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Density and flow structure in the Clyde Sea front
Continental Shelf Research 19 (1999) 1833}1848
Density and #ow structure in the Clyde Sea front A. Kasai *, T.P. Rippeth, J.H. Simpson Department of Fisheries, Graduate School of Agriculture, Kyoto University, Kyoto, 606-8502, Japan University of Wales, Bangor, School of Ocean Sciences, Menai Bridge, Anglesey, LL59 5EY, UK Received 12 November 1998; received in revised form 1 July 1999; accepted 8 July 1999
Abstract A well-de"ned front in temperature and salinity separates the strati"ed Clyde Sea water from the vertically well mixed water of the North Channel. The detailed structure of the front was observed in autumn 1990 by a combination of, repeated crossings of the front using a shipborne ADCP and a towed undulating CTD system, and the deployment of a "xed mooring system with temperature, salinity and velocity sensors for a period of 12 days. The results show that the front was situated on the Great Plateau near a contour of log (H/; )"2.7&3.7 where H is the water depth and ; the amplitude of M tidal velocity. The temperature structure in the Clyde Sea was inverted and the Clyde Sea surface temperature was lower than that of the vertically well mixed water in the North Channel. Since the salinity gradient was stronger than the temperature gradient with fresher water on the surface, the density structure was predominantly controlled by salinity. There were indications of warm and saline bottom water upwelling on the mixed side of the front during spring tides. This upwelling disappeared and the salinity and temperature structure at the front was more di!use during the neap tide period. A jet-like along-front residual current was observed #owing to the northwest in the surface layer with a counter #ow to the southeast in the bottom layer. The vertical di!erence in velocity was about 9 cm s\ and was approximately consistent with the shear determined from the thermal wind relation. Both cross- and along-front components of the current observed at the mooring station varied in response to the advection of the front, although both components had large variations with periods of less than one day and several days. The front was advected past the mooring system by a mean #ow from the North Channel to the inner basin, while oscillating 3}5 km back and forth with the tidal currents. From the velocity at a current meter mooring and CTD data, the front was estimated to have moved up to 20 km during the observational period and the cross frontal velocity was inferred to be 3}4 cm s\. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Tidal mixing fronts; Strati"cation; Fjord dynamics; Flow structure; Clyde Sea
* Corresponding author. Tel.: #81-75-753-6216; fax: #81-75-753-6227. E-mail address: [email protected] (A. Kasai) 0278-4343/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 9 9 ) 0 0 0 4 2 - 4
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1. Introduction The Clyde Sea (Fig. 1) is a deep, partially enclosed basin, on the west coast of Scotland. Its entrance is marked by a sill where there is a depth minimum of 45 m which restricts communication between the deep water (h'150 m) in the basin and the North Channel (h'150 m). Freshwater in#ow enters the basin from the Clyde and other rivers at a rate of 60}700 m s\ (Poodle, 1986) and produces a typical fjordic regime with the salinity of the out#owing surface water reduced by up to 1.5 relative to the bottom water, which is maintained at a higher salinity by in#ow over the sill. The buoyancy input by freshwater, augmented by surface heating in the spring and summer months, tends to maintain a stable strati"cation which is not seriously eroded by tidal stirring on account of the generally weak tidal #ows (;(0.2 m s\) in the deep water of the basin. Even in autumn and winter, when surface cooling results in the formation of a temperature inversion (*t(13C) which acts to destabilize the water column, the structure remains stable, except for infrequent episodes of complete mixing (Rippeth and Simpson, 1996). By contrast, tidal currents reach 1 m s\ in the adjoining North Channel, so that its stirring is extremely vigorous and promotes complete vertical mixing to the full depth of the channel. The strong tidal currents in this region #ow predominantly parallel to the sill and, thus, ensure the rapid removal of waters leaving the Clyde basin in the exchange #ow over the sill. Conditions at the mouth of the Clyde Sea system are, therefore, those of the well-mixed waters of the North Channel (Howarth, 1982). Between these strati"ed and well-mixed regimes, there is a frontal zone with large horizontal gradients located close to the entrance sill. In early summer, the temperature gradient reaches 10\C3 m\ and is large enough to identify the front clearly in satellite infrared imagery (Fig. 1(b)). However, it is the salinity contrast (*s&5;10\ m\) which is responsible for the main contribution to the large density gradient (*.&10\ kg m\). This density gradient across the front, which is among the most intense in the European shelf seas, tends to drive an exchange #ow between the two very di!erent regimes on either side. The dynamics are complicated by the in#uence of the earth's rotation, as the horizontal scale of the entrance (20 km) is large compared with the internal Rossby radius (usually less than 5 km). We are concerned, therefore, with the rather subtle problem of baroclinic #ow over steep topography under the in#uence of the Coriolis forces, with strong frictional e!ects induced in the relatively large tidal #ow over and outside of the sill. Although the existence and general character of the front have been reported previously (Gmitrowicz, 1981; Edwards et al., 1986), rather little is known of its internal structure and the associated #ow "eld which determines the net exchange over the sill. A better understanding of the residual #ow over the sill is needed to improve models of the seasonal cycle of strati"cation, like that of Simpson and Rippeth (1993) which employed a simpli"ed parameterisation of the exchange in terms of the density di!erence across the sill. Improved models of the physical processes are essential for studies on the nutrient balance (Jones et al., 1995) and other water quality questions in the Clyde Sea which has been increasingly a!ected by inputs from industry and land run-o!.
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Fig. 1. (a) Study area of the Clyde Sea front. Open triangles indicate CTD stations and the solid circle mooring position. ADCP and SEAROVER observations were carried out along the thick straight line. GP and NC denote the Great Plateau and North Channel respectively. (b) Satellite thermal image in the Clyde Sea and the North Channel taken on 29 May 1997. Darker and brighter tones indicate warmer and colder areas, respectively. A strong front can be detected at the mouth of the Clyde Sea. (c) Contours of strati"cation Parameter S "log (H/; ), where H is the water depth and ; is the amplitude of M tidal & velocity.
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As with other tidal mixing fronts, the problems of observing the detail of the structure and #ow are complicated by the tidal #ow. Large displacements of the structure occur during the sampling period and limit the resolution of synoptic surveys with a conventional CTD, thus making it di$cult to obtain a de"nitive view of the frontal structure. In this paper, we report on new observations which attempt to overcome this limitation by using a combination of an undulating CTD and a shipborne ADCP to provide high-resolution quasi-synoptic pictures of the density structure over the tidal cycle and allow determination of the mean #ow "eld. These rapid-survey results are complemented by time series data from a mooring deployed within the frontal zone which allows us to focus on the variations in temperature, salinity and velocity in relation to the movement of the front. In addition, a new method is proposed to estimate the cross frontal velocity based on the Hansen} Rattray estuarine circulation.
