- Email: [email protected]
A boundary front in the summer regime of the Celtic Sea
Estuarine and Coastal Marine Science (1976) 471-81
A Boundary Front in the Summer Regime of the Celtic Sea
J. H. Simpson Marine Laboratory, Universiy College of North Wales, Menai Bridge, Anglesey, North Wales Received 25 March I975
Sections through a region of low tidal stream energy in the Celtic Sea in August show a high degree of stratification, which breaks down abruptly at the entrance to the Irish Sea and the Bristol Channel in agreement with the h/USS criterion. Radio tracked parachute drogues have been used to observe the flow both along the front and at the centre of the low energy region. A TSD section of the front, combined with the drogue observations, indicates that the front contains a strongly baroclinic zone implying velocity components parallel to the front of up to 30 cm/s. The residual velocities observed in the low energy region were found to be in approximate geostrophic balance with the density field and in a westerly direction, with a maximum amplitude of -9 cm/s.
Introduction The limited extent of knowledge of the oceanography of the Celtic Sea is made clear in a critical review by Cooper (1967). In spite of more than 50 years of hydrographic observations in this area, there is little that can be stated with confidence about the form and variability of the circulation. The large scale distribution of temperature and salinity was first described by Matthews (1914) whose work has formed the basis of many subsequent studies. Very few of these have included direct observations of residual currents, so that ideas on the circulation are largely based on indirect methods. Two essentially different approaches have been used in interpreting the temperature, salinity and chemical data from the area. In the first, dynamic height computations have been made on the basis of the observed density field. For example Dietrich (1951) has suggested that the differences in dynamic height along the axis of the English Channel in the summer months is evidence of a cyclonic circulation. The obvious difficulty here is that there is no reference level and therefore the relative isobaric slope cannot be meaningfully converted into surface slope and a corresponding geostrophically balanced current. Cooper (1961) seems to have reached this conclusion in attempting to understand geopotential topographies in this area. The second approach has been to make use of more or less conservative properties as tracers to try and elucidate the direction if not the magnitude of the circulation. Matthews (19x4) inferred the existence of a cyclonic circulation from the salinity distribution. The essential idea of his argument, followed by many later workers, is that the highest salinity water in the area south of Ireland is in a region off the coast of Cornwall, and that this 71
J. If. Simpson
72
identifies it as a north-going stream, part of which turns westwards towards southern Ireland. WC now know from the study of advection-diffusion models of circulation (Hunter, 1975) that such simple inferences are not justified. For example, the occurrence of the high salinity water cited by Matthews, may be due to enhanced diffusion, which one would expect on account of the large tidal streams off the coast of Cornwall. A sounder interpretation of the data would be based on an advection-diffusion model but this requires knowledge of the diffusion tensor which remains an ill-defined function of the tidal stream amplitude. To further compound the confusion, the sparse current meter data available are sometimes in conflict with these indirect inferences. -Measurements at the Seven Stones light vessel (Carruthers et al., 1951) showed a residual of ~3.5 cm/s to a direction S 20’E in contrast to the generally cyclonic circulation favoured by other workers.
Strategy of the observations The programme of observations reported here had the aim of investigating the density field and residual velocities in two areas of particular interest. The water movements were observed directly by the use of radio-tracked parachute drogucs. Under the conditions of light wind experienced in these experiments the drogues used have an almost negligible error due to windage, so they should give residual vectors of high reliability. The two areas of particular interest chosen for the drogue observations were suggested by the predicted form of vertical stratification for the region. It may bc shown (Simpson & Hunter, 1974) that if all vertical mixing is due to bottom stress in the tidal motion, the occurrence of stratification will be determined by the dimensionless parameter Q@ = /..Jk c .z-_:3na 8kpcus3 us3 ’ where
a Q h k
is the linear expansion coefficient of seawater, is the rate of heat input, is the bottom depth, is the constant in the quadratic function law, C is the specific heat, p is the density, II, is the amplitude of the tidal stream. For a limited region at a given time of year the quantity A will be essentially constant. The front in the northern Irish Sea, for example, is found to occur in June whenlog,,$/u,3z 1.9 (h and u, in m.k.s.). A plot of log,&~,~ for the Celtic Sea, shown in Figure I, predicts the occurrence of such a transition across the entrances to the Irish Sea and the Bristol Channel. To establish the existence of this front and investigate its velocity field were the central objectives of these observations. A second interesting region is the maximum in h/us3 found just south of the Kymphe Bank, where there is a near stagnation point in the tidal flow. Matthews (1914) commented on this stable region and Cooper (1967) h as suggested that it may be the centre of a weak cyclonic bottom circulation during the summer months. Indications of a closed circulation around a stagnation point have previously been observed in parachute drogue tracks in the northwestern Irish Sea (Davies, 1972). As it is also a location where gcostrophic adjustment of the current is most likely (on account of the weak tidal streams) this area (station 7) was chosen as the centre for the T-S sections and drogue releases.
