wind intensities clearly determine current high speeds and cohesive directions, on the contrary, during light wind periods the energy applied to surface water is no sufficient to cause a main current direction. 3.3.2.2. Some nonbasic cases. But many situations recorded during the studying period may be referred to the above-described basic
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along depth. This nonbasic case it present at several occasions (as another example, between time 740 and 780, from 23 to 25 December 2004). 3.4. Current data arise to boarded ADCP
Fig. 5. Distribution of standard deviation of current direction (degrees) versus mean current speed (cm s 1). Variability of current direction reduces when current speed rises.
Fig. 6. Distribution of standard deviation of current direction (degrees) versus mean wind speed (m s 1). Variability of current direction reduces when wind speed rises.
cases. The first nonbasic case is occurred between 540 and 630 h (from 15 to 19 December 2004, Figs. 3(2) and 4(2), unshaded area). Two strong NW wind peaks were recorded but if the current speed follows the first wind peak (with the respective directions of 3001 for the wind and 1201 for the current) it did not occurred during the second peak. The current speed remained constant in the water column with a weak standard deviation value. And current direction was inverted from the previous peak (orientation around 3001 and constant along the water column), whereas the wind direction has not changed (oriented by 3001, west–east), as compared to the first peak. We may consider these to opposite current evolutions under the same wind condition as the consequence of probable pressure currents. The second type of situation with a no-coupled wind and current evolution was recorded, as example, between time 890 and 920 or from 29 to 30 December 2004) (Figs. 3.2 and 4.2). Surface current speed was strong with low wind speed. Since this surface current is uncoupled to wind forcing and the tidal currents being previously removed, we may consider that these situations are the consequence to water mass balance between the two basins, through the Bonifacio strait, mainly caused by the general circulation of N–W Mediterranean Sea and enhanced by the strait narrowness. This explanation is accorded to the facts that there was low variability of current speed shows a low variability in water column and the current directions remain constant
Two views of ADCP current profiles, covering the whole study area on both sides of the strait, were presented in Fig. 7, each of them in two figures. These maps are not instantaneous views; then they are realised during the several hours of each ship transit. Their spatial information complements the time information provided by the moored ADCP located at one point. The two figures for each transit correspond, respectively, to the current surface at 16 m (bin 3 of ADCP) on the left and to the current at 40 m depth (corresponding to bin 10 of ADCP—around the deepest bottom in the middle of the strait) on the right. In the two upper figures (21 July 2005) a wind of 20 knots (about 35 km/h) is blowing with a direction between of 2601 and 2901 and swell sea; in the two lower maps (11 March 2005), a wind of 10 knots was established from 951 to 1681 with calm sea. The main feature of this comparison is the differences appearing between surface and deep currents on the two set of maps. On the upper figures the high speed of surface currents strongly decreases with depth and many changes in direction exist between the two levels (particularly in the west part of the strait and by the strait itself). On the opposite, on the two lower figures (light winds), the speed decrease is much weaker between surface and deep current; and if some changes in current directions can also be noted, in this case they are rarer. In the first case, the influence of strong wind induced ‘‘wind currents’’ on surface waters which are rapidly decreasing with depth. During the second episode, all the water layers are concerned by the same flow which is probably by the general circulation around Corsica and exchanges though the SoB. These two types of situation were previously deduced from the wind and current time-series of the bottom-moored ADCP data. The second noticeable features of these maps are the frequent inversions of the current directions when offshore-to-coast perpendicular recording transits are examined, and in the surface as well as deep layers. As evidence this is the consequence of the various perturbations which are induced by changes in bathymetry and coastline upon the water masses flow. By the way, this is particularly true in the middle of the strait, which is its narrowest and shallowest part.
