OBSERVATION OF A SHALLOW WATER INTERNAL OSCILLATION EVENT

Chinese Journal of Oceanology and LimnologyVol.20,No.1 ,P. 86- 90 , 2002OBSERVATION OF A SHALLOW WATER INTERNALOSCILLATION EVENTGAO Shu(高抒)( Institute of Oceanology，Chinese Academy of Sciences , Qingdao 266071 , China )A. F. Velegrakis ,M. B. Collins( School of Ocean and Earth Sciences , The University , Southampon S014 3ZH , UK)Received July 8 , 2000 ; revision accepted Jume 16 ,2001AbstractAt a shallow water station( 6 m in depth )，an internal oscillation event which con-sisted of one or two wave- like features , with a period of3 h and a height of 1.5 m , was observed. Thevelocities within the water column were modified by the event during the flood phase of the tide ia multi-layered velocity structure and intense shear were generated. Further investigations are required to under-stand fully the mechanism for the formation of such an event.Key words : Intermal oscillation , velocity structure , coastal waters , Christchurch HarbourINTRODUCTIONIntemal waves had been recorded in marine environments with water depths from 10 m to morethan 200 m( Roberts , 1975 ip. 1- 15 ; Huthnance , 1989 ). Such events have been recorded ma-inly in terms of salinity and temperature structures . Disturbances caused by the interaction betweentidal flow and seabed topographic features , density stratification( for internal gravity waves ) and at-mospheric forcing are identified as some of the mechanisms for the formation of such waves ( Light-hill ,1978 ,p.284 - 290 ; Thorpe , 1992 ). However , observations associated with internal oscil-lations in very shallow( < 10 m ) coastal waters are rare .In this short note , we report the current ve-1°16W144locity , salinity and temperature structures that-. 50~4’'are related to an internal oscillation event at 8shallow water( 6 m in depth ) station. The ob-servations were made during a geological/ oceano-graphic survey offshore of a tidal inletStn1Chrischumch( Christchurch Harbour ) in southem England. .HarbourWe are not sure whether or not this event repre-sents internal waves , because the causes for itsformation are not understood fully.Stn2●、14Over the westermn part of Christchurch BayPoole Bay( Fis 1). snuthem Fnoland , is an area whereLerisEehurch中国煤化工Christchurch HarbourMYHC NM H Gnts interact. Here , theFig.1 Location map showing stations for tidal cycle mea-tides are semidiurmal in character , with a tidalsurements( bathymetry in metres , in terms of Chart Darange of 1.2 m on springs ( Tyhurst , 1978 ),and maximum depth-mean tidal current speedstum )range between 0.3 and 0.8 m/s. ChristchurchLedge is万海熬掘nt bathymetric feature over the area( Fig. 1 ), extending from the shore towardsNo.1GAO et al. :A SHALLOW WATER INTERNAL OSCILLATION EVENT87the southeast for 6 km. To the south of the ridge , the water depth increases to more than 15 m overa 0.5 km horizontal distance ; to the north , the water depth is generally shallower than 10 m._a6-s-4-Q223-2-0.2 -E 0.11-o⊥7-b亘5-3933.90!771S1+0中国煤化工23.01 2.THCNMHG 11 T2a. current speed( m/s , negative values are in the ebb( westerly ) directions );b. salinity ic. temperature( °C)Spring andneap tidal cycle measurements of curents , salinity , and temperature were under-taken at twd samas ( Fig. 1 ) under exceptionally calm sea conditions . Current speed and direction88CHIN. J. OCEANOL. LIMNOL. ,20( 1 ),2002Vol.20were measured using a( Valeport ) direct reading current meter( with accuracy of土2 cm/s ) , salini-ty and temperature by a direct reading salinometer , and water depth by an echo-sounder. The cur-rent velocities recorded were time- averaged values over one minute ( hence , oscillations in currentspeeds with shorter periods than one minute were not recorded ). Within the water column , thehydrographic variables were measured at levels of 0.5 , 1.5 ,2.5 and 3.5 m below the water sur-face. Further downwards , the readings were taken at 0.5 m intervals until the near-bed layer( 0.3to0.5 m above the seabed ) was reached. The measurements were undertaken every half hour andthe data were recorded according to GMT( Greenwich Mean Time ).Station 1 was located approximately 1.4 km to the east of the entrance to Christchurch Har-bour. The mean water depth here was approximately 5.8 m. At this station , the surface water layer(1 to2 m in thickness ) was occupied almost continuously by waters with relatively low salinity andhigh temperature , during both spring and neap tides. Fig. 2 shows the current velocity , salinity andtemperature structures within the water column , recorded over the spring tidal cycle. Freshwater dis-charge from Christchurch Harbour during the measurements ranged between 21 and 22 m'/s ( dataprovided by the National Water Authority( Wessex Region ), UK ). Due to the influence of thefreshwater plume , the surface waters had a salinity of between 32.0 and 34. 