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Some oceanographic studies on the west coast of England
Deep-Sea Research, 1970, Vol. 17, pp. 647 to 654. Pergamon Press. Printed in Great Britain.
Some oceanographic studies on the west coast of England
S. H .
SHARAF EL
DIN*'{"
(Received 14 October, 1969) Abstract--The project is a complete survey of the water circulation along the west coast of England from North Wales to Morecambe Bay area. Current, salinity and temperature measurements were taken at seven anchored stations between 12 and 21 July, 1965. In order to study the effect of horizontal and transverse gradients of salinity on the mixing processes, measurements were taken from sampling stations. The resultant components of the residual flow at each station at the surface and the bottom were drawn. In the upper layer the residual flows give a near-surface flow away from the shore, at the bottom layer it was directed towards or parallel to the shore. The isohalines showed a systematic increase towards the sea. The thermocline may be constructed as a result of different flows from the rivers and the sea. INTRODUCTION
DURING the period 1903-1914, the Fisheries Branch of the Department of Agriculture and Technical Instruction for Ireland, as well as other organizations, carried out sampling programmes for measuring temperature and salinity at various depths. In MATrHEWS paper (1914), the salinity observations at all depths showed that the vertical distribution of salinity resembles that of temperature. From 1920 to 1936, the S.S. Helga, later named the Muirehu, was used for collecting samples in the southern entrance of the Irish Sea. Since 1935, the Oceanography Department at Liverpool University reviewed the data on the variation of surface salinity of the Irish Sea. This review was published later by PROUDMAN (1940). In 1955 K. F. BOWDEN published his paper about " The physical oceanography of the Irish S e a " The data presented in that paper were collected from different cruises working in the Irish Sea since 1903. The only measurements by current meter up to that time would appear to be those described by PROUDMAN (1939) which were obtained at the Skulmartin lightship, about 3.7 km from the Irish Coast in latitude 54 ° 32'N, longitude 5 ° 26'W. Also, some trials had been done for measuring the current velocity using drift bottles. In BOWDEN and SHARAFEL DIN (1966a), a study was made in which the density current circulation in the Mersey estuary was correlated to the river discharge. After that current and salinity observations were taken from different stations in the Liverpool Bay area, to determine the extent to which the density current effects influenced the water movements and mixing processes seaward of the mouth of the estuary. In general, the current measurements carried out in the area of the northwest coast of England during the period 1959-1965 are considered the first attempts to determine the circulation in this area. The investigation described here is an extension of the Liverpool Bay area work to include Morecambe Bay area, for presenting a real picture of the influence of the flow coming from the Dee, Mersey, Ribble and other rivers On the Irish Sea circulation.
METHODS OF OBSERVATIONS
The current observations were taken using Kelvin Hughes and Roberts current meters. The measurements were taken at one meter below surface, one meter above bottom, and every five meters between these two depths. Using the same technique of observations (BoWDEN and SHARAFEL DIN, 1966b), seven anchored stations were occupied during the period 12 to 21 July, 1965. The current *Faculty of Science, Oceanography Department, Alexandria University tPresently on a study leave at the Bedford Institute, Dartmouth, N.S., Canada. 647
648
Shorter Contributions
investigations'were carried out for 12"5 hours at each station repeating every half hour. About 32 water sampling stations were taken at the longitudinal and transverse direction o f the anchored stations. The position o f the current stations (Fig. 1) along the west coast o f England were located as follows:
Station
Latitude
Longitude
7 8 9 10 I1 12 13
53 ° 54'N 54 ° 0'N 53 ° 59'N 53 ° 42'N 53 ° 27'N 53 ° 28'N 53 ° 22'N
3* 16'W 3 ° 19'W 3 ° 2'W 3" 12'W 3" 24'W 3 ° 17'W 3 ~ 19'W
5o30'W .,
I
i
54*Nt
MO~.ECAMBE~.TBAY+ + " * '~">~'~I] + SAMPLING STATIONS • ANCHOR STATIO~,JS +
/BLACklPCOLI
÷
16 + * I0 2.
4505,~N
55~ ,-40'
UVERPOOL BAY ,1
PT ,12
41 ÷
•
•
LIVERPOOL
3"50'W
+
Fig. 1. Chart o f the West Coast o f England. A n c h o r Stas. [1-3 (1962), 4-6 (1963), 7-13 (1965)]. Water sampling stations only.
