JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. C2, PAGES 2995-3011, FEBRUARY 15, 1998
Sea level variability and surface eddy statisticsin the
Mediterranean
Sea from TOPEX/POSEIDON
data
Daniele Iudicone and Rosalia Santoleri
Istitutodi Fisica dell'Atmosfera,ConsiglioNazionaledelleRicerche,Rome,Italy
Salvatore Marullo
and Paolo Gerosa
CentroRicercheCasaccia,Ente per le Nuove Tecnologie,l'Energia e 1'Ambiente,S. Maria di Galeria (Rome),
Italy
Abstract. Two yearsof TOPEX/POSEIDON altimeterdatahave successfullybeenusedto
studythe mesoscalefield in the Mediterraneanandto investigatethe seasonalandyear-toyear variability of the sealevel andeddy statisticsin this basin.The mesoscalefield described
by TOPEX/POSEIDON revealeda strong,but subbasindependent,seasonalsignal.Year-toyear variationsare alsoevidentin termsof bothintensityandpositionof the main mesoscale
features.Crossoveranalysisindicatedthe existenceof a meridionaltransportof eddy
momentumaway from the Algeriancurrentdueto the northwardmigrationof mesoscale
eddies.A comparisonbetweenmesoscale
featuresdetectedby the altimeterand
contemporaneous
featuresobservedusingadvancedvery highresolutionradiometer
(AVHRR) seasurfacetemperature
hasbeenmadefor the followingMediterraneansubbasins:
theAlgerianbasin,the Levantinebasin,andtheTyrrhenianSea.The resultsdefinitivelyprove
the direct relationbetweensealevel anomaliesand the Mediterraneaneddy field.
only 10 cm. Second, the coarse separationbetween
subsatellite
tracks(240 km at 40øN)in respect
to thetypical
Earth-observing satellites are providing a significant dimensions
of theMediterranean
mesoscale
(30-100km)may
quantityof high-qualitysynopticdatafor the investigation
of significantly
reducethe advantage
of the high-frequency
!. Introduction
oceanographicphenomenaon spatial and temporal scales repeatcycle (-10 days).
unattainable
with in situ measurements.
Altimeter
data have
alreadyprovedto be able to resolveseasurfacevariabilityin
regionscorresponding
to the majorwesternboundarycurrents
(Gulf Stream, Kuroshio, Malvinas, and Angulhas) and the
Antarctic Circumpolar Current. On the contrary,very few
works have been done on studies of ocean areas characterized
by smallsealevel slopesor on smallsemienclosed
seaslike
the Mediterranean.This lack of researcheffort could probably
be due to the poor precisionof radar altimetersoperating
before ERS-1
and TOPEX/POSEIDON.
The launch of the TOPEX/POSEIDON
(T/P) satellite
Howeverthesizeof Mediterranean
Sea,although
relatively
small,is sufficient,so that circulation
maybe governed
by
large-scale
oceandynamics.
TheMediterranean
is amongthe
mostinteresting
semienclosed
seasbecause
of thegreatrange
of processesoccurringwithin it [Robinsonand Golnaraghi,
1994]. Most of the physical processesthat characterizethe
global generaloceancirculation,many of which are not well
known or understood, occur analogously in the
Mediterranean.All major forcing mechanisms,surfacewind,
buoyancyfluxes, and lateral massexchange,are present.Airsea interaction is vigorous, and both deep and intermediate
water massesare formed. Increaseddensity due to saltiness
causedby intensesurfaceevaporationis importantfor water
massformationand for the main thermohalinecell in analogy
to the global conveyorbelt.
usheredin a new era in satellite oceanographyby greatly
improvingthe accuracyof the altimetermeasurements.
The
orbits for T/P are estimatedto have a radial accuracyof 3-4
cm root-mean-square
(RMS) [Tapley et al., 1994]. For this
reason,T/P is the most accuratealtimetersatelliteflown up to
The Mediterranean is located between 30 ø and 45øN and is
date. The main goal of this work is to use T/P data to connectedto the Atlantic by the shallowStrait of Gibraltar.It
contributeto the descriptionof the subbasinand mesoscale is composedof two similar-sizebasins(the westernand the
dynamic features of the Mediterraneancirculationand to easternMediterranean Seas) separatedby the shallow and
comparethe resulting picture against other observational narrow Strait of Sicily (Figures 1 and 2). In the Strait of
evidences.
Gibraltar,at the surfacea comparativelyfresherAtlantic water
Thereare manyreasonsthat suggestthatthe Mediterranean flows into the MediterraneanSea to replace both the water
Sea is a basin where T/P performancesare under critical that evaporatedbecauseof the dry winter winds and the
conditions.First, sealevel anomalies(SLA) are of the order of denser,saltier Mediterraneanwater flowing out at depth into
the Atlantic. The incoming Atlantic water forms a surface
water
Copyright1998by theAmericanGeophysical
Union.
Papernumber97JC01577.
0148-0227/98/97 JC-01577 $09.00
mass
100-200
m thick
that
flows
eastward.
The
Atlanticwater,movingeastward,is heatedby and mixeswith
the saltier surface Mediterranean water becoming the
modified Atlantic water (MAW).
2995
2996
IUDICONE
ET AL.: SEA LEVEL
VARIABILITY
IN TIlE MEDITERRANEAN
SEA
z
40 I>
35 I>
30 I>,,
5 Iong.W
Figure 1. Map of theMediterranean
Sea.A pictorialview of the surfacecirculationin theMediterranean
Sea
hasbeensuperimposed;
permanent
featuresare solidlinesandrecurrentfeaturesaredashedlines.(Modified
fromMillot [1987] andPOEM Group[1992]).Acronyms
usedareasfollows:westernAlborangyre(WAG),
easternAlboran gyre (EAG), Almeria Oran front (AO), Balearicfront (BF), LigurianProvencalCatalan
current(LPCC), northemTyrrhenianeddy(NTE), southernTyrrhenianeddy(STE), Algerianeddy(ALE),
IonianAtlanticstream(IAS), Pelopseddy(PE),Cretangyre(CG),Iera-Petra
anticyclone
(IPA), Rhodesgyre
(RG), AsiaMinor current(AMC), Ciliciancurrent(CC), western
Cyprusgyre(WCG),Mid-Mediterranean
jet
(MMJ), Shikmonaeddy(SE), andMersa-Matruhgyre(MMG).
