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Tidal induced variability of mixing processes on Camarinal Sill (Strait of Gibraltar): A pulsating event
Journal of Marine Systems 60 (2006) 177 – 192 www.elsevier.com/locate/jmarsys
Tidal induced variability of mixing processes on Camarinal Sill (Strait of Gibraltar): A pulsating event Diego Macias a,⁎, Carlos M. García a , Fidel Echevarría Navas a,b , Agueda Vázquez-López-Escobar c , Miguel Bruno Mejías c a
b
Department of Biology, University of Cádiz, Spain Centro Andaluz de Ciencia y Tecnología Marina (CACYTMAR), Cádiz, Spain c Department of Applied Physics, University of Cádiz, Spain
Received 18 July 2005; received in revised form 16 December 2005; accepted 19 December 2005 Available online 21 February 2006
Abstract The tidal cycle and its amplitude appear as an a priori forcing factor causing periodic mixing events on the Gibraltar Strait sill. These mixing events can supply nutrient-enriched deep waters to the surface atlantic layer flowing to the Mediterranean. Therefore these mixing phenomena have both important biological and biogeochemical implications at the regional and even basin level. Fortnightly variations of the effects caused by changes in tidal amplitude have been studied by analysing the characteristics of surface water masses flowing through the Strait of Gibraltar during several cruises carried out in 2002 and 2003. These variations were studied through the analysis of results from 7 diel cycles performed at a fixed station at the eastern side of the Strait monitoring the atlantic jet during several tidal cycles of different amplitude. When strong tidal currents occur in higher amplitude cycles, the occurrence of mixed water is more frequent than during neap tides causing a distinctive TS diagram which lacks a clear NACW signature. It is also a common pattern that the highest level of fluorescence in each cycle coincides with the presence of these mixed waters, appearing as an intermittent flow of phytoplankton biomass to the Mediterranean sector and suggesting a strong influence of the physical forcing on the biological patterns. © 2005 Elsevier B.V. All rights reserved. Keywords: Spain; Alborán Sea; Gibraltar Strait; Mixing processes; Upwelling; Tidal effects; Tidal mixing; Internal waves; Patchiness
1. Introduction The Strait of Gibraltar represents the only connection between the Mediterranean Sea and the Atlantic Ocean. The study of the balance of water and elements through it, as well as its dynamics, has implications not only at
⁎ Corresponding author. E-mail address: [email protected] (D. Macias). 0924-7963/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2005.12.003
the regional level but, also, for large basin scale budget calculations. The exchange through the Strait is usually described as a two-layer circulation, with an superficial Atlantic layer flowing towards the Mediterranean and an outflow of deep high-density Mediterranean waters (Armi and Farmer, 1988). A particular place for the circulation in the Strait is the Camarinal Sill (Parrilla, 1990). This Sill is located in the western sector of the Strait, with only some 300 m in the deepest central channel compared to
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Table 1 Positions and time of beginning and end of each fixed station and the current-meter mooring Phase
Longitude
Latitude
Start
End
Vessel
Tidal amplitude (LW-HW)
N-FS1 N-FS2 J-FS1 H-FS1 H-FS2 Myt-FS1 Myt-FS2 Mooring
36° 02′ N 36° 02′ N 36° 02′ N 36° 4,97′ N 36° 4,97′ N 35° 59′ N 36° 00′ N 35° 56′ N
5° 17′ W 5° 17′ W 5° 17′ W 5° 16,2′ W 5° 16,2′ W 5° 14′ W 5° 18′ W 5° 40′ W
01:15 GMT 07.11.02 23:15 GMT 11.11.02 17:15 GMT 09.06.03 17:15 GMT 06.11.03 20:15 GMT 13.11.03 02:30 GMT 07.11.03 23:30 GMT 14.11.03 14:15 GMT 13.11.03
06:00 GMT 07.11.02 08:00 12.11.02 09:00 GMT 10.06.03 18:15 GMT 07.11.03 21:15 GMT 14.11.03 13:30 GMT 07.11.03 03:00 GMT 15.11.03 21:15 GMT 14.11.03
B.C. Malaspina B.C. Malaspina B.C. Tofiño BIO Hespérides BIO Hespérides BO Mytilus BO Mytilus BO Mytilus
High (0.1–1.4 m) Low (0.5–1.0 m) Low (0.4–1.1 m) Medium-high (0.3–1.4 m) Medium-low (0.4–1.2 m) Medium-high (0.3–1.4 m) Medium-low (0.4–1.2 m) Medium-low (0.4–1.2 m)
depths of 900 m in the eastern section. The interaction of water movement with this sharp topography causes intense undulatory processes, such as bores (Boyce, 1975; Armi and Farmer, 1985) or internal waves (Bruno et al., 2002) which are capable of induce mixing between the different water masses present over the sill (Bray et al., 1995). Three different water masses have been detected in the western sector of the Strait of Gibraltar: Surface Atlantic Water (SAW), North Atlantic Central Water (NACW) and Mediterranean Outflowing Water (MOW) (Gascard and Richez, 1985). The amount of the lessabundant NACW present over the Sill is highly influenced by the tidal height and the amplitude of the internal wave (Gascard and Richez, 1985). Also its
proportion diminishes to the east of the Strait, probably as a consequence of mixing processes with the other two water masses (Bray et al., 1995). The intensity and periodicity of NACW signal are linked with the tidal amplitude (Gascard and Richez, 1985). However, the effect of this tidal influence on the biological patterns observed and ecosystem behaviour still remains unknown. Thus, there have been a number of physical studies on the variability and composition of the water masses flowing through the Strait (Armi and Farmer, 1985; La Violette and Arnone, 1988), but very few biological data have been recorded in this context, mainly during the cruises of the European project CANIGO.
