Jun 9, 1988 - and condition the flow to allow intense vertical mixing. Aspiration reduces the ... Once in the Narrows the flow remains stratified until it passes.
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. C2, PAGES 3451-3472, FEBRUARY
15, 1997
The importance of aspiration and channel curvature in producing strong vertical mixing over a sill HarveyE. Seim1 WoodsHole OceanographicInstitution, Woods Hole, Massachusetts
Michael C. Gregg Applied PhysicsLaboratory and Collegeof Ocean and Fishery Science,University of Washington Seattle
Abstract. On the basisof observationsof the time-dependent, tidally forced flow over a long sill we find that aspirationand channelcurvatureset the flow structure and condition the flow to allow intensevertical mixing. Aspiration reducesthe potential energyof the water column by thinning it while maintaining its density contrast. Channel curvature inducesa cross-channelcirculation that can rapidly overturn a stratified flow. Eighteen Mong-channelsectionsof density, velocity, and dissipationrate of turbulent kinetic energy e were collectedin and around the Tacoma Narrows of Puget Sound, a site suspectedof driving a strong vertical circulationin adjoiningMain Basin. Rapid inflow to the Narrows on flood from one channel of a triple junction reducesdynamic pressure,allowing densewater from below sill depth to be uplifted, or aspirated, into the Narrows. We estimate water from below 150 m, 3 times the sill depth, is drawn into the Narrows on a 3-m flood tide. Once in the Narrows the flow remains stratified until it passes • 50ø bend where • strong secondarycirculation overturnsthe 50-m-deepw•ter columnand generatesintenseturbulent mixing. Cross-channelvelocitiesof up to
0.4 m s-1 are observed, and maximumvaluesof e exceed10-3 W kg-1. Upon leaving the sill, stratificationis reestablished,and turbulencedecays. A similar set of sequences occurson ebb, except that the outflow bypassesthe flood inflow channeland insteaddischargesinto ColvosPassage,the third branch of the triple junction. Colvos Passageultimately dischargesthe ebb effluent back into Main Basin, enhancingthe impact of mixing at the Narrowsby dischargingthe mixed product far from the source. Scalingof the cross-channelmomentum equation suggeststhat, below a thresholdvalue of along-channelvelocity, stratification shouldsuppresssecondarycirculationfor a given vertical shear, radius of curvature and channelwidth. Abovethe thresholdvelocitythe magnitudeof the cross-channel velocity is roughlyconsistentwith predictionsfor unstratifiedflow. We estimate the maximum effectiveeddy diffusivity that aspirationand mixing in the Narrows
canproducein Main Basinto be 10-3 m2 S--1. Introduction Sills or contractions influence the circulation of basins
they adjoin in a variety of ways. Their influenceon
ocean. Most heralded is their ability to lintit the flow through via internal hydraulic constraints. Lesspopularizedbut possiblyequally important is their role in
drivinga verticalcirculationin adjoiningbasins.Knudthe exchangeflow in estuaries[Stommeland Farmer, sen[1900][seeCokeletandStewart,1985]demonstrated 1952, 1953],marginalseas[Garrett et al., 1990],and this effectin the Kattegat wherehe foundthat 2/3 of the deepbasinsof the oceans[Hogg,1983]demonstrates the outflowingBaltic Seawaterdid not exit to the North the crucialrole they play in regulatingcirculationin the Sea but instead reenteredthe Baltic, after being mixed downwardin the Kattegat, as part of the denserinflow. XNow at SkidawayInstitute of Oceanography, Savannah,
Georgia
Horizontal and vertical exchangescompete to determine the fate of water enteringa sill or contractionre-
gion. In the inviscidlimit, no verticalexchange occurs, and horizontalexchangeis controlledby the geometry and reservoirconditions.Mixing diminisheshorizontal exchangeby reducingdifferences in propertiesbetween
Copyright1997by the AmericanGeophysical Union. Paper number 96JCO3415.