2. Observational methods Observations were carried out using the research vessel RRS Challenger. Most of the temperature and salinity distributions reported here were observed with a SEAROVER CTD system, which cycles between the surface and a depth of 70 m while the ship steams at &3.5 m s\. This undulating CTD system is ideal for observing variable frontal structure because it can map rapidly both temperature and salinity with horizontal resolution of &1 km or better (Hill et al., 1993). An initial survey using a conventional CTD was performed at station intervals of 5 or 6 km from 20 to 22 November 1990, to detect the main synoptic features of the front (Fig. 1). Two intensive SEAROVER surveys were carried out on a section across the front, each over a period greater than a full tidal cycle. The "rst survey was around spring tides from 3 : 50 to 21 : 50 (GMT) on 20 November 1990 (JD 323), and the second around neap tides from 21 : 09 on 25th (JD 329) to 4 : 16 on 27th (JD 331). A ship-mounted ADCP was used simultaneously with the SEAROVER to measure horizontal water velocities on the section. The ADCP provided velocity in 4 m depth bins, below a surface layer 10 m deep, and above a bottom `shadow zonea which typically has a thickness of 15% of the total water depth. The raw velocities were resolved into along and cross front components by rotation of the axis by 453 in an anti-clockwise direction. Residual currents were estimated by removing tidal currents using the tidal analysis technique described by Simpson et al. (1990). A mooring system was deployed in 48 m of water depth on the Great Plateau (55313.9 N, 5314.9 W, Fig. 1) during the period 15}27 November 1990 (JD 320}331). The system was equipped with Aanderaa instruments measuring velocity, temperature and salinity continuously with a 10 min interval. Five instruments were "xed on the mooring but two of them were lost as a result of damage caused by "shing nets. Temperature at the surface, temperature and salinity at 33 m above the seabed (about 15 m depth from the surface) and temperature, salinity and velocity data at 8 m above the seabed (about 40 m depth from the surface) were recorded successfully.
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3. Results 3.1. Temperature and salinity structure Fig. 2 shows cross-frontal sections constructed from CTD data. The water in the North Channel (left-hand side in the "gure) was strongly mixed whilst that in the Clyde Sea (right-hand side in the "gure) was strati"ed. Both the thermocline and halocline were detected near the seabed at about 50 m at C1. Between the two regimes, there was a high gradient frontal region. The front apparently intersected the surface between Stn. C3 and C4 and the bottom between C1 and C2. The position of the front was near a contour of the strati"cation parameter S "2.7 and 3.7 at the surface and & bottom, respectively, although the position or shape cannot be detected precisely from the horizontally coarse CTD survey (Figs. 1(c) and 2). S is given by log (H/; ), & where H is the water depth and ; the amplitude of M tidal velocity. The temperature structure in the Clyde Sea was inverted at this time; the temperature was 10.83C in the surface layer whilst nearly 11.23C in the bottom layer. Both the temperature and salinity of the bottom layer at C1 were lower than values in the North Channel. Since the salinity gradient at the front was stronger than the temperature gradient, the density structure was predominantly controlled by salinity
Fig. 2. Cross frontal (a) temperature (3C), (b) salinity and (c) p sections obtained by CTD observation. R
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Fig. 3. Temperature (3C) and salinity sections obtained by SEAROVER in a spring tide period (a and b) and a neap tide period (c and d).
so that the density was higher in the North Channel than that in the Clyde Sea by *. 0.5 kg m\. The same structure with much more detail is evident in the vertical sections of temperature and salinity across the front observed by the SEAROVER. Fig. 3(a) and (b) show examples in a spring tide (7 : 50}9 : 18 20 November) and Fig. 3(c) and (d) in a neap tide (11 : 48}12 : 58 26 November). Both sections were at approximately the same state of the tide (within 2 h of low water). Temperature and salinity fronts resemble each other closely in shape at both the springs and neaps situations. They illustrate a well-de"ned front with the strongest surface gradients around x"16 km (spring tide) and x"15 km (neap tide), where x is the distance from southwestern edge of the "rst ADCP line. For the spring tide sections (Fig. 3(a) and (b)) there is strong indication of warm and saline bottom water uplifted on the mixed side of the front with associated temperature and salinity maxima at the sea surface around x"12 km. During the neap tide (Fig. 3(c) and (d)) this upwelling of warm saline water was not apparent and the front was more di!use. The maximum density gradient at the front also became signi"cantly weaker at the neap tide (1.0;10\ kg m\) than that at the spring (1.5;10\ kg m\). In addition, a well-de"ned sloping interface at springs was absent at neaps when the front was lying horizontally from x"17 to 25 km, bending sharply towards the surface around x"15 km.
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Fig. 4. Examples of density sections across the front during a spring tide period (a}c) and a neap tide period (d}f). Horizontal bars indicate typical tidal excursion estimated from the current meter data.
Examples of changes in the density "eld during one tidal cycle are shown in Fig. 4. They indicate that the front was advected 3}5 km back and forth by tidal currents. Since tidal currents are usually stronger in the upper layer than the lower layer, the displacement of the front should be larger in the upper layer leading to tidal straining of the front, which was clearly steeper at high water than at low water, during the springs survey (Fig. 4(a) and (c)). In addition, density gradients in the upper layer were weaker at low water than at high water, although those in the lower layer showed little di!erence. A similar tendency was detected during the neap tide, but the tidal straining was weaker.
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3.2. Flow xeld from the ADCP measurements Fig. 5 shows the along and cross-frontal components of the residual #ow, measured by the ADCP, obtained at the spring (Fig. 5(a) and (b)) and the neap tide (Fig. 5(c) and (d)). There is some evidence of an along front shear #ow during the spring tide survey in the region 18(x(25 km, where the strongest temperature and salinity gradients were located at this time (see Fig. 3(a) and (b)). The #ow in the low density surface layer was &4 cm s\ to the northwest, while near the bed it was &5 cm s\ to the southeast. The di!erence *u&9 cm s\ is approximately consistent with the thermal wind relation (*u&10 cm s\). The vertical di!erence in the along front velocity at the neap tide was 6 cm s\, smaller than that at the spring tide, which is consistent with weaker density gradients observed at the neap tide (Fig. 3(c) and (d)). This jet-like #ow, in the upper layer, along the front and consequent velocity di!erence were detected in each sectional observation oscillating back and forth with the tidal phase. In the mixed water to the southwest (x(15 km) there was a stronger #ow to the southeast of up to 10 cm s\ at the spring tide. The #ow, however, showed large variations in speed and direction and no consistent tendency.
Fig. 5. ADCP residual velocity obtained by least squares removal of tidal signal at a spring (a and b) and a neap tide (c and d).