4. Conclusion This study is in keeping with long term program to study the SoB circulation, with final target to validate and improve, by in-situ data from moored ADCP and shipboard ADCP, a numerical model, on short space and time scales (in the order of few kilometers). Generally models are validated by observations at long scale from satellite observations for try to validate the functioning of this model at short scale, near coast. In fact, these satellite observations are not specifics, it is necessary to add in-situ and local data. In this paper, a first analyse of a short recorder period is performed. This analysis gives a first general vision on area circulation but also first results on wind–current correlation effect. This study shows: (i) Two prevailing winds in area (west and north–east winds), (ii) Tides have a very light influence on current amplitude, (iii) A first description of general tendency current in SoB, by moored ADCP data, is made by analysed different situation of wind and current, these cases are divided in two categories: ‘‘basic cases’’ and ‘‘nonbasic cases’’. Study shows presence of strong current intensities and the latest decrease with depth. The current directions are continual in water column because of light depth in
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18’
−100 −200 −400
0
−1
41 N 24.00’
30cm/s
−2 0
50
o
0
36’
50
−50
−1−05 00
0 −20 −400
30’
−50
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00
−1
36’
0 −80
30cm/s
−4
0 −10
000 15 −−50
42’
−600
−1
−400
42’
7
48’
9oE
12’
24’
36’
Fig. 7. (a) Maps of current recording 21/07/2005 at 10 m, (b) Maps of current recording 21/07/2005 at 40 m, in strong sea conditions, (c) Maps of current recording 11/03/2005 at 10 m, (d) Maps of current recording 11/03/2005 at 40 m, in calm conditions.
area, and two prevailing current directions are present and corresponded to wind direction nevertheless the correlation between wind/current was not constant since their intensities are correlated on the whole water column only for strong winds ð 4 7 m s1 Þ. In many basic cases, wind and current are strong and orientated in the same direction (correlation between current/ wind directions on the whole water column). Basic cases correspond to cases where firstly the current speed follows the wind speed where some dynamics appear between wind and current probably showing inertial effect between each other, however, this part is not detailed because short record period not permit to do statistic treatment. Other important point of this basic case is that variability of current direction decreases when current speed grows. So the stronger the currents are, the less their directions vary in time. The second basic case process a situation of low wind with erratic currents, for this latter it has be noted a decreased variability current direction when the wind speed increased. The no basic cases present other situation where current dynamic are not generate by wind stress with strong wind with low current or low wind and strong current, probably due to a general circulation at large scale or pressure current (water balance between two basins). (iv) Then, these results are completed and compared at shipboard
ADCP data, which give other vision in totality of the strait but also and mostly gives spatial variability. At the west of the coast, there is an eddy anticyclone probably the result of the movement of water to north, this water comes from Gibraltar and may have an Atlantic origin. In the middle of the strait, currents intensify being probably the consequence of Ventury effect. The tightening of SoB, was did at two levels: horizontally with the reduction of the strait between Corsica and Sardinia and vertically with the reduction of the bathymetry in the middle of the channel. These accelerations in the middle of the strait and strong winds are a cause of difficult boat circulation across the strait. Since tide did not influence the current, so the circulation is governed as well as wind stress that general circulation in Mediterranean Sea. This circulation is forced by geological morphology and topography of this area. First model trials will be quickly compared to cases tackled here, however, the recorded data period being relatively short it shows require increasing of data number as well as in middle of the strait that in other local area. At long time moored ADCP (for more than two years) has been installed in the marine park. A better knowing of area circulation, in particular thank to simulation model, could be beneficial in several fields: notably for dangerous matter transport including a risk of
8
O. Gerigny et al. / Continental Shelf Research 31 (2011) 1–8
pollution of the natural reserve. If pollution should happen (particularity oil pollution) knowledge of the dynamic could improve the struggle strategy; in biologic field about dispersion of fish and crustacean larva could be a potential source of repopulation in occidental Mediterranean Sea; and also management field.
Acknowledgements This study was financed by the Collectivite´ Territoriale de la Corse (CTC). We are grateful to the DDE and also the INSU who have produced the data, as well as all team of Tethys II. References Astraldi, M., Balopoulos, S., Candela, J., Font, J., Gacic, M., Gasparini, G.P., Manca, B., Theocharis, A., Tintore, J., 1999. The role of straits and channels in understanding the characteristics of Mediterranean circulation. Prog. Oceanogr. 44, 65–108. Beranger, K., Mortier, L., Crepon, M., 2005. Seasonal variability of water transport through the Straits of Gibraltar, Sicily and Corsica, derived from a high-resolution model of the Mediterranean circulation. Prog. Oceanogr. 66, 341–364. Bowers, J.A., Morton, I.D., Mould, G.I., 2002. Directional statistics of the wind and waves. Appl. Ocean. Res. 22, 13–30. Fisher, N.I., Lee, A.J., 1983. A correlation coefficient for circular data. Biometrika 70 (2), 327–332. Fisher, N.I., 1993. Statistical Analysis of Circular Data. Cambridge University Press 277pp. Leredde, Y., Denamiel, C., Brambilla, E., Lauer-Leredde, C., Bouchette, F., Marsaleix, P., 2007. Hydrodynamics in the Gulf of Aigues-Mortes, NW Mediterranean Sea: In situ and modelling data. Cont. Shelf Res. 27, 2389–2406.