All the investigations made in the Irish Sea were carried out on board the M.V. William Herdman, the research vessel o f the University o f Liverpool.
Shorter Contributions SALINITY
AND
TEMPERATURE
649
DISTRIBUTION
At all stations the salinity variations showed a maximum value near high water and minimum one near low water. At all stages of salinity variation during the tidal period, there was a vertical gradient, sometimes vanishing at high water and at other times giving a small difference. At one meter above the bottom the range of salinity was found to be smaller than at the surface. In general, this range decreases towards the bottom. At Sta. 11 the vertical gradient of salinity persists through the tidal period. At the other stations the vertical gradient practically disappears between 2 hours before and after high water.
3"30W i
54oN_~
SurfGce Solinil'i
MORECAMBE BA~
i
~C54"N
It~is~"~~KP~L I
/ ?
t.ivERP&,"
"" .~ ~@P" I
/
!
I
3"~O'W Fig. 2.
Surface salinity. [From the mean value over the tidal period at the aachor stations (1962-1965)].
A big difference in salinity between the surface and the bottom found at some stations can be attributed to an excess of river discharge. The isohalines at the surface and the bottom between the current stations were drawn showing a systematic increase towards the sea (Figs. 2 and 3). The mean salinity profiles for Stas. 7-13 (1965) were treated in a similar way (BoWOEN and SHARAF EL DIN, 1966b), giving a positive gradient from surface to bottom (Fig. 4). In Fig. 4 Stas. 1-5 (1962 and 1963) were shown previously in a paper by BOWDEN and SHARAF EL DIN (1966a). It is clear that the increase of salinity with depths is not significant at stations at sea away from the effect of river flow.
650
Shorter Contributions
5o30'W
54°N-
BoffomSolinity
MORECAMBE/BAY
~
54°N
53° N 40
53°
40' N J
LIVERPOOL BAY / / .'
/
/
I
Fig. 3. Bottom salinity. [From the mean value over the tidal period at the anchor stations (1962-1965)].
Shorter Contributions
315
320
651
52.5
0°5
350
\
\
35.5
\
°21
I
\
Fig. 4.
Mean salinity profiles at Stas. 1-13, West Coast of England.
The surface variation of the temperature at any station seems to follow the diurnal meteorological system in summer. Such a variation is to be expected due to heating by solar radiation during the day and cooling by back radiation and evaporation at night. The temperature of the bottom layer is more stable, the surface one giving a small difference during the day. The temperature gradient is very small at stations further from the coast away flora the effect of river flow.
THE
LARGE
SCALE
MOVEMENT
OF
THE
WATER
ALONG
THE
WEST
COAST
OF
ENGLAND
The components of U~, and VT of the residual current over a tidal period were obtained by taking the mean of the values over 12 lunar hours at the seven anchored stations. The resultant component of the residual flow V is given by the equation:
The angle 0 to which V inclined from south to north axis of the flow is obtained from the equation: VT tan 8 = . - - . UT In this discussion the resultant of the residual flow of the thirteen anchored stations (Stas. 1-5 were shown previously by BOWDEN and SHARAF EL DIN, 1966b), were put together to give a good picture for the current variation along the coast. At the surface, in most cases, there is a pronounced near-surface flow away from the shore (Fig. 5). At station 10 the surface current is onshore with small magnitude i n comparison with the other stations. At Stas. 7 and 8, the surface flow shows that the tidal flow coming from the north round the Isle of M a n is more effective than the one coming from the south. At Sta. 9 the same tendency as found at Sta. 10 was observed with a big value of about 9 cm/sec. It seems that the only explanation from the stations at which the surface current is inshore is that the duration of the flood current is more than that during the ebb. In general, the net movement of surface water, at any station, is the net circulation between the sea water and river flow. At the bottom (Fig. 6), the resultant of the residual flow is directed towards, or parallel to, the shore. This can be taken as evidence for the existence of a countercurrent parallel to the coast. The bottom flow at the Morecambe Bay area is difficult to explain due to the limited number of the anchored stations.
652
Shorter Contributions
3"30'W
54°NI
cM..,~.
