Progress in Oceanography 83 (2009) 288–295

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Progress in Oceanography

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Mesoscale eddy activity in the southern Benguela upwelling system fromsatellite altimetry and model data

Anna Rubio *, Bruno Blanke, Sabrina Speich, Nicolas Grima, Claude RoyLaboratoire de Physique des Océans, UMR 6523, CNRS-IFREMER-IRD-UBO, Brest, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 July 2008Received in revised form 23 February 2009Accepted 16 July 2009Available online 26 July 2009

0079-6611/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.pocean.2009.07.029

* Corresponding author. Address: Marine ResearPasaia (Gipuzcoa), Spain.

E-mail address: arubio@azti.es (A. Rubio).

We characterize eddy activity in the southern Benguela upwelling system with absolute sea level datadeduced from remote-sensing measurements and numerical simulations run with a regional model,using a wavelet-based technique for identification and tracking of coherent structures. Statistics on eddycount, size, type and location are obtained along the continental slope of the upwelling system for a com-mon period of interest shared by model and satellite data. We draw a comparison between mesoscalevariability calculated by the regional model and ground truth accounted by satellite altimetry. We showthat the presence of cyclonic eddies along the upwelling front over the South Benguela slope is recurrentthroughout the year. Moreover, the agreement between model and observations is fair and validates themodeling approach. Tracking in the model of volume properties of continental slope, cyclonic eddies ontheir way toward the open ocean suggests that these eddies play a significant role in coastal–open oceanexchanges. With lifetimes of several months, these eddies can export significant volumes of nutrient-richwaters far from the coastal upwelling.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The Benguela region, located along the southwest coast of Afri-ca, is characterized by a strong coastal upwelling primarily linkedto predominant equatorward winds. The upwelling events tend tobe short-lived and seasonal (Risien et al., 2004). They are observedalong the coast in the form of a number of cold-water cells whoseorganization depends on coast orientation and bathymetry (Shan-non and Nelson, 1996; Weeks et al., 2006). Geographically, thesouthern Benguela upwelling starts south of 27�S and extendssouthward to 35�S, with Cape Agulhas as its southern limit (Hard-man-Mountford et al., 2003; Shannon and Nelson, 1996). Anupwelling frontal zone is usually well defined, although variable,and coincides generally with the shelf edge (Shannon, 1985). Themain upwelling cells (from north to south) are the Namaqua cellaround 30�S, the Cape Columbine cell around 32.5�S, the Cape Pen-insula cell around 34�S and the Western Agulhas Bank cell around35�S (Weeks et al., 2006). Seasonal and interannual variability inthe surface intensity and extension of the different cells is signifi-cant and has an impact on the biogeochemical processes and thebiological resources of this high primary production area (Weekset al., 2006).

Substantial mesoscale activity in the form of eddies and fila-ments is a constant feature of the upwelling frontal area, making

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the front strongly irregular (Blanke et al., 2002, 2005; Weekset al., 2006). Other sources of mesoscale activity are the recurrentnorthwestward drift of Agulhas eddies and the shedding of cyclo-nic eddies off Cape Columbine (Hardman-Mountford et al., 2003).This mesoscale activity impacts on the local ecosystem dynamicsand on the cross-slope exchanges between the coastal upwellingsystem and the open ocean. As shown by Hutchings et al. (1998),eddies can contribute to the offshore loss of fish eggs and larvaeduring their transport phase from the Agulhas Bank spawningground to the nursery area located along West coast upwelling.Chlorophyll distribution and standing stocks in upwelling systemsare also influenced by mesoscale eddy activity (Carr, 2002; Dem-arcq et al., 2007; Rossi et al., 2008).

Our study comes within the scope of the French-funded InterUpresearch program that is aimed at studying the cross-shore trans-fers in the four major eastern-boundary upwelling systems (Cali-fornia, Humboldt, Canary and Benguela Current) by gatheringscientists of several national laboratories with abroad collabora-tions in countries along each upwelling system. Mesoscale activityin the upwelling region is expected to have a major role on theshelf–slope transfers in the southern Benguela area, as in the otherlarge eastern-boundary upwelling systems (see for instance Marc-hesiello et al. (2003) for the California upwelling system). The aimof our study is to provide a statistical characterization of the meso-scale variability in the southern Benguela area by focusing on thespatial distribution of the eddy activity over the slope. With thisend in view, we apply a wavelet-based technique, which allowsidentification of coherent structures (Doglioli et al., 2007), using

A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295 289

several years of satellite sea surface height (SSH) observations andmodeled sea level data. The qualitative and quantitative (statisti-cal) comparison of both data sets is used to validate the modeleddy content. Finally, the vertical structure of slope eddies is de-scribed using model results. Their volume characteristics aretracked in time by extending the sequential 2D wavelet analysisto successive underlying vertical levels.