Mananes, R., Bruno, M., Alonso, J., Fraguela, B., Tejedor, L., 1998. Non-linear interaction between tidal and subinertial barotropic flows in the Strait of Gibraltar. Oceanol. Acta 21, 33–46. Millot, C., 1987. Circulation in the western Mediterranean sea. Oceanol. Acta 10 (2), 143–149. Millot, C., 1990. The Gulf of Lions’ hydrodynamics. Continental Shelf Research 10, 885–894. Millot, C., 1999. Circulation in the western Mediterranean sea. J. Mar. Syst. 20, 423–442. Mouillot, D., Culioli, J.M., Pelletier, D., Tomasini, J.A., 2008. Do we protect biological originality in protected areas? A new index and an application to the Bonifacio Strait natural reserve. Biol. Conserv. 141, 1569–1580. Narvaez, D.A., Valle-Levinson, A., 2008. Transverse structure of wind-driven flow at the entrance to an estuary: Nansemond river. J. Geophys. Res.—Oceans 113. Office de l’Environnement de la Corse, 2007. Plan de gestion de la Reserve naturelle des Bouches de Bonifacio, 2007–2011. Office de l’Environnement de la Corse, De´partement Espaces Naturels et Prote´ge´s, 157p + Annexes. Ovchinnikov, I.M., 1974. On the water balance of the Mediterranean sea. Oceanology 14 (2), 198–202. Petrenko, A., 2003. Variability of circulation features in the Gulf of Lion NW Mediterranean Sea. Importance of inertial currents. Oceanol. Acta 26, 323–338. Petrenko, A., Leredde, Y., Marsaleix, P., 2005. Circulation in a stratified and windforced Gulf of Lions, NW Mediterranean Sea: in situ and modelling data. Cont. Shelf Res. 25, 7–27. Pillsbury, R.D., Barstow, D., Botero, J.S., Millero, C., More, B., Pittock, G., Root, D.C., Simpkims III., J., Still, R., Bryden, H.L., 1987. Gibraltar experiment: current measurements in the Strait of Gibraltar. Technical Report 87, 29–284, Oregon State University. Pluquet, F., 2006. Evolution re´cente et se´dimentation des plates-formes continentales de la Corse. Ph.D. Thesis, University of corsica, French, 300pp. Plus, M., Dumas, F., Stanisie re, J.-Y., Maurer, D., 2009. Hydrodynamic characterization of the Arcachon bay, using model-derived descriptors. Cont. Shelf Res. 29, 1008–1013. Valle-Levinson, A., Blanco, J.L., Frangopulos, M., 2007. Depth-dependent overtides from internal tide reflection in a Glacial Fjord. Estuar. and Coasts 30, 127–136.
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The current dynamics inside the Strait of Bonifacio: Impact of the wind effect in a little coastal strait O. Gerigny , B. Di Martino, J.C. Romano UMR SPE CNRS 6134 , Universite´ de Corse, 20250 Corte, France
a r t i c l e in f o
abstract
Article history: Received 22 March 2010 Received in revised form 4 November 2010 Accepted 16 November 2010 Available online 25 November 2010
The dynamics of Bonifacio strait (south of Corsica) are investigated in a comprehensive study at long term from two set of data. First, by moored ADCP put in middle of the strait during six weeks (4 November 2004 to 7 January 2005), at a depth of 40 m and at few Kilometers from coast. Second, by velocity profiles obtained with a 314.4 kHz Acoustic Doppler Current Profiler (ADCP) during seven cruises (Cyrce) covering entirety strait. From moored ADCP data, a study is made on time correlation between wind stress and current. Shipboard ADCP data permits to obtain a general view in totality of strait and mostly to observe spatial variability. This study shows that the circulation is governed both by wind stress and by general circulation of Mediterranean Sea. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Wind Current data Coastal zone Moored ADCP Shipboard ADCP Strait of Bonifacio
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Data collection and processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Tidal current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Current data arising from profiler bottom-moored current meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. General scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Case study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Current data arise to boarded ADCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction The straits and passages of Mediterranean Sea are keys regions because they contribute to the understanding of Mediterranean basin-scale circulation. In fact, they act on the general circulation by controlling the water mass-transport (Beranger et al., 2005). The previously studies and literature showed that the inflow and outflow of water mass from basin to basin are characterised by
Corresponding author. Tel.: +33 4 95 45 00 41.
