Spatial structures of Algerian Eddies

20 Pages·2004·1.28 MB·English

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Deep-SeaResearchII51(2004)3009–3028
www.elsevier.com/locate/dsr2
Spatial structure of anticyclonic eddies in the Algerian
basin (Mediterranean Sea) analyzed using the
Okubo–Weiss parameter
Jordi Isern-Fontaneta,(cid:1), Jordi Fonta, Emilio Garcı´a-Ladonaa,
Mikhail Emelianova, Claude Millotb, Isabelle Taupier-Letageb
aInstitutdeCie`nciesdelMar,CMIMA-CSIC,PasseigMarı´timdelaBarceloneta37-49,08003Barcelona,Spain
bLaboratoired’Oce´anographieetdeBioge´ochimie,COM-CNRS,BP330,83507La-Seyne-sur-Mer,France
Abstract
ThreeAlgerianeddies(openseabiganticycloniceddiesintheAlgerianbasin,westernMediterraneansea)arestudied
in detail using altimetric and in situ data. To characterize their spatial structure we make use of the Okubo–Weiss
parameter, which allows to separate the flow into vorticity-dominated and deformation-dominated regions. The
application of this parameter to geostrophic velocity fields obtained from Sea Level Anomaly maps show that the
observed eddies have a spatial structure in close resemblance to that found in coherent vortices of two-dimensional
turbulence. These eddies appear to have a rather coherent vorticity-dominated core region surrounded by an outer
regiondominatedbydeformationwithaverycomplicatedandvariableintimestructure.Thisanalysisisalsoappliedto
CTDdatacollectedacrossthreeeddiessampledbetween1997and1998.Fromthesedataeddycoresareequivalently
defined as the interior region enclosed by the maximum of azimuthal velocity, and are consistent with the horizontal
structures seen in SLA maps. The distinction between the core and the outer region (circulation cell) is particularly
important to understand the role of transport and stirring of properties associated to eddies, as illustrated by the
observedsalinitystructure,someselectedSSTimages,anddriftertrajectories.Thecoreischaracterizedbyverysmall
isopycnalsalinitygradients,whilethesurroundingcellischaracterizedbyastrongslopeofisohalineswithrespectto
isopycnals. Persistent stagnation points are observed around the eddy and correspond to regions of high values of
deformationwithrespectto vorticity.These regionsallowanalyzingcharacteristicpatterns seeninthecorresponding
SST images.Inaddition, drifterstrajectoriesexhibit verydifferent behavior whencrossingsuchregions.
r2004Elsevier Ltd. Allrights reserved.
1. Introduction
Several studies have investigated the redistribu-
(cid:1) tion of properties in the ocean associated
Correspondingauthor.
E-mailaddress:[email protected](J.Isern-Fontanet). with intense mesoscale activity (e.g. Wilkin and
0967-0645/$-seefrontmatterr2004ElsevierLtd.Allrightsreserved.
doi:10.1016/j.dsr2.2004.09.013
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3010 J.Isern-Fontanetetal./Deep-SeaResearchII51(2004)3009–3028
Morrow, 1994; Abraham and Bowen, 2002). This of temperature to the north. This contrasts with
intense eddy activity increases mixing in the theSSTimageoftheAlgerianbasin(Fig.1A)with
surface layer and is mainly due to the presence of complex patterns associated to three big antic-
energetic and long lived structures generically yclons occupying most of the basin. The effective
named eddies (Robinson, 1983; Kamenkovich et mixing is mainly due to the non-local transport of
al., 1986). In the Algerian Basin, western Medi- theseeddiesastheymovethroughthewholebasin
terranean Sea, large anticyclonic open sea eddies (see Fig. 2 and Olson, 1991; Bouzinac et al., 1998;
(see Fig. 1A) have been repeatedly observed Puillat et al., 2002).
