Seismic reflectionalongthepathoftheMediterraneanUndercurrent

摘要

Seismicreflectionprofilingisappliedtothestudyoflargescalephysicaloceanographicprocessesinthe Gulf ofCadiz andwesternIberiancoast,coincidingwiththepathoftheMediterraneanUndercurrent. The multi-channelseismicreflectionmethodprovidesclearimagesofthermohalinefinestructurewith a horizontalresolutionapproximatelytwoordersofmagnitudehigherthanCTDcasting.Theseismic data arecomparedwithco-locatedhistoricaloceanographicdata.Threeseismicreflectivityzonesare identified:NorthAtlanticCentralWater,MediterraneanWaterandNorthAtlanticDeepWater.Seismic evidenceforthepathoftheMediterraneanUndercurrentisfoundinthenear-slopereflectivitypatterns, with risingreflectorsbetweenabout500and1500m.However,thecoreoftheundercurrentislargely transparent. Seismicimagesshowthatcentraland,particularly,intermediateMediterraneanWaters havefinestructurecoherentoverhorizontaldistancesofseveraltensofkilometers.However,the intensity ofthereflectors,andtheirhorizontalcoherence,decreasesdownstream.Thischangein seismicreflectivityisprobablytheresultofdiminishedverticalthermohalinecontrastsbetween adjacentwatermasses,sothatdouble-diffusionprocessesbecomeunabletosustaintemperatureand salinity staircases.Comparisonofroot-mean-squareseismicamplitudeswithtemperatureandsalinity differences betweentheMediterraneanUndercurrentandtheoverlyingcentralwaterssuggestsacausal relationship betweenobservedthermohalinefinestructureandtrueseismicamplitudes.Weestimate that, withinthisintermediatewaterstratum,impedancecontrastsaremainlycontrolledbysoundspeed contrasts(afactorbetween3.5and10timeslargerthandensitycontrasts),whicharemainlycontrolled by temperaturecontrasts(afactorbetween1.5and5timeslargerthansalinitycontrasts).