E-mail address: [email protected] (O. Gerigny). 0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.11.005
1 2 2 2 3 3 3 3 3 4 6 6 8 8
low frequency and seasonal variability (Astraldi et al., 1999) with high variability of the current through these passages, particularly for little coastal area where there was many coastal meanderings (Plus et al., 2009). Circulation in strait could be governed by different process: atmospheric, tidal action, wind stress (the complex role of wind which mainly controls the surface circulation), topographic features and coastal geometry (which are obstacles to the mean circulation) and thermohaline boundary forcing. So the circulation in strait was complex and many studies were required to understand all the physical process playing a crucial role in water mass transport with a direct impact on ecological issues. This dynamics of circulation in coastal area and particularly inside
−800
50
−4
00
−50 0
−100
−1 00
0 −2−0150
−1 00
00
0
−5
−4
−1 50
0
0 −5 Lavezzi Islands
Bottom−mounted −50 ADCP
Sardinia
−5
48’
−15
Pertusato 0
12’
00
0
−100
Moines Islands −5
18’
−6
−20
41oN 24.00’
9oE
12’
24’
36’
Fig. 1. Strait of Bonifacio, south of Corsica.
pattern. Two types of current data were available: the first arise from profiler bottom-mounted ADCP and second arise from shipboarded ADCP. Profiler bottom-mounted ADCP, was deployed by the CETMEF-DDE (Direction Departemental de l’Equipement) during one and a half month on winter (to 24 November 2004 to 07 January 2005), and collected about 63 390 data scans. This bottom-mounted ADCP was installed at 40 m of depth, in the middle of the strait (near the Lavezzi Island—41.201N and 9.151E) and recorded, each 30 min, vertical profiles (30 levels) of current speed (in m s 1) and direction (in degree) along the water. The first level was 1.4 m above current meter and each level were separated by 1 m. ADCP system was equipped with pressure sensor that gave the height water in meter used for study the tide. In the major part of the figures and analysis, any filter was used for processing data. When statistical analysis was necessary, a specific tools concerning angular data. Indeed, the classical statistics developed for the analysis of linear data are not directly applicable to directional data. A standard average between several directions would not be right because these directions are taken between (an arbitrary) 0 and 3591. The same problem exists also when the standard deviation is calculated from a current direction distribution. To solve this problem, the averages and standard deviations of angular dataset was determined using a statistical analysis of circular data (Fisher and Lee, 1983; Fisher, 1993; Bowers et al., 2002) that are briefly described. Measured angles are noted yi . The value of the coefficient Cc, Cs and Rc was computed according to the following formulae: Cc ¼
N X
cosðyi Þ,
Sc ¼
i¼1
N X
sinðyi Þ,
Rc ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðCc2 þ Cs2 Þ
i¼1
The circular mean direction mc , the circular variance Varc and circular standard deviation sc were then given by 8 1 if Sc 40, Cc 4 0 > < tan ðSc =Cc Þ, mc ¼ tan1 ðSc =Cc Þ þ p, if Cc o 0 > : tan1 ðS =C Þ þ 2p, if S o0, C 40 c c c c
2.2. Data collection and processing Varc ¼ 1 Wind data were obtained from Meteo-France (Pertusato’s station) between 1981–1990, and during the current record period, with frequency record every hour, in order to describe the wind
−1 −100
−−150 00 0 −1−520 0
30’
50
The Strait of Bonifacio (SoB) is located in the North-western Mediterranean sea, between Corsica and Sardinia islands where the Liguro-provencal and Tyrrhenian basins communicate (between 8.81E and 9.51E and 41.31N and 41.61N, see Fig. 1). Narrowed of 14 km, the strait of Bonifacio dwell on 80.000 ha (Mouillot et al., 2008) and is dotted with many islands and islets. Along the Corsica west coast the continental shelf is relatively narrow with the 20 m isobath near the coastline and with the 100 m one which reached on average at 12 km of the coast. On the opposite, the Corsican east coast presents a wider continental shelf and the 100 m isobath is currently located at 25 km off the coast.