(Millot, 1985; Taupier-Letage and Millot, 1988; While theoretical and numerical simulations
Benzohra and Millot, 1995b, Obaton et al., 2000; have shown the importance of eddies in the local
Salas et al., 2002; Ruiz et al., 2002). These dispersion of particles and their role in the
mesoscale structures, named Algerian Eddies evolution of tracer concentrations (e.g. Proven-
(AEs), are generated, a few times per year, as the zale, 1999), less is known on the mixing and the
result of an instability of the along slope Algerian stirring properties associated to these individual
current. They have variable diameters of about structures in the ocean. Dewar and Flierl (1985)
100–200km,verticalextentsfrom100sto1000sof analyzed with some detail the link between the
m,andcanhavelifetimesofseveralmonths,upto stream function topology and the dispersion of a
near 3 years (Millot et al., 1997; Puillat et al., passive tracer for simple eddy models, and
2002). The common formation and presence of compared with them some features seen in SST
these big eddies has a large impact on the surface images.Ingeneral,eddieshavedifferentdynamical
and intermediate circulation in the western Med- properties between the inner and outer parts of
iterranean(Millot,1987;Sendetal.,1999;Fudaet theirstructure.Whiletheinteriorpartoftheeddy,
al.,2000).Theycaninteractwithother AEsinthe or core, transports tracers within it, the outer
mid-basin,orwiththeirparentcurrent,sometimes produces intense stirring of the ambient environ-
leading it to be diverted seaward from the coast ment (Olson, 1991). Analysis of numerical results
(Taupier-Letage and Millot, 1988; Benzohra and ontwo-dimensionalturbulentflowshaveproposed
Millot, 1995a). the use of a general criterion based on the
The role of these eddies on the redistribution of Okubo–Weiss parameter (Okubo, 1970; Weiss,
properties inthe Algerian basin isrelated with the 1991) to determine the detailed spatial structure
northwardspreadingofwatermasspropertiesthat of an eddy and to identify the regions with
should initially be transported eastwards by the different mixing properties (McWilliams, 1984;
Algerian current. Evidences of a meridional Elhmaı¨di et al., 1993).
momentumtransporthavebeen observed through Inapreviouspaper(Isern-Fontanetetal.,2003)
velocitiesobtainedfromsatelliteSeaLevelAnom- some of us have proposed to use a criterion to
aly (SLA) maps (Iudicone et al., 1998), mean detect eddies from SLA maps based on the
climatological maps of the surface circulation partition of the flow using the Okubo–Weiss
(Ovchinikov, 1966; Tziperman and Malanotte- parameter. In the present paper, we show,
Rizzoli, 1991) and statistical analysis of drifters combining several data sets, the contribution of
(Salasetal.,2001).Thesignatureisameridionally this criterion to understand the associated stirring
enlarged and smoothed current pattern towards properties. Data are described in Section 2 and
the central part of the Algerian basin. Similar consist on SLA maps, SST images, hydrographic
features are observed in all climatological atlas of measurements, and trajectories of drifters buoys.
water properties (Picco, 1990; Brasseur et al., ThedefinitionoftheOkubo–Weissparameterand
1996) and in numerical simulations of the mean itsapplicationtothesedataareexplainedindetail
circulation (Herbaut et al., 1997). As an example, in Section 3. The relation between stirring and
in Fig. 1B a mean Sea Surface Temperature (SST) mixing properties of the regions partitioned given
image corresponding to the first semester of 1998 by the parameter is analyzed in Section 4. We
showstherelativelysmoothedmeridionalgradient discuss the results and limitations of the approach
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Fig.1. (A)SSTimageforMay7,1998withthelocationof96-1,97-1and98-1eddies.(B)MeanSeaSurfaceTemperatureforthefirst
semesterof1998.Temperatureincreasesfrombluetored,thecontrastisadjustedspecificallyfortheimagetoenhancethesignaturesof
theeddies.