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The cross-frontal #ow was landward almost everywhere on both sections (Fig. 5(b) and (d)) with speeds up to 10 cm s\ and the barotropic component, rather than the baroclinic, was predominant, especially in the neap case. The spatial structure of the barotropic component is not apparently related to the density "eld and may be a result of the incomplete removal of the tidal signal (;&35 cm s\). 3.3. Temperature, salinity and velocity variations at the mooring We consider next the data from the mooring C3 that recorded temperature and salinity changes for a period when the front was in the vicinity of the mooring. Salinity and temperature time series measured at the surface, 15 and 40 m depth at the mooring are given in Fig. 6. Temperature at the surface and at 15 m depth was nearly the same during the whole observational period which indicates that the upper water, at least shallower than 15 m, was well mixed even in the strati"ed Clyde Sea. A clear tidal signal in salinity was present continuously in the upper layer (15 m) and intermittently in the near bed (40 m). This indicated that the front was situated near the mooring system and was oscillated by tidal currents through the mooring, when the signal was strong. A similar signal was recognizable in the temperature series, although it was weaker than that in salinity. Fig. 7 displays the time sequences of the current observed at the mooring site. On the basis of velocity measurements presented in this "gure, the amplitude of the cross-sill tidal currents was 32 cm s\ in springs and 23 cm s\ in neaps at the mooring site. This indicates that the cross-frontal displacement was about 4.5 and 3 km at springs and neaps, respectively. These values are consistent with the movement of the front observed by the SEAROVER (Fig. 4). The displacement of the front is short compared to the width of the sill, which is &30 km (h(50 m). The tidal currents themselves will not, therefore, cause the abrupt intrusion of North Channel water into the Clyde Sea through the bottom layer, unless the front is situated at the northeastern edge of the sill. The along front residual #ow, in the bottom layer at the mooring site, was generally directed to the southeast (indicated by negative values in Fig. 7(b)). On the other hand, the cross frontal component was directed into the basin (positive values in Fig. 7(d)) in the lower layer for most of the deployment period. This tendency in both components is consistent with the results of the ADCP measurements. However, it is worth noting that both components have large variations with periods of less than one day and of several days. Each component showed an apparently di!erent pattern around JD 328 from the other periods. This event happened simultaneously with the whole water column becoming vertically uniform as shown in Fig. 6, and indicates the occurrence of North Channel water at the mooring site. The front is subject to advection by the cross frontal currents caused by the tides, wind-induced currents and so forth. Assuming that the cross-frontal component at the mooring site (u) and that at the front are equal, the position of the front (x) can be estimated by integrating the time series of u. The result is shown in Fig. 8 (thin line). The time sequence indicates the front has been advected about 40 km into the basin during the observational period. The estimated movement is, however, inconsistent with the real movement. It is biased by using the measured velocity near the bed and
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Fig. 6. Time sequences of (a) temperature and (b) salinity observed at mooring station. Horizontal bars indicate the period of ADCP observation.
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Fig. 7. Time sequences of (a) the along front component of tidal current (3153) and (b) the along front component of tidal residual current, (c) the across front component of tidal current (453) and (d) the across front component of tidal residual current obtained at the mooring station. Horizontal bars indicate the period of ADCP observation.
the method takes no account of variation of u with depth. To allow for the latter, the depth mean cross-frontal displacement is considered instead of u as
x" (u!u ) dt, CQ
(1)
where u is the cross-frontal current caused by the estuarine circulation. By analogy CQ with the Hansen}Rattray estuarine circulation, u is taken to be proportional to the CQ density di!erence between the North Channel water (. ) and the basin water in the ,! Clyde Sea (. ): !1 (2) u "C(. !. ), CQ ,! !1 where C is a constant. It is also known from Fig. 3 that the distance from southwestern edge of the ADCP line to the position where the front penetrated the surface, were x"16 km at ¹"324.4 JD and x"15 km at ¹"330.0 JD. These values can be used
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Fig. 7. (Continued).
to determine an integral constant in Eq. (1) and the constant C in equation Eq. (2), and then to recalculate the movement of the frontal position. The thick line in Fig. 8 is the position of the surface front estimated using Eq. (1). It is shown from the estimation that after a sudden movement into the basin on JD 322, there was little frontal movement for two days after which it was gradually advected towards the basin after JD 324. It reached the northeastern edge of the ADCP line (x"25 km), the position of the mooring station at JD 326. It then returned seaward for the subsequent two days and stayed at around x"17 km for the remaining three days. The distance moved was 20 km, from x"5 to 25 km (Fig. 8), and indicates that the surface front was advected from S "2.7 to 3.7 (Fig. 1(c)). The estimated value of & cross frontal velocity, u , is 3}4 cm s\ with variations of about 0.5 cm s\. CQ This estimated frontal movement explains satisfactorily the temperature and salinity variations observed at the mooring site. Fig. 9 illustrates a schematic view of the frontal movement. The distance between surface and bottom fronts appeared about 12 km (Fig. 2). Around JD 321 the surface front was at x"8 km (Fig. 8), so that the bottom front should have been located at about x"20 km and the position of the mooring inshore of the bottom front (position A in Fig. 9). Both temperature and
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Fig. 8. Position of surface front estimated from the cross-front velocity, u, at the mooring station. Thin line is the position estimated from a time series of u and thick line is from u!u . Two solid circles show the CQ positions of the front observed by the SEAROVER observations.
Fig. 9. Schematic view of the movements of the front.
salinity in the lower layer measured at the mooring recorded minima of 11.73C and 33.7 around JD 322 (Fig. 6). In addition, the along front velocity in the lower layer was large in this period (Fig. 7). The average speed over the two days (JD 321}322) was about 18 cm s\, and nearly 10 cm s\ faster than the subsequent period. The preceding and subsequent large changes in salinity and temperature at 40 m depth varying with the tidal periods indicate the bottom front passed the mooring, moving towards the North Channel on JD 321, and moved back landward on JD 323 (position A and B in Fig. 9). On the other hand, both salinity and temperature variations in the upper layer were small without the tidal signal because the surface front was situated 7}18 km away from the mooring.