In the Levantine basin, Levantine intermediatewater (LIW)
of relativelyhigh temperatureand salinityis formedduring
winter. This water mass circulatesthroughboth the eastern
and western Mediterraneanbasin in a generally cyclonic
fashionbefore arriving to the Atlantic throughthe Strait of
Gibraltar.LIW is usuallyobservedbetween200 and 800 m of
depth.Deep wateris producedfor the westernbasin in the
Gulf of Lions and for the eastern basin in the south Adriatic
and northeasternLevantinebasin [Robinsonand Golnaraghi,
1994].
Since
the
middle
of
the
1980s
several
international
are the Westem MediterraneanCirculation Experiment
(WMCE) [WMCE Consortiumof investigators,1989] the
PhysicalOceanography
of the EasternMediterranean
(POEM)
[POEM Group, 1992], and the GibraltarExperiment[Kinder
and Bryden,1987]. In the frameworkof theseprograms,
numerousoceanographic
campaignswere made.The resultof
thisinternational
effortis a completely
changed
pictureof the
Mediterraneancirculationcharacterized
by a more complex
generalpatternthan obtainedfrom earlier studies.The newest
concepts of the Mediterranean circulation focus on the
existence of three scales of motion, i.e., the basin scale
researchprogramswere performedin thisbasin;amongthem defined by the overall thermohalinecells, the subbasinscale
4'5N/I
I I I I I I I I' I I I 1 I I I I I I /I
•
I
1 1 I I I I I I l"l I I I 1 1 I I I I
I
I
/
• •_
,ION
N•
3,5
6W
-
ß
•)•0
5E
) ,
10E
MAX
DEPTH
16E
= 4636.8
20E
M
26E
Figure 2. Mediterranean
bottomtopography.
30E
IUDICONE ET AL.: SEA LEVEL VARIABILITY IN THE MEDITERRANEAN SEA
2997
defining the gyres of the main thermoclineconnectedby
intensejets and meanderingcurrents,and the ubiquitousand
very energetic mesoscale eddy field modulating the two
previousscalesthroughmultiple and nonlinearinteraction.A
pictorial view of the Mediterraneancirculation,as resulting
from WMCE and POEM experiments,is given in Figure 1.
The contrastsin sea surfacetemperatureand the small cloud
cover make the satelliteinfrared imageryan appropriatetool
to studythis basin.
In the last yearsseveralstudieswere carriedout usingsea
surface temperature (SST) data. These works revealed the
the MediterraneanSea. They found that T/P inversebarometer
correctionsometimefails in this basin, occasionallyyielding
SLA shiftsover singletracks.They overcomethis problemby
adjusting the SLA bias of these tracks with respectto the
background.They demonstratethe importanceof the steric
circulation.
Sea,startingfrom November22, 1992 (i.e., from cycle7),
wereusedfor this study(Figure3). Previouscycleswerenot
usedbecauseof attitudeproblems[Neremet al. 1994].
Datawereinitiallyeditedon thebasisof qualityflagsand
effect
on the mean
Mediterranean
sea level
variation
and
show the potentiality of the T/P altimeter data to catch the
main features of the circulation.
The purpose of this paper is to examine the sea level
mesoscalevariabilityof the MediterraneanSeaduringthe first
2 yearsof the T/P mission.The paperis organizedas follows.
richness of mesoscale features in the basin and indicated the
Section 2 describesthe processing.Section 3 deals with
importanceof thesefeaturesfor the Mediterraneandynamics repeat-trackanalysisand large-scalecirculationas observed
[e.g., Philippe and Harang, 1982; La Violette et al., 1990, on T/P. Cross-overanalysisand Reynoldsstressare discussed
Millot, 1991]. In particular,recently,IR imageswere used to in section4. SLA along some particularlyinterestingtracks
infer the seasonaland interannualvariability of this basin that cross the main features of the Mediterranean circulation
[Santoleri et al., 1994; Marullo et al., 1998]. These works are analyzed and discussedin section 5. The results are
demonstratethat a strongseasonalsignal is presentin the summarized in the conclusions.
whole basin with different regional characteristicsand is
mainly due to the seasonalvariability of the main mesoscale
2. Altimeter Data Processing
structures.They showan interannualvariabilityof seasurface
temperaturepattern causedby the variability of the surface
Two years of T/P altimeter data over the Mediterranean
The altimeterdata are particularlyuseful for studiesof the
ocean circulation [Wunschand Goposchkin,1980] because
the measurementof the surfacetopographycan be converted
into surface geostrophiccurrent velocity. Therefore the
analysisof altimeterdata of the Mediterraneancan be useful
to studythe variabilityof the circulation on the global scale
and identify its main characteristics.
B6hm et al. [1992] first
tried to use altimeter data in this basin. Their analysis of
Geosat data was limited by the capability of the Geosat
altimeter and by the lack of knowledgeof the tidal correction
to apply in the Mediterraneanbasin. Nevertheless,they
demonstratethe effectiveness of altimeterdata to investigate
the Mediterranean
circulation.
parameter ranges as recommended in the user handbook
[AVISO, 1992]. A secondqualitycontrol(outliersdetection
and elimination)basedon a procedurespecializedfor the
Mediterranean
casewas alsoappliedafterenvironmental
and
geophysicalcorrections.The following correctionswere
appliedto bothTOPEX and POSE]DONdata:watervapor
from the onboard radiometer,TOPEX dual-frequency
ionosphericcorrection,dry troposphere,
invertedbarometer,
electromagnetic
bias,centerof gravity,solidEarthtide,pole
tide, and net instrument correction. The ocean tide model
Recently, Larnicol et al. [1995] used T/P data to
investigatethe circulationand the meansealevel variationsin
ß: .',.:.• "," .:,5:'::;:..::•""!,'!:;::::
;'.,i'?.',:'•':•::..:";•..:;"i•
,
,.(
suggestedby the user handbookfor the MediterraneanSea
(P. Canceilet al., Barotropictidesin the MediterraneanSea
'::.'
:- j
':
.... •
.: .,., ' '•.:,:•.'";'"'"; :.'.•i/;,..,' ,.?-.••:•5''
ß::(•-
35 •>
....