0148-0227/97/96JC-03415509.00 3451
3452
SEIM AND GREGG: STRONG
VERTICAL
MIXING
OVER A SILL
layers;in the limiting caseof completemixing the horAspiration and secondarycirculation act in concert izontal exchangeflow tendsto zero, to be replacedby to adiabaticallyreducethe energyrequiredto vertically meridional circulationsin each of the adjoining basins, homogenizea stably stratified water column. By comwith little or only one-waycommunicationbetweenthe pressingthe height of the water column while maintaining its vertical densitycontrast,aspirationreduces basins[CokeletandStewart,1985]. In this study, aspiration and secondarycirculation the amount of energy required for subsequentmixing. around bends are shown to greatly contribute to a Cross-channelcirculationinducedby flow around bends strongvertical circulation. Observationsfrom the Ta- producesa large, site-specificbuoyancyflux that can coma Narrows of Puget Sound, a site that has long overturn the water column. Intense turbulence accombeen suspectedof driving a remarkably strong intra- panies the overturning and completesthe processof basinmeancirculation[BarnesandEbbesme;ter, 1978], mixing. Shear instability is not the principal mixing are used to demonstrate the importance of aspiration mechanism,and thus the gradient Richardsonnumber and channel curvature in effectingstrong vertical ex- is not a goodpredictorof turbulent mixing wherecrosschange. Hydrographicprofilesare supplemented with channelcirculationis strong. microstructureand current profilesthat allow us to deThere are two goals of this paper: to describethe fine the flow structure during flood and ebb tide. We time-dependent flow over the sill and to identify the identify specificlocationsof turbulencegenerationthat processesthat determine the flow structure. We first implicate flow curvature as the processresponsiblefor provide a context for the observationsby describing generatingnearly well mixed conditions.The analysis Puget Sound. After introducingour instrumentation, herein focuseson the role of thesetwo processes in deter- we present typical flow conditionson flood and ebb in miningthe flow structure.A future paperwill examine the third section.Simplepredictionsand scalingsof asthe energeticsof turbulencein this flow. piration and cross-channelcirculation are given in the Aspirationis a term coinedby Stommelet al. [1973] fourth section,and the implicationsfor the flow regime to describehow high flow speedsover a sill reducethe in the Narrows and Puget Sound are discussedin the dynamic pressureto such an extent that densewaters fifth section. from below sill depth are uplifted on to the sill. They also referred to this processas Bernoulli suction. The Background processgoesby differentnamesin other fieldsof fluid Our observationswere collectedin Puget Sound, a dynamics. In limnology it is referred to as selective
withdrawal[e.g.,Monismithet al., 1988]andaddresseslargefjord-likeestuaryin westcrnWashingtonState, of four deep(> 150 m) basinsinthe vertical extent of inflow to a point or line sink in a USA, that consists stratified fluid. In atmosphericsciencesthe term block- terconnectedby shallowsillsand tidal channels.Main directlyto the Strait of ing is usedto describehowstratifiedfluid belowa ridge Basinis largestandis connected
canbe preventedfromflowingoverit [e.g.,Gill, 1982]. Juande Fucaby AdmiraltyInlet (Figurela). The basin In all cases the thickness of the inflow in excess of the
is over50 km longandholdsroughly77 km3 of water