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The salinity in the upper layer increased gradually from JD 324 to 327 as did the amplitude of the semi-diurnal variation (Fig. 6), in spite of the fact that the tidal currents were decreasing at this time (Fig. 7). This variation indicates that the surface front was gradually approaching the mooring site, as it moved shoreward (position B and C in Fig. 9). During this period, the observed temperature and salinity in the lower layer varied little because the bottom front had already moved landward of the mooring. The estimated position of the surface front supports this prediction, showing a movement from x"15 to 25 km (Figs. 8, 9B and C). Between 20 h on JD 327 and 14 h on JD 328, completely mixed water was observed at the mooring (Fig. 6). Vertical di!erence in temperature and salinity dropped to (0.23C and (0.1 respectively, for a period of 18 h, while the front was to the northeast of the mooring (Fig. 9C). Conspicuous strong northeastward currents also suggested that the mooring was in the North Channel water in this period (Fig. 7(b)) because the residual current in the North Channel is usually northward and highly variable (e.g., Dickson et al., 1987). Soon after this, the front moved back and the mixed water was replaced by a strati"ed column, and the along-frontal component returned to the normal southeastward #ow. Pronounced oscillations in both the surface and bottom layers at this time suggest that a steeply inclined front was being advected close to the mooring position.
4. Discussion In this study, the Clyde Sea front was observed near the contour of S 3 with & a distance of 12 km between surface and bottom fronts. The position of the surface front was consistent with previous descriptions (Edwards et al., 1986) but the use of the SEAROVER system has provided a more detailed picture of the density "eld in the frontal zone. The results con"rmed the existence of large near-surface density gradients with values, on a scale of 1 km, of up to 1.5;10\ kg m\, during the springs survey. Maximum gradients of comparable magnitude extend down through much of the water column although the near-bed values have not been sampled by the SEAROVER because of the hazards of near-bed operation. Tidal mixing fronts produced by the heating}stirring competition occurring in the European shelf seas at S 2.7. In the case of the Clyde Sea front, however, & the buoyancy input is due to both surface heat exchange and fresh water input. During the summer season when run-o! is relatively low and heating predominant, the front is expected to be located close to S 2.7 as indicated by the infrared & image of Fig. 1(b). By contrast in November, heat is being lost from the sea surface and the front is maintained by freshwater buoyancy input alone so we would expect the front to be located at a di!erent value of S . The observed value of & S 3, i.e. at a lower level of tidal mixing, suggests that the net buoyancy supply & in November is less than during the summer heating period. It is also notable that, at this time, the bottom front was inside the basin and situated near the contour of S 4, which is one order of magnitude bigger than that of the surface & front.
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The apparent uplift of warmer saltier water on the mixed side of the front at springs has not been observed here previously, but analogous uplifting of cold heavier water at tidal mixing fronts has been reported (Allen et al., 1980). This feature and the intensi"cation of the gradients at spring tides has been explained by two models. First, Bowers and Lwiza (1994) reported in terms of a vertical mixing model that the net heat #ux into the sea is greater on the mixed side of the front than on the strati"ed side during the heating season. The formation of a minimum temperature zone was attributed to the di!erence in the heat #ux when the water is mixed with increased tidal stirring at spring tides. This model, however, is not appropriate to our case because it was the cooling season as shown by the temperature inversion (Figs. 2 and 3) and the strati"cation was maintained by the salinity. In the other model proposed by Garrett and Loder (1981), the vertical shear of the cross-frontal #ow is important; the upper layer tends to #ow out towards the mixed region, whilst the lower layer #ows into the strati"ed region by the frictional e!ect when the Ekman number is large. Since the across-front currents were more baroclinic, with a strong landward #ow in the lower layer in the vicinity of the front, during the springs than the neaps (Fig. 5), the latter explanation would seem to o!er a more convincing explanation for the observed structure. The residual #ow "eld deduced from the ADCP observations, for both springs and neaps surveys, shows some evidence of intensi"ed shear #ow parallel to the front, associated with the strong horizontal gradients. To a "rst order, the shear involved is consistent with that calculated from the thermal wind relation. The velocity di!erences involved are, however, not much greater than the other apparent variability in this "eld, which may result from imperfect removal of the much stronger tidal #ow. Such uncertainty also applies to the cross-frontal component, although the observed mean landward #ow is consistent with current measured at the mooring. While the mooring data was limited by the loss of instruments, it has been possible to convincingly relate the observed changes in temperature and salinity in the upper and lower layers to the e!ects of horizontal displacement by the mean and tidal #ow. In summer the front is often situated on the North Channel side of the sill, as shown in Fig. 1(b), because the heating and a large amount of fresh water input intensify the strati"cation in the Clyde Sea. The overall mean displacement (&20 km) is comparable to the range for the summer}winter di!erence reported by Edwards et al. (1986) and indicative of the scale of possible shorter period movements of the front. It is interesting that the curved shape of the surface front in Fig. 1(b) resembles the contour lines of S at the mouth of the basin & (Fig. 1(c)). As pointed out by Fujiwara et al. (1997) short time series of ADCP observations provide only a snapshot of the #ow "eld which may be unrepresentative of the longer term #ow pattern and its variability. In this study, we have combined snapshots obtained from the ADCP with the limited mooring data to obtain encouraging results. In future studies it would be desirable to develop this approach with more extensive ship surveys using ADCP and undulating CTD in combination with an array of moorings.
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Acknowledgements We are grateful to Dr. A.J. Elliott for providing us with the current data in Fig. 1(c) calculated by his model. We are also grateful to University of Dundee for providing the satellite image. This work was undertaken during the visit of A. Kasai to the University of Wales, Bangor under the program of Postdoctoral Fellowships for Research Abroad by Japan Society for the Promotion of Science.