Fig. 1. Position of the different fixed stations.
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Fig. 2. Time of each sampling related to the high-water in Tarifa.
Fig. 3. Current velocity prediction over the Camarinal Sill at 45 m depth for the four cruises. Positive values mean currents toward the Mediterranean and negative towards the Atlantic.
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Fig. 4. T-S diagrams for the different samplings. The colour scale is chlorophyll in mg/m3.
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In 2001, Gómez et al. showed how the short-term variability of the water column composition presented a tidal-cycle-related periodicity in the presence of NACW signal in the inflowing water layer. The patterns of biological variables also showed a relationship with the tidal cycle; the chlorophyll distribution appeared to be related with the pulsating presence of the nutrient-rich NACW (see also Ruiz et al., 2001). Our work presents a wide set of observations of diel and tidal cycles, aiming to describe in more detail the dynamics of water masses and of several variable profiles in a fixed point on the Atlantic jet entering the Mediterranean. We have focused our work on semidiurnal tidal cycle variability as well as on the analysis of variations linked to changes in the tidal amplitude in a fortnightly scale. 2. Material and methods Data were collected during four different cruises carried out from November 2002 until November 2003 (Table 1) in the Strait of Gibraltar. In each cruise one or two fixed stations were sampled in the eastern side of the Strait (Table 1 and Fig. 1). In each fixed station (3 to 26 h total observation time, Fig. 2) several CTD profiles were made following an interval from 0.25 to 1 h. In each profile, salinity, temperature and fluorescence distribution were sampled from surface to 300 m depth by using a combined CTD probe. This limit of depth was chosen to ensure observation of the AtlanticMediterranean-Interface (AMI) and MOW but increasing time resolution. Fig. 2 also shows the different time-length of each sampling in relation to the phase of the tidal cycle. Time has been expressed which referred to tidal cycle using the reference of predicted high-water in Tarifa, just in the middle of the Strait (HW), thus HW-2 means time 2 h before high-water and HW+1 1 h later than HW has been reached. The dates of the fixed stations were selected to include different tidal amplitude cycles, from spring to neap tides. The current velocity over the Camarinal Sill for these dates is shown in Fig. 3. The strongest currents were present during N-FS1 (N 2.5 m/s) and the weakest during the lower amplitude cycles J-FS1 and N-FS2 (0.7–0.9 m/s). 3. Results The characteristics of the water column in the sampling site can be analysed through the TS diagrams
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derived from the CTD profiles. In these diagrams the three types of waters (SAW, MOW and NACW) involved in mixing processes on the Camarinal Sill can be easily detected (Fig. 4, Table 2). The CTD cast could be classified using the percentage of NACW in the upper 150 m of the water column. When no NACW is present, the TS diagram shows a mixing between SAW and MOW, thereby, these samples will be arbitrarily named in this paper as “Mediterranean Mixed Waters (MMW)”, and its presence will be marked with triangles in Figs. 4–11 to facilitate the interpretation of the figures. If, on the contrary, NACW is present in the upper layers of water the TS diagram shows a mixing between SAW and NACW; these stations will be called in this paper “Atlantic Mixed Waters (AMW)”, and, similarly, will be marked with squares in Figs. 4–11. Tidal induced dynamics can be studied by analysing the time evolution of basic descriptive variables (temperature, salinity, chlorophyll fluorescence and tidal current speed) recorded during the seven sampling events (Figs. 5–11). The details about the prediction of the tidal current shown in plate d could be revised in Alonso del Rosario et al. (2003). Following the same symbols proposed for Fig. 4, the triangles mark the CTD cast when MMW was found, and the squares show the time when AMW was detected. These two types of mixed water masses appear clearly separated in time during each cycle at the fixed stations. The presence of MMW is usually detected around HW−4 (i.e. 4 h before highwater), when the current velocity over the Sill changes from inflowing (positive values in plate d Figs. 5–11) to outflowing (negative values in plate d Figs. 5–11). There are however, some deviations from this general pattern. During N-FS1 (Fig. 5) the presence of MMW was detected a little bit earlier, around 2 h before high-water (HW-2). Also, in J-FS2 two “zero crossing” happened but MMW was only associated to the first shift event. It is also noticeable
Table 2 Salinity and temperature characteristics for the three types of water masses (derived from Gascard and Richez, 1985)
Tmax Tmin Smax Smin
SAW
MOW
NACW
20 19 36.4 36.2
13.5 13 38.5 38.2
14 13.5 36 35.6
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Fig. 5. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during N-FS1. Squares showed the presence of mixing of AMW and the triangles the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
that no AMW was detected in the CTD cast carried out in a northernmost position (H-FS1 and H-FS2, Figs. 8 and 9), a fact that will be analysed in discussion. Several undulatory phenomena were detected during the different sampling events, generally associated with the presence of MMW (Figs. 5a, 7a,
b, 8a,b, 9a,b) . These disturbances can be related with the internal waves generated over the Camarinal Sill. However, sometimes these undulations appear to be associated with the presence of AMW, as in N-FS2 (Fig. 6a) and in J-FS1 (Fig. 7a). This fact could be related to the lower velocity of the outflowing currents just before the sampling.
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Fig. 6. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during N-FS2. Squares showed the presence of AMW and the triangles the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
In Fig. 10a the record of the zonal current velocity registered by the ADCP during H-FS2 is shown. The maximum inflowing velocities (positive values) coincide in time with the presence of the undulatory processes (see Fig. 9a and b). These undulatory processes are also clearly visible in the record of a current-meter mooring deployed in the nearby of the
Camarinal Sill (Fig. 10b), being detected around 9 h before being sampled at the fixed station. Another common pattern observed is that the highest levels of chlorophyll coincide with the presence of MMW (Figs. 5c, 6c, 7c, 8c, 9c and 11c), suggesting a strong coupling of the physical forcing and the biological patterns that will be discussed later.