Corsica
36’
−1
2.1. Study area
00 −−1−25500
−400
2. Materials and methods
42’
00 −1 0 −40
straits were complex but essential for many questions concerning environmental management (fisheries, tourism, conservation of marine fauna and flora, possible environmental accident, larva retention and transport Narvaez and Valle-Levinson, 2008). The main hydrodynamic features of currentology in the Northwestern Mediterranean sea were previously described and discussed in many studies (Millot, 1990, 1999) but at large scales of space and time. At the west of south Corsica coastline, a current comes from Atlantic water, enter by Gibraltar strait (with colder characteristic), and goes up to north, while along the east coast a current comes from the Tyrrhenian current goes up to south (water more cold and salt). At a smaller scale some current studies have included the dynamic and water balance between the two basins through the Strait of Bonifacio (SoB, south of Corsica) (Millot, 1987; Ovchinnikov, 1974) and the marks left by deep currents near the sea-floor on the sediments in the SoB by using sonar imagery (Pluquet, 2006). These informations confirm the presence of a north current along the west part of the strait and of a south current on the east. The main current circulation crosses the middle of the strait from west to east. But, there was not available data about water circulation and winds which are the main forcing factor for marine currents. Moreover in this study area, many critical environmental problems exist. On one hand, the presence of the biggest natural park in France which high level of protection, and on the other an intense commercial ship movements cruising by this international maritime way, and most of them carrying dangerous materials. For a policy of environmental management such as, it is very important to know the general current circulation and to have tool for management decision. In order to improve knowledge and data base, several investigations were developed. Two are reported in this paper. First: the UMR SPE CNRS no 6134 (University of Corsica) started in 2004 the Cyrce programme (collection of currentologic data with boarded ADCP (Acoustic Doppler Current Profiler) and hydrological data. The second collect of currentologic data was provided by bottommounted ADCP which was installed in middle of Strait by DDE (Direction Departemental de l’Equipement). In relation to an exhaustive collection of wind conditions, our aim was to describe the main features of water circulation in SoB by (1) determining what were the main influences over this circulation, (2) understanding the wind/current relationships, in surface and in the water column (3) by illustrating this circulation and interactions by some case studies.
−600
O. Gerigny et al. / Continental Shelf Research 31 (2011) 1–8
−200 −150
2
Rc , N
sc ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2logð1Varc Þ:
All the treatments proposed in this paper use these previous definitions for the statistical analysis circular data.
O. Gerigny et al. / Continental Shelf Research 31 (2011) 1–8
All boarded ADCP measurements were carried out within the CYRCE framework (the Current and hYdrology programme of the Corsican Region) and seven cruises were performed onboard the OV-Thethys II (INSU). Measurements were taken using the boarded ADCP (Teledyne RD Instruments—broadband VMBB 314.4 kHz) at a cruise speed of eight knots as recommended by Petrenko et al. (2005) supported by differential-GPS positioning. In all cruises, the same configuration was adopted: 60 cells (4-m depth each), with an averaging process every minute (in realtime automatic control procedures) and bottom tracking when possible. Precision of the ADCP data is given as higher than 0.02 m s 1. For each CYRCE cruise, a first raw-data analysis was performed by the INSU’s Technical Division and all measurement errors were eliminated (Petrenko, 2003; Petrenko et al., 2005; Leredde et al., 2007). The current data was processed using Matlab 7.5.0 (R2007b), in association with the data-treatment software ‘‘V.4.lite’’ (created by INSU), and Microsoft Excel (with Analysis Tool kit for statistical analysis). In consequence of sensors precision, the wind and current speed values were rounded off. 3. Results and discussion 3.1. Winds Subject at Mediterranean climate (sub-humid at temperate winter), SoB is a particularly windy region where winds blow 328 days by year (171 days with a wind 4 16 m s1 , (Office de l’Environnement de la Corse, 2007, Mouillot et al., 2008).The wind regime in this region of South Corsica is first dominated (about 50 percent of wind regime) by westerly wind during all the year (called Libecciu, blowing between 2601 and 3001, with speeds generally superior at 8 m s1 ), and secondly dominated by easterly wind (about 30 percent of wind regime) blowing during winter (called Gregale, include between 601 and 1001, in majority between 5 and 8 m s 1). These violent winds (only 2 percent were smaller at 2 m s 1) are the main cause of strong currents in this area. During period record of bottom-mounted ADCP, mean velocity was 7 m s 1, standard deviation of 4.09, and the most-frequent direction is Libecciu. We supposed that Pertusato wind is represented of the wind on area to disregarding the local effect due to the topography same they could be tender (sensible) in area.