A boundary front
Experimental
in the Celtic Sea
73
details
The positions of the TSD stations and front section together with the drogue release positions are shown in Figure 2. The operational procedure, designed to maximize the use of the limited ship time (6 days), was to work TSD and a meter (optical beam transmittance) stations during daylight hours, reserving the night time for fixing the radio parachute drogues.
Longitude
Figure I. Contours available tidal data.
of the quantity
log&/US3
(“W)
based on Admiralty
charts and
The drogues were tracked by using a 27 MHz transmitter in the surface floats, which are modified aluminium beer barrels ballasted with batteries to have minimal freeboard. The I W transmitter feeds a quarter wave aerial which gives a useful range of ~30 miles. All buoys use the same frequency, but their transmissions are controlled by clocks which cause them to transmit for IO min in sequence on a one hour cycle. Directional bearings are taken by observing the signal strength from the buoy as the ship is rotated through 360”. The presence of the ship’s superstructure imposes an asymmetric polar response which can be used to give a bearing accurate to a few degrees. Positions are fixed by crossed bearings or, more accurately, by homing in on the buoy and using the ship’s Decca navigator. This is best accomplished at night when the buoys are easily located at
J. H. Simpson
74
short range by a Xenon discharge coded light. In daylight the buoys are difficult to see at any distance as only the aerial and light housing protrude significantly above the sea surface. The low windage results in a small correction to the observed velocity vectors. This is estimated as uD-uO = 0~00130(u0-V) where un and ua are the drogue and water velocities and V the wind velocity vector at IO m. The corrections applied in this case, when the mean wind was “3.5 m/s, are -0.5 cm/s which is an order of magnitude less than the observed residuals.
5
_.x* :
x.0.,, . . . .“:
. ..
0 l.5
.,
: :
E
1:
... . :
‘._
‘t\
:
. :’
10
..
: _’ .‘P’
;;;b:
70 1
:. 7 . . ..“Z
:
:
:.
:
:
5
Figure 2. Station positions in the Celtic Sea, August 1972. The vectors indicate the residual currents determined by.parachute drogues at depths given in mctres. The zig-zag features represent posrtrons of the boundary front. The extent of the section through the front (Figure 4) gcneratcd by the tidal excursion is also indicated. Depth contours are for jo fathoms (dotted) and 20 fathoms (dashed).
Ideally the drogues are fixed at intervals corresponding to multiples of the semidiurnal tidal period. Where this is not the case a correction has been applied on the basis of the best available tidal stream data. Temperature and salinity data were obtained with a conventional TSD (Hitich 9040) supplemented by water bottles for calibration. The data is recorded on punched paper tape and processed to give calibrated temperature and salinity which are used to compute the dynamic height anomaly D at 5 db intervals.
A bounaizry front in the Celtic Sea
75
As an alternative to the TSD a profiling optical beam transmittance meter, equipped with a high resolution temperature sensor, was also used. T-S stratification Results of the two main sections of the survey in August 1972 are shown in Figure 3. These sections were centred on the low energy region south of the Eymphe bank where the column is highly stable with a surface to bottom temperature difference of N 5.5 “C. This stability is preserved to the south and west on account of the increasing depth, but to the east a transition to vertically mixed conditions occurs close to station 31 where log,Jz/u,3 = I*~--z*o in accord with prediction. To the north the transition was not observed because it was not possible to extend the observations into Irish waters where the front should occur at about 52”N on the 7’W section. Salinity plays only a minor role in the stratification with a total range of 0.30%~ at the central station 7. Generally the salinity gradient is stabilizing with higher values in the bottom water to the south and west. There are no salinity data for stations 3o-34 which were profiled with the optical beam transmittance meter only. A section of the frontal region showing the contrast in optical transmittance (Tr) is given in Figure 3(c). To the west of station 31 the transmittance is generally high with values mostly >95. The gradients here are weak and predominantly due to vertical structure. To the east of the front, however, there is a strong horizontal gradient in Tr indicating a large increase in suspended load in the entrance to the Bristol Channel. STATION
NUMBERS
DEPTH Cm) M
to. CELTIC SEA E-W SECTIDN AUGUST 24-29
1912
I
Figure 3. (a) E-W temperature and salinity section through the region of maximum Xo salinity data is available for stations 3~2-34.