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4. Discussion 4.1. NACW signal and tidal amplitude The NACW signature corresponding to different tidal amplitude cycles can be studied by a combined analysis of results from all the diel data series obtained in the Strait. In previous works based on a compilation
of information from different cruises (Gascard and Richez, 1985) a negative relationship between the tidal amplitude and the quantity of NACW in the incoming Atlantic layer was found. In the present paper, a common time-period (from HW-5 to HW) has been chosen to compare the amount of NACW present in the water column for each sampling series and only the data of the upper
Fig. 7. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during J-FS1. Squares showed the presence of AMW and the triangles the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
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Fig. 8. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during H-FS1. Triangles show the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
150 m have been used. In this comparison we use the data of N-FS2, J-FS1, Myt-FS1 and Myt-FS2. The sampling in N-FS1 was not included in this analysis because it was very short (only 4 h) and was performed between HW-1 to HW+2, just at the end of the chosen period. The sampling events carried out on board BIO Hespérides were also not included because they did not show the NACW signature.
The percentage of NACW present in the upper layer of the water column can be estimated according to Bray et al. (1995). Using this, a clear inverse relationship between NACW percentage and the tidal outflowing velocity (towards the Atlantic) of the upper layer over the Sill is obtained (Fig. 13). The largest amount of NACW was detected during N-FS2 (tidal current velocity of 0.7 m/s, Fig. 6c) and the lowest quantity detected during Myt-FS2 (tidal velocity of
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Fig. 9. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during H-FS2. Triangles show the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
nearly 1.1 m/s, Fig. 12c). These results agree with the statement of Gascard and Richez (1985). The lack of NACW signal in the sampling performed on board BIO Hespérides (Fig. 4d and e) could be related with the displacement to the north of the sampling site (Fig. 1). Simulations of mixing and advection of water masses in the strait show that the northern half of the channel will be affected by the residual cyclonic circulation present eastwards Gibraltar (Izquierdo et al., 2001), in which no signal of NACW can be found
(Rodríguez et al., 1998). Also, the Coriolis force would tend to accumulate the NACW in the southern half of the Strait (Bray et al., 1995). The relationship between the tidal amplitude and the presence/absence and intensity of the NACW signal would be connected with the mixing processes over the Camarinal Sill. With strong tidal currents, the undulatory disturbances would be more frequent and more energetic (Armi and Farmer, 1988) and consequently, the mixing between the three water masses would be
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Fig. 10. a) Temporal variation of current velocity profiles (east component), recorded by a vessel-mounted ADCP, during H-FS2 sampling and time series of current velocity at 28 m depth. b) Current velocity intensity (at 160 m depth) in the nearby of Camarinal Sill, arrows indicate the arrival time of the internal waves previously generated around the Sill.
more effective. As was discussed by Bray et al. (1995), the signal of the less abundant NACW will be eroded away, making only SAW and MOW detectable in the TS diagram. However, mixing alone could not explain the absolute lack of NACW signal found in some of the CTD cast, being necessary other processes to account for that phenomenon. So, an alternative explanation would be related with the upper layer velocity direction over the Sill. This layer is, on average, flowing towards the Mediterranean, however with enough tidal amplitude and around HW; part of this layer could revert its movement and flow to the Atlantic (La Violette and Arnone, 1988; Candela, 1990; Bruno et al., 2002; Izquierdo et al., 2001). In this phase the NACW, situated below the SAW layer, could not overflow the sill, and, thereby, the composition of the water column shows only SAW and MOW (Gascard and Richez, 1985; Gómez et al., 2004). The absolute value of this western velocity has a very good
correlation with the quantity of NACW present in the water column (Fig. 13). 4.2. Trends in the temporal evolution In the Camarinal Sill region, tidal induced arrested internal waves are generated mainly from HW-3 to HW +2. These initially stationary waves are released to the eastern sector and start to move when the tidal current changes from outflowing to inflowing (Bruno et al., 2002; Alonso del Rosario et al., 2003). The arrival of these internal waves and associated MMW to the fixed station is expected around HW-4 (e.g. Farmer and Armi, 1986) which is in good agreement with the results showed in this work (Figs. 5–11). However, as mentioned above, the MMW appears earlier in N-FS1 (around HW+2), this being a consequence of quick advection from the Sill due to the strong incoming tidal current (N 2.5 m/s, Fig. 5d). There is another deviation of the general pattern
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Fig. 11. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during Myt-FS1. The arrows and circles mark the moments when CTD profiles were made.
during N-FS2, when the undulatory events are associated with the presence of AMW. This also happens in J-FS1 when the MMW only appears associated with the first HW-4 time, but appearing a clear signal of NACW during the second. The presence of undisturbed NACW signal is probably due to the weakness of the previous outflowing event, in which the tidal current barely reach − 0.5 m/s in NFS2 and − 0.2 m/s in J-FS1. That will lead to a lower
mixing over the sill or even, to avoid the reversion of the upper layer velocity, reducing the presence of MMW as, under these conditions, the NACW will flow over the sill throughout all the tidal cycle with a low degree of mixing and retaining its distinctive signature. It is also noteworthy that the first event detected during H-FS2 happens a little bit later than HW-4, probably due to the slower advection from the western side of the Strait caused by the small
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Fig. 12. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during Myt-FS2. The arrows and circles mark the moments when CTD profiles were made.