2. Data and numerical tools

2.1. Satellite sea surface height

Weekly maps of absolute dynamic topography (ADT), i.e., SSHabove the geoid height obtained from the sum of weekly sea levelanomalies and the Rio05 mean dynamic topography (see Ducetet al., 2000; Rio and Hernandez, 2004), and corresponding absolutesurface geostrophic velocities are obtained from the Archiving, Val-idation and Interpretation of Satellite Oceanographic database(AVISO, produced by Ssalto/Duacs with support from the FrenchCentre National d’Études Spatiales). For our study, we analyze a13-year period, from January 1993 to December 2005, of delayed(corrected after acquisition) ADT maps based on data from TO-PEX/POSEIDON, Jason-1, ERS1-2, ENVISAT satellites that followthe same ground track and lead to a sampling stable with time.The maps used here are gridded at 1/3� resolution, covering theoceanic region from 8�W to 26�E and from 20�S to 35�S.

Fig. 1. SSH fields in the southern Benguela region where the wavelet analysis is performodel simulation (2-day mean, February 2004). In both cases the black line shows the pofield derived from the SSH and an example of eddy detection is shown (b) for the observaNAM used for eddy census (see Table 1) and introduced at the end of Section 2.4. Isobathlocations of Lüderitz and Cape Town (CT) are also indicated.

Satellite SSH first illustrates the well-known major patterns inthe Agulhas area (not shown): the Agulhas Current (hereafter AC)flowing southwestward along the eastern coast of Africa and theAgulhas Retroflection (hereafter AR) around 15–20�E and 37–42�S. At smaller scales, intense eddy activity is also observed inconnection with the AC and AR. All year round, intense eddies(the so-called Agulhas rings), mainly anticyclonic, are shed fromthe area of the AR and drift northwestward along the southernBenguela slope region (Fig. 1a). In the Benguela area, a zone oflow SSH values between Cape Town (33.9�S) and Lüderitz(26.6�S) shows the propensity to the coastal waters to upwell overthe shelf (Fig. 1a). This signal is observed almost all year round, andduring the 13 years of analyzed satellite SSH. The most intenseupwelling tendency is observed within the Namaqua shelf cell, be-tween 28 and 33�S. Observations show high variability in the dif-ferent upwelling cells, with intensification and attenuation thatdo not always occur simultaneously. Cyclonic eddies are seen togenerate over the continental and shelf slope along the upwellingfront all year round (see Fig. 1b). These eddies, with diametersaround 100 km, drift westward during several weeks.

2.2. Model data

Realistic numerical simulations of the ocean circulation aroundSouthern Africa are obtained using the IRD version of the UCLA Re-gional Ocean Model System (ROMS), free surface, primitive-equa-

med from (a) satellite data altimetry (7-day mean around March 20, 2001) and (c)sition of the cross-sections drawn in Fig. 4. The surface geostrophic relative vorticitytions and (d) for the model. Black lines in (b) and (d) show the slope regions SLP ands corresponding to 200, 500, 1500, 2500, 3500 and 4500 m are plotted in white. The

290 A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295

tion ocean model (see Shchepetkin and McWilliams (2005), Speichet al. (2006), Doglioli et al. (2007), for a detailed description of thenumerical code). The whole model domain extends from 10�W to34�E and from 25.4�S to 50�S (see Fig. 1 in Speich et al., 2006) witha horizontal resolution of 1/10� (441 � 317 grid points). The gridresolves with an appropriate accuracy the fastest growing scalesfor baroclinic instability, seeing that the local first baroclinic Ross-by radius of deformation is about 30 km in the area (Chelton et al.,1998). The model has 32 vertical terrain-following levels (Song andHaidvogel, 1994) that allow increased resolution in areas of inter-est, such as the thermocline and bottom boundary layers. The bot-tom topography is derived from the 20 resolution Smith andSandwell (1997) database. Although the numerics in ROMS are de-signed to reduce the pressure gradient errors associated to the ver-tical coordinate system (Shchepetkin and McWilliams, 2003), thebathymetry field is filtered to keep the ratio of the gradient ofthe bathymetry to the depth of the ocean under 0.3 (Beckmannand Haidvogel, 1993). At the surface, wind forcing is obtained fromdaily QuikSCAT satellite scatterometer data gridded at ½� resolu-tion (IFREMER/CERSAT, 2002), and the heat and fresh waterssources are extracted from the COADS climatology (Da Silvaet al., 1994). At the lateral open boundaries, the model solutionis connected to the surroundings by an active, implicit, up-stream-biased radiation condition (Marchesiello et al., 2001). Un-der inflow conditions, the model solution is forced by the WorldOcean Atlas 1998 (Levitus et al., 1998) climatology within a nudg-ing border of 150 km. These configuration and forcings have beensuccessfully used in previous studies about the Agulhas currentsystem and the southern Benguela upwelling system (Speichet al., 2006; Doglioli et al., 2007).

