In-situ Submesoscale observations during the formation of a mesoscale eddy in the ACC

138 Pages·2012·2.63 MB·English

In-situ Submesoscale observations during the formation of a
mesoscale eddy in the ACC
KATEADAMS1,PHILHOSEGOOD1,JOHNR.TAYLOR2,JEAN-BAPTISTESALLEE3
1SMSE,PlymouthUniversity,UnitedKingdom
2DAMTP,UniversityofCambridge,UnitedKingdom
3L’Ocean,France
Investigations within various large-scale current systems have shown the impor-
tance of submesoscale frontal dynamics on the vertical exchange between the atmosphere
and the mixed layer. There is a dearth of observations, however, which resolve subme-
soscale variability along the strongly strained, eddy-rich frontal regions in the Southern
Ocean. We present here observations from the Surface Mixed Layer Evolution at Subme-
soscales(SMILES)cruiseconductedinMay2015thatconstitutethefirstin-situ,submesoscale-
resolving measurements of the formation and evolution of a closed core eddy within the
ACC.
Our observations, primarily consisting of towed CTD (Seasoar), vessel-mounted
ADCP, and drogued drifters, are concentrated on a prominent northward meander of the
ACC within the frontal region east of Drakes Passage where the Subantarctic and Polar
fronts converge. The Seasoar survey began at precisely the moment that the meander sep-
arated from the ACC and formed a closed, cold-core mesoscale eddy. The eddy crossed
theScotiaRidgethroughanarrowgaptotheeastofBurwoodBankandmovednorthwards
towardstheFalklandIslandshelfsea. InconjunctionwiththetowedCTDsurvey,adrifter
tripletwasreleasedwithinanarrow(<5km)coldwaterfilamentembeddedwithinthefront.
The drifters completed three revolutions before being ejected from the eddy in a streamer
that was clearly visible in a rare cloud-free SST image to emanate from the south-eastern
sector of the eddy. Additional drifters released at various positions within the eddy also
exitedatasimilarpositionsuggestingalocalizeddegradationinfrontalintegrity.
The ship-based measurements ultimately encompassed the entire eddy with an
across-front horizontal resolution of O(2 km) down to 200-m water depth and thus, along
with the drifter trajectories, provide insight into the variations in lateral frontal structure
in different sectors. The northern sector where we began the survey was composed of the
frontal region associated with the original meander and was defined by a strong cyclonic
circulation and well-defined frontal system. The newly formed southern portion of the
eddywasdistinctlydifferent,withdecreasedalong-frontvelocitiesandawiderfrontalzone
composedofseveralfilamentsthatfurthercomplicatethefrontaldynamicswithinthissector
of the eddy. We highlight the close correspondence of the drifter trajectories during the
severalrevolutionsaroundtheeddy. Streamlinesderivedfromgeostrophicsurfacecurrents
computedfromseasurfaceheightanomalyagreewellwithobserveddriftertrajectories.
Hysteresis Behaviour of the Antarctic Circumpolar Current
Identifed in a Quasi-Geostrophic Model.
ADEKUNLEAJAYI1
1EarthSystemPhysics,TheAbdusSalamInternationalCentreforTheoreticalPhysics,Trieste.
Italy
In this research work, the hysteresis behavior of the Antarctic Circumpolar Cur-
rentwithrespecttowindstressperturbationisinvestigatedusinganidealizededdy-resolving
quasi-geostrophicwinddrivenmodel. Theresponseofcircumpolartransport,potentialen-
ergy and kinetic energy to changes in wind is quantified. The model uses three quasi-
geostrophic layers with variable wind stress forcing and no buoyancy effect. The analysis
of the model results is divided into two stages with each stage having three regimes; eddy
driven, eddy poor and wind driven in order to show the transition from an eddy saturated
flowtoawinddrivenflowwithoutstrongeddyeffects. Theenergybalanceofthesystemis
diagnosedandthemechanismbehindthehysteresisbehaviorisproposed.