inflowing current velocity (maximum of 0.6 m/s, Fig. 9d). In previous works (e.g. Gómez et al., 2000) the highest levels of chlorophyll had been related with the presence of the nutrient-rich NACW which could enhance the primary production by injecting nutrients in the upper layers of water. However, phytoplankton populations will need some time to grow using these new resources and, thereby, a noticeable increase in the phytoplankton concentration will only be detectable after a time enough to accumulate biomass, which is
likely to occur only out of the Strait, in the north-western Alborán Sea. In fact, some evidences of this have also been observed in this area (Packard et al., 1988; Ruiz et al., 2001) or even as far as the Almería-Orán front (Arnone et al., 1990) at the eastern end of the Alborán Sea basin. Surprisingly, in the results presented in this work there is a general pattern of coincidence between the maximum levels of chlorophyll and the presence of MMW (not NACW) as is presented in Table 3. The absolute values of the mean concentration in each
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Fig. 13. a) Percent of NACW in the upper 150 m for the different samplings. b) Relation of the quantity of NACW and the tidal outflowing velocity.
type of mixed water change from one sampling to another, as a consequence of either the annual chlorophyll cycle in the area, which has the minimum levels during summer and the maximum at the end of the winter season (García-Gorriz and Carr, 1999; Reul, 2000) or to the different position of the fixed station during each sampling, as the chlorophyll concentration is lower in the south section of the channel than in the north (Gómez et al., 2000; Echevarria et al., 2002; Reul et al., 2002). However, the percentage of increase within each sampling remains appreciably constant (Table 3), with a mean of 28.32% more chlorophyll in presence of MMW with respect to AMW. It is also interesting to note that, though in the two sampling events carried out in a northern position on Table 3 Mean superficial chlorophyll concentration in the stations with AMW and MMW (0–100 m) and the percentage of increase registered in the latest for each sampling Phase
Average chlorophyll in the upper 100 m in AMW stations (mg/m3)
Average chlorophyll in the upper 100 m in MMW stations (mg/m3)
% of increase in the chlorophyll concentration in presence of MMW
N-FS1 N-FS2 J-FS1 Myt-FS1 Myt-FS2
0.87 0.95 0.59 0.27 0.7
1.17 1.32 0.86 0.38 0.96
25.6% 28% 32% 29% 27% Mean: 28.32% (±2.4%)
board Hespérides there was a continuous presence of MMW, the highest levels of chlorophyll were related particularly with the presence of undulatory disturbances (Figs. 8c and 9c). The development of a modelling tool, a 1D physical– biological coupled model of the channel of the Strait that analyse the problem in a 3-layer scheme (Macías et al., submitted for publication), has confirmed the initial impression that the appearance of these pulsating chlorophyll-rich patches is unlikely to be explained by solely growth as the time needed to travel from the Sill to the eastern entrance of the Strait is not long enough to allow for a noticeable increase of the phytoplankton population. We have hypothesized that this intermittent increase in chlorophyll concentration could alternatively be explained by horizontal advection of water masses with high concentrations of chlorophyll coming from the coast to the centre of the channel forced by tidal hydrodynamics. As commented above, the Atlantic layer over the Camarinal Sill will, sometimes, change the direction of its flow. However, the whole surface layer does not flow towards the Atlantic (as shown by the model of Izquierdo et al., 2001), but the eastern half flows eastwards throughout the tidal cycle. This situation creates a horizontal divergence over the Camarinal Sill which would force a replacement of surface water with coastal waters coming from the shallower marginal areas. These coastal waters tend to be chlorophyll-rich and could be incorporated as discrete periodical patches to the main Atlantic current flowing to the Mediterranean. This hypothesis could
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explain the coincidence of high chlorophyll values with periods of intense interfacial mixing which dilutes the NACW signal. Currently, further investigation is being carried out to test this hypothesis.
5. Conclusions Tidal amplitude has been observed to explain the NACW presence/absence patterns in the surface water masses flowing to the Mediterranean. This influence can be linked to changes induced in the extension and intensity of the mixing processes or related with modifications of the water masses circulation scheme over the Camarinal Sill. The advection velocity of the undulatory disturbances is in quite good agreement with that predicted by theoretical estimations. The maximum levels of fluorescence coincide with the presence of MMW and more specifically with the existence of undulatory disturbances. The amount of chlorophyll involved cannot be explained solely by in situ growth driven by the fertilization of the superficial waters, but rather could be related with 2D complex coastal–channel interactions that will be investigated in future cruises and modelling exercises.
Acknowledgements This work has been funded by the Project of the National Research Program REN-2001-2733-C02-02. Diego Macías and Agueda Vázquez were supported by grants of the FPI fellowship program. We acknowledge also the Spanish Navy “Instituto Hidrográfico de la Marina” that offered support for sampling and several cruises on board BO Malaspina and BO Tofiño. Dr. Edward P. Morris made a kind revision of the English version of the manuscript. References Alonso del Rosario, J.J., Bruno Mejías, M., Vázquez-Escobar, A., 2003. The influence of tidal hydrodynamic conditions on the generation of lee waves at the main sill of the Strait of Gibraltar. Deep-Sea Research I 50, 1005–1021. Armi, L., Farmer, D., 1985. The internal hydraulics of the Strait of Gibraltar and associated sill and narrows. Oceanologica Acta 8 (1), 37–46. Armi, L., Farmer, D., 1988. The flow of Mediterranean Water through the Strait of Gibraltar. Progress in Oceanography 21, 41–82.
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the size structure of the phytoplankton community in the deep fluorescence maximum of the Alboran Sea (southwestern Mediterranean). Deep-Sea Research I 45, 1577–1593. Ruiz, J., Echevarría, F., Font, J., Ruiz, S., García, E., Blanco, J.M., Jiménez-Gómez, F., Prieto, L., González-Alaminos, A., García, C. M., Cipollini, P., Snaith, H., Bartual, A., Reul, A., Rodríguez, V., 2001. Surface distribution of chlorophyll, particles and gelbstoff in the Atlantic jet of the Alborán Sea: from submesoscale to subinertial scales of variability. Journal of Marine Systems 29, 277–292.