h/F,.
J. H. Simpson
76
STATION
L---
.--.
---
Figure
-.-
.~--
.-
3. (b) N-S
deployed
temperature
and salinity
section.
Parachute
drogues
were
at station 7 as shown. Station 2
3
NUMBERS
numbers
Front
I
32
33
IOOJ Figure
3. (c) E-W section showing contours
Structure
of optical beam transmittance.
of the front
To further investigate the structure of the boundary front, a series of observations were made in the mid-channel position at the entrance to the Irish Sea (see Figure 2). Repeated TSD profiles were taken at a fixed point as the frontal structure was advccted past by the tidal stream. At the same time two parachute drogues were deployed in the baroclinic region to estimate the current structure. The corrected residuals for the 20 m and 60 m levels are shown in Figure 2. The flow is along the direction of the front, which was inclined at about 55’ to the axis of the tidal streams. The section generated by the tidal motion therefore makes an angle of 35’ with the normal to the front.
A boundary front in the Celtic Sea
77
78
J. II. Simpson
The TSD observations are shown in the detailed section of Figure 4. The mean position of the region of maximum surface gradient is again consistent with log,Jz/~,~ = 1.5~2-o. On the stratified side of the front there is a surface layer of warm lower salinity water about IO m thick. The underlying water is 3-4 “C cooler and of higher salinity (w 35x0). In the region of maximum horizontal gradient there is a characteristic slope of the isotherms (and isohalines) of N I : 400 extending for N 4 km. Figure j shows the dynamic height anomalies computed from the T-S data, together with the current velocities normal to the section, dctcrmined from the drogue vectors.
IO dB
olstonce C’m) Figure 5. Velocity structure of the frontal region deduced from the dynamic height anomalies and the XI db drogue vector. The computation assumes a uniform surface slope and geostrophic balance. V&cities arc in cm/s.
A tentative interpretation of these data in terms of a geostrophically balanced current field is also given. Whether or not the front is fully balanced in this way is not clear. The velocity difference between the drogues is of the same sign as the geostrophic shear but greater in magnitude (6.8 cm/s versus 2.8 cm/s). Strictly, the observed shear in the current should be compared with the isobaric heights averaged over a tidal period. The use of values from a single section here implies that the front is advected tidally as a ‘frozen field’ and is not subject to internal tides. In addition to these assumptions thevelocityfield of Figure 5 also depends on our being able to extrapolate the surfaccslope across the front. If this is the case, wemay infer thcexistence of an energetic flow to the west in the baroclinic region, with currents in excess of 30 cm/s.
Residual currents
in the low energy region
Three parachute drogues were also released from station 7 and tracked for a period of 62 kms (5 semidiurnal tidal cycles). The parachutes were deployed at depths of IO, 40 and 70 m after reference to the T-S structure (Figure 3).
A boundary front in the Celtic Sea
79
The resulting vectors, averaged over the observation period and corrected for the effects of wind, are shown in Figure 2. There was a steady residual to the west in all caseswith the 40 m vector (9.3 cm/s at 267’) significantly stronger than the 70 m (4.4 cm/s x 282”) and IO m (6.5 cm/s x 297’). This vertical structure in the velocity may be compared with the geostrophic shear deduced from the slope of the isobaric surfaces. Figure 6 shows the smoothed slopes relative to a level sea surface. The isobaric slopes are weak and may be contaminated by noise, both from observational errors (equivalent to AD&O-I dyn cm) and internal waves. Nonetheless a level change of ~0.8 dyn cm in the 40 db surface along the section is clearly indicated. Current components for the IO db and 70 db surfaces have been computed from the isobaric slopes using the 40 db drogue as a reference. It is seen that in both cases the computed velocities are not inconsistent with those observed directly if allowance is made for possible errors in isobaric slope. Station 7 I 5.8 (5.8)
IO db
4.3 (5.8)
70 -0 [ I dyn. cm.
