Progress in Oceanography 106 (2012) 62–79

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Progress in Oceanography

journal homepage: www.elsevier .com/locate /pocean

Sub-surface small-scale eddy dynamics from multi-sensor observationsand modeling

J. Bouffard a,⇑, L. Renault a,b, S. Ruiz a, A. Pascual a, C. Dufau c, J. Tintoré a,b

a IMEDEA (CSIC-UIB), TMOOS Dept., Mallorca, Spainb SOCIB, Coastal Ocean Observing and Forecasting System, Spainc CLS Space Oceanography Division, Ramonville, France

a r t i c l e i n f o

Article history:Received 4 February 2011Received in revised form 6 June 2012Accepted 15 June 2012Available online 20 July 2012

0079-6611/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.pocean.2012.06.007

Abbreviations: BC, Balearic Current; Corr, correlacance > 95%; DH, dynamic height derived from theGeophysical Data Record; IMOS, Integrated Marine ODynamic Topography; MOOSE, Mediterranean Oceanment; (M)SLA, Map of Sea Level Anomaly; NC, NoObservatories Initiative; SLA, Sea Level Anomaly; SOand Forecasting System (translated from Spanishdeviation; SWOT, Surface Water and Ocean Topograp⇑ Corresponding author. Present addresses: Aix-M

Institute of Oceanography (MIO), UMR 7294, CNRS/Marseille, Cedex 09, France and Université du SudGarde cedex, France. Tel.: +33 491829108.

E-mail address: jerome.bouffard@univ-amu.fr (J. B

a b s t r a c t

The study of mesoscale and submesoscale [hereafter (sub)mesoscale] hydrodynamic features is essentialfor understanding thermal and biogeochemical exchanges between coastal areas and the open ocean. Inthis context, a glider mission was conducted in August 2008, closely co-located and almost simulta-neously launched with a JASON 2 altimetric pass, to fully characterize the currents associated with regio-nal (sub)mesoscale processes regularly observed to the north of Mallorca (Mediterranean Sea). A synopticview from satellite remote-sensing fields, before and during the glider mission, provided a descriptivepicture of the main surface dynamics at the Balearic Basin scale. To quantify the absolute surface geo-strophic currents, the coastal altimetry-derived current computation was improved and cross-comparedwith its equivalent derived from glider measurements. Model simulations were then validated bothqualitatively and statistically with the multi-sensor observations. The combined use of modeling andmulti-sensor observational data reveals the baroclinic structure of the Balearic Current and the NorthernCurrent and a small-scale anticyclonic eddy observed northeast of the Mallorca coast (current � 15 cm/s,<30 km in extent and >180 m deep). This mesoscale structure, partially intercepted by the glider andalong-track altimetric measurements, is marked by relatively strong salinity gradients and not, as is moretypical, temperature gradients. Finally, the use of the validated model simulation also shows that thegeostrophic component of this small-scale eddy is controlled by sub-surface salinity gradients. Wehypothesize that this structure contains recently modified Atlantic water arriving from the strait of Ibizadue to a northerly wind, which strengthens the northward geostrophic circulation.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Mesoscale and submesoscale [hereafter (sub)mesoscale] hydro-dynamic features are particularly important in establishing andunderstanding the horizontal and vertical transport of heat (Volkovet al., 2008) and biogeochemical tracers (McGillicuddy et al., 1998;Lévy, 2008 and references therein). Indeed, 90% of the kinetic en-ergy of ocean circulation is contained in small-scale features (e.g.,

ll rights reserved.

tion with statistical signifi-glider CTD; IGDR, Interim

bserving System; MDT, MeanObserving Site for Environ-

rthern Current; OOI, OceanCIB, Coastal Ocean Observing

to English); STD, standardhy.arseille Univ., MediterraneanINSU, UMR 235, IRD, 13288,Toulon-Var (MIO), 83957, La

ouffard).

eddies, fronts and filaments), whereas 50% of the vertical exchangeof water mass properties between the upper and the deep oceanmay occur at the (sub)mesoscale (Fu et al., 2010a, 2010b). Extend-ing our knowledge of the formation, evolution and dissipation ofeddy variability is therefore critical to understanding the ocean’sroles in the earth’s climate. This challenge highlights the impor-tance of describing complex ocean dynamics using both theoreticaland modeling approaches (Klein and Lapeyre, 2009; Capet et al.,2008a; 2008b; 2008c; Hu et al., 2009) and observational ap-proaches adapted to the coastal domain (Nencioli et al., 2011;Bouffard et al., 2010; Hu et al., 2011; Garreau et al., 2011).

Eddy signatures in terms of sea surface height have revealedthat multi-satellite altimetry is highly effective in observing andtracking eddies in the global ocean due to its ability to provide al-most synoptic and periodic measurements of the sea surfacetopography (Chelton et al., 2011; Morrow and Le Traon, 2011).However, given their relatively limited resolution, the use of stan-dard geostrophic current maps derived from multi-satellite altim-etry alone (Ducet et al., 2000) is not always sufficient tocharacterize the spatial and temporal variability of (sub)mesoscale

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 63

features on coastal and regional scales (Dussurget et al., 2011). Seasurface properties related to geostrophic and ageostrophic(sub)mesoscale motions are clearly observed on satellite sea sur-face temperature and ocean color images (Lehahn et al., 2007),but such surface signatures do not convey much quantitative infor-mation on associated currents and sub-surface structures. In addi-tion, the relatively sparse distribution of conventional in situmeasurements at depth (such as ADCP or Argo floats) and at thesurface (such as drifters) only enable a limited quantification of(sub)mesoscale processes and the associated mechanisms alongthe water column.

In this context, complementary high-resolution monitoringtechnologies (e.g., autonomous underwater vehicles or gliders)are also being implemented in ocean observatories, such as IMOSin Australia,1 OOI in the USA,2 and SOCIB3 and MOOSE4 in Europe.By collecting high-resolution observations of temperature, salinityand biogeochemical variables both at the surface and through thewater column, gliders have led to major advances in the understand-ing of key scientific questions related to (sub)mesoscale physical andbiogeochemical processes (e.g., Sackmann et al., 2008; Nie-wiadomska et al., 2008; Perry et al., 2008; Martin et al., 2009a;2009b; Hodges and Fratantoni, 2009; Testor et al., 2010).

Moreover, numerous studies have indicated that the combina-tion of gliders and altimetry yield additional benefits when moni-toring transports (Gourdeau et al., 2008) and characterizingmesoscale structures in the upper ocean (Hátún et al., 2007; Martinet al., 2009a, 2009b; Ruiz et al., 2009a, 2009b; Pascual et al., 2010).Bouffard et al. (2010) recently developed innovative strategiescombining improved coastal along-track altimetry (rather thanstandard altimetric maps) and glider data to quantify horizontalflows more precisely, specifically in terms of the current velocityassociated with filaments, eddies or shelf-slope flow modificationsin the Balearic Sea. The use of these two datasets improves the sep-aration of small-scale dynamics from noise. This increased clarityhas revealed the presence of relatively intense eddies, as previ-ously observed with satellite infra-red images and in situ data, con-firming that (sub)mesoscale variability is a dominant factoraffecting the local circulation and water exchanges between adja-cent Balearic sub-basins (Pinot et al., 1995). As a step forward, thispaper proposes scientifically exploiting such datasets in support ofregional modeling with the main objectives of improving the char-acterization of small mesoscale features and investigating the for-mation process and associated forcing. The study area is theBalearic Basin (Fig. 1), located in the western Mediterranean,where the circulation is rather complex due to the presence ofmultiple interacting scales, including basin scale, sub-basin scaleand mesoscale structures.