Tidal induced variability of mixing processes on Camarinal Sill (Strait of Gibraltar): A pulsating event Diego Macias a,⁎, Carlos M. García a , Fidel Echevarría Navas a,b , Agueda Vázquez-López-Escobar c , Miguel Bruno Mejías c a
b
Department of Biology, University of Cádiz, Spain Centro Andaluz de Ciencia y Tecnología Marina (CACYTMAR), Cádiz, Spain c Department of Applied Physics, University of Cádiz, Spain
Received 18 July 2005; received in revised form 16 December 2005; accepted 19 December 2005 Available online 21 February 2006
Abstract The tidal cycle and its amplitude appear as an a priori forcing factor causing periodic mixing events on the Gibraltar Strait sill. These mixing events can supply nutrient-enriched deep waters to the surface atlantic layer flowing to the Mediterranean. Therefore these mixing phenomena have both important biological and biogeochemical implications at the regional and even basin level. Fortnightly variations of the effects caused by changes in tidal amplitude have been studied by analysing the characteristics of surface water masses flowing through the Strait of Gibraltar during several cruises carried out in 2002 and 2003. These variations were studied through the analysis of results from 7 diel cycles performed at a fixed station at the eastern side of the Strait monitoring the atlantic jet during several tidal cycles of different amplitude. When strong tidal currents occur in higher amplitude cycles, the occurrence of mixed water is more frequent than during neap tides causing a distinctive TS diagram which lacks a clear NACW signature. It is also a common pattern that the highest level of fluorescence in each cycle coincides with the presence of these mixed waters, appearing as an intermittent flow of phytoplankton biomass to the Mediterranean sector and suggesting a strong influence of the physical forcing on the biological patterns. © 2005 Elsevier B.V. All rights reserved. Keywords: Spain; Alborán Sea; Gibraltar Strait; Mixing processes; Upwelling; Tidal effects; Tidal mixing; Internal waves; Patchiness
1. Introduction The Strait of Gibraltar represents the only connection between the Mediterranean Sea and the Atlantic Ocean. The study of the balance of water and elements through it, as well as its dynamics, has implications not only at
⁎ Corresponding author. E-mail address: [email protected] (D. Macias). 0924-7963/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2005.12.003
the regional level but, also, for large basin scale budget calculations. The exchange through the Strait is usually described as a two-layer circulation, with an superficial Atlantic layer flowing towards the Mediterranean and an outflow of deep high-density Mediterranean waters (Armi and Farmer, 1988). A particular place for the circulation in the Strait is the Camarinal Sill (Parrilla, 1990). This Sill is located in the western sector of the Strait, with only some 300 m in the deepest central channel compared to
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Table 1 Positions and time of beginning and end of each fixed station and the current-meter mooring Phase
Longitude
Latitude
Start
End
Vessel
Tidal amplitude (LW-HW)
N-FS1 N-FS2 J-FS1 H-FS1 H-FS2 Myt-FS1 Myt-FS2 Mooring
36° 02′ N 36° 02′ N 36° 02′ N 36° 4,97′ N 36° 4,97′ N 35° 59′ N 36° 00′ N 35° 56′ N
5° 17′ W 5° 17′ W 5° 17′ W 5° 16,2′ W 5° 16,2′ W 5° 14′ W 5° 18′ W 5° 40′ W
01:15 GMT 07.11.02 23:15 GMT 11.11.02 17:15 GMT 09.06.03 17:15 GMT 06.11.03 20:15 GMT 13.11.03 02:30 GMT 07.11.03 23:30 GMT 14.11.03 14:15 GMT 13.11.03
06:00 GMT 07.11.02 08:00 12.11.02 09:00 GMT 10.06.03 18:15 GMT 07.11.03 21:15 GMT 14.11.03 13:30 GMT 07.11.03 03:00 GMT 15.11.03 21:15 GMT 14.11.03
B.C. Malaspina B.C. Malaspina B.C. Tofiño BIO Hespérides BIO Hespérides BO Mytilus BO Mytilus BO Mytilus
High (0.1–1.4 m) Low (0.5–1.0 m) Low (0.4–1.1 m) Medium-high (0.3–1.4 m) Medium-low (0.4–1.2 m) Medium-high (0.3–1.4 m) Medium-low (0.4–1.2 m) Medium-low (0.4–1.2 m)
depths of 900 m in the eastern section. The interaction of water movement with this sharp topography causes intense undulatory processes, such as bores (Boyce, 1975; Armi and Farmer, 1985) or internal waves (Bruno et al., 2002) which are capable of induce mixing between the different water masses present over the sill (Bray et al., 1995). Three different water masses have been detected in the western sector of the Strait of Gibraltar: Surface Atlantic Water (SAW), North Atlantic Central Water (NACW) and Mediterranean Outflowing Water (MOW) (Gascard and Richez, 1985). The amount of the lessabundant NACW present over the Sill is highly influenced by the tidal height and the amplitude of the internal wave (Gascard and Richez, 1985). Also its
proportion diminishes to the east of the Strait, probably as a consequence of mixing processes with the other two water masses (Bray et al., 1995). The intensity and periodicity of NACW signal are linked with the tidal amplitude (Gascard and Richez, 1985). However, the effect of this tidal influence on the biological patterns observed and ecosystem behaviour still remains unknown. Thus, there have been a number of physical studies on the variability and composition of the water masses flowing through the Strait (Armi and Farmer, 1985; La Violette and Arnone, 1988), but very few biological data have been recorded in this context, mainly during the cruises of the European project CANIGO.