Fig.2. Observedtrajectoriesof96-1(solidline),97-1(dot-dot-dot-dashline)and98-1(dot-solid)eddies.Circlesindicatetheinitial
positionobservedinSLAmaps.ThesetrajectorieswereobtainedusingthemethodologydescribedinIsern-Fontanetetal.(2003).
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in Section 5 and finally we summarize the main sites, or are plain AVHRR channel4 of AVHRR,
results. corresponding to relative brightness temperatures
(see details on the processing in the above web
site).The imageshereused areselected picturesto
2. Data sets be almost simultaneous with the SLA maps. They
arepartoftheseriesofimagesusedtotrackeddies
2.1. Satellite data in the Algerian basin by Puillat et al. (2002).
In the Mediterranean Sea, despite the weak 2.2. In situ data
signal intensity and the coarse space and time
resolution of the altimetric tracks, several studies In connection with the EU MATER (MAss
have shown thereliability ofthe altimetric datato Transfer and Ecosystem Response) project, two
analyze its dynamics and particularly in the field programs were carried out in the Algerian
Algerian basin (Vazquez-Cuervo et al., 1996; basin to study the dynamics of AEs. ELISA
Vignudelli, 1997; Ayoub et al., 1998; Iudicone et (Eddies and Leddies Interdisciplinary Study in the
al., 1998; Bouzinac et al., 1998; Larnicol et al., Algerianbasin)intheeasternpart(Taupier-Letage
2002; Font et al., 2004). Here we use SLA maps etal.,2003)andALGERS(theuseofERSsensors
regularly produced by Collecte Localisation Sa- for the study of the dynamics of Atlantic water in
tellites (CLS) in Toulouse, France (http:// the ALGerian basin) in the western part (Font et
www.cls.fr) from a combination of both ERS al., 1998). Three big anticyclonic eddies, labeled
and TOPEX/Poseidon altimeters measurements. 96-1, 97-1 and 98-1 (Puillat et al., 2002), were
These maps are processed including usual correc- detected and sampled with CTD casts between
tions (tides, inverse barometer, etc.), a Sea State 1997 and 1998 during ELISA1, ALGERS98 and
empirical model with four parameters derived ELISA4 cruises, respectively. These three eddies
from a global crossover analysis (Gaspar et al., were particularly long lived and coherent struc-
1994) and with improved ERS orbits using tures with a clear signature in infrared images
TOPEX/Poseidon as a reference (AVISO User (Puillat et al., 2002; Salas, 2003) and altimetry
Handbook, 1996; Le Traon and Ogor, 1998). Sea (Isern-Fontanet et al., 2003; Font et al., 2004).
level anomalies are used instead of absolute sea Casts during ELISA1 and ELISA4 campaigns
surfaceheightduetothestillunprecisedetermina- wereperformedfromtheFrenchRVLeSuroitwith
tion of the marine geoid. SLA are regularly a SBE 911 plus CTD profiler from Sea-Bird
producedbysubtractinga4-yearmeanvaluefrom Electronics,Inc.DuringtheALGERS98campaign
theactualmeasurementsand,priortotheanalysis, theCTD datawereobtainedfrom theSpanish RV
data are low-pass filtered using a 35km median Hespe´rides with a Neil Brown Mark-III probe. All
filterandaLanczosfilterwithacutoffwavelength data were preprocessed with the SEASOFT (v.
of 42km in order to reduce altimetric noise 4.233,Sea-BirdElectronics,Inc.)softwarepackage.
