Deep structure of an Algerian Eddy

17 Pages·2002·0.8 MB·English

JournalofMarineSystems33–34(2002)179–195
www.elsevier.com/locate/jmarsys
Deep structure of an open sea eddy in the Algerian Basin
S. Ruiza,*, J. Fonta, M. Emelianova, J. Isern-Fontaneta, C. Millotb,
J. Salasa, I. Taupier-Letageb
aInstitutdeCie`nciesdelMar(CMIMA-CSIC),P.Maritim37-49,08003Barcelona,Spain
bLaboratoired’Oce´anographieetdeBioge´ochimie,COM-CNRS,BP330,83507LaSeyne-sur-Mer,France
Received16November2000;accepted1August2001
Abstract
TheAlgerianBasindynamicsisdominatedbythepresenceofveryenergeticmesoscalestructures.Deepopenseaeddiesare
keyfeaturesthatinfluencetheregionalcirculationofwatermassesatalldepths.InMay1998,anopenseaeddywassampled
near38jN2jEbymeansofCTD,AcousticDopplerCurrentProfiler(ADCP)andsurfaceLagrangiandrifters.Forthefirsttime,
ithasbeenpossibletocharacteriseoneofthesemesoscalestructuresinitsfulldepth,downto2800m.Thedensitydistribution
indicated the presence of lighter water in the centre of the eddy from the surface to the bottom. The direct velocity
measurementsinthesurfacelayer,plusthedeterminationofthebaroclinicshearfromCTDdata,evidencedthattheanticyclonic
motionwas present in thewhole waterdepth. D2002Elsevier Science B.V.All rights reserved.
Keywords: Mesoscale eddies; Hydrographic measurements; Lagrangian buoys; ADCP; Altimetry; Deep circulation; Mediterranean Sea;
AlgerianBasin
1. Introduction Gibraltar, to the rest of the western and eastern
MediterraneanBasins(Millot,1985).Duetocomplex
The Algerian Basin occupies most of the southern hydrodynamicalprocesses,baroclinic/barotropicinsta-
part of the western Mediterranean Sea. Due to its bility mainly, this alongslope current generally devel-
geographiccharacteristics,theAlgerianBasindynam- opsmeandersassoonas0–1jE,creatinganupwelling
ics is dominated by the interaction between light cell and an anticyclonic eddy inside. Usually, a shal-
watersofrecentAtlanticoriginandtheresidentdenser lowandshort-livedcycloniccirculationalsodevelops
waters. The major feature of its circulation is the upcrest of the meander (Millot, 1999), while the
Algeriancurrent(AC),thealongslopeflowthatdrives meander itself is the most energetic part of the meso-
the Modified Atlantic Water (MAW), incoming from scale structure. Anticyclonic eddies can rapidly grow
up to 50–100 km in diameter, and drift eastwards
along the slope at a few km/day. In some cases, they
areassociatedtoabiggerdeepanticycloniccirculation
* Correspondingauthor.Presentaffiliation:InstitutoMediterra´-
(Millot,1994;Millotetal.,1997;Obatonetal.,2000).
neo de Estudios Avanzados, (CSIC-UIB), Palma de Mallorca,
They are then called events, and are generated only a
Spain.Tel.:[phone];fax:[phone].
E-mailaddress:[email protected](S.Ruiz). few times a year. Most of these energetic events are
0924-7963/02/$-seefrontmatterD2002ElsevierScienceB.V.Allrightsreserved.
PII:S0924-7963(02)00058-1
180 S.Ruizetal./JournalofMarineSystems33–34(2002)179–195
forced by topography, at the entrance of the Sardinia Letage, 2000)ofthebasin.Thefirst ALGERS cruise,
channel,todriftseawardswherebothsurfaceanddeep carried out in October 1996, focused on sampling a
eddiesmergeandbecomelargeopenseaanticyclonic coastal instability (see Mora´n et al., 2001 for major
eddies (100–200 km diameter). Subsequently, they details),whilethesecondone,presentedhereaimedat
tend to drift back to the west, hence depicting a large sampling an open sea eddy. The main goal of the
counterclockwise circuit. These mesoscale structures presentpaperistostudythesurfaceanddeepstructure
(Algerian Eddies, AE) have been observed since the oftheeddyusinganexhaustivemulti-sensorsampling
1980sinsatelliteinfraredimages(Taupier-Letageand strategy. The work is structured as follows: Section 2
Millot, 1988), and can last for many months or even includesthedescriptionofdataset;Section3describes
years (Puillat et al., 2002) while strongly interacting the methodology of analysis performed on the data.