Fig. 1. Position of the different fixed stations.
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Fig. 2. Time of each sampling related to the high-water in Tarifa.
Fig. 3. Current velocity prediction over the Camarinal Sill at 45 m depth for the four cruises. Positive values mean currents toward the Mediterranean and negative towards the Atlantic.
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Fig. 4. T-S diagrams for the different samplings. The colour scale is chlorophyll in mg/m3.
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In 2001, Gómez et al. showed how the short-term variability of the water column composition presented a tidal-cycle-related periodicity in the presence of NACW signal in the inflowing water layer. The patterns of biological variables also showed a relationship with the tidal cycle; the chlorophyll distribution appeared to be related with the pulsating presence of the nutrient-rich NACW (see also Ruiz et al., 2001). Our work presents a wide set of observations of diel and tidal cycles, aiming to describe in more detail the dynamics of water masses and of several variable profiles in a fixed point on the Atlantic jet entering the Mediterranean. We have focused our work on semidiurnal tidal cycle variability as well as on the analysis of variations linked to changes in the tidal amplitude in a fortnightly scale. 2. Material and methods Data were collected during four different cruises carried out from November 2002 until November 2003 (Table 1) in the Strait of Gibraltar. In each cruise one or two fixed stations were sampled in the eastern side of the Strait (Table 1 and Fig. 1). In each fixed station (3 to 26 h total observation time, Fig. 2) several CTD profiles were made following an interval from 0.25 to 1 h. In each profile, salinity, temperature and fluorescence distribution were sampled from surface to 300 m depth by using a combined CTD probe. This limit of depth was chosen to ensure observation of the AtlanticMediterranean-Interface (AMI) and MOW but increasing time resolution. Fig. 2 also shows the different time-length of each sampling in relation to the phase of the tidal cycle. Time has been expressed which referred to tidal cycle using the reference of predicted high-water in Tarifa, just in the middle of the Strait (HW), thus HW-2 means time 2 h before high-water and HW+1 1 h later than HW has been reached. The dates of the fixed stations were selected to include different tidal amplitude cycles, from spring to neap tides. The current velocity over the Camarinal Sill for these dates is shown in Fig. 3. The strongest currents were present during N-FS1 (N 2.5 m/s) and the weakest during the lower amplitude cycles J-FS1 and N-FS2 (0.7–0.9 m/s). 3. Results The characteristics of the water column in the sampling site can be analysed through the TS diagrams
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derived from the CTD profiles. In these diagrams the three types of waters (SAW, MOW and NACW) involved in mixing processes on the Camarinal Sill can be easily detected (Fig. 4, Table 2). The CTD cast could be classified using the percentage of NACW in the upper 150 m of the water column. When no NACW is present, the TS diagram shows a mixing between SAW and MOW, thereby, these samples will be arbitrarily named in this paper as “Mediterranean Mixed Waters (MMW)”, and its presence will be marked with triangles in Figs. 4–11 to facilitate the interpretation of the figures. If, on the contrary, NACW is present in the upper layers of water the TS diagram shows a mixing between SAW and NACW; these stations will be called in this paper “Atlantic Mixed Waters (AMW)”, and, similarly, will be marked with squares in Figs. 4–11. Tidal induced dynamics can be studied by analysing the time evolution of basic descriptive variables (temperature, salinity, chlorophyll fluorescence and tidal current speed) recorded during the seven sampling events (Figs. 5–11). The details about the prediction of the tidal current shown in plate d could be revised in Alonso del Rosario et al. (2003). Following the same symbols proposed for Fig. 4, the triangles mark the CTD cast when MMW was found, and the squares show the time when AMW was detected. These two types of mixed water masses appear clearly separated in time during each cycle at the fixed stations. The presence of MMW is usually detected around HW−4 (i.e. 4 h before highwater), when the current velocity over the Sill changes from inflowing (positive values in plate d Figs. 5–11) to outflowing (negative values in plate d Figs. 5–11). There are however, some deviations from this general pattern. During N-FS1 (Fig. 5) the presence of MMW was detected a little bit earlier, around 2 h before high-water (HW-2). Also, in J-FS2 two “zero crossing” happened but MMW was only associated to the first shift event. It is also noticeable
Table 2 Salinity and temperature characteristics for the three types of water masses (derived from Gascard and Richez, 1985)
Tmax Tmin Smax Smin
SAW
MOW
NACW
20 19 36.4 36.2
13.5 13 38.5 38.2
14 13.5 36 35.6
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Fig. 5. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during N-FS1. Squares showed the presence of mixing of AMW and the triangles the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
that no AMW was detected in the CTD cast carried out in a northernmost position (H-FS1 and H-FS2, Figs. 8 and 9), a fact that will be analysed in discussion. Several undulatory phenomena were detected during the different sampling events, generally associated with the presence of MMW (Figs. 5a, 7a,
b, 8a,b, 9a,b) . These disturbances can be related with the internal waves generated over the Camarinal Sill. However, sometimes these undulations appear to be associated with the presence of AMW, as in N-FS2 (Fig. 6a) and in J-FS1 (Fig. 7a). This fact could be related to the lower velocity of the outflowing currents just before the sampling.
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Fig. 6. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during N-FS2. Squares showed the presence of AMW and the triangles the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
In Fig. 10a the record of the zonal current velocity registered by the ADCP during H-FS2 is shown. The maximum inflowing velocities (positive values) coincide in time with the presence of the undulatory processes (see Fig. 9a and b). These undulatory processes are also clearly visible in the record of a current-meter mooring deployed in the nearby of the
Camarinal Sill (Fig. 10b), being detected around 9 h before being sampled at the fixed station. Another common pattern observed is that the highest levels of chlorophyll coincide with the presence of MMW (Figs. 5c, 6c, 7c, 8c, 9c and 11c), suggesting a strong coupling of the physical forcing and the biological patterns that will be discussed later.