80
I 20
I 40
I 60 N-S
I 100
I 80 distance
I 120
.
(km)
Figure 6. Isobaric slopes for the N-S section with the westerly flow components at station 7 determined by drogues. The bracketed velocities are the geostrophic values based on the 40 db drogue.
Discussion
and Conclusion
The distribution of stratification observed, during a period of relatively calm weather, in this region of the Celtic Sea, is in accord with the h/us3 criterion. A well-defined front was
80
J. II. Simpson
observed at the entrance to the Irish Sea and the Bristol Channel at positions where log,,h/u,3~ 1.5-2.0. It is probable therefore but not yet confirmed by observation, that this front is continuous between these crossings along the contour of Iz/us3 (Figure I). A detailed T-S section of this front reveals that it contains a narrow region (-4 km wide) which is strongly baroclinic. Using the parachute drogue results together with the TSD date, we have constructed a uniform surface slope geostrophic model which indicates regions of strong flow along the direction of the front. Would such a simple structure be stable ? Hart (1974) has recently shown that for an inviscid two layer model with a flat bottom, the two parameters governing the stability are the internal Froudc number F” and the layer to depth ratio 6”. Estimates for the baroclinic region in Figure 4 give Fz I and ~320.1, which, according to Hart, would make the flow only marginally stable However, it seems doubtful that this inviscid theory fully describes the stability of such a front. The tidally driven mixing, which is responsible for its formation, may also exert a significant stabilizing influence on the front. More detailed studies of the structure of the front both theoretical and observational are clearly needed. In a more ambitious version of the programme described here, it is intended to track large numbers of drogues in the front while performing more rapid surveys of the T-S structure using an undulating CTD. In the region of maximum h/~,~ a well-defined westerly residual was observed at all depths. There was no evidence of strong curvature in the flow, so the centre of a hypothetical cyclonic circulation, suggested by both sets of droguc observations, must lit further to the south. The isobaric slopes were generally small and implied geostrophic shears of the same order as those observed by the parachute drogues. “Defined where
as F = s &
0” t
and 6 = I&/I&,,
zx density difference between layers, Coriolis parameter, characteristic width of the baroclinic and bottom layers.
zone and H, and Hb are the depths of the top
Acknowledgement The author is pleased to acknowledge the generous support given during the course of these observations, both by his colleagues from the Marine Science Laboratories and by the officers and crew of the R.V. John Murray.
References Carruthers, J. N., Lawford, A. L. & V&y, V. F. C. 1951 Studies of water movcmcnts and winds at various lightvessels: II. At the Seven Stones Liahtvessel near the Scillv Isles. ‘formal of the Marine Bkgical Association of the United Kingdim 29, 587 -608. ” . Cooper, L. H. N. 1961 The oceanography of the Celtic Sea. II. Conditions in the spring of 1950. Journal of the Marine Riologikzl Association of the United Kingdom 41, 235-270. Cooper, L. II. N. 1967 The physical oceanography of the Celtic Sea. Oceanography and Marim Biology, Annual Review 5, 99 I IO. Davies, A. G. 1972 Aspects of the circulation of the western Irish Sea. M.Sc. Thesis, IJniversity of Wales. Dietrich, G. r9jr Influences of tidal streams on oceanographic and climatic conditions in the sea as exemplified by the English Channel. Nuture, Londonm168, 8-1 I. IIart, J. E. 1974 On the mixed stability problem for quasi-geostrophic ocean currents. rournal of Physical Oceanography 4 (3), 349-356.
A boundary
front in
the Celtic
Sea
81
Hunter, J. R. 1975 The determination of current velocities from diffusion/advection processes in the Irish Sea. Estu&ne afzd Coastal lMn~ine SC&IC~ 3 (I), 43-56. Matthews, D. J. 1914 The salinity and temperature of the Irish Channel and the waters south of Ireland. Scientific Investigations, Fisheries Branch, Ireland IV. Simpson, J. I-I. & IIunter, J. R. 1974 Fronts in the Irish Sea. Nature, London 250,404-406.