This article is organized as follows: First, we briefly present thestudy area characteristics. Second, we describe the experimentalcoastal altimetric data, the glider data and the model configuration.Third, we proceed to a description of the dynamic patterns ob-served in August 2008 from multi-sensor data both at the BalearicBasin scale (with remote-sensed sea surface temperature and alti-metric current maps) and at the coastal scale, north of Mallorca(with glider and along-track altimetry observations). The resultsobtained from the multi-sensor dataset are then compared to arealistic numerical simulation both at the surface and along thewater column. Finally, the validated simulation is exploited toidentify the potential mechanisms associated with the small-scale

1 http://www.imos.org.au.2 http://www.oceanleadership.org/programs-and-partnerships/ocean-observing

ooi/.3 http://www.socib.es/.4 http://www.insu.cnrs.fr/environnement/atmosphere/moose-mediterranean-

ocean-observing-system-on-environment (in French).

/

structure simulated north of Mallorca and previously observedalong the altimetric and glider transects.

2. Study area

The general surface circulation of the Balearic Sea is mainly con-trolled by the presence of two fronts and their associated currents(Font et al., 1988; Font, 1990). The Catalan front is a shelf/slopefront that separates old Atlantic Water (AW) in the center of theBalearic sub-basin from the less dense water transported by theNorthern Current (NC), which is also old AW but is fed into the Gulfof Lion and the Catalan shelves by continental fresh water (refer toFig. 1). The NC is a density coastal current flowing southwestwardfrom the Ligurian Sea to the Balearic Sea. There, it passes the IbizaChannel or retroflects cyclonically over the insular slope formingthe Balearic Current (BC). The Balearic front is also a slope front re-lated to the presence of more recently modified AW that has en-tered the basin through the channels to the south (La Violetteet al., 1990). Both BC and NC have widths on the order of 50 kmand are in good geostrophic balance, as winds only seem to pro-duce transient perturbations in terms of near-inertial oscillations(Font, 1990). In addition to the general basin-scale circulation,the Balearic sub-basin is characterized by frontal dynamics nearthe slope areas and developing between the BC and the NC, suchas mesoscale eddies (Tintoré et al., 1990; Pinot et al., 2002;1994; Rubio et al., 2009), filaments and shelf-slope flow modifica-tions (Wang et al., 1988; La Violette et al., 1990). These dynamicshave been found to modify not only the local dynamics, with asso-ciated significant vertical motions (Pascual et al., 2004), but alsothe large-scale patterns, as shown by Pascual et al. (2002) in a de-tailed study of the blocking effect of a large anti-cyclonic eddy. Thesubmarine topography associated with these complex interactionsbetween the surface and sub-surface waters plays a key role incontrolling the transport between the northern and southern re-gions (Astraldi et al., 1999) and may also enhance the (sub)meso-scale activity in the Balearic Sea (Alvarez et al., 1996).

Despite several previous studies, the characterization of(sub)mesoscale dynamics in the Balearic Sea is difficult given thewide spectrum of temporal and spatial variability of the processeswith which they interact. Moreover, the signal-to-noise ratio inobservations is generally low because the eddy kinetic energy overthis area (Pascual et al., 2007) is an order of magnitude weakerthan that observed by altimetry in the global ocean (Pascualet al., 2006).

3. Materials and methods

Due to the scales of wavelengths and magnitudes involved (fewcentimeters over few tens of kilometers in terms of sea surfaceslope); it is difficult to differentiate small-scale dynamic featuresfrom noise in sea surface observations, specifically over the coastaldomain of the north western Mediterranean Sea (Bouffard et al.,2008, 2011). Cross-comparisons between model and multi-sourcedatasets should therefore increase confidence in our dynamicalstructure characterization. Within this framework, a glider missionwas conducted, well co-located and almost simultaneouslylaunched with a JASON 2 satellite pass (see Fig. 1, right), with themain objective being characterizing the 3D horizontal currentsassociated with the oceanic small mesoscale features. In additionto a more robust error budget assessment, using in situ and re-mote-sensing observations as well as modeling has the advantageof providing complementary information in terms of resolution(glider, along-track altimetry) and coverage (gridded altimetry,model). The following sections provide a detailed description ofthe dataset and the model used.

Fig. 1. (a) The bathymetry of the western Mediterranean Sea with ROMS and WRF domains (in red and black, respectively). (b) Magnified image of the ROMS domain, glidernorthward transect (the southward transect is equivalent) and J2 altimetric track 70 (yellow) overlapped by the main permanent currents (white arrows) and typicalmesoscale events (dashed white arrow). NC stands for Northern Current, BC stands for Balearic Current and AW stands for Atlantic water. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)

64 J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79

3.1. Satellite altimetry

Radar altimetry is a key tool for observing the complexity of theopen ocean circulation (Le Traon and Morrow, 2001). However,coastal and regional dynamics in the Balearic Sea, where horizontalspatial scales can be on the order of 10 km (Send et al., 1999), aremuch more complex to observe with altimetry. Until recently, itwas difficult to capture the dynamics associated with coastalsmall-scale processes with along-track data because the signal-to-noise ratio in the coastal band is rapidly degraded as the altim-eter and radiometer signals are perturbed at distances of 10 and50 km from the coast, respectively. In addition to land contamina-tion, the data quality in these regions suffered from a lack of coast-al zone algorithms (Anzenhofer et al., 1999; Vignudelli et al., 2005).

In this respect, new altimetric post-processing methods andquality control procedures have been developed and can now beexploited for regional and coastal applications (Emery et al.,2011, Kouraev et al., 2011; Ginzburg et al., 2011; Lebedev et al.,2011; Bouffard et al., 2008, 2010, 2011; Roblou et al., 2011; Birolet al., 2010). These studies indicate that new strategies, such asthe use of high-frequency, along-track sampling (20 Hz data), com-bined with a coastal-oriented editing strategy, better constrain thesurface geostrophic current computation in the coastal domain andfor dynamical structures smaller than 50 km.

In the present study, the post-processing of the coastal altime-ter Sea Level Anomaly (SLA) is similar to that used in Bouffard et al.(2010) for ENVISAT but is applied to the JASON 2 experimentalalong-track data provided by the PISTACH project (see Table 1 fora summary of the main characteristics). Across-track altimetricgeostrophic current anomalies are then derived from fully cor-rected altimetric SLA. The along-track altimetric gradient is esti-mated by the optimal filter developed by Powell and Leben

Table 1Main altimetric data characteristics.

Products/satellite Resolution/spatialfiltering

Mean s

PISTACH/JASON-2 (track 70)(Coastal and Hydrology Altimetry producthandbook, 2010)

20 Hz � 350 m MSS Clwww.aMDT (R

1 Hz � 7 km/15 km

Gridded/Multi-missions(SSALTO/DUACS User Handbook)

1/8�/42 km MSS Clwww.aMDT (R

(2004) with a spatial window of 15 km, which is about the firstRossby radius in the Mediterranean Sea (Send et al., 1999). Theacross-track surface geostrophic current is then calculated by add-ing the interpolated Mean Dynamic Topography (MDT) of Rio et al.(2007). By construction, this current is perpendicular to the satel-lite track that, in this case, intercepts the main components of thedynamics related to the BC and/or NC systems (see Fig. 1). Thistype of measurement will therefore be used to precisely quantifythe surface geostrophic current intensity associated with the ob-served coastal and (sub)mesoscale patterns.

Additionally, the 2D surface geostrophic current derived fromregional AVISO Maps of Sea Level Anomalies ((M)SLA on a 1/8� � 1/8� grid, updated delayed-time product, refer to SSALTO/DUACS User Handbook) added to the MDT will be also used toidentify the main surface dynamic structures and provide a quali-tative assessment of the model simulation at the Balearic Basinscale.

3.2. The coastal glider

As a complement to altimetric observations, glider sub-surfacemeasurements will show the baroclinic structure of related surfacepatterns. To this end, the glider transects were almost co-locatedwith the JASON 2 (hereafter J2) altimetric track along a northward(period: 13/08/08–20/08/08) and southward (period: 21/08/08–27/08/08) transect (see Fig. 1 for the glider transect location atnorthward transect; the southward transect is almost identical).

