The strengthening East Australian Current, its eddies and biological effects - an introduction and overview

The polewardflowingEastAustralianCurrent(EAC)ischaracterisedbyitsseparationfromthecoast, 100–200nauticalmilesnorthofSydney,toformtheeastwardflowingTasmanFrontandasouthward flowingeddyfield.Theseparationzonegreatlyinfluencescoastalecosystemsfortherelativelynarrow continentalshelf(only15–50kmwide),particularlybetween32–341S. Inthisregionthecontinental shelfhasamarkedshiftintheseasonaltemperature-salinityrelationshipandelevatedsurfacenitrate concentrations.ThiscurrentparallelstheportionofthecoastwhereAustralia’spopulationis concentratedandhasalonghistoryofscientificresearch.However,understandingofphysicaland biologicalprocessesdrivenbytheEAC,particularlyinlinkingcirculationtoecosystems,islimited.In this specialissueof16papersontheEAC,weexaminetheeffectsofclimaticwind-stressforcedocean dynamicsonEACtransportvariabilityandcoastalsealevel,fromENSOtomulti-decadaltimescales; eddy formationandstructure;finescaleconnectivityandlarvalretention.Comparisonswiththe poleward-flowingLeeuwinCurrentonAustralia’swestcoastshowdifferencesinecosystem productivitythatcanbeattributedtotheunderlyingphysicsineachregion.Onaveragethereis double thechlorophyll a concentrationontheeastcoastthanthewest.IncomparisontotheLeeuwin, the EACmayhavelesslocalretentionoflarvaeandactasapartialbarriertoonshoretransport,which may alsoberelatedtothelocalspawningandearlylifehistoryofsmallpelagicfishoneachcoast.Inter- annualvariationsintheEACtransportproduceadetectablesea-levelsignalinSydneyHarbour,which could provideausefulfisheriesindexasdoestheFremantlesealevelandLeeuwinCurrentrelationship. The EAC’seddystructureandformationbytheEACareexamined.Aparticularcold-coreeddyisshown to havea‘‘tilt’’towardsthecoast,andthatduringarotationtheflowofparticlesmayriseuptothe euphoticzoneandthendownbeneath.Inawarm-coreeddy,surfacefloodingisshowntoproducea new shallowersurfacemixedlayerandpromotealgalgrowth.Anassessmentofplanktondatafrom 1938–1942showedthatthelocal,synopticconditionshadtobeincorporatedbeforeanycomparison with thepresent.ThereareusefulrelationshipsofwatermasscharacteristicsintheTasmanSeaand separationzonewithlarvalfishdiversityandabundance,aswellaswithlong-linefisheries.These fisheries-pelagichabitatrelationshipsareinvaluableforfisheriesmanagement,aswellasforclimate changeassessments. There isfurtherneedtoexaminetheEACinfluenceonrainfall,stormactivity,dustdeposition,and on themovementsbyfish,sharksandwhales.TheAustralianIntegratedMarineObservingSystem (IMOS)hasprovidednewinfrastructuretodeterminethechangingbehaviouroftheEACandits bio-physicalinteractionwiththecoastsandestuaries.Theforecastingandhindcastingcapability developedundertheBluelinkprojecthasprovidedanewtoolfordatasynthesisanddynamical analysis.TheimpactofastrengtheningEACandhowitinfluencesthelivelihoodsofoverhalfthe Australianpopulation,fromBrisbanetoSydney,HobartandMelbourne,isjustbeingrealised.