References Allen, C.M., Simpson, J.H., Carson, R.M., 1980. The structure and variability of shelf sea fronts as observed by an undulating CTD system. Oceanologica Acta 3, 59}68. Bowers, D.G., Lwiza, K.M., 1994. The temperature minimum at tidal fronts. Annales Geophysicae 12, 683}687. Dickson, R.R., Durance, J.A., Howarth, M.J., Hill, A.E., 1987. Irish sea status report of the Marine Pollution Monitoring Management Group. In: Dickson, R.R. (Ed.), Aquatic Environment Monitoring Report 17 MAFF, pp. 9}10. Edwards, A. Baxter, M.S., Ellett, D.J., Martin, J.H.A., Meldrum, D.T., Gri$ths, C.R., 1986. Clyde Sea hydrography. Proceedings of the Royal Society of Edinburgh, vol. 90B, pp. 67}83. Fujiwara, T., Sanford, L.P., Nakatsuji, K., Sugiyama, Y., 1997. Anti-cyclonic circulation driven by the estuarine circulation in a gulf type ROFI. Journal of Marine Systems 12, 83}99. Garrett, C.J.R., Loder, J.W., 1981. Dynamical aspects of shallow sea fronts. Philosophical Transactions of Royal Society of London A 302, 563}581. Gmitrowicz, E.M., 1981. Some aspects of the hydrography of the Clyde Sea, North Channel. M.Sc. Thesis, University of Wales, 56 pp. Hill, A.E., James, I.D., Linden, P.F., Matthews, J.P., Prandle, D., Simpson, J.H., Gmitrowicz, E.M., Smeed, D.A., Lwiza, K.M.M., Durazo, R., Fox, A.D., Bowers, D.G., 1993. Dynamics of tidal mixing fronts in the North Sea. Philosophical Transactions of Royal Society of London 343, 431}446. Howarth, J., 1982. Non-tidal #ow in the North Channel of the Irish Sea. In: Nihoul, J.C.J. (Ed.), Hydrodynamics of Semi-enclosed Seas, Proceedings of the 13th International Liege Colloquium on Ocean Hydrodynamic. Elsevier, Amsterdam, pp. 205}242. Jones, K.J., Grantham, B., Ezzi, I., Rippeth, T., Simpson, J., 1995. Physical controls on phytoplankton and nutrient cycles in the Clyde Sea, a fjordic system on the west coast of Scotland. In: Skjoldal, H.R., Hopkins, C., Erikstad, K.E., Leinaas, H.P. (Eds.), Ecology of Fjords and Coastal Waters. Elsevier, Amsterdam, pp. 93}104. Poodle, T., 1986. Freshwater in#ows to the Firth of Clyde. Proceedings of the Royal Society of Edinburgh 90B, 55}66. Rippeth, T.P., Simpson, J.H., 1996. The frequency and duration of episodes of complete vertical mixing in the Clyde Sea. Continental Shelf Research 16, 933}947. Simpson, J.H., Rippeth, T.P., 1993. The Clyde Sea: Model of the seasonal cycle of strati"cation and mixing. Estuarine, coastal and shelf science 37, 129}144. Simpson, J.H., Mitchelson-Jacob, E.G., Hill, A.E., 1990. Flow structure in a channel from an acoustic Doppler current pro"ler. Continental Shelf Research 10, 589}603.
Density and #ow structure in the Clyde Sea front A. Kasai *, T.P. Rippeth, J.H. Simpson Department of Fisheries, Graduate School of Agriculture, Kyoto University, Kyoto, 606-8502, Japan University of Wales, Bangor, School of Ocean Sciences, Menai Bridge, Anglesey, LL59 5EY, UK Received 12 November 1998; received in revised form 1 July 1999; accepted 8 July 1999
Abstract A well-de"ned front in temperature and salinity separates the strati"ed Clyde Sea water from the vertically well mixed water of the North Channel. The detailed structure of the front was observed in autumn 1990 by a combination of, repeated crossings of the front using a shipborne ADCP and a towed undulating CTD system, and the deployment of a "xed mooring system with temperature, salinity and velocity sensors for a period of 12 days. The results show that the front was situated on the Great Plateau near a contour of log (H/; )"2.7&3.7 where H is the water depth and ; the amplitude of M tidal velocity. The temperature structure in the Clyde Sea was inverted and the Clyde Sea surface temperature was lower than that of the vertically well mixed water in the North Channel. Since the salinity gradient was stronger than the temperature gradient with fresher water on the surface, the density structure was predominantly controlled by salinity. There were indications of warm and saline bottom water upwelling on the mixed side of the front during spring tides. This upwelling disappeared and the salinity and temperature structure at the front was more di!use during the neap tide period. A jet-like along-front residual current was observed #owing to the northwest in the surface layer with a counter #ow to the southeast in the bottom layer. The vertical di!erence in velocity was about 9 cm s\ and was approximately consistent with the shear determined from the thermal wind relation. Both cross- and along-front components of the current observed at the mooring station varied in response to the advection of the front, although both components had large variations with periods of less than one day and several days. The front was advected past the mooring system by a mean #ow from the North Channel to the inner basin, while oscillating 3}5 km back and forth with the tidal currents. From the velocity at a current meter mooring and CTD data, the front was estimated to have moved up to 20 km during the observational period and the cross frontal velocity was inferred to be 3}4 cm s\. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Tidal mixing fronts; Strati"cation; Fjord dynamics; Flow structure; Clyde Sea
* Corresponding author. Tel.: #81-75-753-6216; fax: #81-75-753-6227. E-mail address: [email protected] (A. Kasai) 0278-4343/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 9 9 ) 0 0 0 4 2 - 4
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1. Introduction The Clyde Sea (Fig. 1) is a deep, partially enclosed basin, on the west coast of Scotland. Its entrance is marked by a sill where there is a depth minimum of 45 m which restricts communication between the deep water (h'150 m) in the basin and the North Channel (h'150 m). Freshwater in#ow enters the basin from the Clyde and other rivers at a rate of 60}700 m s\ (Poodle, 1986) and produces a typical fjordic regime with the salinity of the out#owing surface water reduced by up to 1.5 relative to the bottom water, which is maintained at a higher salinity by in#ow over the sill. The buoyancy input by freshwater, augmented by surface heating in the spring and summer months, tends to maintain a stable strati"cation which is not seriously eroded by tidal stirring on account of the generally weak tidal #ows (;(0.2 m s\) in the deep water of the basin. Even in autumn and winter, when surface cooling results in the formation of a temperature inversion (*t(13C) which acts to destabilize the water column, the structure remains stable, except for infrequent episodes of complete mixing (Rippeth and Simpson, 1996). By contrast, tidal currents reach 1 m s\ in the adjoining North Channel, so that its stirring is extremely vigorous and promotes complete vertical mixing to the full depth of the channel. The strong tidal currents in this region #ow predominantly parallel to the sill and, thus, ensure the rapid removal of waters leaving the Clyde basin in the exchange #ow over the sill. Conditions at the mouth of the Clyde Sea system are, therefore, those of the well-mixed waters of the North Channel (Howarth, 1982). Between these strati"ed and well-mixed regimes, there is a frontal zone with large horizontal gradients located close to the entrance sill. In early summer, the temperature gradient reaches 10\C3 m\ and is large enough to identify the front clearly in satellite infrared imagery (Fig. 1(b)). However, it is the salinity contrast (*s&5;10\ m\) which is responsible for the main contribution to the large density gradient (*.&10\ kg m\). This density gradient across the front, which is among the most intense in the European shelf seas, tends to drive an exchange #ow between the two very di!erent regimes on either side. The dynamics are complicated by the in#uence of the earth's rotation, as the horizontal scale of the entrance (20 km) is large compared with the internal Rossby radius (usually less than 5 km). We are concerned, therefore, with the rather subtle problem of baroclinic #ow over steep topography under the in#uence of the Coriolis forces, with strong frictional e!ects induced in the relatively large tidal #ow over and outside of the sill. Although the existence and general character of the front have been reported previously (Gmitrowicz, 1981; Edwards et al., 1986), rather little is known of its internal structure and the associated #ow "eld which determines the net exchange over the sill. A better understanding of the residual #ow over the sill is needed to improve models of the seasonal cycle of strati"cation, like that of Simpson and Rippeth (1993) which employed a simpli"ed parameterisation of the exchange in terms of the density di!erence across the sill. Improved models of the physical processes are essential for studies on the nutrient balance (Jones et al., 1995) and other water quality questions in the Clyde Sea which has been increasingly a!ected by inputs from industry and land run-o!.