MORECA
\
E BAY
o
-54°N
/ / 5 3 ° ,,
4503~N ~
4 0 'J
z ,
3*3O W Fig. 5.
The surface residual current vectors at Stas. 1-13, West Coast of England.
653
Shorter Contributions
3"30'W t
,/
54°N
5
0
CM/$£C
MORECAMBE~BAY
/,
/
/
/
/
/
54°N
/.7
/
@ "!
Io\ ,~t~i?. LWERPOOL BAY .i
L5
e...
Fig 6.
~9
53 °
,,
401 l~
FORMBY PT
4
The bottom residual current vectors at Stas. 1-13, West Coast of England.
654
Shorter Contributions
Acknowledgements--The author wishes to acknowledge the valuable assistance of Prof. K. F. BOWDEN of Liverpool University for his supervision on these investigations. He is also pleased to acknowledge the help of his colleagues in the Oceanography Department of Liverpool University, particularly in making observations at sea. REFERENCES
BOWDEN K. F. (1955) Physical oceanography of the Irish Sea. Fishery lnvest., Lond., 18 (8), 67 pp. BOWDEN K. F. and S. H. SHARAFEL DIN (1966a) Ch'culation, salinity and river discharge in the Mersey Estuary. Geophys. J. R. astr. Soc., 10 (4), 383-400. BOWOEN K. F. and S. H. SHAgAFEL DIN (1966b) Circulation and mixing processes in the Liverpool Bay area of the Irish Sea. Geophys. J. R. astr. Soc., 11 (3), 279-292. MArTh'EWS D. J. (1914) The salinity and temperature of the Irish Channel and the waters south of Ireland. Scient. lnvest. Fish. Brch. 1re. 1913 (4), 26 pp. PROUDMAN J'. (1938) On tidal variations of temperature in the Irish Sea. Mon. Not. R. astr. Soc. geophys. SuppL, 4 (5), 373-386. PROUDMAN .L (1939) On. the currents in the Nolth Channel of the Irish Sea. Mon. Not. R. astr. eeophys. SuppL, 4 (6), 387-403. PROtrOMAN .1".(1946) On the salinity of the surface waters of the Irish Sea. Phil. Trans. R. Soc. A, 239 (812), 579-594.
Some oceanographic studies on the west coast of England
S. H .
SHARAF EL
DIN*'{"
(Received 14 October, 1969) Abstract--The project is a complete survey of the water circulation along the west coast of England from North Wales to Morecambe Bay area. Current, salinity and temperature measurements were taken at seven anchored stations between 12 and 21 July, 1965. In order to study the effect of horizontal and transverse gradients of salinity on the mixing processes, measurements were taken from sampling stations. The resultant components of the residual flow at each station at the surface and the bottom were drawn. In the upper layer the residual flows give a near-surface flow away from the shore, at the bottom layer it was directed towards or parallel to the shore. The isohalines showed a systematic increase towards the sea. The thermocline may be constructed as a result of different flows from the rivers and the sea. INTRODUCTION
DURING the period 1903-1914, the Fisheries Branch of the Department of Agriculture and Technical Instruction for Ireland, as well as other organizations, carried out sampling programmes for measuring temperature and salinity at various depths. In MATrHEWS paper (1914), the salinity observations at all depths showed that the vertical distribution of salinity resembles that of temperature. From 1920 to 1936, the S.S. Helga, later named the Muirehu, was used for collecting samples in the southern entrance of the Irish Sea. Since 1935, the Oceanography Department at Liverpool University reviewed the data on the variation of surface salinity of the Irish Sea. This review was published later by PROUDMAN (1940). In 1955 K. F. BOWDEN published his paper about " The physical oceanography of the Irish S e a " The data presented in that paper were collected from different cruises working in the Irish Sea since 1903. The only measurements by current meter up to that time would appear to be those described by PROUDMAN (1939) which were obtained at the Skulmartin lightship, about 3.7 km from the Irish Coast in latitude 54 ° 32'N, longitude 5 ° 26'W. Also, some trials had been done for measuring the current velocity using drift bottles. In BOWDEN and SHARAFEL DIN (1966a), a study was made in which the density current circulation in the Mersey estuary was correlated to the river discharge. After that current and salinity observations were taken from different stations in the Liverpool Bay area, to determine the extent to which the density current effects influenced the water movements and mixing processes seaward of the mouth of the estuary. In general, the current measurements carried out in the area of the northwest coast of England during the period 1959-1965 are considered the first attempts to determine the circulation in this area. The investigation described here is an extension of the Liverpool Bay area work to include Morecambe Bay area, for presenting a real picture of the influence of the flow coming from the Dee, Mersey, Ribble and other rivers On the Irish Sea circulation.