with the AC. They play a major role in the config- TheresultsarepresentedinSection4,andfinally,the
uration of the general circulation and the distribution summaryand discussion aregiven inSection 5.
of water masses and biogeochemical parameters and,
hence, of the ecosystems, in the Algerian Basin. For
example, due to interactions between an AE and the 2. Data set
AC,thelattercanflowperpendicularlytothecoastfor
months(Millotetal.,1997;Taupier-LetageandMillot, From18to27May1998,anoceanographiccruise
1988),while,withoutanymajorinteractions,it‘‘stead- wascarriedoutonboardtheSpanishR/VHespe´rides.
ily’’ flows alongslope (Font et al., 1998a). This gives A large surface eddy, nearly 140 km in diameter, had
the unique character of the Algerian Basin dynamics beendetectedbytheendofAprilinNOAASSTimages
comparedtootherMediterraneanregions:thepathand centred at 38jN 2jE. Its thermal signature depicted a
thestructureoftheflowcanchangesuddenlyandfora clear anticyclonic structure (Fig. 2). Successive satel-
long time, being then dominated by the presence of liteimagesindicateditwasmovingtothewestatabout
veryenergetic structuresat mesoscale (Fig. 1). 2–3km/day.Thankstoanonboard satellitereceiving
One of the objectives of the Mass Transfer and station (Sea Space Terascan HRPT), the surface ther-
Ecosystem Response (MATER) project (1996–1999) malstructure waspreciselylocatedduringthesurvey.
intheframeworkoftheEUMASTprogrammewasthe Thestrategyofthecruisewasdesignedinordertocross
interdisciplinary study of the mesoscale instabilities the eddy in different directions with underway meas-
generatedbytheAC.Thisincludedaseriesofcruises urements (thermosalinograph, ADCP, XBTs and a
inthewest (ALGERS)and theeast (ELISA, Taupier- towedundulatingCTDinthetop120m).
Fig.1.InfraredimageoftheAlgerianBasininOctober1996.Severalcoastalandopenseaeddiescanbeidentified(fromSalas,1999).
S.Ruizetal./JournalofMarineSystems33–34(2002)179–195 181
Fig. 2. Sea surface temperature of the Alboran Sea and western Algerian Basin as measured by NOAA/AVHRR on 7 May 1998. The
anticycloniceddycanbeobservedsouthoftheBalearicIslands.ImagedownloadedfromtheDLRGISISpublicserver.
2.1. Hydrographic data: CTD averaged in 10-min intervals. Using differential GPS
positioning, as well as precise heading correction by
Three CTD transects (Neil Brown Mark III probe, meansofa3DGPS,wehavecomputedhighaccuracy
resolution DT=0.001 jC, DS=0.001) were per- absolute current profiles in the top 200 m (see for
formed perpendicularly to the Balearic islands south- instance Griffiths (1994) for a detailed description of
ern slope (Fig. 3) with stations 5nautical miles apart: the impact of heading error on ADCP velocities).
one along the diameter of the eddy (with casts down Below this depth, the quality of the echo signal is
f
to a few metres from the bottom), and two 45 km not good enough to obtain precise current velocities.
apart on both sides (casts alternatively to the bottom The mean error in ADCP velocities induced by gyro-
and to 1000 dbar). Salinity/density calculations, data compass heading dependence error was estimated in
processing and more technical details are the same 4–5 cm s(cid:1)1 (Ruiz, 2000). Similar errors on ADCP
than for other cruises on board R/V Hespe´rides, as data have been found in other cruises using the same
given in Font et al. (1998b). Final profiles were configuration acquisition and equivalent processing
interpolated with a vertical resolution of 1 m. methodology (see Allen et al., 1997). It should be
pointed out that in this part of the Mediterranean, the
2.2. Velocity data: VM-ADCP and surface drifters tidal current and inertial oscillations are weak (a few
centimetres per second). Therefore, no specific filter-
Direct velocity measurements in the surface layer ingof tidal and inertial oscillation was performed.
weremadebytwomeans:aVesselMountedAcoustic Fifteensurface driftingbuoyswerereleasedacross
Doppler Current Profiler (VM-ADCP 150 kHz from the eddy following the almost north–south diameter
RDInstruments)andsurfacedrifters.TheVM-ADCP, along the central CTD transect at stations #4 to #18.