Observations of submesoscale features in the Canada Basin
MARIONALBERTY1,JENNIFERMACKINNON1,JANETSPRINTALL1
1ScrippsInstitutionofOceanography,UniversityofCaliforniaSanDiego,USA
TheCanadaBasinupperoceanheatbudgetiscomplicatedbytheunusualvertical
distribution of the water properties, namely cold, fresh water lying over warm, salty wa-
ter. Buoyancy forcing here is driven by solar radiation, ice melt in spring/summer, brine
rejection in fall/winter, terrestrial freshwater inputs, and the advection of Pacific water ed-
dies shed off of Point Barrow, Alaska. These mechanisms are inherently spatially patchy,
resulting in complicated sea surface temperature and salinity fields of eddies and fronts.
Additionally the surface velocity field can strain these mesoscale features and lateral gra-
dients into thin filaments with short spatial and temporal scales making them challenging
to observe. The resulting lateral gradients can potentially slump and restratify the surface
layers or generate submesoscale instabilities and turbulence, enhancing local mixing. A
cruise during the 2015 seasonal sea ice minimum to the Beaufort and Chukchi Seas of the
CanadaBasinmadeuseofnovelinstrumentationwiththeintentofresolvingtheheatbudget
oftheupperArcticOceanandobservingthesubmesoscaleprocessesthatmodulatevertical
heatfluxesbelowthesurface. Wewillpresentobservationsofsubmesoscalefeaturesinthe
upper25metersfromachainofthermistorsdeployedthroughundisturbedwaterwithhigh
spatialresolution, 1mverticalandhorizontalspacing. Simultaneouslyobservedturbulent
dissipation, background shear and stratification provide evidence of the potential mecha-
nisms responsible for enhancing or suppressing both mixing at the halocline and vertical
heatfluxes.
Measuring and modelling a shallow coastal sea area
PEKKAALENIUS1,SIMOSIIRIA¨1,LAURATUOMI1,ANTTIWESTERLUND1,KIMMOTIKKA1
1FinnishMeteorologicalInstitute,Finland
TheGulfofFinland(GoF)intheBalticSeaisashallowandnarrowseaareathat
hasveryintensivelongitudinalandtransversalshiptraffic. Theendofthegulfreceivesthe
largest single fresh water river flow and the gulf opens to the Baltic Sea proper. There are
coastal archipelagos and larger islands that affect the circulation in the gulf. The overall
hydrographyoftheGoFischaracterizedbylargehorizontalandverticalvariationsinsalin-
ity and also seasonal variation in temperature. The dynamic scales of the GoF are rather
small,somekilometresonly. Furthermore,someofthedeeperareasoftheGoFsufferfrom
oxygendepletion.
The challenge in the modelling of the 3D hydrodynamics of the GoF is its com-
plicated dynamics that have been demonstrated in many numerical studies. The lack of
sufficientvalidationdataformodelstudieshashinderedthedevelopmentofmodelsinthis
area. Though the gulf is only 400 km long and 60 - 120 km wide, the sea areas belong to
territorial waters and economic zones of three countries and the routine monitoring obser-
vations are limited only to small number of stations. In 2013 and 2014 related to the Gulf
of Finland Year, intensive measurement campaigns were done in the GoF by three coun-
tries. FMI organised three research cruises in the GoF to obtain good validation data sets
formodelstudies.
ManymodelstudiesoftheGulfofFinlandexistsalready. However,theresolution
ofthemodelshasbeentoocoarsetosolvethemesoscaleprocessesproperlyuntilrecently.
The meso-scale horizontal processes and strong vertical mixing are essential parts of the
dynamicsofthegulf. Weused3Docean-icemodelNEMOwithNordicconfigurationwith
1 and0.25 NM resolutions. Temperaturevalidationsof the high-resolutionresults seem to
bebetter,asexpected,butthereisstilldevelopmentstobedonetoproperlymodelthestudy
area.