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Fig. 1. (a) Study area of the Clyde Sea front. Open triangles indicate CTD stations and the solid circle mooring position. ADCP and SEAROVER observations were carried out along the thick straight line. GP and NC denote the Great Plateau and North Channel respectively. (b) Satellite thermal image in the Clyde Sea and the North Channel taken on 29 May 1997. Darker and brighter tones indicate warmer and colder areas, respectively. A strong front can be detected at the mouth of the Clyde Sea. (c) Contours of strati"cation Parameter S "log (H/; ), where H is the water depth and ; is the amplitude of M tidal & velocity.
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As with other tidal mixing fronts, the problems of observing the detail of the structure and #ow are complicated by the tidal #ow. Large displacements of the structure occur during the sampling period and limit the resolution of synoptic surveys with a conventional CTD, thus making it di$cult to obtain a de"nitive view of the frontal structure. In this paper, we report on new observations which attempt to overcome this limitation by using a combination of an undulating CTD and a shipborne ADCP to provide high-resolution quasi-synoptic pictures of the density structure over the tidal cycle and allow determination of the mean #ow "eld. These rapid-survey results are complemented by time series data from a mooring deployed within the frontal zone which allows us to focus on the variations in temperature, salinity and velocity in relation to the movement of the front. In addition, a new method is proposed to estimate the cross frontal velocity based on the Hansen} Rattray estuarine circulation.
2. Observational methods Observations were carried out using the research vessel RRS Challenger. Most of the temperature and salinity distributions reported here were observed with a SEAROVER CTD system, which cycles between the surface and a depth of 70 m while the ship steams at &3.5 m s\. This undulating CTD system is ideal for observing variable frontal structure because it can map rapidly both temperature and salinity with horizontal resolution of &1 km or better (Hill et al., 1993). An initial survey using a conventional CTD was performed at station intervals of 5 or 6 km from 20 to 22 November 1990, to detect the main synoptic features of the front (Fig. 1). Two intensive SEAROVER surveys were carried out on a section across the front, each over a period greater than a full tidal cycle. The "rst survey was around spring tides from 3 : 50 to 21 : 50 (GMT) on 20 November 1990 (JD 323), and the second around neap tides from 21 : 09 on 25th (JD 329) to 4 : 16 on 27th (JD 331). A ship-mounted ADCP was used simultaneously with the SEAROVER to measure horizontal water velocities on the section. The ADCP provided velocity in 4 m depth bins, below a surface layer 10 m deep, and above a bottom `shadow zonea which typically has a thickness of 15% of the total water depth. The raw velocities were resolved into along and cross front components by rotation of the axis by 453 in an anti-clockwise direction. Residual currents were estimated by removing tidal currents using the tidal analysis technique described by Simpson et al. (1990). A mooring system was deployed in 48 m of water depth on the Great Plateau (55313.9 N, 5314.9 W, Fig. 1) during the period 15}27 November 1990 (JD 320}331). The system was equipped with Aanderaa instruments measuring velocity, temperature and salinity continuously with a 10 min interval. Five instruments were "xed on the mooring but two of them were lost as a result of damage caused by "shing nets. Temperature at the surface, temperature and salinity at 33 m above the seabed (about 15 m depth from the surface) and temperature, salinity and velocity data at 8 m above the seabed (about 40 m depth from the surface) were recorded successfully.
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3. Results 3.1. Temperature and salinity structure Fig. 2 shows cross-frontal sections constructed from CTD data. The water in the North Channel (left-hand side in the "gure) was strongly mixed whilst that in the Clyde Sea (right-hand side in the "gure) was strati"ed. Both the thermocline and halocline were detected near the seabed at about 50 m at C1. Between the two regimes, there was a high gradient frontal region. The front apparently intersected the surface between Stn. C3 and C4 and the bottom between C1 and C2. The position of the front was near a contour of the strati"cation parameter S "2.7 and 3.7 at the surface and & bottom, respectively, although the position or shape cannot be detected precisely from the horizontally coarse CTD survey (Figs. 1(c) and 2). S is given by log (H/; ), & where H is the water depth and ; the amplitude of M tidal velocity. The temperature structure in the Clyde Sea was inverted at this time; the temperature was 10.83C in the surface layer whilst nearly 11.23C in the bottom layer. Both the temperature and salinity of the bottom layer at C1 were lower than values in the North Channel. Since the salinity gradient at the front was stronger than the temperature gradient, the density structure was predominantly controlled by salinity
Fig. 2. Cross frontal (a) temperature (3C), (b) salinity and (c) p sections obtained by CTD observation. R
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Fig. 3. Temperature (3C) and salinity sections obtained by SEAROVER in a spring tide period (a and b) and a neap tide period (c and d).
so that the density was higher in the North Channel than that in the Clyde Sea by *. 0.5 kg m\. The same structure with much more detail is evident in the vertical sections of temperature and salinity across the front observed by the SEAROVER. Fig. 3(a) and (b) show examples in a spring tide (7 : 50}9 : 18 20 November) and Fig. 3(c) and (d) in a neap tide (11 : 48}12 : 58 26 November). Both sections were at approximately the same state of the tide (within 2 h of low water). Temperature and salinity fronts resemble each other closely in shape at both the springs and neaps situations. They illustrate a well-de"ned front with the strongest surface gradients around x"16 km (spring tide) and x"15 km (neap tide), where x is the distance from southwestern edge of the "rst ADCP line. For the spring tide sections (Fig. 3(a) and (b)) there is strong indication of warm and saline bottom water uplifted on the mixed side of the front with associated temperature and salinity maxima at the sea surface around x"12 km. During the neap tide (Fig. 3(c) and (d)) this upwelling of warm saline water was not apparent and the front was more di!use. The maximum density gradient at the front also became signi"cantly weaker at the neap tide (1.0;10\ kg m\) than that at the spring (1.5;10\ kg m\). In addition, a well-de"ned sloping interface at springs was absent at neaps when the front was lying horizontally from x"17 to 25 km, bending sharply towards the surface around x"15 km.
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Fig. 4. Examples of density sections across the front during a spring tide period (a}c) and a neap tide period (d}f). Horizontal bars indicate typical tidal excursion estimated from the current meter data.