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4. Discussion 4.1. NACW signal and tidal amplitude The NACW signature corresponding to different tidal amplitude cycles can be studied by a combined analysis of results from all the diel data series obtained in the Strait. In previous works based on a compilation
of information from different cruises (Gascard and Richez, 1985) a negative relationship between the tidal amplitude and the quantity of NACW in the incoming Atlantic layer was found. In the present paper, a common time-period (from HW-5 to HW) has been chosen to compare the amount of NACW present in the water column for each sampling series and only the data of the upper
Fig. 7. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during J-FS1. Squares showed the presence of AMW and the triangles the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
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Fig. 8. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during H-FS1. Triangles show the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
150 m have been used. In this comparison we use the data of N-FS2, J-FS1, Myt-FS1 and Myt-FS2. The sampling in N-FS1 was not included in this analysis because it was very short (only 4 h) and was performed between HW-1 to HW+2, just at the end of the chosen period. The sampling events carried out on board BIO Hespérides were also not included because they did not show the NACW signature.
The percentage of NACW present in the upper layer of the water column can be estimated according to Bray et al. (1995). Using this, a clear inverse relationship between NACW percentage and the tidal outflowing velocity (towards the Atlantic) of the upper layer over the Sill is obtained (Fig. 13). The largest amount of NACW was detected during N-FS2 (tidal current velocity of 0.7 m/s, Fig. 6c) and the lowest quantity detected during Myt-FS2 (tidal velocity of
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Fig. 9. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during H-FS2. Triangles show the presence of MMW. The arrows and circles mark the moments when CTD profiles were made.
nearly 1.1 m/s, Fig. 12c). These results agree with the statement of Gascard and Richez (1985). The lack of NACW signal in the sampling performed on board BIO Hespérides (Fig. 4d and e) could be related with the displacement to the north of the sampling site (Fig. 1). Simulations of mixing and advection of water masses in the strait show that the northern half of the channel will be affected by the residual cyclonic circulation present eastwards Gibraltar (Izquierdo et al., 2001), in which no signal of NACW can be found
(Rodríguez et al., 1998). Also, the Coriolis force would tend to accumulate the NACW in the southern half of the Strait (Bray et al., 1995). The relationship between the tidal amplitude and the presence/absence and intensity of the NACW signal would be connected with the mixing processes over the Camarinal Sill. With strong tidal currents, the undulatory disturbances would be more frequent and more energetic (Armi and Farmer, 1988) and consequently, the mixing between the three water masses would be
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Fig. 10. a) Temporal variation of current velocity profiles (east component), recorded by a vessel-mounted ADCP, during H-FS2 sampling and time series of current velocity at 28 m depth. b) Current velocity intensity (at 160 m depth) in the nearby of Camarinal Sill, arrows indicate the arrival time of the internal waves previously generated around the Sill.
more effective. As was discussed by Bray et al. (1995), the signal of the less abundant NACW will be eroded away, making only SAW and MOW detectable in the TS diagram. However, mixing alone could not explain the absolute lack of NACW signal found in some of the CTD cast, being necessary other processes to account for that phenomenon. So, an alternative explanation would be related with the upper layer velocity direction over the Sill. This layer is, on average, flowing towards the Mediterranean, however with enough tidal amplitude and around HW; part of this layer could revert its movement and flow to the Atlantic (La Violette and Arnone, 1988; Candela, 1990; Bruno et al., 2002; Izquierdo et al., 2001). In this phase the NACW, situated below the SAW layer, could not overflow the sill, and, thereby, the composition of the water column shows only SAW and MOW (Gascard and Richez, 1985; Gómez et al., 2004). The absolute value of this western velocity has a very good
correlation with the quantity of NACW present in the water column (Fig. 13). 4.2. Trends in the temporal evolution In the Camarinal Sill region, tidal induced arrested internal waves are generated mainly from HW-3 to HW +2. These initially stationary waves are released to the eastern sector and start to move when the tidal current changes from outflowing to inflowing (Bruno et al., 2002; Alonso del Rosario et al., 2003). The arrival of these internal waves and associated MMW to the fixed station is expected around HW-4 (e.g. Farmer and Armi, 1986) which is in good agreement with the results showed in this work (Figs. 5–11). However, as mentioned above, the MMW appears earlier in N-FS1 (around HW+2), this being a consequence of quick advection from the Sill due to the strong incoming tidal current (N 2.5 m/s, Fig. 5d). There is another deviation of the general pattern
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Fig. 11. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during Myt-FS1. The arrows and circles mark the moments when CTD profiles were made.
during N-FS2, when the undulatory events are associated with the presence of AMW. This also happens in J-FS1 when the MMW only appears associated with the first HW-4 time, but appearing a clear signal of NACW during the second. The presence of undisturbed NACW signal is probably due to the weakness of the previous outflowing event, in which the tidal current barely reach − 0.5 m/s in NFS2 and − 0.2 m/s in J-FS1. That will lead to a lower
mixing over the sill or even, to avoid the reversion of the upper layer velocity, reducing the presence of MMW as, under these conditions, the NACW will flow over the sill throughout all the tidal cycle with a low degree of mixing and retaining its distinctive signature. It is also noteworthy that the first event detected during H-FS2 happens a little bit later than HW-4, probably due to the slower advection from the western side of the Strait caused by the small
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Fig. 12. Temporal evolution of salinity (p.s.u.), temperature (°C), chlorophyll (mg/m3) and current velocity prediction over the Camarinal Sill (m/s) during Myt-FS2. The arrows and circles mark the moments when CTD profiles were made.