The coastal glider has provided high-resolution hydrographicdata between depths of 10 and 180 m at a horizontal resolutionof approximately 500 m. The glider data processing includes thethermal lag correction (Garau et al., 2011) for unpumped CTD sen-sors installed on Slocum gliders. Hydrographic profiles have been

ea surface/MDT Quality controlprocedure

Time sampling/cycles

s01 (http://viso.oceanobs.com)/io et al., 2007)

As in Bouffard et al.(2010)

10 days/cycle 4 andcycle 5

sO1 (http://viso.oceanobs.com)/io et al., 2007)

Standard 7 days/time-averaged

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 65

averaged vertically to 1 m bins. Relative geostrophic currents canbe estimated by the thermal wind equation from the glider dy-namic height (DH) obtained from temperature and salinity fields(see Ruiz et al., 2009b) with respect to an arbitrary reference depth,which is here represented by the maximum depth of glider mea-surements (in our case, 180 m).

The issue of reference level correction has been recently ad-dressed in Bouffard et al. (2010) and Gourdeau et al. (2008) andis applied here by combining the relative geostrophic currents withdepth-averaged currents retrieved from the GPS glider positioning.These depth-averaged currents are obtained using a hydrodynamicmodel and on the basis of the assumption that the main differencebetween glider GPS surfacing points and dead-reckoned positionsis due to horizontal currents. Thus, the difference between the rel-ative depth-average geostrophic currents (from DH) and the abso-lute depth-average currents (from GPS glider positioning) shouldapproximately correspond to the absolute geostrophic current at180 m because horizontal ageostrophic motion, tide currents, andbarotrophic high-frequency contributions can be considered negli-gible (Bouffard et al., 2010). By adding this value to the relativegeostrophic current at each depth level, the absolute geostrophiccurrent, both at the surface and along the entire 180 m water col-umn, can therefore be estimated, as expressed in the followingequations:

ugðzÞ ¼ urgðzÞ þ uref ð1Þ

uðzÞ ¼ urgðzÞ þ uref þ uagðzÞ ð2Þ

where u corresponds to the total current at depth z (such that uðzÞ isthe glider-based depth-average velocity); urg and uag are the relativegeostrophic and ageostrophic currents, respectively; and uref is theunknown current at the reference level depth.

By vertically averaging and combining Eqs. (1) and (2), we ob-tain the following:

uref ¼ uðzÞ � urgðzÞ � uagðzÞ ð3Þ

Because the vertical average horizontal ageostrophic motion isconsidered here to be negligible compared to total horizontal cur-rents, we obtain from Eqs. (1) and (3) the absolute geostrophic cur-rent from the sea surface to the reference level depth:

ugðzÞ � urgðzÞ þ uðzÞ � urgðzÞ ð4Þ

At the surface (z = 0) and setting aside the issue of synopticity,this quantity should therefore be directly comparable with theinstantaneous, co-located, absolute geostrophic current derivedfrom altimetric measurements (when an MDT is also used).

3.3. Model description

The coastal and (sub)mesoscale processes north of Mallorcashould be partially observed by the glider and along-track altimet-ric measurements, but they are space–time sub-sampled by thesetwo observing systems. The interpretation of such data does not al-low for a complete identification of the mechanisms and forcingsassociated with the observed patterns. This challenge is the mainreason why a regional oceanic model has also been used to bettercharacterize the water masses’ dynamical interactions at the Bale-aric Basin scale and their relative positions with respect to the gli-der and altimetric track.

The oceanic model used is the Regional Ocean Modeling System(ROMS) (Haidvogel et al., 2000; Shchepetkin and McWilliams,2005), a 3D free-surface, sigma coordinate, split-explicit equationmodel with Boussinesq and hydrostatic approximations. The read-er is referred to the work of Shchepetkin and McWilliams (2005)for a more complete description of the numerical code. Note that

the simulation has been implemented over the Balearic Sea anddeveloped within the framework of the Balearic Islands CoastalObserving and Forecasting System (SOCIB, www.socib.eu and Tin-toré et al., 2012) and, therefore, is not specifically designed for thisstudy. The model domains extends from 1�W to 5�E and from 38�Nto 44�N (see Fig. 1b). The vertical discretization considers 30 sigmalevels, and the horizontal grid is 192 � 224 points with a resolutionof 1 km, which enables a comprehensive sampling of the first baro-clinic Rossby radius of deformation throughout the entire area(10–15 km, Send et al., 1999). The bottom topography is derivedfrom Smith and Sandwell (1997). The simulation, lasting 2 years,is initialized on 1st May 2007 using data on temperature, salinity,horizontal velocities, and sea surface elevation derived from theMediterranean Forecasting System (MFS, Pinardi et al., 2003). Atthe three lateral open boundaries (north, east and south) an active,implicit, upstream-biased radiation condition connects the modelsolution to the surrounding ocean (Marchesiello et al., 2001). Thedaily MFS fields are used to infer the thermodynamics and the cur-rents at the open-boundary conditions. The connection betweenthe MFS fields and the model did not create artificial features;therefore, we can focus on features situated close to theopen-boundary conditions. The regional configuration of theatmospheric model Weather Research and Forecasting (WRF,Skamarock et al., 2008), described in Ruiz et al. (2012), providesROMS with the following atmospheric fields every 1 h: 2 m airtemperature, relative humidity, surface wind vector, net shortwaveand downwelling, long wave fluxes, and precipitation. A bulk for-mulate based on Fairall et al. (2003) is used to compute turbulentheat and momentum fluxes.

4. Results

4.1. Hydrodynamics from observations

4.1.1. Synoptic view from remote sensingBefore the glider mission, from 01/08/2008 to 12/08/2008

(Fig. 2a), the AVHRR SST show a warm and relatively homogenoustemperature of approximately 27–28 �C in the center of the do-main, whereas cooler water, below 26.5 �C, is only present to thenorth and south. This configuration is typical, as described in Sec-tion 2. Indeed, a marked temperature front, corresponding to theCatalan front, is positioned to the north of the domain, separatingcolder (by 1.5 �C) northern water from the warmer water of theBalearic Sea. This front (indicated by the red dashed line onFig. 2a) is remarkably well co-localized with the main surface abso-lute geostrophic current patterns derived from altimetry sea levelelevation (see vectors on Fig. 2).

During the glider mission, from 13/08/2008 to 27/08/2008(Fig. 2b), the synoptic situation becomes quite different. The SSTcools throughout the western part of the domain, where a meandecrease of 1.5 �C is observed, with reference to the previous15 days. The map also indicates a northward current flowingthrough the Ibiza Channel into the Balearic Basin along the temper-ature fronts (red dashed line); one branch of this geostrophic flowpropagates north until 41�N, whereas another branch generates ameander after flowing along the north coast of Ibiza. This meanderexpands at the time of the glider mission and evolves into an anti-cyclonic, mesoscale eddy (hereafter called ‘‘OBELIX’’, indicated bythe black arrow in Fig. 2b) with a mean current of �15 cm/s, cen-tered southwest of the glider transect (2�E, 40.5�N). This mesoscalestructure is located at the interface between the relatively warmwater to the south and the cooler water to the north; it corre-sponds to the northwestern border of the new temperature frontreplacing the previously observed Catalan front (Fig. 2a). A branchof the eastern border of this eddy partially feeds the BC, whichflows and accelerates northward along the coast of Mallorca until

Fig. 2. Time-averaged SST (AVHRR) overlapped by the gridded altimetric geostrophic current (a) before (01/08/2008–12/08/2008) and (b) during (13/08/2008–27/08/2008)the glider mission. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Comparisons of absolute surface currents (in cm/s) interpolated at the glider location along the (a) northward and (b) southward transects (large curves correspond to15-km-smoothed signals). The blue curve corresponds to the glider rebuilt across-track absolute surface geostrophic current (using a reference level correction as described inSection 3.2). The red curve corresponds to the instantaneous altimetric across-track absolute surface geostrophic current derived from along-track PISTACH data (+MDT, (a):cycle 4 of JASON2; (b): cycle 5 of JASON2). The gray curve corresponds to the space/time interpolated altimetric across-track absolute surface geostrophic current derivedfrom AVISO ((M)SLA + MDT). The large black curve corresponds to the space/time interpolated model across-track absolute surface geostrophic current and the thin curve tothe model across-track total surface current. BC, NC and the associated separations represent the supposed mean position of the Balearic Current and of a retroflection branchof the Northern Current, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2Statistical comparisons between across-track absolute surface geostrophic current(mean in cm/s) from along-track altimetry (PISTACH) and glider observations.