(Larnicol et al., 1995). SLA maps are finally built Final profiles were interpolated with a vertical
onaregulargridof0:2(cid:1)0:2degreesevery10days resolution of 1dbar: More details on data proces-
using an improved space/time objective analysis singcanbefoundinRuizetal.(2002)andTaupier-
method,whichtakesintoaccountlongwavelength Letage et al. (2003). From all the available CTD
errors (Le Traon et al., 1998). profileswehaveselectedthosethatcrossedaneddy
The infrared satellite images used here come close to one of its diameters (Fig. 3). While during
from the NOAA/AVHRR sensor, have been ELISA4 and ALGERS98 the sections crossed
processed at the German Remote Sensing Data completely the eddy, in ELISA1 the CTD section
Center, and are freely available through its cut the eddy through a sector at the north and not
database server (http://www.dfd.dlr.de). Images passing close to the eddy center (Taupier-Letage
are either processed up to SST and combined etal.,2003).Cross-sectionsofgeostrophicvelocities
temporally to daily, weekly and monthly compo- for all the three transects were derived from
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Fig.3. GeostrophicvelocitiesderivedfromSLAmapsclosetoCTDsampling(diamonds)ofeddies96-1,97-1and98-1.
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dynamicheightswithareferencelevelat1000dbar, different regions: a vorticity-dominated region
similar to previous works in the region (Benzohra (Wo(cid:4)W0), a strain-dominated region (W4W0)
andMillot,1995b).Resultinggeostrophicvelocities and a background field, characterized by small
showed that the Rossby numbers for these eddies positiveandnegativevaluesofW(jWjpW0),where
were Ro¼0:13 for 96-1, Ro¼0:11 for 97-1 and W ¼0:2s with s being the standard deviation
0 W W
Ro¼0:09 for 98-1. Then, the gradient flow inthewholedomain,inourcasetheMediterranean
approach was used instead of direct geostrophic Sea (Bracco et al., 2000; Pasquero et al., 2001).
velocities following Olson (1991). Whenapplyingthiscriterionofflowpartitionitcan
In ALGERS98 cruise, 15 surface drifting buoys be observed that the general structure of an eddy
(drogued with a 10m long holy sock at a mean consists in a vorticity-dominated inner region, the
depthof10m)werereleasedacrosstheeddyatthe eddycore,surroundedbyastrain-dominatedregion,
positions of the CTD casts (Fig. 3). The drifters the circulation cell (Elhmaı¨di et al., 1993). The core
were tracked during a variable number of days, edge can then be identified as the closed lines with
from 12 to 148, until transmission stopped due to W ¼0: This separation of the field in terms of the
failure of the system at open sea or after having sign of W has been proven to be a robust criterion
gone ashore (Salas, 2003; Font et al., 2004). for educing eddy cores from complex fluid flows
(Jeong and Hussain, 1995; Pasquero et al., 2001;
Isern-Fontanet et al., 2003). Furthermore, the
3. The Okubo–Weiss parameter boundary of the core constitutes a barrier to the
exchangeofparticleswiththesurroundingcell,then
3.1. Definition and properties particles trapped inside the eddy core remain there
unless the eddy is destroyed or they are ejected
Several studies in the past have revealed the through a filamentation process. In the circulation
close link between the local topology of the flow cell,ontheotherhand,thereexistanintensestirring
field, the stirring and mixing processes, and the and exchange of particles with the background
dispersion of Lagrangian particles in complex field.Duringthedispersionprocess,neutralparticles
flows. Okubo in 1970 analyzed the dispersion in exhibit a tendency to concentrate in such region
the ocean surface by looking at the shape of the (Provenzale, 1999).