2.3. First validation

Using this model configuration, 5 years of realistic simulation,from January 2000 to December 2004, are obtained. Model outputsare averaged and stored every 2 days of simulation. A first compar-ison between model fields and satellite data based on the multi-year average of the sea surface height (not shown) gives a goodagreement between real and simulated large-scale patterns, withthe signature of the AC flowing along the southeastern coast ofAfrica and the signal of the AR around 15–20�E. As observed forthe satellite SSH, simulations show significant eddy activity inthe Benguela zone, dominated at its southern edge by the north-westward drift of intense Agulhas rings.

In general, model mesoscale structures have realistic time andspace scales, although point-to-point misfits between model andobservations can be noted. Indeed, no data assimilation is used inthe numerical simulation and a significant proportion of the oceanmesoscale activity is related to internal variability. Therefore, themodel cannot reproduce observed eddies at the right time andlocation. On the other hand, it can represent, in a satisfactoryway, the large-scale ocean circulation and account properly forthe physical mechanisms related to the generation, evolution anddissipation of mesoscale structures. Agulhas rings generate in thearea of the AR and show realistic dimensions, with diameters be-tween 200 and 300 km. Their northwestward drift matches pathsfor observed equivalent structures (Van Ballegooyen et al., 1994).

Along the Benguela coast, modeled low SSH values are consis-tent with the observed sea level (Fig. 1c). The model reproducescorrectly the mean extension and intensity of the different upwell-ing cells in this area, keeping in mind that the horizontal resolutionof the satellite altimetry prevents observation of mesoscale struc-tures with diameters under 60 km and that onboard instrumentcorrections may lead to jumps in the estimated SSH at the ap-proach of the limit between sea and land. This results in the lackof reliable measurements over a band large of about a few tens

of km, which corresponds to a significant fraction of the continen-tal shelf in the case of the southern Benguela upwelling system.Generation of eddies, mainly cyclonic, over the slope and alongthe upwelling front is spotted all year round. Model eddies withdiameters around 100 km have dimensions similar to those ob-served by satellite altimetry (Fig. 1b and d). Their abundance andspatial distribution also show convincing similarities (see below),except for the northernmost part of the model domain wheremesoscale activity is partially damped in the model by the use ofnumerical open boundaries with an upstream-biased radiationcondition.

2.4. Wavelet analysis

We use the WATERS wavelet-based utility program (Doglioliet al., 2007) for identification of mesoscale eddies in the surfacegeostrophic velocity fields provided by satellite data and thenumerical simulation. The wavelet analysis allows 2D decomposi-tion of a signal into wave packets that have a position localized inspace. The main advantage of wavelets compared to Fourier trans-forms is that the former give information about a data set with re-spect to scale and location in contrast to the latter, which describethe global frequency content. For more details on the algorithmused here, the reader is referred to Doglioli et al. (2007).

In this new work, geostrophic surface velocities are computedfrom model data and satellite-derived observations with finite dif-ferences using:

Ug ¼ �gf@1@y

; Vg ¼gf@1@x

ð1Þ

where Ug and Vg are the zonal and meridional components of thegeostrophic surface velocity, respectively, g the gravity and f isthe Coriolis parameter. The sea elevation, f is deduced either fromobserved SSH maps or model free-surface height maps. Relativevorticity is calculated with finite differences using:

fg ¼@Vg

@x� @Ug

@yð2Þ

Then, the wavelet technique is used following two approaches:(i) a quantitative approach that consists in using the wavelet anal-ysis to identify coherent eddy-like structures in successive maps ofobserved and modeled surface relative vorticity fields and (ii) aqualitative approach that allows us to track with time and in 3Dsome of the modeled eddies. The first approach allows quantitativeestimates of eddy distribution, polarity and size and comparison inan objective way of the model eddy activity with that observed bysatellite altimetry. Then, the qualitative approach is applied to the3D model fields to gain knowledge of the vertical structure of thecyclonic eddies generated in the vicinity of the upwelling frontover the slope and of their main volume characteristics on theirway toward the open sea.