opening(e.g., belowsill depthor ridgeheight)is pro- [Lavelieet al., 1988].WhidbeyBasinemptiesinto the portional to U/N, where U is a measureof the flow north end of Main Basin,and Hood Canal emptiesdispeedand N is the ambientstratification[Gill, 1982]. rectlyinto AdmiraltyInlet. The southernendof Main In our casethis is the depth of water belowthe sill that Basin is separatedinto two passages,ColvosPassage is upliftedand passesoverthe sill. The flow aspirates and East Passage,by VashonIsland. TacomaNarto greaterdepthsfor strongerflowsand weakerstrati- rows, the focusof our study, connectsthe Main and fication. In the limit of no stratification, fluid from all Southern Basins. More than a dozen rivers empty into PugetSound,supplyingeachbasinwith a directinput depthspassesup and over the sill. Channelcurvaturegeneratesa cross-channel circu- of fresh water. The Narrowsformthe southernlegof a triplejunction lation in unstratifiedopen-channelflows. A force im-
at the southend of Main Basin (Figurelb). Dalco is the eastlegof thejunctionand connects the bottom,reducingthe centrifugalacceleration, allowing Passage balance arises because friction slows the flow near the
the cross-channelpressuregradient to acceleratethe Narrowsto 200 rn depthsin East Passage.The north near-bottom flow toward the inside of the bend. The leg is ColvosPassage, a narrow,100-mchannelthat strengthof the cross-channel circulationcan be more rejoinsMain Basin20 km to the north. The Narrowsis of two than the 10%of the streamwise flow [Geyer,1993].In a 10-km-long,1.5-km-widechannelthat consists joinedby a 500bend. The fast moving,weakly stratifiedflowslike the Narrows relativelystraightsegments
the cross-channel circulationproducesa largebuoyancy TacomaNarrowsBridge crossesthe Narrowsnear the flux that stronglyimpactsverticalmixing. Sufficiently midpointof the southernsegment.Waterdepthsof 50strongstratificationcansuppress the cross-channel cir- 70 rn in the Narrowsare half or lessof the depthsin the passages. The Narrowsis thereforea culation. An estimate of the location, time, and con- threesurrounding obstacle to flowbetweenMain andSouthern ditions when this transition takes place is discussedin significant the analysissectionof the paper.
Basins.
SEIM AND GREGG: STRONG VERTICAL MIXING OVER A SILL Strait of Juan de
3453
Whidbey Basin
48.1
47:40N Main Basin
48 Admiralty Inlet
47.9
47.8 > landward
=•
seaward
47.7
47.6
Passage
Passage
47.5 Colvos
Passage 47.4
DalcoPassage
......
47.3
47.2
--
Southern. Basin
......
Southern
47'12N
122.9
122.7 122.5 Longitude
122.3
...............................
122'40W
(b) 122:18W
Figure 1. (a) Map of PugetSoundshowingthe bathymetryand the principalbasins. (b) Detailed chart of south Main Basin includingplace namesusedin the text and arrowsshowing the senseand strengthof the mean circulation.The strongclockwisecirculationaroundVashon Islandmakesexchangeratesin southMain Basintwicethosein North Main Basinand Southern Basin.
The strengthof the time-averagedestuarinecirculation varieswidely betweenthe basinsof Puget Sound. The mean exchangerates estimatedby Cokeletet al. [1990a]are greatestin Main and SouthernBasins,the basinsjoinedby the Narrows(Table1), and smallestin Whidbey Basin, the basinthat lacksa pronouncedsill. The meanexchangerates includethe density-drivenestuarine circulation
that deepwaterfromEast Passage wateris "pumped upwardby floodtides"and mixedwith surfacewater
in theNarrows.(CannonandEbbesmeyer [1978]suggestthat aspiration may accountfor uplift nearthe ebb some of this water returns to Main Basin through
ColvosPassage whereit sinksbeneaththe local river
and the rectified tidal circulation.