Mean Altimetry GliderCorrelation Correlation

AltimetryCycle 4 7.9 1 0.91Cycle 5 6.8 1 0.73

GliderNorthward 5.6 0.91 1Southward 15.0 0.73 1

66 J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79

reaching the glider transect, at its southern edge (near the Minor-can coast). Farther north (>40.5�N along the glider transect), apartfrom the return branch of the NC located at the northern edge ofthe glider transect (also refer to Fig. 1), no clear sea surface signa-ture is observed.

4.1.2. Surface currents from along-track altimetry and glidermeasurements

Fig. 3 shows the absolute across-track surface geostrophic cur-rents derived from the glider data, satellite altimetry data (bothgridded and along-track product) and model output (used later,in Section 4.2.2.2) to precisely identify surface signatures associ-ated with dynamical features during the glider mission.

The northward (southward) glider transects, corresponding tothe 13/08/08–20/08/08 (21/08/08–27/08/08) period, were com-pared with across-track altimetric measurements of J2 cycle 4:13/08/2008 (J2 cycle 5: 23/08/2008). The two measurements arespatially well co-located (Fig. 1; right), but the time delay betweeninstantaneous altimetry sampling and the glider data may be apotential source of differences and should be considered (not

specifically discussed here). Nevertheless, the correlations are0.91 and 0.73 for the northward and southward transects, respec-tively (see Table 2). However, even if the spatial variations ofcurrent are well phased, an important bias (8.2 cm/s) is observedon the southward transect. This bias is not found during the north-ward transect, where the mean difference is only 2.3 cm/s (Fig. 3band Table 2). This unrealistic bias is likely due to instrumentalerrors in the glider GPS positioning system and induced glider

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 67

compass errors (Merckelbach et al., 2008). To correct this error, thespatial average altimetric absolute surface geostrophic currentcould be used as a reference despite the potential inconsistenciesdue to temporal lags between the (instantaneous) altimetric and(non-synoptic) glider measurements.

As in Bouffard et al. (2010) (for ENVISAT), comparisons alsomade with standard 1 Hz altimetry (not shown here) confirm thatthe PISTACH 20 Hz along-track sampling using the new editingstrategy (described in details in Bouffard et al., 2010) improvesthe altimetry–glider statistical consistency: along the northwardtransect, the correlations are 0.90 (for edited 20 Hz data) and 0.78(for standard 1 Hz data), with 55% of STD explained for the edited20 Hz data and 35% for the standard 1 Hz data. The same conclu-sions are obtained when the glider surface absolute currents arecompared with corresponding currents derived from standard AVI-SO (M)SLA. Despite weaker current amplitudes, this product (seethe gray curves in Fig. 3) however provides realistic surface currentswhen qualitatively compared to glider and along-track altimetry.

The previous comparisons show a relatively good agreementbetween the glider and PISTACH altimetry (see Fig. 3 and Table 2),particularly at the southern edge of the track, where the currentintensity is approximately 15 cm/s. This finding corresponds withthe BC position (see Figs. 1 and 2), whereas at the opposite edge,the two datasets also captured major dynamical features (cur-rents > 10 cm/s), which may correspond to a return branch of theNC (see Figs. 1 and 2). Between the potential positions of the BCand NC, a small-scale oscillation is also revealed (hereafter called‘‘ASTERIX’’), whereas this oscillation was not clearly observed inthe previous SST and gridded altimetry-derived map (Fig. 2b). Toinvestigate the associated baroclinic structure, we now analyzethe hydrographic fields from the glider measurements. This analy-sis should provide insight into the potential forcings associatedwith the observed surface geostrophic flow.

Fig. 4. (a) The potential temperature, (b) salinity and (c) density glider profiles along the nof depth (m) and latitude (�N). (d) The potential density computed by removing the sagradients. The white dashed lines in (a)–(e) correspond to 60 m depth. (f) Black curve: tcurve: the percentage of the STD explained by (e) in (c) as a function of depth (in m). (Forthe web version of this article.)

4.1.3. Hydrography from glider measurementsFig. 4 shows the potential temperature (a), salinity (b) and den-

sity (c) profiles captured by the glider for the northward transect,co-located at the surface with the altimetric track (the resultsalong the southward transect, not shown here, are equivalent).

Concerning the potential temperature (Fig. 4a), sub-mesoscaleoscillations, less than 10 km in extent, can be observed in the first50 m layer, whereas no marked signals appeared below 50 m.Examining the salinity profiles (Fig. 4b), small-scale features(marked by relatively low salinity values of 38.0–38.1 psu) are alsoobserved in the top 50 m of the water column. These structures arevery different from the signals at greater depth (>60 m), wheremarked horizontal salinity gradients appear. These (sub)surfacesalinity gradients are well phased with the surface altimetric andglider currents previously analyzed. Table 3 shows that below50 m, the mean correlations between these salinity gradients andthe across-track surface absolute geostrophic currents are 0.67and 0.81 for altimetry and glider data, respectively (�0.13 and0.06, respectively, in the first 50 m). Because such significant(anti)correlations are not obtained with temperature gradients,this finding may indicate that local geostrophic currents are drivenmainly by salinity and not temperature gradients.

To quantitatively confirm or reject this assessment, two virtualdensity fields have been computed in two different ways (Fig. 4dand e) and compared to the density field derived from the ‘‘real’’temperature (T) and salinity (S) profiles (‘‘real’’, Fig. 4c). The firstdensity field (Fig. 4d) was built using the real potential tempera-ture measurements but by considering, at each depth level, a con-stant salinity value (the horizontal spatial mean from the glider). Incontrast, the second density field (Fig. 4e) was built using the realsalinity field but considering at each depth a constant temperaturevalue (the spatial mean from glider). These approaches imply thatthe first density field (‘‘S fixed’’, Fig. 4d) does not consider effects

orthward transect (the result for the southward transect is equivalent) as a functionlinity gradients. (e) The potential density computed by removing the temperaturehe percentage of the STD explained by (d) in (c) as a function of depth (in m). Blueinterpretation of the references to color in this figure legend, the reader is referred to

Table 3Mean spatial correlations (function of depth) between across-track surface absolute geostrophic currents (from glider northward transect and altimetry cycle 4) and temperatureand salinity gradient from glider measurements.

Absolute surface geostrophic current Temperature (from glider) Salinity (from glider)

0 to 50 m 50 m to 180 m 0 to 50 m 50 to 180 m

Glider 0.05 �0.06 0.17 0.81Altimetry �0.04 �0.13 0.11 0.67

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caused by the salinity gradients, whereas the second density field(‘‘T fixed’’, Fig. 4e) does not consider effects related to the temper-ature gradients. By comparing the horizontal gradients of thesetwo virtual density fields to the observed density field (Fig. 4c), itshould be possible to evaluate the respective contributions ofsalinity and temperature to the density gradient (and thereforeon the across-track geostrophic flow).