The results of the wavelet analysis are dependent on the vari-able used for eddy detection, on the analysis grid size and positionand on some other predefined parameters (Doglioli et al., 2007).We note here that the number of spectral coefficients kept for sig-nal reconstruction is a critical parameter of the analysis and ourbest results were obtained with a percentage of about 10%. Severaltests showed that the wavelet technique can work on maps ofeither SSH anomalies, potential vorticity or Okubo–Weiss criterion,but that processing relative vorticity gives excellent results interms of eddy identification (Doglioli et al., 2007). As shown inFig. 1, the Benguela upwelling system is under the influence ofthe northwestward drift of intense Agulhas eddies. They capturethe most energetic wavelets (those that are kept in the analysis),making the Benguela upwelling eddies, which are relatively smalland slowly rotating, partly disregarded. The best results are ob-

A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295 291

tained by using the wavelet analysis on observations- and model-derived surface geostrophic relative vorticity fields (from the sur-face geostrophic velocities provided by AVISO and the model freesurface slope, respectively) and an analysis grid centered on theBenguela continental slope (see Fig. 1). To make the analysis ofboth types of data equivalent, we modified the spatial resolutionand grid point spacing of both data sets: satellite fields (initiallyat 1/3� of resolution) have been oversampled to get one value every1/9� and model fields (initially at 1/10� of resolution) have beendegraded by applying a 2D running mean with a window size of3 � 3 grid points.

Fig. 1c and d show an example of eddy identification in ob-served and modeled fields. We note that most structures that aredetected are well-defined eddies, although a few patterns look likefilaments or meanders. To derive maps of eddy activity (see Section3), all eddy-like structures are delimited in space and are assigneda size and a vorticity sign, noting that only structures with diame-ters over 60 km are kept (again with the objective of making theanalysis equivalent for both types of data since satellite altimetryhas not enough resolution to sample smaller structures). Maps ofeddy activity including eddies with diameters under 60 km are alsoderived from the model fields to discuss the presence and role ofsmaller mesoscale structures in the slope area. The relative abun-dance of cyclones and anticyclones is addressed by means of eddypolarity maps, with eddy polarity defined as:

EP ¼ ðPant � PcycÞ=ðPant þ PcycÞ ð3Þ

where Pant (Pcyc) is the probability to find an anticyclonic (cyclonic)eddy.

For the statistics, eddy census (Section 3) is done for eddies over60 km in diameter located on the shelf slope, over depths between200 m (to avoid the lack of satellite data near the coast) and4500 m. Two areas for eddy census are considered: one (SLP) coversthe slope along the coast from 27 to 34�S and a second one (NAM) iscentered on the Namaqua cell (see black lines in Fig. 1a and c).

Finally, in order to track model slope cyclonic eddies in timeand volume, the 2D wavelet analysis is used every 50 m alongthe vertical direction and for successive time steps until the eddysignal is lost with depth or time (Doglioli et al., 2007). Trackingidentified eddies forward in time requires that the following crite-rion be satisfied:

Ct;z¼surface 2 et�Dt;z¼surface ð4Þ

where Ct,z=surface is the eddy center at the time t and at the sea sur-face and et�Dt is the set of grid points of the same eddy at the pre-vious time step (with Dt = 2 days in our analysis). The repetition ofthe analysis at successive depths is done with an equivalent crite-rion in order to ensure vertical tracking till the eddy signal in rela-tive vorticity becomes too weak to be detected (Doglioli et al.,2007).

Table 1Mean number of eddies and mean eddy size in the slope area (SLP, Fig. 1b) between 27 and 3obtained with the wavelet analysis applied to the full time series of satellite observations (satellite (OBS5y) and model data (MODEL).