A Boundary Front in the Summer Regime of the Celtic Sea
J. H. Simpson Marine Laboratory, Universiy College of North Wales, Menai Bridge, Anglesey, North Wales Received 25 March I975
Sections through a region of low tidal stream energy in the Celtic Sea in August show a high degree of stratification, which breaks down abruptly at the entrance to the Irish Sea and the Bristol Channel in agreement with the h/USS criterion. Radio tracked parachute drogues have been used to observe the flow both along the front and at the centre of the low energy region. A TSD section of the front, combined with the drogue observations, indicates that the front contains a strongly baroclinic zone implying velocity components parallel to the front of up to 30 cm/s. The residual velocities observed in the low energy region were found to be in approximate geostrophic balance with the density field and in a westerly direction, with a maximum amplitude of -9 cm/s.
Introduction The limited extent of knowledge of the oceanography of the Celtic Sea is made clear in a critical review by Cooper (1967). In spite of more than 50 years of hydrographic observations in this area, there is little that can be stated with confidence about the form and variability of the circulation. The large scale distribution of temperature and salinity was first described by Matthews (1914) whose work has formed the basis of many subsequent studies. Very few of these have included direct observations of residual currents, so that ideas on the circulation are largely based on indirect methods. Two essentially different approaches have been used in interpreting the temperature, salinity and chemical data from the area. In the first, dynamic height computations have been made on the basis of the observed density field. For example Dietrich (1951) has suggested that the differences in dynamic height along the axis of the English Channel in the summer months is evidence of a cyclonic circulation. The obvious difficulty here is that there is no reference level and therefore the relative isobaric slope cannot be meaningfully converted into surface slope and a corresponding geostrophically balanced current. Cooper (1961) seems to have reached this conclusion in attempting to understand geopotential topographies in this area. The second approach has been to make use of more or less conservative properties as tracers to try and elucidate the direction if not the magnitude of the circulation. Matthews (19x4) inferred the existence of a cyclonic circulation from the salinity distribution. The essential idea of his argument, followed by many later workers, is that the highest salinity water in the area south of Ireland is in a region off the coast of Cornwall, and that this 71
J. If. Simpson
72
identifies it as a north-going stream, part of which turns westwards towards southern Ireland. WC now know from the study of advection-diffusion models of circulation (Hunter, 1975) that such simple inferences are not justified. For example, the occurrence of the high salinity water cited by Matthews, may be due to enhanced diffusion, which one would expect on account of the large tidal streams off the coast of Cornwall. A sounder interpretation of the data would be based on an advection-diffusion model but this requires knowledge of the diffusion tensor which remains an ill-defined function of the tidal stream amplitude. To further compound the confusion, the sparse current meter data available are sometimes in conflict with these indirect inferences. -Measurements at the Seven Stones light vessel (Carruthers et al., 1951) showed a residual of ~3.5 cm/s to a direction S 20’E in contrast to the generally cyclonic circulation favoured by other workers.
Strategy of the observations The programme of observations reported here had the aim of investigating the density field and residual velocities in two areas of particular interest. The water movements were observed directly by the use of radio-tracked parachute drogucs. Under the conditions of light wind experienced in these experiments the drogues used have an almost negligible error due to windage, so they should give residual vectors of high reliability. The two areas of particular interest chosen for the drogue observations were suggested by the predicted form of vertical stratification for the region. It may bc shown (Simpson & Hunter, 1974) that if all vertical mixing is due to bottom stress in the tidal motion, the occurrence of stratification will be determined by the dimensionless parameter Q@ = /..Jk c .z-_:3na 8kpcus3 us3 ’ where
a Q h k
is the linear expansion coefficient of seawater, is the rate of heat input, is the bottom depth, is the constant in the quadratic function law, C is the specific heat, p is the density, II, is the amplitude of the tidal stream. For a limited region at a given time of year the quantity A will be essentially constant. The front in the northern Irish Sea, for example, is found to occur in June whenlog,,$/u,3z 1.9 (h and u, in m.k.s.). A plot of log,&~,~ for the Celtic Sea, shown in Figure I, predicts the occurrence of such a transition across the entrances to the Irish Sea and the Bristol Channel. To establish the existence of this front and investigate its velocity field were the central objectives of these observations. A second interesting region is the maximum in h/us3 found just south of the Kymphe Bank, where there is a near stagnation point in the tidal flow. Matthews (1914) commented on this stable region and Cooper (1967) h as suggested that it may be the centre of a weak cyclonic bottom circulation during the summer months. Indications of a closed circulation around a stagnation point have previously been observed in parachute drogue tracks in the northwestern Irish Sea (Davies, 1972). As it is also a location where gcostrophic adjustment of the current is most likely (on account of the weak tidal streams) this area (station 7) was chosen as the centre for the T-S sections and drogue releases.