0 , whilst the bottomwaters had a value in excess of 34.2. The temperature pattern was less significant than that of salin-ity ,but it was clear that the warmer estuarine water was underlain by the cooler English Channelwater. Spring tide curent speeds reached a maximum of more than 0.30 m/s.8-一4 25 cms-!6F4-3?2 239101112Timc (h. GMT)Fig. 3 East- west components of the current velocities observed at Station 1( Fig.1 2)An intemal oscillation event can be identified on the basis of the salinity and temperature struc-tures( Fig.2). One or two internal oscillationsflood phase of the tide中国煤化工( in terms of water level ), with a period of aroof around 1.5 m( Fig.2b ). The wave forms were almost sinusoidal in:TYHC N M H Ghase , the flow structurewas disturbed significantly by the internal waves. The velocity structure for the ebb phase had thepattern associated with quasi-steady tidal flows( Fig.2a ). However , such a patterm was replaced bythat of oscillatory currents during the flood tidal phase , between 04. 00 and 11.00 hrs CMT. Withinthe water calymn }high- and low-speed zones occurred altermatively ( such a multi- layered structureextended beyona碗halocline , reaching the sea bottom ). The layered-structure was considered toNo.1GAO et al. :A SHALLOW WATER INTERNAL OSCILLATION EVENT89be significant because the resolution of the measurements was sufficiently high with 8 to 10 velocityreadings throughout the water column.7-a6-5-4+.。2-S2, 0.40.3、nb仓“十3-1-77310-121314151617中国煤化工0 iTYHCNM HGFig.4 Tidal cycle observations at Station2 ,on 15- 16 May , 1991 ( spring tide )a. current speed( m/s , negative values are in the ebb( westerly ) directions );b. salinity ic. temperature(°C)Modificatign o normal” flow patterns can be demonstrated clearly using the east-west compo-nents of the路护velocities( Fig.3 ). Here , the ebb tidal currents were directed towards the west90CHIN. J. OCEANOL. LIMNOL. ,20( 1 ),2002Vol.20and those during the flood flow towards the east. During the flood , the easterly-directed currentswere inter-layered with smaller or even reverse currents. The observed feature may be interpreted asinternal waves , taking into account that topographic protrusion and density stratification were present. However , the puzzle is that the curents had a much higher frequency than 3 hr ,as shown bythe salinity structure .During the neap tide measurements , the water column was also stratified. However , the magni-tude of the current velocity was lower , with a maximum of 0.19 m/s. In this situation , no internaloscillations were identified from the current velocity , salinity and temperature records.Nor were these observed at Station 2 located approximately 1.7 km to the southeast of the en-trance to Christchurch Harbour and 1.0 km to the northeast of Christchurch Ledge( Fig.1 ). The .mean water depth here was around 5.5 m. The tidal currents were directed towards the northeastduring the flood and towards the southwest during the ebb. Over the spring tidal cycle , currentspeeds up to 0.86 m/s were reached. The currents were rectilinear in character , with the westerly-flowing ebb currents being stronger( Fig. 4a ). The water here was influenced by the estuarine watersfrom Christchurch Harbour only towards the final stage of the ebb tide phase , as shown by the salini-ty and temperature data( Fig. 4b ;4c ). The influence of the estuarine waters here was similar during theneap tidal cycle , except that the temperature stratification was more significant than that during the spri-ng tide. The currents were rectilinear and weak , with maximum speeds of less than 0.2 m/s.ACKNOWLEDGEMENTSThe authors wish to thank the skippers of the MV Satina for their assistance with the measure-ments. Discussions with Dr. D. X. Jiang and Dr. M. Oikonomou ( University of Southampton )have been helpful. Kate Davis is thanked for her assistance with the preparation of the figures .ReferencesHuthnance ,J. M. , 1989. Intemal tides and waves near the continental shelf edge. Geophysical and AstroplhysicalFluid Dynamics 48 :81- 106Lighthill ,J , 1978. Waves in Fluids. Cambridge University Press , Cambridge ,504p.Roberts ,J , 1975. Internal Gravity Waves. Marcel Dekker , New York , 274p.Thorpe,S. A. ,1992. The generation of intemal waves by flow over the rough topography of a continental slope.Proceedings of the Royal Sociely of London A439 :115- 130.Tyhurst ,M. F. , 1978. Tidal conditions in Poole Bay. Joural of the Society of Civil Engineering Technicians 5( 6):6- 11.中国煤化工MHCNMHG