3
3.2. Tidal current In Mediterranean Sea, the tidal flows could be considered almost rectilinear, with a clear predominance of the west–east component of current velocity over the south–north component (Mananes et al., 1998; Pillsbury et al., 1987) with non-linear phenomena affecting tidal. By virtue of his configuration (west– east), it was possible that currents in SoB are influenced by the tidal movement. In spite of low tide in Mediterranean Sea, the height of water was studied to understand the role of tide in current circulation. Fig. 2 presented (a) the Fast Fourier Transform filter (FFT) on water height, (b) the FFT on speed wind data and (c) the FFT on speed current data through water column. On first FFT (2a), two harmonics are observed corresponding at classical semidiurnal tidal (M2) and diurnal tidal (K1) (with for respective amplitude lower 0.1 and lower 0.05). On the current speed FFT (2c), the more important frequency are lower than 1 by day, and a significant signal can be observed around the 2 by day. At low frequency (lower to 1 cycles per day), a mass of energetic peaks were observed which were due to a low frequency forcing coming from circulation at big scale in western Mediterranean and Tyrrhenian basins. The influence of non-linear effects in their dispersions was accentuated by coastal phenomena (coastal morphology and bottom proximity, (Valle-Levinson et al., 2007). Even if the peak at 2 by day could be associated with the diurnal tide, doubt persist because there is any significant peak associated to a semidiurnal tide (which should be around 4 by day) though the more important than one. On the other hand, this peak could not be associated with barometric phenomenon (record during winter season) or direct wind effect since the FFT of wind data (2b) do not presents significant peak at this frequency. In every cases, the tide effect can be associated to a maximum current velocity from 3 to 5 cm s 1. Later in this study, the tidal impact will be neglected compared to other physical phenomena. 3.3. Current data arising from profiler bottom-moored current meter 3.3.1. General scope Surface speed average was of 50 cm s 1, resulting from strong currents but with high standard deviation 28 cm s 1: over all the measurement period, (the lowest speed of 2 cm s 1 and the
Fig. 2. (a) The Fast Fourier Transform filter (FFT) on water height, (b) the FFT on wind data and (c) the FFT on current data through water column.
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Fig. 3. (1) To 1 at 500 h of recording: wind and current speed, (a) wind and current surface speed, (b) current speed in column water. (2) To 500 at 1056 h of recording: wind and current speed, (a) wind and current surface speed, (b) current speed in column water.
strongest current of 146 cm s 1). Figs. 3 and 4 show the speed and direction of currents and winds (with Fig. 3(1) and Fig. 4(1) to 1 at 500 h of recording and Fig. 3(2) and Fig. 4(2) to 500 at 1056 h of recording): (a) in the surface layer and (b) current data in the whole water column. The curve of Fig. 3(a) showed the strong variability of surface current dynamic. Deeper, with an average value of 16 cm s 1 in association with an high standard deviation value (12 with a minimum of 0 cm s 1 and a maximum of 81 cm s 1) the no-normal distribution of deep current speed exhibits also a high variability of depth current speed. As expected, the average current speed decreased with depth (Fig. 3). This decrease is of linear type and the difference between surface and deep-layer current speed is significant at p o 0:01 level when tested by a one-factor ANOVA analysis. Two major directions of current were identified: the first is centred on 701 and second on 2301 (see Fig. 4), in correspondence of wind direction dominance. The directions have only few light variations in water column and appeared to be strongly constrained by the bathymetry and the topography of the studied
marine area and did not show the classical pattern of Eckman spiral. Current direction variability decreased when current speed increased (Fig. 5) so the more the current were strong, the least their direction varied. 3.3.2. Case study To understand the general viewing of this hydrodynamic system, we analysed data by study of some particular cases. Links can be identified (Fig. 3a) between surface current and wind speed, but not always by the same way: strong winds are associated to high current intensities, but also with low currents, and, on the contrary, strong currents may appear with low winds. We identified two categories: the ones may be considered as basic cases because they concern all the studied area and the second no basic cases, can be detected at a local space scale. Of course, inside the basic cases, the correlation between wind and current speed was obvious, but a finer analyse of these temporal datasets showed well the complexity of hydrodynamic area.
O. Gerigny et al. / Continental Shelf Research 31 (2011) 1–8
5
Fig. 4. (1) To 1 at 500 h of recording: wind and current direction, (a) wind and current surface direction, (b) current direction in column water; (2) To 500 at 1056 h of recording: wind and current direction, (a) wind and current surface direction, (b) current direction in column water.
