View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Electronic Publication Information Center 27 Harmsetal. Salt intrusions in Siberian river estuaries: Observations and model experimentsinObandYenisei I.H.Harms1*,U.Hübner1,J.O.Backhaus1,M.Kulakov2,V.Stanovoy2,O.V.Stepanets3,L.A. Kodina3,R.Schlitzer4. 1InstituteforOceanography,UniversityHamburg,Germany 2ArcticandAntarcticResearchInstitute(AARI),St.Petersburg,Russia 3VernadskyInstituteofGeochemistryandAnalyticalChemistry,RussianAcademyofSciences,ul.Kosygina19, Moscow,119991,Russia 4AlfredWegenerInstituteforPolarandMarineResearch,Columbusstrasse2,27568Bremerhaven, Germany Abstract Observations in Siberian river estuaries show a very pronounced vertical stratification duringsummer.InparticularintheYeniseiEstuary,salinityprofilesarestronglyaffected,not onlybyfreshwaterrunoff,butalsobybottomfollowingsaltintrusionsthatpenetrateactively intotheestuary. Inordertostudytheestuarinevariabilityandtoinvestigatethephysicsbehindthesesalt intrusions, two different numerical circulation models are applied to the Kara Sea and the estuariesofObandYenisei.The3-D,baroclinicmodelsarebasedontherealistictopography, forcedwithwind,tidesandriverrunoff. Modelresultsfromsummerrevealageneralnorthwardflowofriverwateratthesurface out of the estuaries. Near the bottom, however, a south-westward transport of saline water from the Taymyr coast towards theestuariesprevails.Asaltintrusionoccursduringstrong runoffwhenthedirectionofthewind-inducedoffshoretransportisalignedwiththeaxisof theestuary.Inthiscase,theenhancedsurfaceflowhastobecompensatedbyanonshorenear bottomflowthatmaypenetrateintotheestuary. Salt intrusions occur frequently in the Yenisei Estuary in connection with north-easterly winds, a prevailing wind direction in summer in the Kara Sea. In the Ob Estuary, salt intrusionsarealmostabsentandthehalinestratificationisweakerthanintheYenisei.Thisis duetoenhancedtidalmixingintheObbutalsoduetotheorientationanddepthprofileofthe estuary. 1Introduction TheKaraSeaisamarginalshelfsealocatedintheEurasianpartoftheArcticOcean.Itis influenced by oceanic water masses from the Arctic Ocean, the North Atlantic and the Barents Sea. Additionally, the Kara Sea receives large amounts of freshwater through the Siberian rivers Ob and Yenisei. The rivers drain acatchmentareainSiberiaandRussiaof morethan5mill.km2.ThetotalannualamountoffreshwaterinputintotheKaraSeaequals roughly1200km3/yofwhich80%isdischargedinspring(May-June)(PavlovandPfirman, *correspondingauthor:[email protected] Harmsetal. 28 1995). Peak freshwater discharge rates of more than 100,000 m3/s lead to a pronounced vertical stratification in the estuaries of Ob and Yenisei. Under specific meteorological conditions,highsalinemarinewatersmaypenetrateasabottomfollowingcounterflowinto theestuarywhichadditionallyenhancesthestratification. OnegoalofthebilateralGerman/RussianprojectSIRROistostudythedispersionandfate of biogeochemical tracers emerging from Ob and Yenisei rivers. In numerical transport models, most of these river tracers cannot be regarded as dissolved and conservative, dispersed only by the circulation. Horizontal and vertical gradients between freshwater and brackish water affect the transport behaviour and form a so called'marginal filter’. The mixtureofriverandseawaterleadstoasettlementofsuspendedmaterialaswellastothe formationofprecipitates.Knowingthetimeandspacevariabilityoftheverticalstratification andthetransitionzonebetweenfreshwaterandmarinewatersisthereforeapriorcondition fortheinvestigationofestuarinetransportprocesses. Severalpublicationsweredealingsofarwiththeareaandtheabovedescribedphenomena. Sinceacompletereviewofthepreviousworkandtheexistingliteratureisoutofscopeofthe presentpaperwereferonlytosomeimportantwork.Thepenetrationofhighsaline,marine watersintotheObandYeniseiestuarieshasalreadybeennotedinearlydescriptionsofthe hydrological regimes of these areas byAntonov(1962)andAntonovandMaslaeva(1965). More recent papers investigated the magnitude of salt water intrusions depending on tides, windsurgesandriverdischargeintheObGulf(Ivanova,1984;Stanovoy,1984,Ivanovand Sviyatsky, 1987) and in the Yenisei Gulf (Graevsky, 1997). During intense field studies in summer 1993-1996 in the Kara Sea, different types of data from the region of freshwater influence(ROFI)ofObandYeniseiwereobtained.Resultsfromthesecampaignsaregiven e.g.inJohnsonetal.(1997)andMcClimansetal.