Marine algae are taught the basics of angular momentum
JOHNALLEN1
1SOCIB,PalmadeMallorca,Spain
Advanced modelling studies and high resolution observations have shown that
‘sub-mesoscale’ flows (5-20 km scale) may provide both the fertilisation mechanism for
nutrient depleted (oligotrophic) surface waters and a subduction mechanism for the rapid
exportofphytoplanktonbiomasstothedeepocean. Here,adetailedmultidisciplinaryanal-
ysis of the data from an example of these studies, is presented. The data provide direct
observations of the sub-mesoscale transport of phytoplankton. Furthermore, the data con-
firm that this transport is constrained by the requirement to conserve angular momentum,
expressed in a stratified water column as the conservation of potential vorticity. This con-
straintisclearlyseentoproducelongthinstreaksofphytoplanktonpopulationsstrainedout
alongisopycnalvorticityannuliassociatedwithmesoscalefrontalinstabilities.
Water circulation inside the Bay of Calvi (Corsica, France):
historical review and future perspectives
AIDAALVERA-AZCARATE1,ALEXANDERBARTH1,ALAINNORRO2,SALIMDJENIDI1,
GENEVIE`VELACROIX3,SVETLANAKARIMOVA1,PIERRELEJEUNE4,MICHE`LELEDUC4,
SYLVIEGOBERT5
1AGO/GHER,UniversityofLie`ge,Belgium
2MARECO,ODNature,RoyalBelgianInstituteofNaturalSciences
3REMSEM,ODNature,RoyalBelgianInstituteofNaturalSciences
4STARESO,Corsica,France
5LaboratoryofOceanology,UniversityofLie`ge,Belgium
The bay of Calvi, located on the North Western coast of Corsica (Mediterranean
sea) is a small bay (8 km wide) featuring complex bathymetry characterized by a subma-
rinecanyonwithverysteepsideslopes, andaninnershallowzonewithagentleslopeand
that is home to an important ecosystem, notably Posidonia oceanica meadows. Submarine
canyons act as a connection between coastal areas and the open sea, facilitating upwelling
anddownwellingevents,andtheexchangeofnutrientsandsedimentsbetweentheshelfand
thedeepwatermasses. Theeffectof thesecoldandnutrient-richwatersincoastalecosys-
tems is of high relevance in the oligotrophic Mediterranean Sea ecosystem. Fronts and
eddiesalsoinfluencethemarineecosystemthroughupwellinganddownwellingdynamics,
fromthelargescaletothesubmesoscale.
TheBayofCalvihasbeenstudiedsincethe1970sbyresearchersattheUniversity
of Liege thanks to the research station STARESO (Station de Recherches Sous-Marines et
Oce´anographiques). ResearchatSTARESOincludeslong-termmonitoringofvariableslike
sea water temperature, salinity, chlorophyll-a concentration, and meteorological variables.
Ocean currents have been sparsely measured, and therefore our knowledge about the sub-
mesoscaledynamicswithintheBayofCalviandtheinfluenceofthesubmarinecanyonon
theecologyofthebayarenotwellknown.
In this work a historical review of the ocean currents measured in the Bay of
Calviduringthelast25yearsisperformed,withtheaimofcharacterisingthemainpatterns
of variability as well as the spatial and temporal scales resolved so far with the existing
measurements. This study will serve to optimise the locations inside the bay for future,
long-term currents measurements efforts. Plans for increasing the infrastructure dedicated
tothemeasurementofsubmesoscalecurrentsintheBayofCalviwillbepresented.
Mechanisms of Eddy Formation in the Bay of Bengal
ANANDHTS1,BIJANKUMARDAS1,SUMITDANDAPAT1,J.KUTTIPPURATH1,ARUN
CHAKRABORTY1
1CentreforOceans,Rivers,AtmosphereandLandSciences,IndianInstituteofTechnology
kharagpur,India
Bay of Bengal (BOB) is a unique region in the Indian Ocean with seasonally re-
versing currents, large freshwater influx from several rivers and significant influence from
bothsurfaceandremoteforcings. Numerouseddiesforminthisregion,playingakeyrolein
productivity and weather of the coastal East India. In order to understand the mechanisms
of eddy formation, a high resolution (1/12 degree) regional coupled model, the Coupled
Ocean Atmosphere Wave Sediment Transport (COAWST) system, is set up for the BOB.