.%
15 Iong.W10
5
0
5
10
15
20
25
Figure3. TOPEX/POSEIDON(T/P) groundtracksin theMediterranean
Sea.
30 Iong.
E 35
2998
IUDICONE
ET AL.: SEA LEVEL VARIABILITY
IN THE MEDITERRANEAN
SEA
The SLA obtained using the conventional repeat-track
using a finite element numerical model, submittedto the
Journal of GeophysicalResearch,1994) wasusedto subtract analysismethod were used to computesea level variability
the main ocean tide constituents within the Mediterranean
(SLV) mapsof the entireMediterranean.The RMS of the sea
Sea, while outside the Mediterranean, "modified enhanced level residualwas computedat eachT/P grid point usingall
Schwidersky"tides [AVISO, 1992] have been used. Finally, the SLA data.
the JointGravity Model (JGM-2) preciseorbit wassubtracted.
Eddy kinetic energy(EKE) for the entire data set was also
In order to evaluatethe mean sea level, altimeter passes computedat eachgrid point alongthe T/P track.Maps of
were interpolatedto a fixed grid definedby eachfirst satellite SLV andEKE werecomputed
by objectiveanalysis
usingthe
track free of gaps.The resultingmean grid size was -6 km. correlationfunction(1). As a consequence
of the combined
Interpolatedaltimeterdata were usedto computea mean sea effect of the data pointsdistributionand the usedcovariance
level that was subtracted from each single altimeter pass function,the interpolationerror rangesfrom a minimumof
8% along tracks to a maximum of 75% in the centerof the
obtainingSLA data.
Becauseof the very low sea level signalof oceanographic T/P grid.
featuresin the MediterraneanSea a particularattentionhas
Apart from these considerationsabout the statistical
been paid to the outliersdetectionphase.We adoptedan ad interpolationerror,the large spatialgapsbetweenthe tracks
hoc algorithm based on the objective analysis theory (dueto samplingcharacteristics
of the T/P orbitrepeatcycle)
[Brethertonet al., 1976]. Objective analysis(OA) has been imply that small-scalefeaturesalwaysexistingnearthe center
widelyusedto interpolatein spaceandtime sparsedataontoa of T/P mashescannotbe detectedby this altimeter.Therefore
regular grid but rarely applied to individuate outliers as any interpolationmethodthat can revealtheir presencedoes
suggested
by Breterthonet al. [1976]. FollowingBretherton not exist. This is particularlyimportantin the Mediterranean,
et al. [1976], we use the discrepancybetween OA estimate where mesoscaleeddiesare characterized
by a diameterof
and the measuredvalue as an indicatorof whethera particular few tensof kilometers.
OA horizontalmapsshouldsimplybe
observationis grosslyincorrect.We optimallyinterpolatedthe consideredas a two-dimensional
representation
of the along
SLA along the satellitetrack over measurement
points,and track signals,which on the contraryhave enoughspatial
we calculated the difference between the estimated and the
measured SLA. Measurements that deviate from the estimate
for more than 3 times the standard deviation of the differences
were eliminated.
The covariance function used for the
objectiveinterpolation
wasdirectlydeduced
fromthedataand
is
F(x)= A1e-(x/B•
) + A2e-(X/B
2)
where A•=0.825, A2=0.175, B•=90 km, B2=20 km, and x is the
distance.
The objective analysismethod for outliers detection,even
if rather time-consuming,worked extremely well in all the
cases.A visual inspectionof data samplesconfirmedthat the
algorithm is really able to individuate the sameoutliersthat a
"manual editor" would individuate but can definitively
individuatemore quickly (and more objectively).
Finally, slope and bias have been subtractedfrom each
track in order to separatethe variability, causedby steric
effectsinducedby seasonalwarming, and seasonalvariations,
caused by large-scale quasi-stationary currents, from
mesoscalevariability.The removalof slopeand bias, even if
not strictly necessaryowing to preciseorbit determinationof
T/P, also contributed to remove the relative bias found
resolution.
In thefollowingdescription
of thehorizontalmaps
of SLV and EKE, we will focus on features observed in areas
of relativelysmallinterpolation
error.
3. Variability of the Dynamic Ocean
Topography
The map of SLV for the first 2 yearsof the T/P mission
(from cycle 2) have alreadybeenproducedby Larnicol et al.
[ 1995]. We computethe samemap (seePlate 1) using2 years
of T/P data(from cycle7) andafterbiasandslopeadjustment.
Besidethe different analysismethod and the almost2 month
shift betweenthe two data setsthe two SLV patternsare very
similar.
In our case the bias adjustment applied to T/P data
considerablyreducedthe meanSLV by removingthe seasonal
signal due to the steric effect and other basin scaleeffects
[Larnicol et al. 1995]. The SLV ranges from a minimum
value of 1-2 cm, located in the area south of Cyprus, in
several small cells in the Ionian Sea, and in the area offshore
the Gulf of Lions, and maximum values of 8-10 cm southeast
of Crete, mainly causedby the variability of Iera-Petra(IPA,
Figure 1), southwestof the islandof Sardiniabecauseof the
Algerian current eddies (AE, Figure 1) along the north
between TOPEX and POSEIDON altimeters (for instance,
African coast, and in the Alboran Sea because of the Alboran
Nerem et al. [1994]) and to eliminate biasesdue to occasional
gyressystem(WAG, EAG, AO, Figure 1).
failure of the inverse barometer correction [Larnicol et al.,
Note that the mentionedfeaturesare crossedby T/P tracks
1995].
The residual
sea surface velocities
can be calculated
from
along track sea surfaceslope using the geostrophicequation;
that is, for ascendingtrack
_Va
=gdh
f dx
(2)
The componentof the geostrophicvelocity perpendicularto
each track were estimatedfrom the SLA slopes.Slopeswere
obtainedusinga linear fit over sevenT/P datapoints.
(superimposed
on Plate 1) where the interpolationerror is
small.
In order to investigate the seasonal and interannual
variability of the SLV and EKE fields, maps were computed
by dividing our data set into four parts, each of one half a
year. From the meteorologicalpoint of view, spring and
autumn in the Mediterraneanarea are very short and are
characterizedby a rapid sequenceof summerand winter
situations[Meteorological Office, 1962]. Recent works by
Marullo et al. [1998] and Santoleri et al. [1994] suggested
IUDICONE
ß.