A boundary front
Experimental
in the Celtic Sea
73
details
The positions of the TSD stations and front section together with the drogue release positions are shown in Figure 2. The operational procedure, designed to maximize the use of the limited ship time (6 days), was to work TSD and a meter (optical beam transmittance) stations during daylight hours, reserving the night time for fixing the radio parachute drogues.
Longitude
Figure I. Contours available tidal data.
of the quantity
log&/US3
(“W)
based on Admiralty
charts and
The drogues were tracked by using a 27 MHz transmitter in the surface floats, which are modified aluminium beer barrels ballasted with batteries to have minimal freeboard. The I W transmitter feeds a quarter wave aerial which gives a useful range of ~30 miles. All buoys use the same frequency, but their transmissions are controlled by clocks which cause them to transmit for IO min in sequence on a one hour cycle. Directional bearings are taken by observing the signal strength from the buoy as the ship is rotated through 360”. The presence of the ship’s superstructure imposes an asymmetric polar response which can be used to give a bearing accurate to a few degrees. Positions are fixed by crossed bearings or, more accurately, by homing in on the buoy and using the ship’s Decca navigator. This is best accomplished at night when the buoys are easily located at
J. H. Simpson
74
short range by a Xenon discharge coded light. In daylight the buoys are difficult to see at any distance as only the aerial and light housing protrude significantly above the sea surface. The low windage results in a small correction to the observed velocity vectors. This is estimated as uD-uO = 0~00130(u0-V) where un and ua are the drogue and water velocities and V the wind velocity vector at IO m. The corrections applied in this case, when the mean wind was “3.5 m/s, are -0.5 cm/s which is an order of magnitude less than the observed residuals.
5
_.x* :
x.0.,, . . . .“:
. ..
0 l.5
.,
: :
E
1:
... . :
‘._
‘t\
:
. :’
10
..
: _’ .‘P’
;;;b:
70 1
:. 7 . . ..“Z
:
:
:.
:
:
5
Figure 2. Station positions in the Celtic Sea, August 1972. The vectors indicate the residual currents determined by.parachute drogues at depths given in mctres. The zig-zag features represent posrtrons of the boundary front. The extent of the section through the front (Figure 4) gcneratcd by the tidal excursion is also indicated. Depth contours are for jo fathoms (dotted) and 20 fathoms (dashed).
Ideally the drogues are fixed at intervals corresponding to multiples of the semidiurnal tidal period. Where this is not the case a correction has been applied on the basis of the best available tidal stream data. Temperature and salinity data were obtained with a conventional TSD (Hitich 9040) supplemented by water bottles for calibration. The data is recorded on punched paper tape and processed to give calibrated temperature and salinity which are used to compute the dynamic height anomaly D at 5 db intervals.
A bounaizry front in the Celtic Sea
75
As an alternative to the TSD a profiling optical beam transmittance meter, equipped with a high resolution temperature sensor, was also used. T-S stratification Results of the two main sections of the survey in August 1972 are shown in Figure 3. These sections were centred on the low energy region south of the Eymphe bank where the column is highly stable with a surface to bottom temperature difference of N 5.5 “C. This stability is preserved to the south and west on account of the increasing depth, but to the east a transition to vertically mixed conditions occurs close to station 31 where log,Jz/u,3 = I*~--z*o in accord with prediction. To the north the transition was not observed because it was not possible to extend the observations into Irish waters where the front should occur at about 52”N on the 7’W section. Salinity plays only a minor role in the stratification with a total range of 0.30%~ at the central station 7. Generally the salinity gradient is stabilizing with higher values in the bottom water to the south and west. There are no salinity data for stations 3o-34 which were profiled with the optical beam transmittance meter only. A section of the frontal region showing the contrast in optical transmittance (Tr) is given in Figure 3(c). To the west of station 31 the transmittance is generally high with values mostly >95. The gradients here are weak and predominantly due to vertical structure. To the east of the front, however, there is a strong horizontal gradient in Tr indicating a large increase in suspended load in the entrance to the Bristol Channel. STATION
NUMBERS
DEPTH Cm) M
to. CELTIC SEA E-W SECTIDN AUGUST 24-29
1912
I
Figure 3. (a) E-W temperature and salinity section through the region of maximum Xo salinity data is available for stations 3~2-34.
h/F,.