METHODS OF OBSERVATIONS
The current observations were taken using Kelvin Hughes and Roberts current meters. The measurements were taken at one meter below surface, one meter above bottom, and every five meters between these two depths. Using the same technique of observations (BoWDEN and SHARAFEL DIN, 1966b), seven anchored stations were occupied during the period 12 to 21 July, 1965. The current *Faculty of Science, Oceanography Department, Alexandria University tPresently on a study leave at the Bedford Institute, Dartmouth, N.S., Canada. 647
648
Shorter Contributions
investigations'were carried out for 12"5 hours at each station repeating every half hour. About 32 water sampling stations were taken at the longitudinal and transverse direction o f the anchored stations. The position o f the current stations (Fig. 1) along the west coast o f England were located as follows:
Station
Latitude
Longitude
7 8 9 10 I1 12 13
53 ° 54'N 54 ° 0'N 53 ° 59'N 53 ° 42'N 53 ° 27'N 53 ° 28'N 53 ° 22'N
3* 16'W 3 ° 19'W 3 ° 2'W 3" 12'W 3" 24'W 3 ° 17'W 3 ~ 19'W
5o30'W .,
I
i
54*Nt
MO~.ECAMBE~.TBAY+ + " * '~">~'~I] + SAMPLING STATIONS • ANCHOR STATIO~,JS +
/BLACklPCOLI
÷
16 + * I0 2.
4505,~N
55~ ,-40'
UVERPOOL BAY ,1
PT ,12
41 ÷
•
•
LIVERPOOL
3"50'W
+
Fig. 1. Chart o f the West Coast o f England. A n c h o r Stas. [1-3 (1962), 4-6 (1963), 7-13 (1965)]. Water sampling stations only.
All the investigations made in the Irish Sea were carried out on board the M.V. William Herdman, the research vessel o f the University o f Liverpool.
Shorter Contributions SALINITY
AND
TEMPERATURE
649
DISTRIBUTION
At all stations the salinity variations showed a maximum value near high water and minimum one near low water. At all stages of salinity variation during the tidal period, there was a vertical gradient, sometimes vanishing at high water and at other times giving a small difference. At one meter above the bottom the range of salinity was found to be smaller than at the surface. In general, this range decreases towards the bottom. At Sta. 11 the vertical gradient of salinity persists through the tidal period. At the other stations the vertical gradient practically disappears between 2 hours before and after high water.
3"30W i
54oN_~
SurfGce Solinil'i
MORECAMBE BA~
i
~C54"N
It~is~"~~KP~L I
/ ?
t.ivERP&,"
"" .~ ~@P" I
/
!
I
3"~O'W Fig. 2.
Surface salinity. [From the mean value over the tidal period at the aachor stations (1962-1965)].
A big difference in salinity between the surface and the bottom found at some stations can be attributed to an excess of river discharge. The isohalines at the surface and the bottom between the current stations were drawn showing a systematic increase towards the sea (Figs. 2 and 3). The mean salinity profiles for Stas. 7-13 (1965) were treated in a similar way (BoWOEN and SHARAF EL DIN, 1966b), giving a positive gradient from surface to bottom (Fig. 4). In Fig. 4 Stas. 1-5 (1962 and 1963) were shown previously in a paper by BOWDEN and SHARAF EL DIN (1966a). It is clear that the increase of salinity with depths is not significant at stations at sea away from the effect of river flow.
650
Shorter Contributions
5o30'W
54°N-
BoffomSolinity
MORECAMBE/BAY
~
54°N
53° N 40
53°
40' N J
LIVERPOOL BAY / / .'
/
/
I
Fig. 3. Bottom salinity. [From the mean value over the tidal period at the anchor stations (1962-1965)].
Shorter Contributions
315
320
651
52.5
0°5
350
\
\
35.5
\
°21
I
\
Fig. 4.