ET AL.' SEA LEVEL
VARIABILITY
IN THE MEDITERRANEAN
SEA
2999
/
ß
Plate 1. Sea level variability (SLV) for the period from November 1992 to November 1994 Contour
intervalsare 0.5 cm. T/P tracksare superimposed.
that also in the sea, intermediateseasonsare short(1 month). slightlystrongerin winter-springthanin summer.This means
Therefore we have decided to divide the year into the that the differencein the eddy kinetic energyand sea level
following two periods:"winter" from Novemberto May and variability, observedduring 1994, should only be due to a
"summer" from June to October. The time series of the 2
different level of intermittenceof the two-gym (sometimes
main seasonsfor the 2 years(Plate 2) revealsevidentseasonal three-gyre)systemof the AlboranSea.Unfortunately,we do
vailabilities as well as differencesbetween corresponding not have data to supportthis hypothesis.Someanswercould
seasonsin the 2 years.
be providedby a systematicanalysisof the infraredimagesof
Seasonal variability is more evident in those areas the 1993-1994 period.A large set of imagesis not availableat
characterizedby a higher energy level. In the western present,and suchanalysisis out of the scopeof this work.
Mediterranean Sea the Algerian basin clearly shows the
East of the Alboran Sea, a seasonalchangeis more evident
higher seasonal variability, the winter being generally
in 1994 rather than in 1993
with an absolute
maximum
of
characterizedby a higher level of EKE and SLV. In 1993 the activity in summer 1994ß In the Ligurian-Provencalbasin
winter to summerchangeis evidentin termsof extensionand (northwest part of the western Mediterranean Sea) the
intensityof the high-energyarea,while in 1994 the seasonal seasonality
is markedby a deeperwinterminimumin the Gulf
variability is limited to a decreaseof the energylevel in the of Lions.
cell southwestof Sardinia. It is worth noting that SLV and
In the TyrrhenianSea the seasonalsignalis not so evident.
EKE are more intense in 1994 than in 1993.
A more detailed investigationabout the seasonalityin this
In the Alboran Sea, while energy levels are fairly basinwill be donein section5.3 analyzingthe time evolution
equivalentduring the summerand winter of 1993, in 1994 we of SLA alongsingletracks.
observean increasedenergy level in summerrather than in
In the Ionian basin we always observea very low and
winter. On the basis of our data we cannotgive a definitive patchysignal with a minimum during summer.Immediately
explanationof this particularbehavior.The variabilityof the southwest of Greece, where two T/P tracks cover the area of
sea level should,in any case,be related to the variability of Pelops (PE, Figure 1), a more pronouncedvariability is
water mass exchangesbetween the Atlantic Ocean and the observedin 1994 rather than in 1993. In 1993, Pelops
Mediterranean Sea throughoutthe Strait of Gibraltar and to produceda small signal (SLV) only in winter, southwestof
the intermittent presenceof the two-gym system in the Greece.During 1994 a maximumof SLV appearssouthof
Alboran Sea. In situ sea level measurements[Bormans et al,
Greecein winter and westof Greecein summer,while a peak
1986] suggestthat flux intensitiesand flux variability are of EKE is visibleonly in winter.The 1994 mapssupportthe
3000
IUDICONE
ET AL.: SEA LEVEL
VARIABILITY
Eddy Ki-'neticF•nergy
IN THE MEDITERRANEAN
SEA
St,and,xrd Deviation
.,
!
0.6
3
55
'75
95
1.3
.I
•,.8
3.6
>
43
:':'' .3
Plate2. Seasonal
mapsof eddykineticenergy(EKE)(leftpanel)andSLV (rightpanel).Unitsarein
centimeters
for SLV andcm2/s
2 for EKE.
5
IUDICONEET AL.: SEALEVEL VARIABILITY IN THE MEDITERRANEANSEA
3001
that the originaltime seriesof altimetervelocities
hypothesis
thatPelopscanbe foundin twodifferentpositions Assurning
givesnearlyindependent
estimates,
eachcrossover
pointwill
depending
ontheseason
[Theocharis
et al., 1993].
The area southeast and east of Crete shows a strong
seasonal and interannual variation of SLV and EKE. This
have a maximum of 74 independentvelocity estimates
meridional front [Marullo et al., 1998].
The existenceof the Mersa-Matruh gyre (MMG, Figurel;
POEM Group, [1992]) is revealedduring wintertimeby the
is
(degrees
of freedom)overthe 2 yearperiod.On the basisof
effectis mainlydueto theIera-Petraanticyclone
(IPA, Figure the numberof degreesof freedoma statisticalF-testcan be
1) activityas alreadypointedout by Larnicolet al. [1995] usedto estimatehow closewe are to the expectedvarianceof
analyzingmeanseasonal
SLA maps.A detailedanalysisof an infinite numberof samples.On the averagewe have 60
degreesof freedomat eachcrossover
point.This meansthat
the T/P data in this area will be done in section 5.
In the Levantinebasin,alongthe Africancoasts,the north with a 95% confidence level we are within 32% of the
variancefor aninfinitetimeseries.In thecaseof no
Africancurrentmarksits presence
by enhanced
variabilityand expected
EKE in proximityof the Libyancoastsonly duringwinter datagapsthispercentwouldbe 29%.
The magnitude
andthedirectionof theeddyvariabilityare
1993.Duringsummer,waterexchanges
betweenthe Ionian
by thevariance
ellipses.
Thedirection
of theaxis
basin and the Levantine basin are stronglyreducedby the represented
counterclockwise
fromeast
increaseddirnensionof the west Cretangyre and by the 22øE of principalvariabilitymeasured
tanO
= 0.11<u'u'>
presenceof an area of increasedvariabilityalongthe African
coast south of the Rhode gyre (RG, Figure 1) area. This is
(4)
more intense in 1993 rather than in 1994.
whereO']]isthevariance
alongthemajoraxisandisgivenby
4. Crossover Analysis
0'11
=•(<u'u'>
+<v'v'>
The EKE maps discussedabove are obtainedunder the
(5)
assumption
thatvariancesalongthe ascending
anddescending
tracksare similar. To quantify anisotropy,we computedthe
whilethe variancealongtheminoraxisis
principalaxesof variancesof thevelocityfluctuations.