trajectories of two particles initially located close For two-dimensional flow fields, as those ob-
to each other. By means of a linear analysis he tained from SLA maps, the criterion is straightfor-
found that the shape of the possible particle ward implemented by computing the velocity field
trajectories is characterized by the eigenvalues of and its derivatives assuming geostrophic balance
the velocity gradient tensor. Later Weiss in 1981
g @h0 g @h0
arrived to an equivalent criterion analyzing the u¼(cid:4) ; v¼ ;
f @y f @x
enstrophy dynamics in two-dimensional turbulent
flows.Inbothapproaches,thetopologyofparticle whereh0 istheSeaLevelAnomaly,ggravity,andf
trajectories and the behavior time evolution the Coriolis parameter. However, for velocity
vorticity gradients is determined by the sign of transects, as those obtained from aligned CTD
W ¼s2 þs2(cid:4)o2; (1) measurements, the criterion is not so easily
n s implemented since it requires some assumptions
where sn; ss and o are the normal and the shear on the shape of the eddy. In polar coordinates the
components of strain and the relative vorticity of Okubo–Weiss parameter, assuming a horizontally
the flow defined respectively by non-divergent flow, is given by
@u @v @v @u @v @u " #
sn ¼@x(cid:4)@y; ss ¼@xþ@y; o¼@x(cid:4)@y: W ¼4 (cid:1)@vr(cid:2)2þ1(cid:1)@vr(cid:2)(cid:1)@vy(cid:2)(cid:4)1(cid:1)@v2y(cid:2) ;
@r r @y @r r @r
The parameter W, named after them as Okubo–
Weiss parameter, allows to separate the flow into (2)
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where v and v are the radial and azimuthal instance compare it with Fig. 3 of Elhmaı¨di
r y
velocities, r is the radius and y is the azimuthal et al., 1993). To provide an evidence of the
angle. A common assumption is that the eddy has evolution of these eddies from the point of view
axial symmetry. Then the velocity field is given by of W, Figs. 5A–C show the spatial distribution of
the parameter for three consecutive SLA maps
v ¼0; v ¼VðrÞ
r y
close to ELISA1 cruise. It can be appreciated that
and then the Okubo–Weiss parameter is while eddy cores are rather coherent structures in
1(cid:1)@V2(cid:2) both space and time the circulation cells are much
W ¼(cid:4)4 : (3) more irregular in space and variable in time.
r @r
The diameter of eddy cores can be estimated
Thisimpliesthattheboundaryofthecore(W ¼0) from SLA by calculating the diameter of an
correspondstotheradiallocationofthemaximum equivalent circular core with the same area.
ofvelocities.Inageneralcase,thelineofmaximum Observations show that 96-1 has a diameter that
kineticenergyisclosetothelineofW ¼0butthey rangesfrom95kmon9July1997to116kmon19
donotnecessarilycoincide,asisseenfromEq.(2). July 1997, 97-1 ranges from 116km on 1 July to
96kmon11July1998and98-1from97to77km.
3.2. Satellite observations Notice, however, that the core of 97-1 was very
elongated on 1 July 1998 as observed from Fig. 3.
As illustration, in Fig. 4A we have represented For96-1thenorthernboundaryislocatedbetween
the horizontal distribution of W derived from a CTD 31 and 32, and the southern limit is beyond
SLA map corresponding to July 7, 1996. A color the CTD casts. The limits for 97-1 are close to
scale has been selected so that blue colors CTD 235 in the north and between CTD 244 and
represent eddy cores (Wo(cid:4)0:2sW), green repre- 245 in the south. For 98-1 the limits are close to
sents the background field (jWjp0:2sW) and CTD5inthenorthandbetweenCTD14and15in
yellow and red colors the deformation-dominated the south.
regions(W40:2s ).Whenthisfigureiscompared From a sequence of maps as the one shown in
W
withaSSTimage(Fig.4D)itcanbeobservedthat Fig. 5 the trajectories of these eddies can be
the core region of strong negative values of W is recovered(asexplainedindetailbyIsern-Fontanet
consistentwithsurfacepatternslabeledas96-1,as et al., 2003). Once identified in one map they are
wellasthepositionofothereddies.Oneexception tracked by looking at nearby regions of strongly
is the eddy located south of Mallorca island negative values of W in the next map. Using this
(38:5(cid:9)N; 3(cid:9)E) in SST image, which is not seen in method, 96-1 was first identified in SLA maps on
the W field. However, SLA maps contain more 19/2/1996 and lived until 21/7/1998. Eddy 97-1
eddy cores than those identified by visual inspec- from 20/5/1997 until 18/9/1998 and 98-1 from 22/
tion in infrared images, although it is not possible 2/1998 until 7/11/1998. These resultsgive lifetimes
to confirm whether or not are real eddies (see of 883, 486 and 139 days, respectively. In situ
Isern-Fontanet et al. (2004) for a detailed discus- measurements were made when 96-1 was 512 days
sion). The structure of 96-1 in terms of W (Fig. old (ELISA1), 97-1 was 410 days old (ELISA4)
4A) displays a nearly axisymmetrical core sur- and 98-1 was 88 days old (ALGERS98).