Mean salinity profiles at Stas. 1-13, West Coast of England.
The surface variation of the temperature at any station seems to follow the diurnal meteorological system in summer. Such a variation is to be expected due to heating by solar radiation during the day and cooling by back radiation and evaporation at night. The temperature of the bottom layer is more stable, the surface one giving a small difference during the day. The temperature gradient is very small at stations further from the coast away flora the effect of river flow.
THE
LARGE
SCALE
MOVEMENT
OF
THE
WATER
ALONG
THE
WEST
COAST
OF
ENGLAND
The components of U~, and VT of the residual current over a tidal period were obtained by taking the mean of the values over 12 lunar hours at the seven anchored stations. The resultant component of the residual flow V is given by the equation:
The angle 0 to which V inclined from south to north axis of the flow is obtained from the equation: VT tan 8 = . - - . UT In this discussion the resultant of the residual flow of the thirteen anchored stations (Stas. 1-5 were shown previously by BOWDEN and SHARAF EL DIN, 1966b), were put together to give a good picture for the current variation along the coast. At the surface, in most cases, there is a pronounced near-surface flow away from the shore (Fig. 5). At station 10 the surface current is onshore with small magnitude i n comparison with the other stations. At Stas. 7 and 8, the surface flow shows that the tidal flow coming from the north round the Isle of M a n is more effective than the one coming from the south. At Sta. 9 the same tendency as found at Sta. 10 was observed with a big value of about 9 cm/sec. It seems that the only explanation from the stations at which the surface current is inshore is that the duration of the flood current is more than that during the ebb. In general, the net movement of surface water, at any station, is the net circulation between the sea water and river flow. At the bottom (Fig. 6), the resultant of the residual flow is directed towards, or parallel to, the shore. This can be taken as evidence for the existence of a countercurrent parallel to the coast. The bottom flow at the Morecambe Bay area is difficult to explain due to the limited number of the anchored stations.
652
Shorter Contributions
3"30'W
54°NI
cM..,~.
MORECA
\
E BAY
o
-54°N
/ / 5 3 ° ,,
4503~N ~
4 0 'J
z ,
3*3O W Fig. 5.
The surface residual current vectors at Stas. 1-13, West Coast of England.
653
Shorter Contributions
3"30'W t
,/
54°N
5
0
CM/$£C
MORECAMBE~BAY
/,
/
/
/
/
/
54°N
/.7
/
@ "!
Io\ ,~t~i?. LWERPOOL BAY .i
L5
e...
Fig 6.
~9
53 °
,,
401 l~
FORMBY PT
4
The bottom residual current vectors at Stas. 1-13, West Coast of England.
654
Shorter Contributions
Acknowledgements--The author wishes to acknowledge the valuable assistance of Prof. K. F. BOWDEN of Liverpool University for his supervision on these investigations. He is also pleased to acknowledge the help of his colleagues in the Oceanography Department of Liverpool University, particularly in making observations at sea. REFERENCES
BOWDEN K. F. (1955) Physical oceanography of the Irish Sea. Fishery lnvest., Lond., 18 (8), 67 pp. BOWDEN K. F. and S. H. SHARAFEL DIN (1966a) Ch'culation, salinity and river discharge in the Mersey Estuary. Geophys. J. R. astr. Soc., 10 (4), 383-400. BOWOEN K. F. and S. H. SHAgAFEL DIN (1966b) Circulation and mixing processes in the Liverpool Bay area of the Irish Sea. Geophys. J. R. astr. Soc., 11 (3), 279-292. MArTh'EWS D. J. (1914) The salinity and temperature of the Irish Channel and the waters south of Ireland. Scient. lnvest. Fish. Brch. 1re. 1913 (4), 26 pp. PROUDMAN J'. (1938) On tidal variations of temperature in the Irish Sea. Mon. Not. R. astr. Soc. geophys. SuppL, 4 (5), 373-386. PROUDMAN .L (1939) On. the currents in the Nolth Channel of the Irish Sea. Mon. Not. R. astr. eeophys. SuppL, 4 (6), 387-403. PROtrOMAN .1".(1946) On the salinity of the surface waters of the Irish Sea. Phil. Trans. R. Soc. A, 239 (812), 579-594.