which was continuously operating, was configured They were of the classical TOGA-WOCE standard
witha4-mdepthcellandatotalof80cells(maximum type, drogued with a 10-m long holy sock at a mean
depth profile of 328 m). The velocity profiles were depth of 10 m. The drifters’ positions, tracked by
182 S.Ruizetal./JournalofMarineSystems33–34(2002)179–195
Fig.3.StudyareasouthofIbizaandFormenteraIslands,withthebathymetry,CTDstations(castdepthindicatedontherightandnumerationon
theleft)andpositionoftheeddyaccordingtoitsSSTsignature.
satellite, were received directly on board with the AlgerianBasinisanexceptiontothissincetheopensea
f
HRPTstation, and through the ARGOS service after eddies at this area are big ( 100 km in radius) and
the end of the cruise (more technical details can be strong(50cm/sasamaximumspeed)enoughtoallow
found in Salas, 1999). acorrectaltimeteridentification(Iudiconeetal.,1998;
Bouzinac et al., 1998; Fuda et al., 2000). Although
2.3. Altimeter data tides and atmospheric effects can be modelled and
filtered out from altimeter data, the lack of a precise
Radaraltimeterscanmeasuretheinstantaneoussea geoidknowledgedoesnotallowtheuseofabsolutesea
levelwithanerrorofafewcentimetres.Thisaccuracy levelmeasurementsbutonlyanomaliesinrespecttoa
is adequate to observe big eddies in the open oceans, temporalaverage.TheenergeticcharacteroftheAlger-
but close to the threshold for detecting mesoscale ian Basin, and the consequent absence of permanent
structuresinmostoftheMediterranean.However,the flows off the Algerian coast, allow the sea level ano-
S.Ruizetal./JournalofMarineSystems33–34(2002)179–195 183
Fig.4.SealevelanomalymapgeneratedwithERS2andTopex/Poseidonaltimeterdataon22May1998.
malies(SLA)tofullydescribethecirculationfeatures. observed data (figure not shown). The method
Fig. 4 is the SLA map of the Algerian Basin closest assumes no correlation between error variance and
tothecruisedates(18–27May1998),wheretheeddy isotropic correlation over the scattered data distribu-
sampledduringthecruisecanbeidentifiednear38jN tion. A horizontal filter is applied in order to remove
1.5jE. non-resolved structures. This filter consists in a con-
A series of SLA maps of combined ERS plus volution of the Gaussian correlation model with a
Topex/Poseidon (T/P) have been used. As ERS has normal-errorfilter.Weusedacutoffwavelengthof90
a repeat cycle of 35 days and Topex/Poseidon of 10 km that is imposed by the mean distance between
f
days, the analysed SLA maps are produced at inter- adjacent transects ( 45 km).
vals of 10 days. The interpolated SLA maps have The same objective analysis scheme used with
been provided by CLS, Toulouse. The maps of SLA, CTD data has been applied to construct gridded
combining T/P and ERS data, have been interpolated horizontal fields of velocity from ADCP measure-
on a regular 0.2j(cid:2)0.2j grid using a suboptimal ments, as well as from geostrophic computations.
space/time objective analysis (Bretherton et al., Drifters’ velocities, computed from successive satel-
1976)whichtakesintoaccountalong-trackcorrelated lite locations, have also been filtered and interpolated
errors(foramoredetaileddescriptionofthismapping to the same grid.
technique see Le Traon et al. (1998)).
4. Analysed fields
3. Univariate objective analysis
4.1. Hydrographic results
Thehigherprecisionmeasurementsmadeinthe49
fixed CTD stations were objectively interpolated Fig. 5a shows the potential temperature, salinity
using a successive correction scheme (Pedder, 1993) and potential density (sigma–theta) in the top 500
to a regular grid of 8(cid:2)8 km. The scheme uses a dbar along the central section. The structure of the
simple Gaussian correlation model with a character- isolines is typical of an anticyclonic eddy. Due to the
isticlengthscaleof25km,whichwascomputedfrom development of the seasonal thermocline (May is
184 S.Ruizetal./JournalofMarineSystems33–34(2002)179–195
Fig.5.Verticaldistributionofpotentialtemperature,salinityandsigma–thetameasuredwithCTDacrossthecentreoftheeddyinacross-slope
(almostN–S)direction:(a)inthetop500dbar,(b)between500and1000dbarand(c)from1000to2800dbar.Theseparationbetweenisolines
isnotconstant,tobetterdisplaythegradients.