rounded by a region of intense deformation and
irregularshape.Azonalcross-sectionalongoneof 3.3. In situ observations
its diameters (Fig. 4B and C) shows that W (red
line) takes strong negative values in the inner part The consistency between the spatial distinction
(the eddy core) and becomes positive (the circula- between eddy regions in SLA maps as determined
tion cell) after crossing the radial position of the by W, and the characteristics of the vertical
kineticenergy(blueline)maximum.Thisstructure structure of these eddies, is checked by represent-
is very similar to the structure of two-dimensional ing vertical sections of s and gradient velocity
t
vortices as those observed in turbulence (for from the CTD measurements that approximately
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Fig. 4. The Okubo–Weiss parameter computed for the 96-1 eddy from SLA maps. (A) General view of the distribution of the
Okubo–Weissparameteron7/7/1996.ThecolorscalehasbeenchosentoenhancethevaluesaboveandbelowthelineW ¼0:2s
W
(s ¼2:44(cid:1)10(cid:4)11=s2).(B)Sealevelanomalies(cm)correspondingtothe96-1eddywiththetransectdrawninfigureC(dashedline).
W
(C) Longitudinal profile of the Okubo–Weiss parameter (red line) and kinetic energy (blue line) across the eddy. (D) SST image
correspondingtoJuly9,1996.Temperatureincreasesfrombluetored,thecontrastisadjustedspecificallyfortheimagetoenhancethe
signaturesoftheeddies.
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Fig.5. DistributionoftheOkubo–Weissparameterandstreaklines(black)derivedfromSLAmapsfor96-1closetoELISA1cruise:
(A)July9,1997(s ¼2:64(cid:1)10(cid:4)11=s2),(B)July19,1997(s ¼3:39(cid:1)10(cid:4)11=s2),(C)July29,1997(s ¼3:31(cid:1)10(cid:4)11=s2).(D)SST
W W W
imagecorrespondingtoJuly30,1997withlinesofW ¼0(black).(E)DistributionoftheOkubo–Weissparameterandstreaklines
derivedfromSLAmapsfor98-1closetoALGERS98cruise,May22,1998(s ¼3:42(cid:1)10(cid:4)11=s2)andinitialtrajectoriesofdrifters
W
(redlines)launchedduringthecruise.NumbersindicatesCTDcasts.Temperatureincreasesfrombluetored,thecontrastisadjusted
specificallyfortheimagetoenhancethesignaturesoftheeddies.
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crossed 96-1, 97-1 and 98-1 cores (Figs. 6 and 7). salinity (see Ruiz et al., 2002; Taupier-Letage
The vertical structures are characteristic of anti- et al., 2003) except for the presence of a shallow
cyclonic eddies with the center located between thermocline located within the upper 50m, clearly
casts 27–29, 237–238 and 8–10 for 96-1, 97-1 reflected in the s sections of 96-1 and 97-1, and
t
and 98-1, respectively. As expected the vertical characteristic of the Mediterranean summer sea-
structures are similar for both temperature and son. The vertical extent of the baroclinic structure
Fig.6. Transectofs (kg=m3)correspondingtotheCTDcastsshowninFig.3.DottedlinesaredrawnattheCTDlocation.Numbers
t
identifyCTDcasts.