Number

Total Anticyclones Cyclones

Mean Std. Mean Std. Mean

SLP OBS13y 7.90 2.17 3.99 1.50 3.92OBS5y 8.43 2.21 4.28 1.48 4.15MODEL 6.80 2.20 3.09 1.56 3.71

NAM OBS13y 3.57 1.66 1.56 1.10 2.01OBS5y 3.88 1.64 1.73 1.06 2.14MODEL 2.87 1.42 1.20 0.93 1.67

3. Mapping southern Benguela eddy activity

On average, for the 5 years of overlapping observational andmodel data, the mean number of eddies (cyclonic or anticyclonic)present over the slope at each time step is around 8.4 for the obser-vations and 20% lower (6.8) for the simulation (Table 1). The com-parison between the complete series of satellite SSH (13 years) andthe 5-year-long series of satellite SSH over the period shared bymodel data suggests that the period 2000–2004 is slightly more ac-tive (by 6.7%) in terms of cyclonic and anticyclonic activity. Themean numbers of cyclones and anticyclones are equivalent forboth 13- and 5-year-long series of observations whereas cyclonesin the simulation are slightly more frequent than anticyclones.Mean diameters around 100 km are obtained with all data sets.For the area around the Namaqua cell, the mean number of eddiesover the slope is around 4 for the observations and 2.8 for the sim-ulation, with a larger abundance of cyclones. Eddy diameters areslightly smaller than those obtained over the entire slope zone.The equatorward increase of the internal Rossby radius does nothave, here, a signature in the meridional organization of eddy size,because of the existence of larger Agulhas rings (200–300 km) atthe south of the slope domain in the model and in the satelliteobservations. No significant differences in size can be diagnosedbetween cyclones and anticyclones in the satellite SSH fields,whereas cyclonic eddies are about 10% smaller than anticyclonesin the simulation. The standard deviations account for varioustypes of variability: the size of eddies is latitude-dependent, andthis variability mixes with time fluctuations on different scales.This is the case of the larger slope region (SLP) that covers a widelatitude band. In the Namaqua zone (NAM), which covers a narrowlatitude band, most of the variability in eddy size is likely related totemporal effects.

The distribution of eddies over the slope varies indeed with lat-itude and depth and the concentration of eddies presents signifi-cant variability with time. Fig. 2 shows the eddy probabilitymaps for the 5-year common period of model and satellite data(January 2000–December 2004). The maps corresponding to the13-year series of satellite SSH have also been computed to get anestimate of eddy distribution over a longer term (not shown). Sinceno significant differences have been found between the 5- and 13-year-long series of observations, we only use the former for thiswork in order to set up a fair comparison between model and data.

Model and data eddy probability maps show reasonable agree-ment in terms of total, cyclonic and anticyclonic activity with theexception of the northern limit of the domain (between 27 and25.5�S). In this region (the southern part of the northern Benguelaupwelling system) the proximity of the numerical open boundaryprevents the model from representing appropriately mesoscaleactivity. In both cases (model and data), the highest activity is ob-served between 32 and 35�S. In this region, eddies originating in

4�S and in the slope area around Namaqua cell (NAM, Fig. 1d) between 28 and 32�S, asOBS13y) and the 5-year-long series, from January 2000 to December 2004 common to

Size (km)

Total Anticyclones Cyclones

Std. Mean Std. Mean Std. Mean Std.

1.28 101.5 14.4 101.5 20.8 103.8 19.11.43 102.3 12.2 102.7 18.4 104.3 17.11.30 102.5 15.0 103.7 23.5 102.4 19.5

0.99 94.6 17.9 94.2 24.0 96.3 21.51.06 95.9 15.9 96.8 23.0 96.5 19.20.94 94.7 22.3 101.3 32.3 93.3 24.7

Fig. 2. Eddy probability distribution within the southern Benguela upwelling region. (a, d, and g) Total, (b, e, and h) anticyclonic, and (c, f, and i) cyclonic eddy activityobtained from the wavelet analysis of satellite (top) and model (center and bottom) surface geostrophic relative vorticity fields. The eddy probability distribution in (a–f)considers only eddies with diameters over 60 km, whereas the distribution in (g–i) considers all eddies with diameters over 20 km. Color bars show the probability of findingan eddy at each grid point within the period 2000–2004. Isobaths corresponding to 200, 500, 1500, 2500, 3500, and 4500 m are plotted in white.

292 A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295

the AR area (cyclones and anticyclones) and drifting northwest-ward contribute for most of the variability. Satellite and modelshow a higher eddy activity over the deepest part of the slope(from 2500 m to 4500 m) and a relative maximum of anticycloniceddy activity south of 34�S along the 4500 m isobath correspond-ing to the drift of large anticyclonic structures propagating fromthe AR region.

In the northern part of the domain (from 26�S to 32�S) the activ-ity is mainly linked to the generation of eddies along the upwellingfront and to their subsequent drift toward the open ocean. Modeland observations show maximum cyclonic activity between 29�Sand 32�S coinciding with the Namaqua cell and a relative mini-mum of anticyclonic activity between 28� and 30�S. Besides thedifferences already mentioned near the northern limit of the anal-ysis window, model eddy activity is slightly less intense on thewhole (with eddy probabilities about 10% lower). Both data sets

present low eddy activity over the shelf and the upper slope, atdepths shallower than 2500 m.