Examples of changes in the density "eld during one tidal cycle are shown in Fig. 4. They indicate that the front was advected 3}5 km back and forth by tidal currents. Since tidal currents are usually stronger in the upper layer than the lower layer, the displacement of the front should be larger in the upper layer leading to tidal straining of the front, which was clearly steeper at high water than at low water, during the springs survey (Fig. 4(a) and (c)). In addition, density gradients in the upper layer were weaker at low water than at high water, although those in the lower layer showed little di!erence. A similar tendency was detected during the neap tide, but the tidal straining was weaker.
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3.2. Flow xeld from the ADCP measurements Fig. 5 shows the along and cross-frontal components of the residual #ow, measured by the ADCP, obtained at the spring (Fig. 5(a) and (b)) and the neap tide (Fig. 5(c) and (d)). There is some evidence of an along front shear #ow during the spring tide survey in the region 18(x(25 km, where the strongest temperature and salinity gradients were located at this time (see Fig. 3(a) and (b)). The #ow in the low density surface layer was &4 cm s\ to the northwest, while near the bed it was &5 cm s\ to the southeast. The di!erence *u&9 cm s\ is approximately consistent with the thermal wind relation (*u&10 cm s\). The vertical di!erence in the along front velocity at the neap tide was 6 cm s\, smaller than that at the spring tide, which is consistent with weaker density gradients observed at the neap tide (Fig. 3(c) and (d)). This jet-like #ow, in the upper layer, along the front and consequent velocity di!erence were detected in each sectional observation oscillating back and forth with the tidal phase. In the mixed water to the southwest (x(15 km) there was a stronger #ow to the southeast of up to 10 cm s\ at the spring tide. The #ow, however, showed large variations in speed and direction and no consistent tendency.
Fig. 5. ADCP residual velocity obtained by least squares removal of tidal signal at a spring (a and b) and a neap tide (c and d).
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The cross-frontal #ow was landward almost everywhere on both sections (Fig. 5(b) and (d)) with speeds up to 10 cm s\ and the barotropic component, rather than the baroclinic, was predominant, especially in the neap case. The spatial structure of the barotropic component is not apparently related to the density "eld and may be a result of the incomplete removal of the tidal signal (;&35 cm s\). 3.3. Temperature, salinity and velocity variations at the mooring We consider next the data from the mooring C3 that recorded temperature and salinity changes for a period when the front was in the vicinity of the mooring. Salinity and temperature time series measured at the surface, 15 and 40 m depth at the mooring are given in Fig. 6. Temperature at the surface and at 15 m depth was nearly the same during the whole observational period which indicates that the upper water, at least shallower than 15 m, was well mixed even in the strati"ed Clyde Sea. A clear tidal signal in salinity was present continuously in the upper layer (15 m) and intermittently in the near bed (40 m). This indicated that the front was situated near the mooring system and was oscillated by tidal currents through the mooring, when the signal was strong. A similar signal was recognizable in the temperature series, although it was weaker than that in salinity. Fig. 7 displays the time sequences of the current observed at the mooring site. On the basis of velocity measurements presented in this "gure, the amplitude of the cross-sill tidal currents was 32 cm s\ in springs and 23 cm s\ in neaps at the mooring site. This indicates that the cross-frontal displacement was about 4.5 and 3 km at springs and neaps, respectively. These values are consistent with the movement of the front observed by the SEAROVER (Fig. 4). The displacement of the front is short compared to the width of the sill, which is &30 km (h(50 m). The tidal currents themselves will not, therefore, cause the abrupt intrusion of North Channel water into the Clyde Sea through the bottom layer, unless the front is situated at the northeastern edge of the sill. The along front residual #ow, in the bottom layer at the mooring site, was generally directed to the southeast (indicated by negative values in Fig. 7(b)). On the other hand, the cross frontal component was directed into the basin (positive values in Fig. 7(d)) in the lower layer for most of the deployment period. This tendency in both components is consistent with the results of the ADCP measurements. However, it is worth noting that both components have large variations with periods of less than one day and of several days. Each component showed an apparently di!erent pattern around JD 328 from the other periods. This event happened simultaneously with the whole water column becoming vertically uniform as shown in Fig. 6, and indicates the occurrence of North Channel water at the mooring site. The front is subject to advection by the cross frontal currents caused by the tides, wind-induced currents and so forth. Assuming that the cross-frontal component at the mooring site (u) and that at the front are equal, the position of the front (x) can be estimated by integrating the time series of u. The result is shown in Fig. 8 (thin line). The time sequence indicates the front has been advected about 40 km into the basin during the observational period. The estimated movement is, however, inconsistent with the real movement. It is biased by using the measured velocity near the bed and
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Fig. 6. Time sequences of (a) temperature and (b) salinity observed at mooring station. Horizontal bars indicate the period of ADCP observation.
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Fig. 7. Time sequences of (a) the along front component of tidal current (3153) and (b) the along front component of tidal residual current, (c) the across front component of tidal current (453) and (d) the across front component of tidal residual current obtained at the mooring station. Horizontal bars indicate the period of ADCP observation.
the method takes no account of variation of u with depth. To allow for the latter, the depth mean cross-frontal displacement is considered instead of u as
x" (u!u ) dt, CQ
(1)
where u is the cross-frontal current caused by the estuarine circulation. By analogy CQ with the Hansen}Rattray estuarine circulation, u is taken to be proportional to the CQ density di!erence between the North Channel water (. ) and the basin water in the ,! Clyde Sea (. ): !1 (2) u "C(. !. ), CQ ,! !1 where C is a constant. It is also known from Fig. 3 that the distance from southwestern edge of the ADCP line to the position where the front penetrated the surface, were x"16 km at ¹"324.4 JD and x"15 km at ¹"330.0 JD. These values can be used
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Fig. 7. (Continued).
to determine an integral constant in Eq. (1) and the constant C in equation Eq. (2), and then to recalculate the movement of the frontal position. The thick line in Fig. 8 is the position of the surface front estimated using Eq. (1). It is shown from the estimation that after a sudden movement into the basin on JD 322, there was little frontal movement for two days after which it was gradually advected towards the basin after JD 324. It reached the northeastern edge of the ADCP line (x"25 km), the position of the mooring station at JD 326. It then returned seaward for the subsequent two days and stayed at around x"17 km for the remaining three days. The distance moved was 20 km, from x"5 to 25 km (Fig. 8), and indicates that the surface front was advected from S "2.7 to 3.7 (Fig. 1(c)). The estimated value of & cross frontal velocity, u , is 3}4 cm s\ with variations of about 0.5 cm s\. CQ This estimated frontal movement explains satisfactorily the temperature and salinity variations observed at the mooring site. Fig. 9 illustrates a schematic view of the frontal movement. The distance between surface and bottom fronts appeared about 12 km (Fig. 2). Around JD 321 the surface front was at x"8 km (Fig. 8), so that the bottom front should have been located at about x"20 km and the position of the mooring inshore of the bottom front (position A in Fig. 9). Both temperature and
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Fig. 8. Position of surface front estimated from the cross-front velocity, u, at the mooring station. Thin line is the position estimated from a time series of u and thick line is from u!u . Two solid circles show the CQ positions of the front observed by the SEAROVER observations.