(Rectifiedflowsare meanflowsgenerated fromoscillating currentsby nonlinearities.)The averageexchange rate in north Main Basin is m 1.5 x 104 m3 s-1 and in
SouthernBasinis m 1.3 x 104 m3 s-1, but the depthaveragedtransportsin ColvosPassageand East Passage are 3 x 104 m3 s-1 twice that to the north or south, indicatinga strongrecirculationaroundVashonIsland (Figurelb) [Cokeletet al., 1990a]. BarnesandEbbesmeyer [1978],p. 218, postulatetha,t the rectified
Narrowsbut makeno attempt to demonstrateit.) On
tidal circulation
associated with the Nar-
rows is the causeof the dramatic differencein exchange rates in Whidbey Basin and Main Basin. They assert
Table 1. ExchangeRate Ratiosfor Puget Sound Basins
Basin Main
Basin
Whidbey Basin Hood Canal Southern Basin
42
4 33 100
Shown is the ratio of transport in the most seaward reach of each basin to the sum of all
riverine inflow to the reach,
3454
SEIM AND GREGG:
STRONG
VERTICAL
MIXING
OVER A SILL
plumes. Experimentswith a hydraulicmodel of Puget dissipation-scale spectrawhene • 10-6 W kg-x, so a
Sound[Ebbesme•ter andBarnes,1980]suggest that the
variance correction scheme is used when e exceeds this
strength of the exchangeflow in Main Basin depends strongly on the tidal circulation through the Narrows. When they blockedoff the flow through the Narrows in the model, the transport rates werereducedby 75%. Their resultsimply that Main Basin wouldbe relatively stagnantand unproductivewithout the tidal circulation
value[Wesson and Gregg,1994]. A 153-kHznarrowbandacousticdopplercurrentpro-
filer (ADCP) measured the currentvelocity,producing
averagedestimatesevery 2 min. The accuracyof the measurements varied due to bottom tracking. In May, no bottom tracking was used, and approximately 240 of the Narrows.In fact, [Cokeletet al., 1990b]suggest pingsare averagedin each2-min ensemble;using4-m that all the upward vertical transport in Main Basin bins,the rmserrorsare 0.014m s-x. In June,bottom occurs at the Narrows. trackingwasused,halvingthe numberof pingsper enPerhaps the most remarkable aspect of the flow in sembleand increasing errorsto 0.02 m s-x. Puget Sound is that a strong clockwisecirculation exThe ADCP had someproblems.The most seriousof ists around Vashon Island, even at tidal frequencies. thesewasits compass.We installeda Humphries2-axis Though the flow reversesdirection in East Passagewith gyrocompasson the Miller, slavedto a fluxgate comthe tide, in Colvos Passagemonth-long current moor- pass, to provide heading information. Unfortunately, ings confirm northward flow throughout the tidal cy- the fluxgate compasswas installed very closeto a vercle overthe depth of the water column[Larsenet al., tical iron tube which severelydistorted the magnetic 1977; National Oceanic and Atmospheric Administra- field. Because the gyrois slavedto the (magnetic)fluxtion (NOAA), 1973].The measurements indicateweak- gate compass,headinginformationwas in generalvery ening of the northward flow as flood progresses, slowing poor, except on due north settings, and unusablefor to almost zero during peak flood current in the triple most of the cruise. junction. Speedsthen increaseagain as flooding curLacking heading information, we have used courserents wane and reach a maximum during ebb. Over the over-ground (cog),computedfrom the LORAN-C fixes, month of current observations,flow was to the south in placeof the headings.Becausewe operatedrubidium only 2% of the time. clockswith our LORAN receivers,we wereable to com-
Becausethe length of the Narrows (10 km) is a significantfraction of the tidal excursion(6-18 km), it is classifiedas a long sill [Farmer and Freeland, 1983]. Mixed tides of up to 4 m producetidal currentsU < 2.5 m s-• andthe stratification is relatively
putehighlyaccuratefixes(4-5m) usingcircularranges
rather than the standardhyperbolicrangesthat are particularly poor in Puget Sound. In a surprisingnumber of instanceswe find cogto be an adequatesubstitutefor heading. However,when hezding changessignificantly weak(A•r• < 1 kg m-a). The internalFroudenum- overan ensembleor whenshipspeedwith respectto the
berFr- U/(#'h)x/2,where#' = Ap/pisthereducedwater is small, typicallylessthan 0.75 m s-x, we can-
gravity and h is the water depth, is greater than one not form absolutevelocities.We have made significant for U > 0.7 m s-•. Owingto the highFroudenumbers, effort to confirm the believability of the velocity data. the Narrows is expectedto be a site of locally enhanced For each section in the cruise we have compared the mixing rather than a generatorof large lee wavesor in- magnitude of the depth-averagedcurrents with those
ternalhydraulicresponses [FarmerandFreeland,1983]. predictedby the tidal model of Lavelleet al. [1988]. We confirmedthe currentdirectionby plottingthe measured velocities on a chart to ensure the flow orientation
Data
is reasonable.