Following Morrow et al. [1994], the componentsof the
geostrophic
velocityperpendicular
to each track (ascending
(6)
0'22
--(< u'u'>2
+ < v'v'>2
)--0'11
anddescending)were calculatedat eachcrossoverpoint.Then
they are interpolatedlinearly to commontime (every 5 days)
Anisotropicflow is represented
by an elongatedellipsis,with
and used to derive the zonal (u') and meridional (v')
the principaldirectionof the velocity variancealignedwith
components
of the geostrophic
velocityfluctuations
the directionof the major axis.
Even if the spatialdistributionof crossover
pointsfor T/P
(-V' a+V'd )
in the Mediterranean basin is very coarse, this type of
2cosgo
analysisgivesan indicationof the anisotropy
of the examined
(3)
area. The principalaxes of the variance(Figure4) show a
+i(<u'u'>-<
v'v'>)
2+4<u'v'>
2
v,=(Va-Vd
)
stronganisotropic
natureof theregion.Thereis a tendency
of
2sintp
the varianceellipsisto be alignedwith the bottomtopography
whereq0is the anglebetweengroundtrackand the north (Figure2) especiallyalongthe basinboundaryeitherin the
meridianandV,,andVaarethecomponents
of thegeostrophic MediterraneanSea or in the Atlantic Ocean. High values of
velocity
perpendicular
to theascending
anddescending
track, eddyvariabilityare foundalongthe Algeriancurrentand in
the crossovereast of Crete, where high valuesof SLV were
respectively.
observed. The crossoverat 0øE 37øN has the major axis
The covariancesof the velocity fluctuations,<u'u'>,
<v'v'>, and<u'v'>, are thencomputed
from the 2 yeartime oriented from NW to SE indicating a strong and highly
seriesof 5 day interpolatedu' and v' velocitycomponents. variabletransportfrom SpaintowardtheAfricancoast.
45
'•
35
3O
,
i
-10
.........
i
0
.........
i
.........
10
i
20
,
i
30
Lon (degrees E)
Figure4. Principalaxesellipsesfromcrossover
variance.
4O
3002
IUDICONE ET AL.: SEA LEVEL VARIABILITY IN THE MEDITERRANEAN SEA
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Figure5. Mapof <u'v'>distribution.
Values
arein cm2/s
2.
In the TyrrhenianSeathe ellipsesfollow the directionof
5. A Closer Look at the Tracks
the main current. This seems to indicate that the main
A way to better understandlinks betweenSLA derived
from T/P data and dynamicalfeaturesof the circulationis to
makea comparison
with othercontemporary
datasets.
We will
utilize
advanced
very
high
resolution
radiometer
(AVHRR)
East of Crete, in an area betweenIera-Petraand the Rhode
gyre the orientationof the major axis in the northwest data to compareSLA anomaliesto thermal signaturesof
subbasins:
quadrant
impliesthatthe<u'v'> is negative
andindicates
the dynamicalfeaturesin the followingMediterranean
the
Algerian
basin,
the
Levantine
basin,
and
the
Tyrrhenian
direction of the residual flow along the oscillating front
circulationis responsible
for the eddyfield advection.In the
Ionian Sea, all the ellipseshave similar intensity,and the
north-southorientationindicateseddyflux in thatdirection.
Sea. These areas have been chosen on the basis of the results
between Iera-Petra and the Rhode gyre.
The computed
Reynoldsstresses
canbe usedto inferthe
energytransferbetweenthe meanflow andthe eddyfield.
Neglectingcontributions
due to vertical velocities,the
exchange
termof kineticenergybetween
theeddiesandthe
meanflow canbe writtenas [e.g.,WilkinandMorrow, 1994]
3U
3V
3U
3V
of the SLV analysis (section 3) that stressedan intense
mesoscaleactivity in the first two areasand a connectedcell
in the north TyrrhenianSea off the Bonifacio Strait.
The National Oceanic and Atmospheric Administration
(NOAA) satellite passescorrespondingto the days of T/P
measurementsover the Mediterranean were acquired. Sea
surface temperature (SST) and channel 4 brightness
temperature maps of the selected areas were produced.
Clearly, the cloud cover often partially obscuredthe view of
surface temperature field of the three selected basins.
Nevertheless, the typical cloud cover condition of the
Mediterraneanpermittedus to have enoughclear or partially
clear imagesto analyze.
The comparisonhas been done by superimposingalong
track sea level anomalies and corresponding residual
geostrophicvelocities on the images. Positive values are to
the fight (east side) and negative values are to the left (west
side) of ground tracks plotted over the images. In the
following sectionthe most representativeexamplesof these
imagesare shown.
<u'u'>
3x
+<u'v'>-b--ffx
+<Uv>3y
+<v'v'>
3y(7)
wherecapitallettersrefer to meanflow. When this term is
negativea transferof kinetic energyfrom the mean flow to
the eddiesoccurs.For a pure zonal flow, only the third term
of the equationis differentfrom zero. Consideringthat the
Algerian currentis mainly a zonal flow, the third term gives
the major contributionto the energytransfer.In the Algerian
basin, <u'v'> is positive (Figure 5), and (assumingthe
maximumvelocityat the coast)3U/3y is negative.It follows
that the current tends to releasekinetic energy toward the
eddy field by means of the main current leaving eddies.
Moreover, consideringthat the typical velocity of the current
is around50 cm s-• andthe currentis -50 km wide,we can
estimatethe horizontal meridional eddy viscositycoefficient
using
3U
- <u'v'>=
Ay3y
andweobtain
avalue
ofAyof106_
107
cm
2s-•.
(8)
5.1. Algerian Basin
In April the Algerianbasinimagesrevealedthe appearance
of a very complexfeaturein the southeastern
part of the basin.