Fig. 9. Schematic view of the movements of the front.
salinity in the lower layer measured at the mooring recorded minima of 11.73C and 33.7 around JD 322 (Fig. 6). In addition, the along front velocity in the lower layer was large in this period (Fig. 7). The average speed over the two days (JD 321}322) was about 18 cm s\, and nearly 10 cm s\ faster than the subsequent period. The preceding and subsequent large changes in salinity and temperature at 40 m depth varying with the tidal periods indicate the bottom front passed the mooring, moving towards the North Channel on JD 321, and moved back landward on JD 323 (position A and B in Fig. 9). On the other hand, both salinity and temperature variations in the upper layer were small without the tidal signal because the surface front was situated 7}18 km away from the mooring.
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The salinity in the upper layer increased gradually from JD 324 to 327 as did the amplitude of the semi-diurnal variation (Fig. 6), in spite of the fact that the tidal currents were decreasing at this time (Fig. 7). This variation indicates that the surface front was gradually approaching the mooring site, as it moved shoreward (position B and C in Fig. 9). During this period, the observed temperature and salinity in the lower layer varied little because the bottom front had already moved landward of the mooring. The estimated position of the surface front supports this prediction, showing a movement from x"15 to 25 km (Figs. 8, 9B and C). Between 20 h on JD 327 and 14 h on JD 328, completely mixed water was observed at the mooring (Fig. 6). Vertical di!erence in temperature and salinity dropped to (0.23C and (0.1 respectively, for a period of 18 h, while the front was to the northeast of the mooring (Fig. 9C). Conspicuous strong northeastward currents also suggested that the mooring was in the North Channel water in this period (Fig. 7(b)) because the residual current in the North Channel is usually northward and highly variable (e.g., Dickson et al., 1987). Soon after this, the front moved back and the mixed water was replaced by a strati"ed column, and the along-frontal component returned to the normal southeastward #ow. Pronounced oscillations in both the surface and bottom layers at this time suggest that a steeply inclined front was being advected close to the mooring position.
4. Discussion In this study, the Clyde Sea front was observed near the contour of S 3 with & a distance of 12 km between surface and bottom fronts. The position of the surface front was consistent with previous descriptions (Edwards et al., 1986) but the use of the SEAROVER system has provided a more detailed picture of the density "eld in the frontal zone. The results con"rmed the existence of large near-surface density gradients with values, on a scale of 1 km, of up to 1.5;10\ kg m\, during the springs survey. Maximum gradients of comparable magnitude extend down through much of the water column although the near-bed values have not been sampled by the SEAROVER because of the hazards of near-bed operation. Tidal mixing fronts produced by the heating}stirring competition occurring in the European shelf seas at S 2.7. In the case of the Clyde Sea front, however, & the buoyancy input is due to both surface heat exchange and fresh water input. During the summer season when run-o! is relatively low and heating predominant, the front is expected to be located close to S 2.7 as indicated by the infrared & image of Fig. 1(b). By contrast in November, heat is being lost from the sea surface and the front is maintained by freshwater buoyancy input alone so we would expect the front to be located at a di!erent value of S . The observed value of & S 3, i.e. at a lower level of tidal mixing, suggests that the net buoyancy supply & in November is less than during the summer heating period. It is also notable that, at this time, the bottom front was inside the basin and situated near the contour of S 4, which is one order of magnitude bigger than that of the surface & front.
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The apparent uplift of warmer saltier water on the mixed side of the front at springs has not been observed here previously, but analogous uplifting of cold heavier water at tidal mixing fronts has been reported (Allen et al., 1980). This feature and the intensi"cation of the gradients at spring tides has been explained by two models. First, Bowers and Lwiza (1994) reported in terms of a vertical mixing model that the net heat #ux into the sea is greater on the mixed side of the front than on the strati"ed side during the heating season. The formation of a minimum temperature zone was attributed to the di!erence in the heat #ux when the water is mixed with increased tidal stirring at spring tides. This model, however, is not appropriate to our case because it was the cooling season as shown by the temperature inversion (Figs. 2 and 3) and the strati"cation was maintained by the salinity. In the other model proposed by Garrett and Loder (1981), the vertical shear of the cross-frontal #ow is important; the upper layer tends to #ow out towards the mixed region, whilst the lower layer #ows into the strati"ed region by the frictional e!ect when the Ekman number is large. Since the across-front currents were more baroclinic, with a strong landward #ow in the lower layer in the vicinity of the front, during the springs than the neaps (Fig. 5), the latter explanation would seem to o!er a more convincing explanation for the observed structure. The residual #ow "eld deduced from the ADCP observations, for both springs and neaps surveys, shows some evidence of intensi"ed shear #ow parallel to the front, associated with the strong horizontal gradients. To a "rst order, the shear involved is consistent with that calculated from the thermal wind relation. The velocity di!erences involved are, however, not much greater than the other apparent variability in this "eld, which may result from imperfect removal of the much stronger tidal #ow. Such uncertainty also applies to the cross-frontal component, although the observed mean landward #ow is consistent with current measured at the mooring. While the mooring data was limited by the loss of instruments, it has been possible to convincingly relate the observed changes in temperature and salinity in the upper and lower layers to the e!ects of horizontal displacement by the mean and tidal #ow. In summer the front is often situated on the North Channel side of the sill, as shown in Fig. 1(b), because the heating and a large amount of fresh water input intensify the strati"cation in the Clyde Sea. The overall mean displacement (&20 km) is comparable to the range for the summer}winter di!erence reported by Edwards et al. (1986) and indicative of the scale of possible shorter period movements of the front. It is interesting that the curved shape of the surface front in Fig. 1(b) resembles the contour lines of S at the mouth of the basin & (Fig. 1(c)). As pointed out by Fujiwara et al. (1997) short time series of ADCP observations provide only a snapshot of the #ow "eld which may be unrepresentative of the longer term #ow pattern and its variability. In this study, we have combined snapshots obtained from the ADCP with the limited mooring data to obtain encouraging results. In future studies it would be desirable to develop this approach with more extensive ship surveys using ADCP and undulating CTD in combination with an array of moorings.
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Acknowledgements We are grateful to Dr. A.J. Elliott for providing us with the current data in Fig. 1(c) calculated by his model. We are also grateful to University of Dundee for providing the satellite image. This work was undertaken during the visit of A. Kasai to the University of Wales, Bangor under the program of Postdoctoral Fellowships for Research Abroad by Japan Society for the Promotion of Science.
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