The eddy polarity map for the observations shows two areasover the slope where cyclones are more abundant (Fig. 3a): thelower slope around 31�S and 29�S (around the Namaqua upwellingcell), where cyclones dominate between 500 and 4500 isobaths,and the upper slope (above ocean bottoms shallower than1500 m) between 34�s and 30�S, where anticyclones are almost ab-sent. For the model simulation, a clear cyclonic signal suggestsequally a dominance of cyclones between 30�S and 27�S abovethe lower slope (Fig. 3b). South of 30�S, the cyclonic signal coversthe slope between isobaths 1500 and 3500 m. For the model, therelative concentration of cyclonic structures seems more impor-tant on the whole, as shown by the statistics given in Table 1.

It is worth noting that these results apply to eddies with diam-eters over 60 km, since the main motivation of this study is the

Fig. 3. Eddy polarity in the study region for (a) the observations and (b and c) the model. Eddy polarity in (a and b) deals only with eddies with diameters over 60 km, whereaseddy polarity in (c) deals with all eddies with diameters over 20 km. Isobaths corresponding to 200, 500, 1500, 2500, 3500, and 4500 m are drawn in white.

A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295 293

comparison of comparable features, in the model and in the obser-vations. An analysis allowing the detection of smaller structures(down to 20 km in diameter), done only for the model, leads ofcourse to increased eddy concentration over the whole domain.Over the slope between isobaths 500 and 1500 m, the concentra-tion of cyclones is for instance significantly increased (Fig. 2g–i).However, the contribution of small eddies to eddy polarity(Fig. 3c) or to the time variability of eddy distribution (not shown)is minor.

More generally, no obvious seasonal signal can be identified inthe observed or modeled time series of eddy concentration andsize for the entire slope. This was an expected result since eddieswith different polarities, origins and characteristics coexist overthe southern Benguela slope. To refine the analysis we looked atseasonal variations in the number of observed and modeled cyclo-nic eddies locally generated over the slope around the Namaquaregion, where we expected to find a seasonal signal linked to vari-ations of the upwelling front intensity. In this case, we do find aweak seasonal signature with a maximum activity at the end ofsouthern summer. This signal is, however, of the same order ofmagnitude as interannual variations (i.e., the seasonal variationof the monthly means is of the same order of magnitude asmonthly standard deviations) and may not be strongly significant.

Fig. 4. Vertical structure of one modeled southern Benguela cyclonic eddy over theslope (structure with a ‘‘+” label in Fig. 1d; cross-section in Fig. 1c). (a) SSH signal incm/s. The dotted curve shows the SSH signal of one equivalent eddy (‘‘+” label inFig. 1b; cross-section in Fig. 1a) observed from satellite altimetry with dimensionssimilar to the modeled eddy. (b) Vertical section of model meridional velocity in m/s. (c) Vertical section of model T (�C) and S (psu) fields. The red line represents thewestern and eastern edges of the eddy for all depths considered by the waveletanalysis.

4. Volume properties of model southern Benguela cycloniceddies

To illustrate the vertical structure of southern Benguela slopecyclones (Fig. 4), a vertical section crossing a model cyclonic eddygenerated a few days beforehand at the upwelling front has beenextracted as indicated in Fig. 1b. The agreement of the model eddySSH signal with that corresponding to an observed cyclonic eddywith similar dimension and position over the slope (see cross-sec-tion in Fig. 1a) assesses the model ability to reproduce coherentstructures (Fig. 4a). Both eddies, which are located over the2500 m isobath, have a surface signature of around 120 km indiameter associated with a negative SSH anomaly. They are bothflanked on their coastal side by an area of low SSH related to thecoastal upwelling and on their open-sea side by an intense anticy-clonic structure that induces an offshore positive SSH signal.

The model velocity field shows that the eddy has a cyclonicstructure that reaches depths down to 1200 m (considering the0.05 m/s isoline as a reference), with maximum velocities around0.4 m/s at 20 m. The cross-shore vertical section of the meridionalvelocity shows that the eddy is interacting with the bathymetry onits coastal side, where it presents weaker dynamics (Fig. 4b). On itsopen-sea side, northward velocities are reinforced by the presence

Fig. 5. Trajectories of the centers of the six cyclonic eddies introduced in Table 2, ascalculated by the wavelet analysis. Isobaths are contoured with a 500 m interval.