The results are reported in Fig. 4f, which shows that for depthsless than 50 m, temperature gradients dominate because the per-centage of standard deviation (STD) explained by the ‘‘S fixed’’(‘‘T fixed’’) field in the ‘‘real’’ density field is greater than 80%(approximately 0%). Below 50 m, the percentage of STD explainedby the ‘‘S fixed’’ field progressively decreases, whereas the percent-age of STD explained by the ‘‘T fixed’’ field increases. Below 60 m,the percentage of STD explained by the ‘‘T fixed’’ field (>70% below100 m) is greater than that explained by the ‘‘S fixed field’’ (<20%below 100 m). Thus, the contribution of the salinity gradients inthe density gradient computation becomes higher than the contri-bution of temperature gradient. Therefore, the across-track geo-strophic flow (proportional to the density gradient) associatedwith the observed dynamical features appears to be driven mainlyby sub-surface salinity gradients.

Fig. 5. The time-averaged sea surface temperature in �C and kinetic energy (KE) in cm2/AVHRR for SST (a), altimetry (AVISO (M)SLA + MDT) for KE (b) and from model (c and d

However, using altimetry and glider data alone does not allowus to draw definitive conclusions about the origins of these strongsalinity gradients. Such conclusions would require the comple-mentary use of modeling simulations. Thus, the next objective ofthis paper is to more clearly identify the origin and mechanismsassociated with the observed small-scale features. To this end, a re-gional model will also be used in conjunction with the previousmulti-sensor observations. However, it will first be necessary todetermine whether the model is able to realistically reproducethe ocean dynamics at the Balearic Basin scale, both before andduring the glider mission. In a second step, it will also be crucialto assess the model’s ability to adequately simulate the observedpatterns along the northern coast of Mallorca, specifically duringthe glider mission. If the simulation is adequate, we can confi-dently use it to explore the processes involved in the altimetryand glider measurements.

4.2. Model validation

4.2.1. Surface comparison with remote sensing4.2.1.1. The synoptic surface view for the July–August 2008 period. Inthis section, we first compare the ROMS model outputs with the

s2 for the period July–August 2008 from remote-sensing data derived from satellite).

Table 4Statistical comparisons between model and AVHRR SST (in �C), KE and EKE (byremoving the 2-month mean current) (in cm2/s2).

Area Mean STD Absolute difference

SSTModel 26.0 0.8 0.5Satellite 26.0 1.0

KEModel 170 35 52Satellite 221 20

EKEModel 58 16 18Satellite 41 18

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remote-sensing data over the July–August 2008 period. In thisanalysis, the surface geostrophic current from altimetric griddedabsolute dynamic topography (ADT = (M)SLA + MDT) can be com-pared to the 2D model geostrophic currents derived from the mod-el sea level height (interpolated on the same grid). Thus, the kineticenergy (KE) has been calculated from these two consistent currentfields and time-averaged over the period July–August 2008 (seeFig. 5b and d). To complement this analysis, we also computed,during the same period, the time-averaged SST from both simula-tion and satellite AVHRR data (see Fig. 5a and c). The temporal evo-lution of these two variables, spatially averaged over the entiremodel domain, is also used for quantitative comparison (Fig. 6).The corresponding statistical results are reported in Table 4.

Fig. 5 illustrates the relatively close general agreement betweenthe model and satellite data. Concerning SST (Fig. 5a and c), theCatalan front is clearly observed in the two SST fields in the north-ern part of the model domain, separating relatively cold water(<25 �C) from the warmer water (>26 �C) located in the BalearicSea. In the southern part of the domain, colder water patches ob-served using remote sensing are also reproduced by the model.The time series of the spatial-averaged SST (Fig. 6a) also showthe model’s ability to correctly simulate the SST variability ob-served by satellite measurements in the period July–August2008. The curves show a general SST decrease of 1 �C between01/07/08 and 20/07/08, an increase of 2.5 �C between 21/07/08and 06/08/08 and a decrease of 1 �C until 31/08/08. The statisticalresults confirm this good agreement (see Table 4): the model andsatellite SST are characterized by the same mean temperature(26 �C), an absolute mean difference of 0.5 �C, a correlation greaterthan 0.9 and similar standard deviations of 0.8 �C and 1 �C for themodel and the satellite observations, respectively.

As expected, the agreement between the model and remote-sensing data in terms of KE is not as clear because the model isnot constrained using data assimilation, and, as noted previously,altimetric (M)SLA are not always adapted for regional studies. Rel-atively high mean KE values (>500 cm2/s2) are observed in the twomaps, especially in the southern part of the domain, but they arenot co-located (cf. Fig. 5b and d). The temporal evolution of thespatially averaged KE, however, exhibits very closely aligned ten-dencies with a general decrease of 20% between 01/07/08 and06/08/08 followed by an equivalent increase between 06/08/08and 27/08/08, exactly coincident with the SST decrease previouslyobserved (see Fig. 6a and b). Statistical results show a mean abso-lute difference between the model and altimetry data, which rep-resents 20% of the mean altimetric KE (see Table 4 and the red

Fig. 6. A time series of spatially averaged (over the model domain) SST and KE derived froto the absolute difference between model and satellite KE. (For interpretation of the refearticle.)

curve in Fig. 6b). When the eddy kinetic energy (EKE) is computedby removing the 2-month mean current both in the model andaltimetry (and therefore the impact of MDT in the altimetric cur-rents) data, the results are quite different (see Table 4; figure notshown). In that case, the mean spatial EKE is 66% and 80% lessfor the model and altimetry data, respectively (refer to Table 4for the associated values), the STD is of the same order of magni-tude, and the mean absolute difference is only 18 cm2/s2.

These initial assessments show that the model is able to ade-quately reproduce the general surface dynamics for the periodJuly–August 2008, both for the spatial patterns and the associatedtemporal variability. In addition, we subdivided the model domaininto four zones of equal surface (as shown in Fig. 5) to discriminatethe relative errors and the contributions of each sub-region interms of KE and SST mean and variability. The obtained statisticalresults are reported in Appendix A and appear to confirm that themodel is able to satisfactorily reproduce the surface ocean dynam-ics at the Balearic regional scale for the period July–August 2008.

4.2.1.2. Surface changes during the glider mission. We now focus ontwo distinct periods of August 2008 to describe potential changesat the Balearic Basin scale immediately before (01/08/2008–12/08/2008) and during (from 13/08/2008 to 27/08/2008) the glidermission. These two periods correspond to a relatively low and highKE, respectively (Fig. 6b), in addition to a significant change in thetrend of SST (Fig. 6a), which tends to decrease after 06/08/2008(both observed and simulated).

As previously indicated, a comparison between Figs. 2 and 7confirms the good agreement between the ROMS simulation andremote-sensing data, specifically over the two periods in August

m model and satellite (AVHRR and AVISO (M)SLA + MDT). The red curve correspondsrences to color in this figure legend, the reader is referred to the web version of this

Fig. 7. The time-averaged SST overlapped by the geostrophic current (a) before (01/08/2008–12/08/2008, top) and (b) during (13/08/2008–27/08/2008, bottom) the glidermission from the ROMS oceanic model. Boxes used to monitor the water mass characteristics (in Figs. 13–15) have also been added.

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2008. The description of dynamical patterns is similar to that ob-served using remote sensing, described in Section 4.1.1. In particu-lar, the temperature appears cooler over the western part of thedomain during the glider mission. An analysis of the model’s atmo-spheric forcing (not shown) indicates this cooler temperature islikely due to a strong cold wind event that increased local heat lossby turbulent heat fluxes and vertical mixing. The local SST cooling,in turn, generates a zonal temperature gradient that reinforces thenorthward geostrophic circulation and also leads to an increase inthe mesoscale activity (as observed in the KE; see the previous sec-tion and Appendix A).