J. H. Simpson
76
STATION
L---
.--.
---
Figure
-.-
.~--
.-
3. (b) N-S
deployed
temperature
and salinity
section.
Parachute
drogues
were
at station 7 as shown. Station 2
3
NUMBERS
numbers
Front
I
32
33
IOOJ Figure
3. (c) E-W section showing contours
Structure
of optical beam transmittance.
of the front
To further investigate the structure of the boundary front, a series of observations were made in the mid-channel position at the entrance to the Irish Sea (see Figure 2). Repeated TSD profiles were taken at a fixed point as the frontal structure was advccted past by the tidal stream. At the same time two parachute drogues were deployed in the baroclinic region to estimate the current structure. The corrected residuals for the 20 m and 60 m levels are shown in Figure 2. The flow is along the direction of the front, which was inclined at about 55’ to the axis of the tidal streams. The section generated by the tidal motion therefore makes an angle of 35’ with the normal to the front.
A boundary front in the Celtic Sea
77
78
J. II. Simpson
The TSD observations are shown in the detailed section of Figure 4. The mean position of the region of maximum surface gradient is again consistent with log,Jz/~,~ = 1.5~2-o. On the stratified side of the front there is a surface layer of warm lower salinity water about IO m thick. The underlying water is 3-4 “C cooler and of higher salinity (w 35x0). In the region of maximum horizontal gradient there is a characteristic slope of the isotherms (and isohalines) of N I : 400 extending for N 4 km. Figure j shows the dynamic height anomalies computed from the T-S data, together with the current velocities normal to the section, dctcrmined from the drogue vectors.
IO dB
olstonce C’m) Figure 5. Velocity structure of the frontal region deduced from the dynamic height anomalies and the XI db drogue vector. The computation assumes a uniform surface slope and geostrophic balance. V&cities arc in cm/s.
A tentative interpretation of these data in terms of a geostrophically balanced current field is also given. Whether or not the front is fully balanced in this way is not clear. The velocity difference between the drogues is of the same sign as the geostrophic shear but greater in magnitude (6.8 cm/s versus 2.8 cm/s). Strictly, the observed shear in the current should be compared with the isobaric heights averaged over a tidal period. The use of values from a single section here implies that the front is advected tidally as a ‘frozen field’ and is not subject to internal tides. In addition to these assumptions thevelocityfield of Figure 5 also depends on our being able to extrapolate the surfaccslope across the front. If this is the case, wemay infer thcexistence of an energetic flow to the west in the baroclinic region, with currents in excess of 30 cm/s.
Residual currents
in the low energy region
Three parachute drogues were also released from station 7 and tracked for a period of 62 kms (5 semidiurnal tidal cycles). The parachutes were deployed at depths of IO, 40 and 70 m after reference to the T-S structure (Figure 3).
A boundary front in the Celtic Sea
79
The resulting vectors, averaged over the observation period and corrected for the effects of wind, are shown in Figure 2. There was a steady residual to the west in all caseswith the 40 m vector (9.3 cm/s at 267’) significantly stronger than the 70 m (4.4 cm/s x 282”) and IO m (6.5 cm/s x 297’). This vertical structure in the velocity may be compared with the geostrophic shear deduced from the slope of the isobaric surfaces. Figure 6 shows the smoothed slopes relative to a level sea surface. The isobaric slopes are weak and may be contaminated by noise, both from observational errors (equivalent to AD&O-I dyn cm) and internal waves. Nonetheless a level change of ~0.8 dyn cm in the 40 db surface along the section is clearly indicated. Current components for the IO db and 70 db surfaces have been computed from the isobaric slopes using the 40 db drogue as a reference. It is seen that in both cases the computed velocities are not inconsistent with those observed directly if allowance is made for possible errors in isobaric slope. Station 7 I 5.8 (5.8)
IO db
4.3 (5.8)
70 -0 [ I dyn. cm.