S.Ruizetal./JournalofMarineSystems33–34(2002)179–195 185
usually the month with the most intense increase in
surface temperature), salinity displays more clearly
thischaracteristic.Inthecentreoftheeddy,analmost
homogeneous salinity layer (36.9–37.0) is found in
the top 80 dbar, while in the outer edge surface,
salinity is about 37.4. Below 300 dbar for temper-
ature, and 400 dbar for salinity, the presence of
Levantine Intermediate Water (LIW) is indicated by
thepresence of relative and absolute maxima, respec-
tively. In this central section, the LIW core values
(h=13.45 jC at 360 dbar, S=38.55 psu at 460 dbar)
are found near the centre (slightly south of it),
although higher salinity values (38.56 at 380–400
dbar) were detected in the eastern section. These
valuesareverysimilartothosemeasuredbyBenzohra
and Millot (1995b) in an open sea eddy around 5j in
May–June 1986, and that appeared to be clearly
higher than in surrounding waters. This is a clear
indication ofa possible ‘‘unstructured’’ LIW distribu-
tion, i.e. it forms filaments everywhere in the interior
sothatthisaccountsfortheeddyactionontheLIWin
the Algerian Basin and contradicts a LIW-westward-
flowing vein.
Inthelayer500–1000dbar(Fig.5b)thedeepening
ofisotherms,increasedbytherelativeLIWmaximum
justabove,isconsiderable:13jisfoundat930dbarin
the centre of the eddy and some 150 dbar above on
both sides. The salinity, due to the presence of LIW
until 500–600 dbar, is now decreasing with depth,
which is the typical situation for the Mediterranean.
As LIW was found mainlyin the centre (core values)
and the southern half of the section, the isohalines
display an asymmetrical deepening: the saltier water
in the centre is found 130 dbar above in the southern
edge and 180 dbar in the northern edge (e.g. 38.50
isohaline). In spite of this salinity distribution acting
to increase the density in the centre, the temperature
effect dominates and the isopycnals continue to indi-
catethepresenceoflightwaterssurroundedbydenser
ones.Thehorizontalgradientshaveevenincreasedin
the southern part of the eddy: from a slope of 1.5 m/
kmforthe28.5isopycnal(at200dbarinthecentre)to
2.3 m/km for 29.1 (790 dbar).
The structure of lighter (warmer, although saltier)
waterinthecentrewithrespecttothesouthernpartis
maintained in the deep layers (Fig. 5c). However,
isolines tend to become horizontal when approaching
Fig.5(continued). theslope,andbelow1300–1400dbarboth isotherms
186 S.Ruizetal./JournalofMarineSystems33–34(2002)179–195
and isohalines appear tilted in a typical shelf/slope for each defined level, without any connection
front position with only an undulation marking the between them, which makes the algorithm to not
respectively maximum/minimum values in the centre exploit the vertical coherence of the data (this limi-
oftheeddy.Theiroppositeeffectsondensityresultin tationhasalsobeenreportedrecentlybyotherauthors
adistributionofsigma–thetacurvesthatstillpresenta (Gomis et al., 2001)). Additionally, in this particular
strongandnotsoasymmetricaldeepening,sothatthe case,wehavetokeepinmindthatatlowlevelsthere
conditions for an anticyclonic motion are maintained. arefewavailabledata.However,itisworthremarking
The slope of isopycnals is 3.5–4 m/km on both sides that horizontal distribution of density at different
when they are near 1200 dbar in the centre. Below depths (Fig. 7) confirms the nearly circular structure
2000 dbar, the centre of the eddy is only 10–15 km with lighter water in the centre. This pattern is main-
from the continental slope, and the thermal structure taineddowntothebottom,inspiteoftheintersection
appears as relatively warm water between the centre with the continental slope.
and the slope, and colder ((cid:1)0.015j) water outside h–S diagrams of stations in the centre of the eddy
separated by a marked front (more than 30 m/km). In comparedtoothersfromtheAlgeriancurrentandopen
density we can only distinguish light water close to AlgerianBasin(Fig.8)indicatethedifferentstagesof
the slope and heavier (+0.002r ) offshore. This is MAW mixing. Like the one measured in the eastern
h
illustrated by a comparison (Fig. 6) of the deep r AlgerianBasinduringELISAin1997(Taupier-Letage,
h
profiles between a station in the centre of the eddy 2000),thesalinityinthesurfacelayerinthecentreof
(#10) and one in the southern edge (#17). Stations our eddy lies between the low values of the Algerian
situated at an equivalent radial distance from the current(36.6inOctober1996)andthehigheratopen
centre in the other two transects (#34, #50 in the sea locations (37.2–37.9 in different cruises). This
southern edge and #26 in the slope) show a deep clearly accounts for the origin of the eddy from the
profile very similar to #17. The evident instability of Algeriancurrentanditsfurtherdriftintheopenbasin.