The image of April 19, 1993, shows an organizedstructure
coveringthe area between6.5ø-8.5øEand 37.2ø-38.5øN (Plate
3). This area is delimited southby the African coastand east
IUDICONE ET AL.' SEA LEVEL VARIABILITY
•,
7
•'
•
IN THE MEDITERRANEAN
SEA
3003
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•
Figure 6. Temporalevolutionof SLA alongtrack 146 (upperpanel),alongtrack094 (centerpanel),and
alongtrack 161 (lowerpanel).
bytherisingof bottom
topography
in theSardinian
channel circulation for B. The system appears as two coupled
(Figure 2). This structureseemsstrictlyconnectedwith the mushroomssharingthe middle eddy (B). North of these
features,a hammer-likestructureis present.
eastwardflowing Algerian current, constitutedby MAW
SLA along track 146 (cycle 21, April 15) shows a
incomingfrom the Straitof Gibraltar.It hasthe shapeof three
of the C eddyanda
highlyconvolutedspiraleddies,two of themzonallyaligned minimumof--12 cm in correspondence
(A centeredat 38.0øN, 7.0øE and B centeredat 38.1øN, 7.9øE, maximumof 14 cm for the B eddy (Plate 3). The SLA also
that the anticyclonic
circulationis largerthen the
respectively)and the third one (C centeredat 37.5øN, 8.1øE) suggest
located southeast of the first two. The eddies have an almost
spiralingfeaturerevealedby the SST field reachingthe
at 38.7øN.Evenif
uniform SST of 14.7øCwith somecolderpatchesof entrained northernboundaryof the warmstructure
water. Their mean radius is about 30 km, even if
A and,
thealtimeter
onlyprovides
theanomaly
relative
to themean,
SLA quiteresembles
whatwe wouldexpectfor
above all, C are elliptical. Their spiralingshapessuggesta the observed
by the SST field.
cyclonic circulation for A and C and an anticyclonic the dipolesuggested
3004
IUDICONE
ET AL.: SEA LEVEL VARIABILITY
IN THE MEDITERRANEAN
SEA
8O
60
40
.2o
0
Jun
Sept
Dec
Mor'
Jun
1992
Sep
Dec
Mor
Jun
1993
Sep
Dec
1994
Figure7, Wind datain the westMediterranean
(ECMWFsquared
windintensity,
averaged
on 1 month).
Unitsarein m2/s2.
38. o
January
38
I
24.8
38. o
28.8
Apr i i
.
32.8
.
ß
34
24.8
38.8
38.
38.
M•y
M•rch
1
0
34
1
38
28.8
32.8
24.8
28,8
32.8
June
I
38.0
I
38
24.8
38.0.
30.
February
38
3o
34.
ß
28.8
July
. ' I
32.8
'
24.8
38.8
28.8
32.8
August
1
24.8
28.8
32.8
September
I
38.8
o
o
24.0
38
-
28.8
32.8
24.8
28.8
24.8
32.8
28.8
Decemben
November
Oc { ober
38.8
32.8
38.0
38 8
i
34.0
38ø
38
38.8
24.•
28.8
32.0
24 . ltl
28.0
32.8
ß
24.0
28.0
32.•
Figure8. Tenyear(1983-1992)monthlyaveraged
SST mapsof theRhodegyreareaandT/P groundtracks.
IUDICONE ET AL.' SEA LEVEL VARIABILITY IN THE MEDITERRANEAN SEA
3005
ß
,.
40N ' -'
5E
'7E
9E
Plate 3. Advancedvery high resolutionradiometer(AVHRR) image of April 19, 1993 (Channel4, NOAA
11). Superimposed
are the 146 and 161 T/P groundtracks,the sealevel anomalies(SLA) (black), and vectors
(white) for the estimatedgeostrophicvelocityorthogonalto the trackfor cycle 21.
The maximum velocity is betweenB and C, suggestinga
The time evolution of the SLA along track 146 is shownin
westward
jet-likeflowof -50 cm s-•. Thevelocity
for the Figure 6 (upper panel). The picture presentsmany striking
othersideof C is 40-50cms-l, whilefor thenorthern
sideof
features.First, the featureformerlydiscussedas anticycloneB
B thevelocityis -30 cms-•. Thislastfeaturehasalsotwo in 1993 has a lifetime of severalmonths.After lasting in the
plateausof uniform velocity,symmetricallydisplaced-20 km
south and north of the center, like a ring, that alters the
expectedfield. This is probablythe result of the entrainment
of outer water, as seen in the image, not yet dynamically
adjusted.The asymmetryin the velocityfield seemsto suggest
that the smaller cyclone (C) is more energetic than the
anticylonic structure (B); therefore C acceleratesB. The
areafor a periodof 4 months,at the end of March it movesto
thenorthwest
ata quiteconstant
phase
speed
of -3 kmday
-•
until it reaches39øN. The phasespeedobtainedfrom the T/P
data is coherent either with the in situ observations [Millot,
1991] or with numerical estimatesof the phase speed for a
baroclinicinstabilityof a coastalflow similarto the Algerian
current (Mortier, 1992). Once the eddy reaches39øN, it
relative
vorticity
estimated
fromthedataare-0.810-5s'• forB almostsuddenlychangesits trajectoryto a more westward
by the SLA of the adjacenttracks
and 1.3 10-5s-• for C, aboutonetenthof the localCoriolis motionas also suggested
parameter.
SLA along track 161 give someadditionalinformationon
the easternboundaryof the complex structuresvisible in the
image. The southern part of the track clearly shows the
eastwardflow of the coastalcurrentwith residualgeostrophic
andby the analysisof AVHRR images.The lack of spatial
resolutionof the groundtracksof the satellitecannotpermit
us to follow it longerwith enoughreliability.CycloneC has
insteaddisappeared
from the T/P data after a few cycles
(Figure6, upper panel), as expectedby the anticyclonic
velocityof 10-15cm s-•. Immediately
north,the westwardrotation
flow is associatedto the local cyclonic tendency of the
circulationsuggestedby the AVHRR image.
of the whole structure.
Sincethe beginningof 1994 we againobservefeatures
with
a behavior
similar
to that found
for the structure
3006
IUDICONE
ET AL.' SEA LEVEL VARIABILITY
IN THE MEDITERRANEAN
SEA
o_-
,/
.:,2H
Plate 4. AVHRR imageof the Levantinebasinof June26, 1993 (Channel4, NOAA 11). Superimposedare
the 094 and 185 T/P groundtracks,the SLA (blue),and vectors(white) for the estimatedgeostrophicvelocity
orthogonalto the track for cycle 28.
discussed
above.They have the samepropagation
speedand
all have left the track around39øN. What is changedis the
intensity,which is much higher in this secondperiod.This
interannualvariability, already seen in the EKE and SLV
maps(section3), is also presentin the wind data (seeFigure
7) that showan absolutemaximumjust beforethe beginning
of the moreactiveperiodsuggesting
a link betweenthe wind
and the observationof energeticeddies at the southeastern
comer of the basin. A possible mechanismcould be the
secondarycirculationdue to the curl of the wind (mistral)that
producesa circulationwith a main flow crossingthe basin
from north-west to southeast[Herbaut, 1994]. This would
constrain the eddies to move close, or closer than in absence
fed for a longer time by the currentand, at last, forced to
moveeastwardas far as topographypermits.