294 A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295

of the anticyclone. In terms of temperature and salinity, the dom-ing observed in the vertical distribution of these variables suggeststhat the eddy core is composed of low temperature and low salin-ity waters with respect to its surroundings (Fig. 4c). The uplift ofthe temperature and salinity structures is observed until 1200 m.The temperature drives the distribution of the density, with theeddy consequently characterized by a high-density core.

This eddy and five other model cyclones (selected among thepopulation of cyclonic eddies generated along the upwelling frontand crossing the 10�E meridian) have been tracked along their pathtoward the open sea using 3D wavelet analysis (see Section 2). Ta-ble 2a shows their mean volume properties calculated within theperiod between their generation in the vicinity of the upwellingfront and their entrance in the open sea region that we associatehere with the crossing of the 10�E meridian. Table 2b shows equiv-alent quantities except for the whole eddy lifetimes. In the model,the vertical extension of eddies is calculated as the average of thesuccessive maximum depths of detection given by the waveletanalysis (note that the extension is accurate to 50 m). Diameter,volume and centre velocity are the time averages of correspondingsuccessive volume-averaged quantities. The model permits thedetermination of instantaneous drift velocities that cannot be diag-nosed easily from the successive maps of sea level obtained byaltimetry (because its coverage has a poorer resolution). The driftvelocities are in the range of direct estimates based on the dis-placement of surface drifters. For instance, Richardson (2007)found a mean translation velocity of 9.6 cm/s for seven of theirdrifters looping in the same region.

The trajectories followed by these eddies are occasionally farfrom being a straight line (Fig. 5). This explains the differences be-tween the mean instantaneous velocity and the average velocityobtained by dividing the beeline-traveled distance by the trackingtime. The main variations in instantaneous velocities are associ-ated with not only interaction with the mean flow or bathymetricfeatures, but also on eddy–eddy interactions (mostly within thehighly energetic corridor formed by Agulhas rings drifting north-westward along the slope edge). For example, an eddy generatedin February 2004 was seen to interact during several days withan Agulhas Ring around 8�E, 32�S, which made it stretch and inten-sify, and thus to present locally larger drifting velocities (notshown). It is worth noting that instantaneous velocities are alsosensitive to possible eddy reshaping, as detected by the wavelet

Table 23D characteristics of southern Benguela model cyclonic eddies along their pathwayfrom the continental slope to the open sea. The mean instantaneous velocity, Vi, iscalculated as the average of instantaneous velocities (computed every 2 days) and themean drift velocity, Vd, is the beeline distance between the initial and final positionsdivided by the eddy lifetime. (a) Time tracking to 10�E. (b) Full time tracking (overeach eddy lifetime).

Startdate

Timetracking(days)

Depth(m)

Diameter(km)

Volume(m3)

Vi

(m/s)Vd

(m/s)

(a)13/06/2000 122 736 68 3.25 � 1012 0.04 0.02711/04/2001 44 444 107 4.52 � 1012 0.12 0.05803/10/2001 78 417 78 2.55 � 1012 0.05 0.04609/02/2002 44 543 100 3.84 � 1012 0.14 0.04626/09/2003 78 496 104 4.39 � 1012 0.05 0.04813/02/2004 116 695 78 3.84 � 1012 0.04 0.037

(b)13/06/2000 284 586 90 4.22 � 1012 0.06 0.02311/04/2001 88 594 115 6.81 � 1012 0.12 0.06903/10/2001 90 402 75 2.37 � 1012 0.05 0.04609/02/2002 272 800 92 6.23 � 1012 0.07 0.03026/09/2003 224 556 110 4.74 � 1012 0.06 0.04713/02/2004 190 645 96 4.65 � 1012 0.12 0.080

analysis, leading to sudden relocation of its center (defined bythe maximum of relative vorticity).

The total tracking time gives an idea of the lifetime of these ed-dies and varies from 1 month and a half to more than 4 months.After they are generated in the southern Benguela upwelling re-gion cyclones drift westward, and mostly towards the south, reach-ing longitudes far away from the coastal region. These eddies havemean vertical extensions between 400 and 800 m and mean diam-eters between 70 and 150 km over the slope. Their structure canendure major changes after they leave the slope as a result ofstrong interactions with other cyclonic and anticyclonic structures.Their volumes over the slope, when they are transported cross-shore at mean velocities between 0.04 and 0.14 m/s, range from2.4 to 6.2 � 1012 m3, suggesting that these eddies contribute signif-icantly to coastal–open sea fluxes.