3.3.2.1. Basic cases. The situation may be considered as basic when, first the current speed follows the wind speed, and second, when low winds are associated with erratic currents. The most-classically cases are when the changes in current speed follow those of wind, as presented in Fig. 3: current speed increase classically follows a wind speed (as example between 200 and 310 h or from to 1 to 3 December 2004), isolated by dotted line in Fig. 3(1)(a), within a slight lag time. After statistical tests performed on current speed and wind (in cases of wind speeds 4 7 m s1 ), the Ho hypothesis can be rejected exhibiting a positive correlation between current speed and wind speed in the whole water column (r ¼0.23 for surface current and r ¼0.35 for depth current, where r was the correlation coefficient, n¼543). In this set of data, surface current and wind directions are strongly linked (Fig. 4, with r ¼ 0.46 for the surface and r ¼ 0.35 for n ¼543, the bottom (the negative sign is only the consequence of the conventional inverse formulation for the directions: ahead for the currents, origin for the winds)). Another case was identified between times 300 and 450 (from 5 to 11 December 2004), where the current speed was directly
correlated with wind speed, the two curves being roughly parallel. The second kind (or type) of situation which may be considered as basic are the cases where low winds are associated with erratic currents (they are named here as erratic since they exhibit strong direction variability at a small scale of time and space, Fig. 6). In these cases, there is none correlation between respective wind and current speed. But, as presented in Fig. 5, lower is the wind speed and higher is the dispersion of current direction. In this figure each point is the average of respective twenty adjacent data for, respectively, mean of wind intensities and standard deviation of current directions; and the exponential fit of relationship is of exponential form and significant at p 0:05 level (n¼ 52, R2 ¼0.23, unilateral test). When strong wind intensities clearly determine current high speeds and cohesive directions, on the contrary, during light wind periods the energy applied to surface water is no sufficient to cause a main current direction. 3.3.2.2. Some nonbasic cases. But many situations recorded during the studying period may be referred to the above-described basic
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along depth. This nonbasic case it present at several occasions (as another example, between time 740 and 780, from 23 to 25 December 2004). 3.4. Current data arise to boarded ADCP
Fig. 5. Distribution of standard deviation of current direction (degrees) versus mean current speed (cm s 1). Variability of current direction reduces when current speed rises.
Fig. 6. Distribution of standard deviation of current direction (degrees) versus mean wind speed (m s 1). Variability of current direction reduces when wind speed rises.
cases. The first nonbasic case is occurred between 540 and 630 h (from 15 to 19 December 2004, Figs. 3(2) and 4(2), unshaded area). Two strong NW wind peaks were recorded but if the current speed follows the first wind peak (with the respective directions of 3001 for the wind and 1201 for the current) it did not occurred during the second peak. The current speed remained constant in the water column with a weak standard deviation value. And current direction was inverted from the previous peak (orientation around 3001 and constant along the water column), whereas the wind direction has not changed (oriented by 3001, west–east), as compared to the first peak. We may consider these to opposite current evolutions under the same wind condition as the consequence of probable pressure currents. The second type of situation with a no-coupled wind and current evolution was recorded, as example, between time 890 and 920 or from 29 to 30 December 2004) (Figs. 3.2 and 4.2). Surface current speed was strong with low wind speed. Since this surface current is uncoupled to wind forcing and the tidal currents being previously removed, we may consider that these situations are the consequence to water mass balance between the two basins, through the Bonifacio strait, mainly caused by the general circulation of N–W Mediterranean Sea and enhanced by the strait narrowness. This explanation is accorded to the facts that there was low variability of current speed shows a low variability in water column and the current directions remain constant
Two views of ADCP current profiles, covering the whole study area on both sides of the strait, were presented in Fig. 7, each of them in two figures. These maps are not instantaneous views; then they are realised during the several hours of each ship transit. Their spatial information complements the time information provided by the moored ADCP located at one point. The two figures for each transit correspond, respectively, to the current surface at 16 m (bin 3 of ADCP) on the left and to the current at 40 m depth (corresponding to bin 10 of ADCP—around the deepest bottom in the middle of the strait) on the right. In the two upper figures (21 July 2005) a wind of 20 knots (about 35 km/h) is blowing with a direction between of 2601 and 2901 and swell sea; in the two lower maps (11 March 2005), a wind of 10 knots was established from 951 to 1681 with calm sea. The main feature of this comparison is the differences appearing between surface and deep currents on the two set of maps. On the upper figures the high speed of surface currents strongly decreases with depth and many changes in direction exist between the two levels (particularly in the west part of the strait and by the strait itself). On the opposite, on the two lower figures (light winds), the speed decrease is much weaker between surface and deep current; and if some changes in current directions can also be noted, in this case they are rarer. In the first case, the influence of strong wind induced ‘‘wind currents’’ on surface waters which are rapidly decreasing with depth. During the second episode, all the water layers are concerned by the same flow which is probably by the general circulation around Corsica and exchanges though the SoB. These two types of situation were previously deduced from the wind and current time-series of the bottom-moored ADCP data. The second noticeable features of these maps are the frequent inversions of the current directions when offshore-to-coast perpendicular recording transits are examined, and in the surface as well as deep layers. As evidence this is the consequence of the various perturbations which are induced by changes in bathymetry and coastline upon the water masses flow. By the way, this is particularly true in the middle of the strait, which is its narrowest and shallowest part.