Instrumentation
and Setting
Bottom tracking was not reliable in 1988 and did not
Allourobservations werecollected fromtheR/V
workwell. Forall bottom-tracked sections wecompared velocities computed frombottomtrackingwith Miller, a 15-m utility boat, in May and June of 1988. absolute computedwith the navigationdata. The Miller was equippedwith LORAN-C for naviga- absolutevelocities tion and carried two instruments. The Advanced MiIn almost all casesthe navigation-corrected velocities crostructureProfiler(AMP) is a looselytetheredfree- were more reasonable,and they have been used in all falling profiler that measuresmicroscaleshearand tem-
but a few cases. To maintainsimilarvelocity(and
peraturegradientand conductivity-temperature-depthshear)errorsthroughoutthe cruise,bottom-trackdata profiles. Most profileswere collectedwhile steaming has beenaveragedover4 min. slowly,AMP being deployedoff the stern. The AMP To examinethe strengthof the cross-channel circulaprofiles were terminated 5-10 m above the bottom.
tion,wedecompose eachvelocityvectorintoan along-
Standardprocessing of the data producesdissipation channel component u• andcross-channel component rate of turbulentkineticenergy(e) anddissipation rate whereu• is alignedwith the directionof the depthof temperaturevariance(X) profileswith 0.5-m reso- averagedflow at that point and un is positiveto the lution and temperature,salinity, and densityprofiles right. This wasa practicalmeansof decomposing the with 0.1-m resolution.The shearprobesresolvehalf of vectorsin a systemwhereit is not practicalto definethe
SEIM AND GREGG:
STRONG
VERTICAL
MIXING
OVER A SILL
3455
47ø25'30N
47ø11 '24N
122ø3636W
122ø27'00W
122ø36'36W
122ø27'00W
Figure 2. Planviewmapsof currentvectors.(a) Depth-averaged currents, showing thedirection usedto decompose velocities into along-and cross-channel components.(b) Near-surface and near-bottomcurrentsfrom the samesectionas in Figure2a showingthe strongveeringof the current with depth.
along-channeldirection at everypoint along the various In all casesthe flow is frownleft to right in the sections. channels. In most casesthe depth-averagedflow that Please note that north is to the left in the flood tide definesus parallelsthe channel(Figure2a); the excep- sectionand to the right in the ebb tide section. Three tions are near times of high and low water when the flow cross sections at the northern entrance to the Narrows is reversingdirection. Plotting near-surfaceand near- (point A), the centralbendat Point Evans(point B), bottom velocity vectorsshowsthe current veering that andthe southernentrance(pointC) aremarkedoneach occursas the flow passesaround bends in the channels graphicfor comparison (seeFigures3 and 4).
(Figure 2b).