5.2. Eastern
Basin
The absolutemaximumin the mesoscale
activityof the
Mediterranean, as derived from the T/P data, is located
southeast of Crete in the Levantine basin near the crossover
point betweentracks094 and 185 (see section3). The time
evolutionof the SLA alongtrack094 is shownin Figure6
(center panel). At the latitude between 34ø and 35øN a
sequenceof positiveand negativeSLA is clearly visible,
suggestingthe seasonalappearanceof a coherentstructure.
of thiswind, to the coastin their initial mainlyeastwardpath, Indeed,in the 2 yearT/P dataseta negativeSLA characterizes
IUDICONE ET AL.: SEA LEVEL VARIABILITY
IN THE MEDITERRANEAN SEA
/////.///// //,////
cms
3007
Ocm
/
////
17
19
21
23
Plate 5. AVHRR imageof theLevantinebasinof November26, 1994 (SST, NOAA 11). Superimposed
are
the 185 T/P groundtrack,the SLA (black),and vectors(black)for the estimatedgeostrophic
velocity
orthogonalto the trackfor cycle 80.
the first half of each year and a positiveone characterizesthe
second half.
To understand
the cause of a similar behavior
for the SLA
time evolution, we compared the altimetric data with the
AVHRR climatology of the Levantine basin reported by
Marullo et al. [1998]. In Figure 8 the SST monthly
climatology (1983-1992) of the AVHRR-derived SST field
are shown together with the altimetric ground tracks. In
autumnand in winter a well-organizedRhodegyre structureis
mainly localized in the centerof a meshof the T/P grid. The
warmer Iera-Petra anticyclone is evident south of the
crossover,and it is separatedfrom the Rhode gyre by a sharp
front. In summera cold plumecomingfrom the AegeanSeais
presentin the area crossedby tracks094 and 185, while the
Iera-Petraanticyclonehasshiftedwestwardandis not crossed
by the tracks.
To further investigatethe relationshipbetween SLA and
SST featuresin thesetwo scenarios,we give two examplesof
3008
IUDICONE
ET AL.: SEA LEVEL
SEFTEMBER
18
VARIABILITY
30 1993
2O
ß"'"":
.....:':":
:"
:
IN THE MEDITERRANEAN
10 cm/s
SEA
10 cm
22
24
'
?
i
.......
"
Plate 6. AVHRR imageof theTyrrhenianSeaof September
30, 1993(SST,NOAA 11). Superimposed
are
the 161and044 T/P groundtracks,theSLA (blue),andvectors(white)for theestimated
geostrophic
velocity
orthogonalto the trackfor cycle38.
the SLA and residual geostrophicvelocities for June and
November superimposedon contemporaryAVHRR images.
In Plate 4, SLA and residual geostrophicvelocitiesof tracks
185 and 94, cycles 28, are superimposedon the AVHRR
image of June 26, 1993. In Plate 5, SLA and residual
geostrophic velocities of tracks 185, cycle 80, are
superimposedon the AVHRR imageof November26, 1994.
The image of June 26, 1993, (Plate 4) indicates the
presenceof a plumeof relativelycold watercomingfrom the
AegeanSea east of Crete and elongatingfor 220 km in the
southeast
direction.At its southeastern
limit, part of the cold
water is spiraling, entrained by two anticyclonic twin
structures of 100 km of diameter each.
The negativeanomaliesobservedin boththeascending
and
descending
tracksat about35øNcorrespond
quitewell to the
areaof relativelycold water. The valuesof the minima are of
about10 cm, and the resultinggeostrophic
velocitiesdescribe
the cyclonic circulation associatedto the cold area with
IUDICONE
ET AL.: SEA LEVEL VARIABILITY
maximum velocities of 20 cm s't. The same value is
associatedto the west of the two anticyclonic features,
crossedby track 094.
The SST image of November 26, 1994 (Plate 5), reveals
the presenceof both Iera-Petraand the Rhode gyre, southeast
and east of Crete, respectively.The warm structure (IeraPetra) has an almost circular shapewith a diameterof -150
km and has a temperature of .•23øC. The cold structure
(Rhode gyre) has an elliptical shape zonally oriented with
major and minor axes of
-220 km and -120 km,
respectively.It hasa seasurfacetemperatureof-• 17øC.
Track 185 crossesthe center of Iera-Petra, while the Rhode
gyre is not crossedby any T/P track. The SLA alongtrack 185
exhibit a regular dome, with an amplitude of more than 25
IN THE MEDITERRANEAN
SEA
3009
and anticyclonicgyre.Plate 6 illustratesa particularcaseof
comparison.
On September30, 1993, in the northernpart of the
Tyrrhenian
Sea an almostelliptic.
al cold patchof waterof
about 150 km of major axis and lessthan 100 km of minor
axisis clearlyvisible.The featurehassomemeandersand is
elongating
alongthe continental
shelf.At the southeast
tip of
Sardinia a filamentous hammer-like structure extends in the
northeast direction for about 100 km.
Along track 161 the SLA has a domingwith a maximum
of 10 cm in goodcorrespondence
with the warmwater,just
southof the cold feature,and a negativetrendgoingtoward
the northernend of the track. The SLA along track 44 better
reproduces
the relative minimumconnectedto the cold
cm, exactly in correspondence
of the warm structure.The structurebut, not having an optimal intersectionwith the
residualgeostrophicvelocitiesdescribethe clockwiserotation anticyclone,showsa reducedmaximumconnectedto the
of theanticyclone
withmaximum
values
of -50 cms'•. The warmpatch.Thevelocityfieldalongtrack161showsa 20 cm
velocityfield is asymmetricwith highervaluesand a stronger
horizontalgradientin the northernflank near the Rhodegyre.
This asimmetry could be due to the interactionof Iera-Petra
andthe Rhodegyre itself.