The air-sea interactions and their influence on eddy generation in the BOB are studied us-
ing the coupling between Weather Research and Forecasting model (WRF) and Regional
OceanModelingSystem(ROMS).Topographicaleffectsareanalysedusingrossbyradiiof
deformation. Thecoupledmodelresultsarethencomparedtostand-aloneWRFandROMS
resultsandobservations. Itshowsthattheair-seainteractionsarebettercapturedinthecou-
pledsimulations. Themesoscaleeddiesstarttoappearduringthepre-monsoonseasonwith
theonset ofmonsoonal winds. The resultsindicate thatboth windforcing andbathymetry
affecttheformationanddevelopmentoftheeddies. Also,thenumberandstrengthofthese
eddiesincreaseduringmonsoon,furtherdevelopbypost-monsoonanddecayduringwinter.
It is found that uneven forcings weaken the central BOB gyre into numerous eddies. Also
theremoteforcings,throughkelvinandrossbywaves,arefoundtoenhance/supressoreven
dissipatetheeddies.
Optimizing wave run-up measurements by video imagery
UMBERTOANDRIOLO1,ELENASANCHEZ-GARCIA2,RAFAELALMAR3,RUITABORDA1
1IDL,UniversityofLisbon,FacultyofScience,Lisbon,Portugal
2PolytechnicUniversityofValencia,DepartmentofCartographicEngineering,Geodesyand
Photogrammetry,Valencia,Spain
3IRD-LEGOS-CNRS/CNES/ToulouseUniversity,Toulouse,France
Video remote sensing technique has been providing high resolution and continu-
ousdatatocoastalengineersandscientistsforthelastthreedecades. Amongallthecoastal
video applications, monitoring high frequency swash motion on the beach face has been
improvingtheanalysisandunderstandingofwaverun-upprocesses. Hydrodynamicanaly-
sisattheshorelineisacrucialfactorforassessingstorm-relatedriskanddesigningcoastal
protectionstructures. Conventionally, maximumwaverun-upexcursionsaremarkedman-
uallyorbyimageprocessingalgorithmsonspace-timeimages(timestack). However,such
practicesrequirecomprehensivehumanefforts,andmuchofdigitalsignalprocessingcodes
arelimitedduetoimagenoiseinducedbybeachrecreationalactivities.
This study presents a simple method of extracting wave run-up properties using
statistical properties of timestack image. Video data set was derived by online-streaming
high-definition cameras, which are installed at three exposed beaches on Portuguese west
coast. Nine cross-shore profiles were considered for producing a series of several hundred
timestacks with a range of different wave characteristics, tide elevation and beach slope.
Relative standard deviation of pixel time series is found to determine horizontal location
ofmaximumandminimum,alongwithmeanand98percentile(R2%)waverun-upvalues
over the transect. Wave run-up elevations were computed combining obtained horizontal
locations with topographic surveys. The method was validated though the comparison of
the results against manually digitalization of wave run-up, performed by different users,
and an automated detection procedure based on threshold intensity value. The presented
innovativemethodology,whichshowedpromisingresults,aimstooptimizehydrodynamic
analysisanddirectmeasurementsintheswashzonethroughvideoimagery.