(2000).Agoodreviewingdescriptionof the variability in Kara Sea estuaries, based on long term observations, can be found in StanovoyandNøst(2002a,b). Hydrodynamicmodellingwaspreviouslycarriedoutbye.g.Pavlov(1996)orHarmsand Karcher(1999)whoinvestigatedmoregenerallythecirculationandhydrographyoftheKara Sea,butalsotheriverrunoffandthetransportofrivertracersfromObandYenisei(Pavlov andTimokhov,1997);(Harmsetal.,2000).ModelstudiesintheObGulfwereperformedby Ivanov and Sviyatsky (1987) and Doronin and Ivanov (1997), mainly for simulating the magnitude of salt water intrusions depending on the river discharge and its inter-annual variability.Alsoalaboratorymodelwasappliedinordertoinvestigatetransportprocessesin theeasternBarentsandKaraSeas(McClimansetal.,1997;McClimansetal.,2000). 2Observations RegularmeasurementsofwatertemperatureandsalinityintheYeniseiEstuarystartedin the1930’s.Datawerecollectedduringsummerandwinterexpeditions,mainlybytheArctic and Antarctic Research Institute (AARI, St. Petersburg) and by the Dikson Hydrometeorological Observatory. Summer observations were carried out from the second half of July until the beginning of October. In wintertime, observations under the ice were carriedoutfromMarchuntilthebeginningofJune. 29 Harmsetal. Ingeneral,thehistoricaldatashowedapermanentlyexistingpycnoclineintheestuarybut only recent CTD1-profiles (from 1993 on) revealed that vertical salinity and temperature gradientsmaybeextremelystrongandreachvaluesofupto5-10per10cmand1.5-2.0°C per10cm.TheaveragemultiyearsalinityatthesurfaceandbottominAugustandthevertical distribution of temperature and salinity in summer 2001 in the Ob and Yenisei Estuary is showninFigures1and2.Thesepicturesexhibitaclearlypronouncedtwo-layerstructureof the water masses which is the typical situation during summer. It is also apparent that the difference between surface and bottom water salinity is more pronounced in the Yenisei Estuary,wheresalinebottomwaterseemstopenetrateintotheestuary. 73.0 73.5 72.5 73.0 72.0 e d u t i t a L 72.5 71.5 72.0 71.0 70.5 71.5 72 73 74 75 78 79 80 81 82 83 73.0 73.5 72.5 73.0 72.0 e d u t i t a 72.5 L 71.5 72.0 71.0 70.5 71.5 72 73 74 75 78 79 80 81 82 83 Longitude Longitude Fig.1:Averagemultiyearsalinityatthesurface(upperpanels)andatthebottom(lowerpanels)in AugustintheestuariesofOb(left)andYenisei(right). 1CTD=Conductivity,temperatureanddepth Harmsetal. 30 Fig.2:Selectedprofilesoftemperature(solidlines)andsalinity(dashedlines)insummer2001inthe estuariesofOb(left)andYenisei(right). 31 Harmsetal. 0 -5 -10 m , h t -15 p e D -20 73 72 -25 71 78 80 82 84 -30 50 75 100 125 150 175 200 225 250 275 Distance,km 0 -5 -10 m , -15 h t 73 p e D -20 72 -25 71 -30 70 78 80 82 84 50 100 150 200 250 300 350 400 Distance,km 0 -10 m , h t 73 p e -20 D 72 71 -30 70 78 80 82 84 50 100 150 200 250 300 Distance,km Fig.3:SalinitysectionsintheYeniseiEstuary,observedin:(a)summer2001,(b)summer2000and (c)summer1995.Distancelabelsstartat73o50'N;80o00'E Harmsetal. 32 ThemostrapidfresheningofestuarinesurfacewatersinObandYeniseioccursinspring when the spring flood wave passes through. At Dikson Island (outer Yenisei Estuary), for example, a sea surface salinity between 0-2 is observed for 20-30 days in June-July. Horizontalsalinitygradientsmayreachvaluesofupto0.5per1km.Thehorizontaltransition zonebetweenfreshwaterandbrackishwaterinsummerislocatedusuallyinthemiddlepart oftheYeniseiEstuary,between71o30'and72o00'N(StanovoyandNøst,2002b).Atypical exampleofsuchadistributionwasobservedduringthelastcruiseofRV"AkademikBoris Petrov"insummer2001(Fig.3a).Thissectionshowsclearlythehorizontalsalinitygradient just above the sill and the strong vertical stratification further seaward. Specific meteorological conditions might enhance the observed stratification shown in Figure 3a considerably. Driven by an on-shore counter flow at the bottom, saline bottom water may penetrate into the estuary, leading to so called'salt intrusions’. Figures 3b and c show this phenomenonontwoYeniseisectionstakeninsummer1995and2000.Inbothfigures,high saline bottom water has moved upstream of the sill, into the river delta, however, with differentintensity. TheapparentinterannualvariabilityofsaltintrusionsisdepictedmoreclearlyinFigure4, which shows the bottom water salinity for August,Septemberandthewinterseasoninthe Yeniseiriverdelta,northofthelargesandbank. 30 25 August 73.5 y 20 t i n i 15 l a S 10 73.0 5 0 1965 1970 1975 1980 1985 1990 1995 72.5 25 September e 20 d 72.0 u t i y t t i 15 a n i L l a 10 71.5 S 5 0 71.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 DELTA 20 Winter 70.5 y t 15 i n i l 70.0 a 78 79 80 81 82 83 84 S 10 Longitude 5 1975 1980 1985 1990 Years Fig.4:ObservedinterannualvariabilityofbottomwatersalinityforAugust,Septemberandwinter season(left)at71o24'N,YeniseiEstuary(rightpanel,dot). All available observations carried out during the last 50 years were used. An extremely highvalueoccurredinAugust1985butthenumberofsaltintrusionsseemstobehigherin September, even if the absolute values are lower compared to August. Also in winter, the bottomwatersalinityvariesconsiderablefromyeartoyear.Thefactthatthewintervaluesare