The whole of the comparisonsbetweenthe SST fields and
-1
s westwardflow connectedto the southernpart of the warm
areaand an almostequalreturnflow northof it, still in the
warm area. In the cold area the estimated velocities are
weaker than in the warmer ones. This asymmetrycan be
explainedby two differenthypotheses.
First, the cyclonic
circulationis a quasi-steady
characteristic
of the northern
1995] that the observedSLA variability is essentiallydue to TyrrhenianSea [Astraldi and Gasparini, 1994], weakly
circulationis
the seasonal appearance of Iera-Petra. Nevertheless, the detectedby T/P SLA. Second,the anticyclonic
availabledata set is not enoughto reject the hypothesisthat reinforcedby the generalcirculationfield of the southern
of a zonaljet between
40øand
the seasonalshift of the westernboundaryof the Rhodegyre basinthatimpliestheexistence
is alsosupported
can also contributeto the SLA variability. Structureslike the 41øN [Perilli et al., 1995]. This hypothesis
cold patch observedin June 1993, and presentin the SST byin situdata[Moen,1984]thatshowin thesamemonthof a
climatology (Figure 8), should also contributeto the SLA different year the presenceof a southernanticyclonic
circulationstrongerthanthe northerncyclonicone [seealso
variability.
Astraldi and Gasparini, 1994]. A similar asymmetric
T/P measurements seems to confirm [see Larnicol et al.,
5.3. Tyrrhenian Case
circulationfield hasbeenalreadyobservedby Trasvinaet al.
[1995] in the Gulf of Tehuantepec,where a symmetric
The northernpart of the TyrrhenianSea is characterized
by
the presenceof a well-known cyclonicstructure[e.g., Marullo
et al., 1994] linked to the funneling of the wind in the
Bonifaciomouths.The historicalanalysisof AVHRR images
showedthat in autumnthis gyrehasa seasonalintensification,
causedby the setupof the mistralwind and the .strongupper
offshorewind producedmainly a large anticyclonicwarm
:ore eddywith only a weakcycloniccounterpart.
Interpolatingthe EuropeanCentre for Medium-Range
layerstratification.
The commonideais thatit is coupledwith
of the wind intensityis similarto that of the westernbasin
(Figure7), themistralbeingthemainforcingfor bothbasins
andthewinddatashowinga clearinterannual
variability,with
an anticyclonicstructureimmediatelysouthof it, againcaused
by the curl of the northwesterlywind [Moen, 1984]. The
propernatureof the structuremakesit almostinvisiblein the
AVHRR images,and the observations
comefrom few in situ
measurements[Artale et al., 1994]. T/P tracks over the
TyrrhenianSea are well locatedto intersectthe anticyclonic
gyreandthencanbe usedto studyits time evolution.
The time series of the SLA along track 161 is shown in
Figure 6 (lower panel). At the beginning of the T/P
measurements
a quite strongcoupleof positiveand negative
anomaliesoccupiedthe areabetween39.5ø and41øN. Then a
positiveanomalyappearsin the datain thesecondhalf of July
1993 at 40.5øN and disappearsat the end of the year. This
anomalyseemsconnectedby a weak positivesignal to the
next positive anomaly, startingagain in July 1994 but at
Weather Forecast(ECMWF) wind data in the lee of the
islands,in front of the BonifacioStrait, we obtain a raw
estimateof the windflowingon thebasin.The time evolution
an absolute maximum in winter 1994. This maximum
corresponds
well to thepositivemaximum
between
40.5ø and
39.5øN observedin the secondyear for track 161 SLA time
series(Figure6, lowerpanel).Duringsummer,
reduced
values
of SLA are relatedto the reduceddynamicalactivityobserved
in the TyrrhenianSea by severalauthors[Astraldiand
Gasparini,1994'Marulloet al., 1994]as a consequence
of
the isolationof the basinduringthatperiod.
6. Conclusions
Two yearsof T/P altimeterdata were analyzed to studythe
sealevel variabilityof the MediterraneanSea.The analysisof
of
samepositionof our first cycle positivemaximum.More at the SLV andEKE mapsclearlyrevealsthe mainstructures
north (-41øN) in the periodJune-September
1994, thereis a the Mediterraneancirculation,indicatingthat T/P altimeter is
able to catch a large part of the Mediterraneanmesoscale
traceof a weaknegative-positive
anomaly.
The comparisonbetweenAVHRR imagesand altimeter circulation.The analysispointsout two areasof strongsea
data shows a good correspondencebetween negative level signal in the easternpart of the Algerian basin and
anomaliesand cyclonicgyre and betweenpositiveanomalies southeastof Crete. SeasonalSLV and EKE mapsstressedthe
39.5øN. Then it moves to the north until it reaches about the
3010
!UDICONE ET AL.: SEA LEVEL VARIABILITY IN THE MEDITERRANEAN SEA
seasonal variation of the Mediterranean
circulation and also
suggested
a differentbehavior
in eachsinglesubbasin.
The comparisonbetween1993 and 1994 datahas clearly
shownan evident year to year changein the SLV and EKE
values, 1994 values being definitively larger than the 1993
ones.This interannualvariationcan be explainedin termsof
interannual variability of the atmosphericforcing. The
analysisof ECMWF winds confirmedthis hypothesis.From
the meteorologicalpoint of view, 1993 is consideredan
anomalousyearsincewinterwascharacterized
by high values
of atmospheric pressure, especially in the western
Mediterranean,where in Januarythe pressurefield was 12
mbar higher than the mean of the last 30 years (M. Conte,
private communication,1996).
information
onthetimeevolution
of thiscomplex
system
is of
greatinterestfor understanding
the role of the interactionof
theIera-Petra
anticyclone
andtheRhodegyrein theformation
andespeciallyin the spreadingof the Levantineintermediate
water.
Acknowledgments.
The presentworkwasfundedby Agenzia
SpazialeIraliana (ASI). The image processing
softwarewas
developedby O. Brown, R. Evans, J. Brown, and A. Li at the
Universityof Miami with Office of Naval Research
funding.
Continuing
support
of theMiamigroupis gratefully
acknowledged.
We thankE. B6hmfor theconstructive
discussion
andfor providing
theAVHRR imageof November26, 1994.
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(ReceivedJuly 6, 1996;revisedMay 16, 1997;
acceptedMay 29, 1997.)