High-resolution airborne observations of ocean color and
thermal signatures of submesoscale eddies in the Southern
California Bight
INGRIDM.ANGEL-BENAVIDES1,BURKARDBASCHEK1,W.DAVIDMILLER2,
GEOFFREYSMITH2,RYANP.NORTH1,GEORGE.MARMORINO2,RU¨DIGER
RO¨TTGERS1
1InstituteforCoastalResearch,Helmholtz-ZentrumGeesthacht,Germany
2RemoteSensingDivision,NavalResearchLaboratory,USA
Highspatialvariabilityinphytoplanktondistributions,fromlargescalesdownto
submesoscales, has been long-observed in satellite ocean color data. Submesoscale eddies
are one of the physical drivers of such variability, as they can alter phytoplankton distri-
bution and growth conditions through horizontal stirring and vertical advection. However,
theirrapiddynamics(hourstodays)andfullrangeofspatialscales(0.1-10km)arenotprop-
erly sampled by traditional ocean color sensors on board of satellites at sun-synchronous
near-polar orbits, such as MODIS and VIIRS. These satellites’ typical spatial resolution
of 1km and temporal resolution of 1 to 2 days, only provide snapshots and are limited
to processes lying at the upper limit of the submesoscale length scales. During the Sub-
mesoscale Experiments (SubEx) in the Southern California Bight, we used airborne ocean
color(VNIR)andthermal(IR)sensorstoobtainhighspatial(1m)andtemporal(10mins)
resolution imagery of the surface signatures associated with submesoscale features. Our
multi-platformsamplingschemealsoincludedrapidinsitumeasurementswithatowedin-
strumentarray,whichprovidedinsightintothefeaturesverticalstructure. Targetedcyclonic
eddies were detected due to their sharp cold-core signature (L = 0.2 to 1 km) and sampled
for several hours. The rotational currents, apparent in the sequence of IR images, extend
to larger regions than the cold-cores, which, according to vertical in situ profiles, are most
likely due to doming of subsurface isopycnals. The eddy signature in the chlorophyll con-
centration field, derived from the ocean color imagery, varied greatly and was sometimes
lower and sometimes higher than the surrounding areas. For eddies with low-chlorophyll
signatures, the spatial distribution of chlorophyll was highly correlated with temperature,
and the gradients in both fields were nearly co-located. In contrast, higher chlorophyll ed-
dies signatures were generally larger than the cold-core area. These results highlight the
large variability in surface phytoplankton responses to the three-dimensional circulation
of submesoscale features, and how these features introduce spatial heterogeneity into the
ecosystem. The possible mechanisms and environmental conditions responsible for such
contrastingresponseswillbediscussed.
Exploring the small cyclones in the Malvinas Current
CAMILAARTANA1,CHRISTINEPROVOST1
1Laboratoired’Oce´anographieetduClimat: Expe´rimentationetApprocheNume´rique
(LOCEAN),UMR7159,Universite´ PierreetMarie. Paris,France
TheAntarcticCircumpolarCurrent(ACC)isorganizedinthreemainfronts. From
south to north: the Southern Antarctic Circumpolar Front (SACCF), the Polar Front (PF)
and the Subantarctic Front (SAF). The last one penetrates 1800 km into the Western Ar-
gentinean Basin forming a major western boundary current, the Malvinas Current. This
swiftandrelativelynarrowcurrentflowsnorthalongthePatagonianshelfbreakfrom55°S
up to 38°S providing a permanent injection of cold, fresh and nutrient-rich waters to the
Southwestern Atlantic Ocean (SWA). The interaction of the MC with the sloping bottom
ispresumablyresponsibleforgeneratingupwellingalongtheshelf-break,whichbringsnu-
trients to the surface. As consequence, the SWA hosts a highly productive ecosystem that
sustainsoneofthelargestfisheriesoftheworld. Apartfromitsregionalinfluence,theMC
contributes to the Meridional Overturning Circulation since it helps the exchanges of heat
andsalt.
Thepopulationofcontrastedfronts,meanders,eddiesandfilamentsintheSWAis
richandcomplex. Theeddykineticenergy(EKE)inthisregioncanreachvaluesashighas
1700cm2/s2. However,theinterioroftheMCpresentsmuchlowervaluesofEKE(around
200 cm2/s2) and mesoscale activity is considered to be low in comparison with the rest of
theSWA.
HighresolutionSeaSurfaceTemperature(SST)dataduringthewinterseasonre-
veal the presence of numerous persistent cyclonic eddies which had not been previously
noticed in the interior of the MC. The SST data allow a precise description of these struc-
tures (size, shape, interaction with topography, coalescing and breaking apart), but cloud
coverage prevents a systematic continuous examination. Even if summer images are less
cloudy, the presence of a seasonal thermocline impedes a proper detection of cold eddies.
WecombinedhighresolutionSSTwithsatellitealtimetrydatatoinvestigatethegeneration
of these coherent structures and their propagation. Low resolution altimetry data allows a
continuousexamination,atthecostofloosingtrackofeddieswithverysmallradii. Several
generatingprocesseshavebeenidentified. Inparticular,therecurrentgenerationofcyclonic
eddiesattheexitofDrakePassage(eastBurwoodBankandtheShagRocksPassage)ispre-
ciselydescribed.