the density field observed on vertical plots (Fig. 5c, TheoriginandtrajectoryspecifiedbyIsern-Fontanetet
potential density) islikelyan artifact produced by the al. (2000) using a retrospective analysis of satellite
analysis scheme because of its 2D character. That is, thermalimageryandaltimetersealevelanomalymaps,
the analysis interpolation is performed independently whichshowedthatthetrajectoryhadnotbeenatypical
one, is confirmed by the present analysis. Instead of
havingbeenreleasedoffshoreinthechannelofSardi-
nia, it detached in February 1998 from another big
anticyclonic eddy that was in contact with the coast
near4–5jE.
Concerning the detected LIW, it is less mixed in
the centre of the eddy than in other areas of the
western Algerian Basin, but more than in open sea
eddies in the eastern basin as those sampled during
ELISA. These results are consistent with the water
masses distribution found in previous studies (Ben-
zohra and Millot, 1995a; Emelianov et al., 1999).
4.2. Velocity results
4.2.1. Surface velocity
The trajectories of the buoys (Fig. 9) immediately
indicated an anticyclonic sense of rotation with the
centre very close to station #9, since the drifter
launched at that position experienced almost no
Fig.6.Comparisonbetweenthedeeppartofonedensityprofilein
thecentreoftheeddy(#10)andoneinitsouteredge(#17). motion. A comparison of the successive positions of
S
.
R
u
iz
e
t
a
l.
/
J
o
u
r
n
a
l
o
f
M
a
r
in
e
S
y
s
te
m
s
3
3
–
3
4
(2
0
0
2
)
1
7
9
–
1
9
5
Fig.7.ObjectivelyanalyseddensitydistributionsatdifferentdepthsfromdatarecordedwithCTDatfixedstations.Fromlefttorightandtoptobottom:50,500,1000,1500,2000
and2500dbar.
1
8
7
188 S.Ruizetal./JournalofMarineSystems33–34(2002)179–195
cm s(cid:1)1 in the south. Only in the southern part of the
transect, where drifters were launched at greater dis-
tances from the centre, the outer part could be char-
acterised (stations #14–#18). At a diameter of 100
km, the period is 8 days and the speed has decreased
to 40 cm/s. Beyond this point, the speeds continue to
decrease (by approximately 5 cm s(cid:1)1 every 10 km)
but the trajectories are not closed. That is, the four
external drifters, in spite of following initially an
anticyclonic motion, tend to leave tangentially the
eddy (Fig. 9). However, two of them will be trapped
againinearlyJuneduetoatopographiceffectandthe
drift of the eddy to the west.
Fig. 11 presents the horizontal distribution of the
velocity vectors from ADCP, averaged every 10 min,
at 10 m depth, recorded during the underway legs
(operation of the undulating CTD). The anticyclonic
gyre is evident but the speed distribution is not sym-
Fig.8.h–SdiagramsfromCTDcastsinsideeddies,intheAlgerian metrical: values appear to be higher (maximum of 50
current,andatopenseaintheAlgerianBasin(fromseveralMATER cm s(cid:1)1) for the eastward component in the northern
cruises1996–1998).
partthanforthewestwardoneinthesouth(maximum
the different drifters presents the image of an almost of 40 cm s(cid:1)1).
solid-body rotation in an inner zone of 80-km diam- We have compared data from drifters and ADCP
eter (nine drifters). The rotation speed (Fig. 10) is along the central transect. The spatial distribution of
slowlyincreasing (periods from3to6days)from the thevelocitycomponentsperpendiculartoshiptrackat
centre to the edge of this inner zone, where the 10 m depth is very similar in both cases (Fig. 10).
tangential velocity is 40 cm s(cid:1)1 in the north and 50 However,theADCPrecordsdisplaylowervaluesand
Fig.9.Releasepoints(w)andtrajectoriesofthe15driftingbuoysdeployedfrom19to26May1998.