The strengthof currentveeringis well
representedby un becausethe decompositionforcesthe depth averageof un to be zero. Errors associatedwith the compassproblemsdetailed above are all barotropic and will only affect the magnitude of the flows, and becausethe along-channelspeedscomparefavorablywith
ing when and where intense turbulence occurs in the Narrows and to quantify its magnitude. Two lengths
Lavelleet al. [1988],we estimatethat un is accurateto
often used to characterize
4-0.05 m s-1.
to addressthe forcingand state of evolutionof the tur-
Turbulent Length Scales
Our observations are particularlyusefulfor identify-
the scales of turbulence
and
Our database consistsof eighteensets of AMP pro- bulenceare the buoyancy (or Ozmidov[1965])scaleLb files (340 total profiles)collectedunderwayor on an- and the root-mean-square overturn(or Thorpe[1977]) chor in the Narrows and two additional sets of profiles from Main Basin. Each AMP profile has been paired with a 2-min or 4-min averagedADCP profile collected at the same time. Together the observationsspan the triple junction and the Narrows, though data coverage in time is poor at any givenlocation. We thereforecombine observationscollectedat a given phase of the tide to form a composite picture. Flood and ebb tides are describedbelowusinga seriesof along-channelsections.
scale Lrms. The buoyancy scale arises from assuming a balance between inertial and buoyancy forces
Lo - (e/N3)1/2. Theoverturn scaleis a measure of the distancethat parcelsfrom a densityprofile must be movedto form a monotonicallyincreasingprofile by resorting. It providesa physicalmeasurementof overturn scalesby assumingthat diffusionis negligible. In most oceanicobservations,Lb • Lrms[e.g., Dillon, 1982;Gregget al., 1986],wherethis relationshipis
3456
SEIM AND GREGG: STRONG
VERTICAL
MIXING
OVER A SILL
07-Jun-88
09-Jun-88
0.0 0.2
0.4 0.6 0.8
oe (kg/m 3)
1.0
I
I
I
I
I
I
0.0 0,2
0.4
0.25
•0.6 0.8
u s (m/s)
1.0 0,0
0.2
(•0.4 I
0,6 0.8
(m/s)
1.0
I
I
I
I
I
le'
0,0 0,2
..5...'""""•'••' "•••"'••.
0ø4 •0o6 0.8 1.0
, 5 6 kilometers
7
8
9
, I f) 10
11
Figure 3. Section f4 collected ona minorfloodtide. (a) Theshiptrack,wherethelargeasterisk marksthe first dropin the section.(b) The predictedtidal height[Lavelleet al., 1988]where shading marksthe timewhenthesection wascollected. (c) Contoured densityfield(•r•) shows aspirationof densewaterfromDalcoPassage andlargehorizontaldensitygradients aroundpoint
A. Thecontour interval is0.1kgm-s. (d) Contours ofalong-channel velocity (every0.25m s-•), and(e) cross-channel velocity, contour intervalof 0.05m s-•. (f) Contours ofloge reveala rapid
increasein dissipation rates as the flow enters the Narrows.
interpretedto indicatethat the outer scalesof turbu- Flood Tide: Dalco Passage to Point Evans lent eventsare limited by the stratification. An exception to this behavioris in the convectingsurfacemixed Inflow to the Narrowsoriginatesentirely from Dalco layer where Lrms> Lb, reflectingthat turbulenceis Passage.Becausea fractionof the inflowentersColvos generatedthroughbuoyancyfluxesat the seasurface Passage on flood (seethe Background Section),there rather than by shearinstability [Brainerdand Gregg, must be a stagnationpoint in the triple junction Rapid
1993].Anotherexception is foundin laboratorystudiesof grid-generated turbulencewhereturbulentenergy is inputat smallscales[e.g.,Rohret al., 1988].Right behindthe grid, L• > Lrm•. Downstream of the grid,
flow within the Narrows causesan uplift of the denser water in Dalco Passage.A sectionfrom Dalco Passage
into the Narrows,collectedon a minorfloodtide (Figure 3b), showsa significantuplift of isopycnals as the turbulentscalesgrow,and e decreases until L• • Lrma, flowmovesintotheNarrows(Figure3c). In particular, after which the two scales tend to decrease in concert. the 23 kg m-• isopycnal risesmorethan50 m in a hor-
SEIM AND GREGG: STRONG VERTICAL MIXING OVER A SILL
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