A major difference between the model and satellite observa-tions, however, occurs near the Iberian coast and more generallyin zone 3 (see Figs. 2 and 7), where a southward coastal currentnot reproduced by the model is observed. In addition to potentialboundary condition issues, this disagreement may be due to stan-dard altimetry limitations in the coastal zone (refer to the intro-duction) and a lack of the data required to constrain the MDTcomputation (refer to Rio et al., 2007). Close to this area, a perma-nent and clearly unrealistic cyclonic eddy, centered at (1�E,40.5�N), is also seen (present in the MDT and not in the (M)SLA).This structure, not reproduced by the ROMS simulation before orduring the glider mission, is relatively far from the glider transectand altimetric track location (>200 km) and should not affect thefollowing interpretations. Farther east, the modeled and altimetricsurface geostrophic circulations are quite similar (as will also be

Fig. 8. The temporal difference in the surface geostrophic current (vectors) and relative vglider mission from (a) altimetric data from AVISO and (b) ROMS model in the closeinterpretation of the references to color in this figure legend, the reader is referred to th

confirmed in Section 4.2.2), which confirms the previous valuesof KE, showing relatively good results (except in zone 3; refer toAppendix A) when the damaging impact of the MDT is notremoved.

With regard to the absolute geostrophic current, using a mapof differences between two time periods allows us to precisely re-move potential issues related to the MDT by dealing directly withthe oceanic signal variability. Fig. 8 shows that the changes dur-ing the glider mission in the model and gridded altimetry spatialpatterns are quite similar. In particular, both the model and satel-lite observations are marked by a reinforcement of the currentintensity and a negative vorticity associated with an anticycloniceddy acceleration (‘‘OBELIX,’’ as noted previously in Section 4.1.1).Fig. 8 also shows that the BC intensity decreases by approxi-mately 5 cm/s, whereas the northward geostrophic current cross-ing the glider transect in the north tends to increase. The middlepart of the glider transect is characterized by low surface vorticityvariability (and KE; see also Fig. 5b and d) both in the model andgridded altimetry. However, weak geostrophic surface currentsignatures may hide more intense sub-surface geostrophic cur-rents (confirmed in the following section). Additionally, even ifcurrents derived from the altimetric map provide a qualitativeassessment of general geostrophic patterns at the Balearic Basinscale, the synoptic representation of the surface circulation at asub-regional scale is compromised by the limitations arising fromthe space/time smoothing effect from the optimal interpolation

orticity before (01/08/2008–12/08/2008) and during (13/08/2008–27/08/2008) thevicinity of the glider transect (the red line indicates the northward transect) (Fore web version of this article.).

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 71

required to merge data from multiple altimeters (Dussurget et al.,2011).

4.2.2. Comparison with glider data4.2.2.1. Hydrography. A comparison of Figs. 9 and 4 shows that theROMS simulation and glider hydrographic profiles are in goodagreement despite some differences (including a weak salinity gra-dient in the model) and spatial lags of approximately 10 km (theBC is farther offshore in glider data). Moreover, the glider sectionexhibits wiggles in the first layer, which may be due to internalwave heaving, that are not reproduced by the model.

Concerning the potential temperature (Fig. 9a), the modelshows less spatial variability in the first 50 m layer, where sub-mesoscale oscillations less than 10 km in extent were observedin the glider profile. As a result, in this area, the temperature STDis 30% lower in the model and low correlations between modeland glider are observed (Fig. 10, top). Outside of the BC location(latitude > 40.2�N) and below a depth of 50 m, the temperaturehorizontal gradients are very weak (STD < 0.5 �C in both the gliderand model), implying that correlations are not highly significant.Despite this weak correlation, the glider and model data showclose general statistical profiles (STD and mean; Fig. 10, top) withdepth-averaged mean temperatures of 15.8 �C and 16.1 �C andSTDs of 0.31 �C and 0.25 �C, respectively.

Concerning salinity, the mean model and glider profile are sim-ilar, with correlations below 50 m of between 0.8 and 0.9 (Fig. 10,middle). However, even if the glider and model salinity profiles arequite clearly phased, the sub-surface salinity STD in the glider datais twice that in the model (see Fig. 10). Indeed, as suggested by thesalinity profile (see Figs. 4 and 9b), the sub-surface salinity gradi-ents are stronger in the glider data than in the model. This discrep-ancy may be the source of an underestimation of the modeled

Fig. 9. (a) The potential temperature, (b) salinity and (c) density model profiles space–ttransect is equivalent) as a function of depth (m) and latitude (�N). (d) The potential densby removing the temperature gradients. The white dashed lines in (a)–(e) correspond tofunction of depth (in m). Blue curve: the percentage of the STD explained by (e) in (c) as alegend, the reader is referred to the web version of this article.)

surface currents with respect to the observed currents (as will benoted in Section 4.2.2.2; see also Fig. 2). As previously observedwith glider measurements, these model salinity gradients are alsowell correlated with the surface absolute geostrophic current (cor-relations > 0.5, see Tables 3 and 5), unlike the temperaturegradient.

To precisely evaluate the relative contributions of salinity andtemperature gradients to the model density gradient computation,the same experiment as that performed with the glider has beenperformed. The results from the model (Fig. 6f) confirm that thesub-surface geostrophic currents are also mainly driven by salinitygradients. Indeed, as with the glider data, the influence of modeltemperature gradients decreases for depths below 50 m. Fig. 6falso shows that the impact of salinity gradients exceeds the impactof temperature gradients at depths above 100 m (whereas thethreshold was at approximately 60 m in depth according to the gli-der data). This finding may explain why the correlations betweenthe model and glider density profiles become significantly betterbelow 100 m in depth, where the salinity gradients dominate (cor-relation > 0.8; refer to Fig. 10, bottom).

Moreover, the water masses identified in Figs. 4 and 9 between40.1�N and 40.4�N and between 40.6�N and 40.8�N have the samehydrographic properties in both the model and glider measure-ments (a low salinity of less than 38.1 psu and temperature of14 �C at 70 m), which suggests that they have the same originbut have followed two different trajectories. This finding is consis-tent with the 2D surface analysis described in Section 4.1.2 (alsoconfirmed in Section 4.2.1.2 with model outputs), in which theOBELIX eddy was shown to partially feed the BC in the south,whereas its northern part interacted with the NC return loop andadvected water northward. The origin of this water mass, charac-terized by a lower salinity value at the source of local salinity

ime interpolated along the northward glider transect (the result for the southwardity computed by removing the salinity gradients. (e) The potential density computed

60 m depth. (f) Black curve: the percentage of the STD explained by (d) in (c) as afunction of depth (in m). (For interpretation of the references to color in this figure

Fig. 10. Statistical comparisons between model and glider hydrographic measurements (potential temperature, salinity and density) as a function of depth (m). Blue curvescorrespond to statistics from the glider (both STD and mean); black curves correspond to statistics from the model. The red curve corresponds to the correlation betweenmodel and glider measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 5Mean spatial correlations (function of depth) between across-track surface absolutegeostrophic currents and temperature and salinity gradient (from model).

Temperature(from model)

Salinity(from model)

Correlation with surface absolute geostrophic current (from model)0 to 50 m 50 to 180 m 0 to 50 m 50 to 180 m0.35 �0.30 0.64 0.53

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gradients (and therefore geostrophic flows), will be discussed inSection 4.3.

Fig. 11. Comparisons between glider (blue curves, raw and 15-km-smoothed data) and spcurrents as a function of latitude (reference level at 180 m) at the time of the (a) northwa2008). (For interpretation of the references to color in this figure legend, the reader is r

4.2.2.2. Currents. The model surface relative geostrophic currenthas been computed with respect to the 180 m reference depth(as in glider) and interpolated to the glider track in space and time.Fig. 11 shows comparisons with the glider hydrographic measure-ments along the northward (13/08/08–20/08/08, Fig. 11a) andsouthward transects (21/08/08–27/08/08, Fig. 11b).

Despite spatial lags of a few kilometers, Fig. 11 shows that themodeled and glider relative surface geostrophic currents are con-sistent, with a mean difference less than 1 cm/s and correlationsof 0.91 and 0.52, respectively, for the northward and southwardtransects (see Table 6 for statistics). For the northward glider tran-sect (Fig. 5a), the model and glider data show a decrease in the cur-rent intensity from 5 cm/s to 0 cm/s in the cross-shore direction at

ace–time interpolated model (black curve) relative across-track surface geostrophicrd (13/08/2008–20/08/2008) and (b) southward transects (right, 21/08/2008–27/08/eferred to the web version of this article.)