5. Conclusions

Small and large eddies define a continuum of scales that all takepart in the structuring of the large-scale circulation such as theSouth Atlantic subtropical gyre. Though they are less easily ob-served and quantified, smaller mesoscale structures are nonethe-less more numerous than larger eddies and are likely tocontribute significantly to large-scale heat and freshwater and bio-geochemical exchanges. The thorough assessment of this process isan essential issue in fine resolution numerical modeling. The maingoal of this study was to characterize eddy activity in the vicinity ofthe slope in the southern Benguela upwelling system. We used awavelet analysis that allowed us to quantify eddy activity from sa-tellite altimetry observations and to validate the eddy activitypresent in a 5-year-long realistic simulation of the ocean regionaround southern Africa.

The model-altimetry comparison shows that model eddy activ-ity has correct spatial distribution and that modeled eddies havecorrect sizes and paths. However, the model slightly underesti-mates the eddy activity (mainly to the north of the domain dueto the proximity of the model open boundaries). Moreover, thisstudy gives an insight into the space and time variability of south-ern Benguela cyclones and anticyclones in relation with two mainphysical features coexisting in this region: the northwestwarddrifting of AC rings along the slope at the southwest edge of thestudy area and the dynamics of the upwelling cells north of 32�S.We have shown that between 26�S and 32�S the eddy activity ismainly linked to the generation of cyclonic eddies along theupwelling front (Namaqua cell) and their subsequent drift towardthe open ocean. The analysis deals with observed and modeled ed-dies with diameters over 60 km and shows significant activity overimmersions deeper than 2500 m. The simulation shows that eddyactivity over shallow ocean floors (between 500 and 2500 m) is re-

A. Rubio et al. / Progress in Oceanography 83 (2009) 288–295 295

lated to smaller structures with diameters over 20 km. The analysisfor smaller eddies shows an increase in the number of structuresover the whole domain. The main differences between probabilitymaps obtained for large eddies and small ones are mainly relatedto the fact that larger (and deeper) eddies are located over deeperfloor.

A visual examination of relative vorticity maps indicates thatthe generation of cyclonic eddies mostly happens along theupwelling frontal area and starts at the level of isobaths 500–2500 m. Eddies are observed to develop there for some days whilestarting to drift westwards until they leave the upwelling zonewith roughly stabilized properties. The presence of cyclonic eddiesover the slope is recurrent throughout the year, with on average 2cyclonic eddies found over the slope around the Namaqua cell. Noclear seasonal signal is observed in the model nor in the observa-tions due to the coexistence over the slope of eddies of differentorigin and dynamics and also to significant interannual variability.Southern Benguela model cyclonic eddies drift westward aftertheir generation with volumes between 2 and 4.5 � 1012 m3, whichsuggests that these eddies may play a significant role in the coast-al–open ocean exchanges. With lifetimes of several months, theyare able to export far from the coast upwelled high productivewaters and may play a major role in structuring the ecology ofthe region. In coastal upwelling systems, nutrient content of thesurface water is largely influenced by the upwelling of cold andnutrient subsurface waters. However, the effect of mesoscale fea-tures such as small eddies and filaments is gaining increasingattention because of their role in shaping the dynamical structureand the biogeochemical functioning of these systems (Capet et al.,2008).

This study provides statistics about eddy activity in the south-ern Benguela upwelling region and shows the skills of the modelconfiguration in simulating the surrounding mesoscale activity.Ongoing work is dedicated to the quantification of the cross-shoreexport actually achieved by these eddies and of the physical pro-cesses related to their generation. The response of the mesoscalefield and cross-shore transports to the different physical forcingsat play is also worth investigating. Lagrangian advective tracingof numerical particles in the velocity field calculated by the regio-nal model together with the automation of the process of eddy 3Dtracking are used for this purpose, and also for a simulation of theocean around Southern Africa at a slightly improved resolution (1/12� instead of 1/10�) and over a domain extended equatorward tolessen the impact of the open boundary conditions on the modeledsouthern Benguela upwelling circulation.

Acknowledgements

The authors wish to thank Guillaume Dencausse, Andrea Dogl-ioli and Guillaume Lapeyre for useful discussions about eddydetection and tracking, and Marie-Paule Friocourt for her assis-tance in correcting sections of the manuscript. For this study, A.Rubio is supported by a CNRS postdoctoral fellowship. This studyis a contribution to InterUp, a project funded by the French LEFE-IDAO program (Les Enveloppes Fluides et l’Environnement, Inter-actions et Dynamique de l’Océan et de l’Atmosphère) and byIFREMER.

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