80
I 20
I 40
I 60 N-S
I 100
I 80 distance
I 120
.
(km)
Figure 6. Isobaric slopes for the N-S section with the westerly flow components at station 7 determined by drogues. The bracketed velocities are the geostrophic values based on the 40 db drogue.
Discussion
and Conclusion
The distribution of stratification observed, during a period of relatively calm weather, in this region of the Celtic Sea, is in accord with the h/us3 criterion. A well-defined front was
80
J. II. Simpson
observed at the entrance to the Irish Sea and the Bristol Channel at positions where log,,h/u,3~ 1.5-2.0. It is probable therefore but not yet confirmed by observation, that this front is continuous between these crossings along the contour of Iz/us3 (Figure I). A detailed T-S section of this front reveals that it contains a narrow region (-4 km wide) which is strongly baroclinic. Using the parachute drogue results together with the TSD date, we have constructed a uniform surface slope geostrophic model which indicates regions of strong flow along the direction of the front. Would such a simple structure be stable ? Hart (1974) has recently shown that for an inviscid two layer model with a flat bottom, the two parameters governing the stability are the internal Froudc number F” and the layer to depth ratio 6”. Estimates for the baroclinic region in Figure 4 give Fz I and ~320.1, which, according to Hart, would make the flow only marginally stable However, it seems doubtful that this inviscid theory fully describes the stability of such a front. The tidally driven mixing, which is responsible for its formation, may also exert a significant stabilizing influence on the front. More detailed studies of the structure of the front both theoretical and observational are clearly needed. In a more ambitious version of the programme described here, it is intended to track large numbers of drogues in the front while performing more rapid surveys of the T-S structure using an undulating CTD. In the region of maximum h/~,~ a well-defined westerly residual was observed at all depths. There was no evidence of strong curvature in the flow, so the centre of a hypothetical cyclonic circulation, suggested by both sets of droguc observations, must lit further to the south. The isobaric slopes were generally small and implied geostrophic shears of the same order as those observed by the parachute drogues. “Defined where
as F = s &
0” t
and 6 = I&/I&,,
zx density difference between layers, Coriolis parameter, characteristic width of the baroclinic and bottom layers.
zone and H, and Hb are the depths of the top
Acknowledgement The author is pleased to acknowledge the generous support given during the course of these observations, both by his colleagues from the Marine Science Laboratories and by the officers and crew of the R.V. John Murray.
References Carruthers, J. N., Lawford, A. L. & V&y, V. F. C. 1951 Studies of water movcmcnts and winds at various lightvessels: II. At the Seven Stones Liahtvessel near the Scillv Isles. ‘formal of the Marine Bkgical Association of the United Kingdim 29, 587 -608. ” . Cooper, L. H. N. 1961 The oceanography of the Celtic Sea. II. Conditions in the spring of 1950. Journal of the Marine Riologikzl Association of the United Kingdom 41, 235-270. Cooper, L. II. N. 1967 The physical oceanography of the Celtic Sea. Oceanography and Marim Biology, Annual Review 5, 99 I IO. Davies, A. G. 1972 Aspects of the circulation of the western Irish Sea. M.Sc. Thesis, IJniversity of Wales. Dietrich, G. r9jr Influences of tidal streams on oceanographic and climatic conditions in the sea as exemplified by the English Channel. Nuture, Londonm168, 8-1 I. IIart, J. E. 1974 On the mixed stability problem for quasi-geostrophic ocean currents. rournal of Physical Oceanography 4 (3), 349-356.
A boundary
front in
the Celtic
Sea
81
Hunter, J. R. 1975 The determination of current velocities from diffusion/advection processes in the Irish Sea. Estu&ne afzd Coastal lMn~ine SC&IC~ 3 (I), 43-56. Matthews, D. J. 1914 The salinity and temperature of the Irish Channel and the waters south of Ireland. Scientific Investigations, Fisheries Branch, Ireland IV. Simpson, J. I-I. & IIunter, J. R. 1974 Fronts in the Irish Sea. Nature, London 250,404-406.