4. Conclusion This study is in keeping with long term program to study the SoB circulation, with final target to validate and improve, by in-situ data from moored ADCP and shipboard ADCP, a numerical model, on short space and time scales (in the order of few kilometers). Generally models are validated by observations at long scale from satellite observations for try to validate the functioning of this model at short scale, near coast. In fact, these satellite observations are not specifics, it is necessary to add in-situ and local data. In this paper, a first analyse of a short recorder period is performed. This analysis gives a first general vision on area circulation but also first results on wind–current correlation effect. This study shows: (i) Two prevailing winds in area (west and north–east winds), (ii) Tides have a very light influence on current amplitude, (iii) A first description of general tendency current in SoB, by moored ADCP data, is made by analysed different situation of wind and current, these cases are divided in two categories: ‘‘basic cases’’ and ‘‘nonbasic cases’’. Study shows presence of strong current intensities and the latest decrease with depth. The current directions are continual in water column because of light depth in
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Fig. 7. (a) Maps of current recording 21/07/2005 at 10 m, (b) Maps of current recording 21/07/2005 at 40 m, in strong sea conditions, (c) Maps of current recording 11/03/2005 at 10 m, (d) Maps of current recording 11/03/2005 at 40 m, in calm conditions.
area, and two prevailing current directions are present and corresponded to wind direction nevertheless the correlation between wind/current was not constant since their intensities are correlated on the whole water column only for strong winds ð 4 7 m s1 Þ. In many basic cases, wind and current are strong and orientated in the same direction (correlation between current/ wind directions on the whole water column). Basic cases correspond to cases where firstly the current speed follows the wind speed where some dynamics appear between wind and current probably showing inertial effect between each other, however, this part is not detailed because short record period not permit to do statistic treatment. Other important point of this basic case is that variability of current direction decreases when current speed grows. So the stronger the currents are, the less their directions vary in time. The second basic case process a situation of low wind with erratic currents, for this latter it has be noted a decreased variability current direction when the wind speed increased. The no basic cases present other situation where current dynamic are not generate by wind stress with strong wind with low current or low wind and strong current, probably due to a general circulation at large scale or pressure current (water balance between two basins). (iv) Then, these results are completed and compared at shipboard
ADCP data, which give other vision in totality of the strait but also and mostly gives spatial variability. At the west of the coast, there is an eddy anticyclone probably the result of the movement of water to north, this water comes from Gibraltar and may have an Atlantic origin. In the middle of the strait, currents intensify being probably the consequence of Ventury effect. The tightening of SoB, was did at two levels: horizontally with the reduction of the strait between Corsica and Sardinia and vertically with the reduction of the bathymetry in the middle of the channel. These accelerations in the middle of the strait and strong winds are a cause of difficult boat circulation across the strait. Since tide did not influence the current, so the circulation is governed as well as wind stress that general circulation in Mediterranean Sea. This circulation is forced by geological morphology and topography of this area. First model trials will be quickly compared to cases tackled here, however, the recorded data period being relatively short it shows require increasing of data number as well as in middle of the strait that in other local area. At long time moored ADCP (for more than two years) has been installed in the marine park. A better knowing of area circulation, in particular thank to simulation model, could be beneficial in several fields: notably for dangerous matter transport including a risk of
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pollution of the natural reserve. If pollution should happen (particularity oil pollution) knowledge of the dynamic could improve the struggle strategy; in biologic field about dispersion of fish and crustacean larva could be a potential source of repopulation in occidental Mediterranean Sea; and also management field.
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