Table 6Statistical comparisons between across-track relative surface geostrophic current (/180 m) from model and glider observations (mean is in cm/s).

Mean Correlation with glider

GliderNorthward 2.2 1Southward 2.7 1

ModelNorthward 1.6 0.91Southward 1.3 0.52

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 73

the BC mean location (40.1–40.5�N). The findings differ for thesouthward transect (Fig. 5b), with a BC relative surface geostrophiccurrent that is larger but half as intense. In the northern BC, the rel-ative geostrophic current is alternatively negative (southward) andpositive (northward), with a current intensity of ±5 cm at thenorthward and southward transects for both the glider and modeldata.

The comparisons between the absolute surface geostrophic cur-rents from the model and observed data show relatively goodagreement along the northward transect (significant correlationsof 0.78 and 0.50 with respect to glider and along-track altimetry),which is not the case for the southward transect, marked by a20 km spatial lag (see Fig. 3). Moreover, the amplitude of absolutecurrents (both geostrophic and total) in the model is much less thanin the observations despite the good agreement previously ob-served in terms of surface relative geostrophic currents. Therefore,this decreased amplitude of absolute currents should also corre-spond to an underestimation of sub-surface currents in the model.

Based on the analysis in Section 4.1.3, it followed that the sur-face altimetric and glider absolute geostrophic current – an ‘‘inte-grated value’’ – reflected the salinity gradient signals of depth andnot the surface layer, where small-spatial-scale disturbances con-ceal more stable deep geostrophic signals. Therefore, we investi-gate and compare the patterns in the distribution of the currentsalong the water column from both the model and the glider mea-surements (Fig. 12).

When the absolute geostrophic current (Fig. 12b and d) and therelative geostrophic current (Fig. 12a and c) are compared, the fea-tures are quite different, both in terms of magnitude and spatialdistribution. In both cases, the BC is located between 40.1�N and40.3�N, with a maximum intensity at a depth of 50 m, but the abso-lute current intensity is twice larger than the relative geostrophiccurrent (10–20 cm/s as compared to 5–8 cm/s). The BC observedby the gliders was wider (up to 40.4�N) and stronger than thecurrent simulated by the model. Between the BC and NC

Fig. 12. Comparisons between glider and ROMS across-track currents (cm/s) as a functcurrent derived from the DH calculated with respect to 180 m depth from (a) the ROMS musing the methodology described in Section 2.2 from (b) ROMS and (d) glider measuremeand a red square, respectively. (For interpretation of the references to color in this figur

return-branch, a sub-surface feature is observed by the gliderand is partially reproduced by the model (see the red square inFig. 12b and d). Although the model structure shows a spatial lagof approximately 10 km toward the south, it shows an alterna-tively positive (northward) and negative (southward) current withmaximum intensity at depths above 50 m for the positive currentvalues in the model and the negative values in glider data (at lati-tude 40.6�N).

The next challenge will be to use the validated model to de-scribe the characteristics and the potential processes associatedwith this structure, intercepted north of Mallorca (previouslycalled ‘‘ASTERIX’’ in Section 4.1.2).

4.3. Spatio-temporal variability from numerical modeling

Three virtual boxes are located at critical sites chosen with re-spect to the previous analyses conducted with the multi-sensordata and oceanic model outputs. The first box (the red box inFig. 7b) is aimed at monitoring the time variability of hydrologicaland dynamical properties of the southern water entering throughthe Ibiza Strait. The two other boxes are located on each side ofthe glider transect to assess the hydrological and dynamicalchanges both near the ASTERIX site (the black box in Fig. 7b) andaround the NC retroflection (the blue box in Fig. 7b).

Fig. 13 shows the T/S diagrams, as a function of depth and hor-izontally space-averaged, at the three locations during the July–August 2008 period. This figure clearly demonstrates that waterin the south (Box 1) shows hydrographical properties very differentfrom those of the water in the north (Box 2 and Box 3). Indeed, thewater mass here appears to be mainly composed of recently mod-ified AW characterized by a salinity of approximately 0.5 psu lessthan the ‘‘older’’, more modified AW of northern origin (see Sec-tion 2). Moreover, the T/S diagram in Box 1 (recent AW) is moredispersed than that in Boxes 2 and 3 (older AW), which suggestsa progressive change of water mass properties during the 2-monthstudy period.

To quantitatively assess the temporal evolution of the watermasses’ characteristics in the three virtual boxes and their poten-tial interactions, we now use a Hovmöller diagram of horizontalspace-averaged key variables – salinity, current magnitude and rel-ative vorticity – as a function of time and depth (see Fig. 14).

Fig. 14 shows that the salinity anomaly (removing the 2-monthmean value) in the Ibiza Strait becomes significantly negative frommid-July to mid-August. This strong decrease in the salinity anom-aly coincides with an increase in the total current at depth (5 cm/sat 50 m depth). This change corresponds to an entrance of less salty

ion of latitude (�N) through the water column (0–180 m). The relative geostrophicodel and from (c) glider measurements. The rebuilt absolute geostrophic current bynts. The BC and a sub-surface mesoscale structure are highlighted by a black squaree legend, the reader is referred to the web version of this article.)

Fig. 13. A T/S diagram from the model (in �C and psu) as a function of depth (m) from July 2008 to August 2008, spatially averaged (but not time averaged) for each of thethree boxes (refer to Fig. 15 for the location of the boxes).

Fig. 14. (a) The modeled spatially averaged salinity anomaly, (b) total absolute current norm and (c) relative vorticity as a function of time and depth in the three boxes. Top:the Ibiza Channel neighborhood (Box 1); middle: the NC retroflection neighborhood (Box 2); bottom: the glider east-side neighborhood (Box 3). The dashed lines correspondto the glider mission period.

74 J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79

AW, which is progressively modified and advected northward bythe general circulation until it reaches the OBELIX location in Au-gust 2008. At the northwest side of the glider transect (Box 2),the conditions in July show a strong current magnitude (+15 cm/s) and alternating relative vorticity (±2.10�5 s�1). This trend corre-sponded to the cyclonic NC position, which moved southward ornorthward as a function of its interaction with southern currents(as also observed in Fig. 16). In August 2008, a positive salinityanomaly of 0.05–0.1 psu is observed, which is associated with a de-crease of current intensity. This finding corresponds with theperiod during which OBELIX progressively expands to the north

and interacts with the NC return loop. Consequently, the less saltyAW entering through the Ibiza Strait is advected northward until itreaches salty water from the NC system at the time of the glidermission (see dashed line on Fig. 14). Indeed, Fig. 14 (Box 3) showsa negative salinity anomaly of 0.1 psu in August 2008 in conjunc-tion with a relatively strong, deep current (approximately 5 cm/sat the surface and 10–15 cm/s from 30 m to 200 m). At the sametime, a negative sub-surface relative vorticity is generated at depthin the neighborhood of the glider transect, with maximum valuesof �3 � 10�5 s�1. This relatively strong negative vorticity istime-correlated (>0.9, see Fig. 15) with the sub-surface salinity

Fig. 15. Time series of the model salinity difference (psu) between Box 2 and Box 3(green curve right axis) and of the model relative vorticity (10�5 s�1) in Box 3 at theASTERIX location. The dashed lines correspond to the glider mission period. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 75

difference observed between Box 2 and Box 3 (�0.2 psu over a fewkm). We will now try to identify the 2D horizontal structure asso-ciated with this sub-surface negative vorticity signature, generatedclose to Box 3, in the neighborhood of the glider transect.

Fig. 16 shows the relative vorticity temporal evolution (fromJuly 2008 to August 2008) to characterize the 2D horizontal shearof the velocity field. For this analysis, the model relative vorticityhas been computed at 2 depth levels to account for the baroclinicstructure of features previously observed.