inflowing current velocity (maximum of 0.6 m/s, Fig. 9d). In previous works (e.g. Gómez et al., 2000) the highest levels of chlorophyll had been related with the presence of the nutrient-rich NACW which could enhance the primary production by injecting nutrients in the upper layers of water. However, phytoplankton populations will need some time to grow using these new resources and, thereby, a noticeable increase in the phytoplankton concentration will only be detectable after a time enough to accumulate biomass, which is
likely to occur only out of the Strait, in the north-western Alborán Sea. In fact, some evidences of this have also been observed in this area (Packard et al., 1988; Ruiz et al., 2001) or even as far as the Almería-Orán front (Arnone et al., 1990) at the eastern end of the Alborán Sea basin. Surprisingly, in the results presented in this work there is a general pattern of coincidence between the maximum levels of chlorophyll and the presence of MMW (not NACW) as is presented in Table 3. The absolute values of the mean concentration in each
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Fig. 13. a) Percent of NACW in the upper 150 m for the different samplings. b) Relation of the quantity of NACW and the tidal outflowing velocity.
type of mixed water change from one sampling to another, as a consequence of either the annual chlorophyll cycle in the area, which has the minimum levels during summer and the maximum at the end of the winter season (García-Gorriz and Carr, 1999; Reul, 2000) or to the different position of the fixed station during each sampling, as the chlorophyll concentration is lower in the south section of the channel than in the north (Gómez et al., 2000; Echevarria et al., 2002; Reul et al., 2002). However, the percentage of increase within each sampling remains appreciably constant (Table 3), with a mean of 28.32% more chlorophyll in presence of MMW with respect to AMW. It is also interesting to note that, though in the two sampling events carried out in a northern position on Table 3 Mean superficial chlorophyll concentration in the stations with AMW and MMW (0–100 m) and the percentage of increase registered in the latest for each sampling Phase
Average chlorophyll in the upper 100 m in AMW stations (mg/m3)
Average chlorophyll in the upper 100 m in MMW stations (mg/m3)
% of increase in the chlorophyll concentration in presence of MMW
N-FS1 N-FS2 J-FS1 Myt-FS1 Myt-FS2
0.87 0.95 0.59 0.27 0.7
1.17 1.32 0.86 0.38 0.96
25.6% 28% 32% 29% 27% Mean: 28.32% (±2.4%)
board Hespérides there was a continuous presence of MMW, the highest levels of chlorophyll were related particularly with the presence of undulatory disturbances (Figs. 8c and 9c). The development of a modelling tool, a 1D physical– biological coupled model of the channel of the Strait that analyse the problem in a 3-layer scheme (Macías et al., submitted for publication), has confirmed the initial impression that the appearance of these pulsating chlorophyll-rich patches is unlikely to be explained by solely growth as the time needed to travel from the Sill to the eastern entrance of the Strait is not long enough to allow for a noticeable increase of the phytoplankton population. We have hypothesized that this intermittent increase in chlorophyll concentration could alternatively be explained by horizontal advection of water masses with high concentrations of chlorophyll coming from the coast to the centre of the channel forced by tidal hydrodynamics. As commented above, the Atlantic layer over the Camarinal Sill will, sometimes, change the direction of its flow. However, the whole surface layer does not flow towards the Atlantic (as shown by the model of Izquierdo et al., 2001), but the eastern half flows eastwards throughout the tidal cycle. This situation creates a horizontal divergence over the Camarinal Sill which would force a replacement of surface water with coastal waters coming from the shallower marginal areas. These coastal waters tend to be chlorophyll-rich and could be incorporated as discrete periodical patches to the main Atlantic current flowing to the Mediterranean. This hypothesis could
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explain the coincidence of high chlorophyll values with periods of intense interfacial mixing which dilutes the NACW signal. Currently, further investigation is being carried out to test this hypothesis.
5. Conclusions Tidal amplitude has been observed to explain the NACW presence/absence patterns in the surface water masses flowing to the Mediterranean. This influence can be linked to changes induced in the extension and intensity of the mixing processes or related with modifications of the water masses circulation scheme over the Camarinal Sill. The advection velocity of the undulatory disturbances is in quite good agreement with that predicted by theoretical estimations. The maximum levels of fluorescence coincide with the presence of MMW and more specifically with the existence of undulatory disturbances. The amount of chlorophyll involved cannot be explained solely by in situ growth driven by the fertilization of the superficial waters, but rather could be related with 2D complex coastal–channel interactions that will be investigated in future cruises and modelling exercises.
Acknowledgements This work has been funded by the Project of the National Research Program REN-2001-2733-C02-02. Diego Macías and Agueda Vázquez were supported by grants of the FPI fellowship program. We acknowledge also the Spanish Navy “Instituto Hidrográfico de la Marina” that offered support for sampling and several cruises on board BO Malaspina and BO Tofiño. Dr. Edward P. Morris made a kind revision of the English version of the manuscript. References Alonso del Rosario, J.J., Bruno Mejías, M., Vázquez-Escobar, A., 2003. The influence of tidal hydrodynamic conditions on the generation of lee waves at the main sill of the Strait of Gibraltar. Deep-Sea Research I 50, 1005–1021. Armi, L., Farmer, D., 1985. The internal hydraulics of the Strait of Gibraltar and associated sill and narrows. Oceanologica Acta 8 (1), 37–46. Armi, L., Farmer, D., 1988. The flow of Mediterranean Water through the Strait of Gibraltar. Progress in Oceanography 21, 41–82.
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