From Fig. 16, it follows that a sub-surface, mesoscale anticy-clonic structure is generated in the northeast of the model domain,at the time of the glider mission, in close proximity to the transectand an ROMS boundary (there are no continuity issues with re-spect to MFS, as noted in Section 3.3). Based on the relatively goodagreement previously established between the model and observa-tion data (both qualitatively and quantitatively), our major

Fig. 16. The relative vorticity (10�5 s�1) in the ROMS model from the beginning of July tocharacteristics in Figs. 13–15) and (b) 75 m depth.

hypothesis is that the same type of structure has been interceptedby the glider and along-track altimetry measurements. Given itssmall-scale extension (<30 km) and its relatively weak signatureat the surface, this structure was not clearly observed with griddedaltimetry. However, it is remarkable that both the model relativevorticity and the associated currents of ASTERIX are significantlystronger at a depth of 75 m than at 10 m, which is in line withthe previous sub-surface glider observations made in its neighbor-hood, north of Mallorca. The intensity of the ASTERIX eddy appearsto be controlled by a salinity gradient (as is the structure observedby the glider profiles) created by the meeting of two distinct, ener-getic water flows. The first water flow mainly consists of recentAW entering through the Ibiza Strait, which is then progressivelymodified and advected by the general circulation until it reachesthe ASTERIX area at the time of the glider mission. This small-scaleanticyclonic eddy, whose residual currents in the south have beenpartially captured by the glider transect and along-track PISTACHdata, seems to result from the baroclinic interaction between thiswater mass and the saltier Mediterranean waters (old AW) comingfrom the NC cyclonic circulation located to the north.

5. Discussion and conclusions

In this study, the potential synergies between the multi-sourceremote-sensing, glider and numerical model data have been dis-cussed in terms of accuracy, resolution and spatial/temporal sam-pling. New methodologies have been tested to improve theconsistency between altimetry and glider datasets using experi-mental altimetry data from the PISTACH project (Coastal andHydrology Altimetry product handbook, 2010) and by computingglider absolute geostrophic currents following the recent strategydescribed by Bouffard et al. (2010). The oceanic model and mul-ti-sensor datasets were then used in conjunction to interpret theobserved physical processes. By increasing the confidence in thedynamical structure interpretation, this approach has led to a fullcharacterization of small-scale processes. We believe that the

the end of August 2008 at (a) 10 m depth (the boxes used to monitor the water mass

76 J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79

worldwide challenge of coastal (sub)mesoscale dynamic character-ization must be addressed locally through such an integrated ap-proach, combining both observations and free numerical runs.This is especially true in areas where small mesoscale structuresare characterized by a low signal-to-noise ratio. The surface signa-tures are particularly weak in the northwestern Mediterranean andare therefore difficult to interpret with the use of isolated measure-ments alone.

Although the obtained results are quite encouraging, our com-parisons have also shown the limitations of these two observationsystems in terms of both coverage and accuracy. Focused on theglider platform, the observed bias at the time of back transectcould be further reduced by applying additional corrections tothe glider depth-averaged GPS currents. In particular, we wouldexpect to achieve better results with a more accurate estimationof the error in the glider displacement assumptions, which wouldentail a more accurate measurement of glider heading. With regardto the gridded altimetric product and the MDT used, we have alsoconfirmed the relatively poor resolution of existing products,which employ both a more comprehensive satellite constellationcoverage and a regional approach to better constrain the represen-tativeness of local mesoscale features (refer to Dussurget et al.,2011). Furthermore, experiments based on glider versus along-track altimetry cross comparisons need to be repeated to improveand consistently assess the impact of new methods dedicated tocoastal zone applications. The promising early results in coastalaltimetry (refer to Vignudelli et al., 2011, for a review) support

Fig. A1. A time series of the spatially averaged (over the four subdivided

the need for continued research and scientific applications withthe opportunity to providing inputs and recommendations to fu-ture satellite missions (refer to the SWOT satellite and last CoastalAltimetry Workshop, San Diego 2011).

In this study, in addition to multi-sensor cross comparisons,particular attention has been paid to the model validation andthe characterization of a small-scale (<30 km) anticyclonic eddynorth of Mallorca, which was observed by glider and along-trackaltimetry and partially reproduced by a ROMS simulation. Thesub-surface salinity gradient associated with this eddy seems to re-sult from a recent Atlantic water inflow through the Ibiza Channelthat is advected northward and interacts with saltier old Atlanticwater flowing with the Northern Current. It has been shown the lo-cal geostrophic currents during the glider mission were mainly dri-ven by these salinity gradients and not, as is typical in the BalearicSea, temperature gradients, which represents a major finding withrespect to previous studies of this area (e.g., Pascual et al., 2002; Pi-not et al., 2002; Rubio et al., 2009).

This study therefore demonstrates the key role of water massexchanges and, in particular, salinity properties between thenorthern Gulf of Lion and the southern Algerian Basin throughthe Balearic Island channels. The Ibiza and Mallorca Channels pro-vide a significant passage for water masses potentially interactingwith the northern general circulation, which emphasizes theimportance of monitoring the entrance of recent Atlantic wateras an indicator of the mesoscale climatology in the northwesternMediterranean Sea.

domains; refer to Fig. 5) SST from model and AVHRR satellite data.

Fig. A2. A time series of the spatially averaged (over the four subdivided domains; refer to Fig. 5) KE associated with the geostrophic currents derived from model andsatellite altimetry (AVISO (M)SLA + MDT). The red curve corresponds to the absolute difference between model and satellite KE. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)

J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79 77

Acknowledgements

We gratefully acknowledge the contribution of M. Martínez-Ledesma and B. Casas during the data acquisition phase of the gli-der missions. Special thanks are also due to B. Buongiorno Nardelli,R. Escudier, F. Nencioli, A. Petrenko and A. Doglioli for fruitful dis-cussions concerning the interpretations of data and simulations.The authors are also grateful to B. Garau for processing the gliderdata, E. Heslop and R. Campbell for highly valuable editing com-ments and G. Vizoso for the ROMS numerical setup and data man-agement. The altimeter (M)SLA was produced by SSALTO/DUACSand distributed by AVISO with support from the Centre National

Table A1Statistical comparisons between model and AVHRR SST (in �C, refer to Fig. 5 for theZone locations).

Area Mean STD Absolute difference

ZONE 1 Model 26.1 0.8 0.4Satellite 26.3 1.1

ZONE 2 Model 26.0 0.8 0.4Satellite 25.9 1.0

ZONE 3 Model 25.8 0.8 0.3Satellite 25.9 1.0

ZONE 4 Model 26.0 0.8 0.4Satellite 26.0 1.0

d’Etude Spatiale (CNES, France). The coastal high-resolution altim-eter products were produced by CLS and distributed by AVISOwithin the PISTACH project supported by CNES. Sea surface tem-perature images were acquired and processed by EUMETSAT. Thiswork was partially conducted during a CNES Postdoctoral Fellow-ship and supported by PERSEUS FP7-OCEAN-2011 (GA 287600)and the European Commission MyOcean Project (SPA.2007.1.1.01– development of upgrade capabilities for existing GMES fast-trackservices and related operational services; Grant Agreement218812-1-FP7-SPACE 2007-1).

Table A2Statistical comparisons between model and AVISO KE (in cm2/s2, ref to Fig. 5 for theZone locations).

Area Mean STD Absolute difference

ZONE1 Model 170 20 21Satellite 169 30

ZONE 2 Model 230 65 53Satellite 270 44

ZONE 3 Model 105 52 135Satellite 240 25

ZONE 3 (EKE) Model 53 27 18Satellite 41 20

ZONE 4 Model 182 41 40Satellite 178 40

78 J. Bouffard et al. / Progress in Oceanography 106 (2012) 62–79

Appendix A

See Figs. A1 and A2 and Tables A1 and A2.

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