Deep-Sea Research II 85 (2013) 182–194

Contents lists available at SciVerse ScienceDirect

Deep-Sea Research II

0967-06

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/dsr2

Direct observations of the upper layer circulation in thesouthern Gulf of Mexico

Paula Perez-Brunius n, Paula Garcıa-Carrillo, Jean Dubranna, Julio Sheinbaum, Julio Candela

Departamento de Oceanografıa Fısica, Centro de Investigacion Cientıfica y de Educacion Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas,

Ensenada, BC 22860, Mexico

a r t i c l e i n f o

Available online 26 July 2012

Keywords:

Bay of Campeche

Wind-driven circulation

Topographic steering

Equivalent barotropic

Cyclonic gyre

Loop Current Eddies

45/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.dsr2.2012.07.020

esponding author. Tel.: þ52 646 175 050024

ail address: brunius@cicese.mx (P. Perez-Brun

a b s t r a c t

The upper layer circulation in the Bay of Campeche is analyzed with three years of data recorded by

surface drifters, current meter moorings, and satellite altimetry. The measurements show that the

mean cyclonic circulation observed by previous authors extends below 1000 m, and that its size and

location are delimited by the particular topography of the region: a deep basin to the west, and a

shallower and gentle sloping submarine fan to the east. An Empirical Orthogonal Function analysis and

large correlations of the surface flow with the deeper currents suggest that the topographic constraint

is the result of potential vorticity conservation for an equivalent barotropic flow. The variability of the

surface currents in the western basin is mostly due to changes in the size, form, position and intensity

of the cyclonic gyre due to its interaction with northern Gulf of Mexico eddies, particularly Loop

Current Eddies traveling the southern route towards the western boundary. By contrast, the eastern

basin is characterized by a weak northward drift, with the occasional generation of anticyclones in the

southeastern boundary, the genesis of which remains to be understood. This suggests that the

variability in the eastern basin is mostly driven by locally generated disturbances, rather than by an

influx of northern Gulf of Mexico eddies. Strong northward flows in the central and eastern basins

result from the flow convergence between locally generated anticyclones and the cyclonic gyre.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The Bay of Campeche (BOC) is the southern-most semi-enclosed region of the Gulf of Mexico, bound by the coast ofMexico to the west, south and east, and open towards the north at221N. It is well known that the dynamics of the upper-layer in thenorthern Gulf of Mexico (NGOM) is dominated by the LoopCurrent and the westward migration of energetic Loop CurrentEddies (LCEs) and associated cyclones (e.g. Elliot, 1982; Hamiltonet al., 1999; Oey et al., 2005; Schmitz, 2005), superimposed on amean wind-driven anticyclonic gyre (e.g. Blaha and Sturges,1981; DiMarco et al., 2005; Sturges, 1993). By contrast, theanalysis of the paths followed by Loop Current Eddies towardsthe western boundary made with 26 years of satellite and in situdata by Vukovich (2007) shows that only 14% of the Loop CurrentEddies follow a southern path (i.e. spend more than 75% of theirtime south of 241N).

Vazquez de la Cerda et al. (2005) review the circulation in theBOC with the oceanographic observations available at the time,

ll rights reserved.

044.

ius).

presenting strong evidence of a mean cyclonic gyre, likely forcedby the positive wind stress curl that prevails in this regionthroughout the year (Blaha and Sturges, 1981; Gutierrez deVelasco and Winant, 1996). The evidence presented is a low inthe mean dynamic height (relative to 800 m) obtained from thescarce hydrographic data available in the region, and the meanflow derived from 10 years of surface drifters deployed in theNGOM that reached the BOC (DiMarco et al., 2005; Vazquez de laCerda et al., 2005).

Enough drifters entered the BOC to estimate the mean velocityand its variance in wintertime, but fewer drifters entered the regionthe rest of the year. Hence, the mean surface circulation, particu-larly in springtime, could not be resolved (DiMarco et al., 2005).Their results show the presence of a western boundary current atleast in winter, and some indication that the cyclonic circulation ismore intense in fall–winter than in spring–summer, in phase withthe seasonal variation of the wind-stress curl (Gutierrez de Velascoand Winant, 1996). From these results, Vazquez de la Cerda et al.(2005) conclude that the cyclonic gyre is wind-driven, and that thegeostrophic transport estimated from the hydrographic data isconsistent with the Sverdrup transport estimated from the magni-tude of the mean wind-stress curl. Nevertheless, Ohlmann et al.(2001, their Plates 6 and 7) show that the effect of eddies are at

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

20

40

60

80

Speed (m/s)de

gree

s of

free

dom

mean [W,E] = [0.31 , 0.24] m/sstand. dev. [W,E] = [0.18 , 0.16] m/smax. registered [W,E]= [1.83 , 1.19] m/s

EastWest

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0

20

40

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Month of the year

98°W 97°W 96°W 95°W 94°W 93°W 92°W18°N

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23°N

<0.20 m/s0.20-0.35 m/s0.35-0.50 m/s0.50-0.65 m/s0.65-0.80 m/s0.80-0.95 m/s>0.95 m/s

P. Perez-Brunius et al. / Deep-Sea Research II 85 (2013) 182–194 183

least equally important as surface wind stresses in driving Gulf ofMexico circulation on long time scales.

In addition, some studies suggest that a LCE collapsing againstthe western boundary influences the cyclonic circulation in theBOC. Based on numerical results, Romanou et al. (2004) mentionthat the generation of cyclonic eddies in the western BOC iscorrelated with the arrival of Loop Current anticyclones. Fromhydrographic data, Vidal et al. (1992) concluded that the collisionof a LCE with the southwestern continental shelf led to a transferof mass (Sutyrin et al., 2003) and angular momentum to thesouth, thus generating a cyclonic eddy in the BOC.

Vazquez de la Cerda et al. (2005) address the non-seasonalvariability of the surface currents in the BOC with eight years ofaltimetry sea-level anomaly data, concluding that it is mainly dueto a net influx of NGOM eddies, with little contribution fromlocally generated perturbations. Their Empirical Orthogonal Modeanalysis suggests that the eddy flux is in the form of smallercyclones and anticyclones entering into the BOC, which theyhypothesized are generated when Loop Current Eddies collidewith the western continental slope.

In this study we analyze the seasonal and intraseasonalvariability of the upper layer circulation in the BOC using threeyears of data from current meter moored arrays and surfacedrifters deployed in the region, complemented with absolute sealevel and geostrophic currents, estimated from altimetry data.First, the seasonal variability of the surface currents and thehorizontal and vertical extension of the cyclonic gyre areexplored. We continue with the intraseasonal variability of thecurrents produced by the presence of three NGOM anticyclonesthat influenced the northern boundary of the BOC: two energeticLCEs (Cameron and Darwin), and a less intense anticyclone (AC1).The role played by locally generated meso-scale structures in thevariability of the BOC currents is also addressed. We finish with adiscussion of how these new results compare to previous studies,and the importance of bottom topography in determining the sizeand position of the cyclonic gyre via conservation of potentialvorticity for an equivalent barotropic flow.

2007-2008

2008-2010

2007-2010

LNK 500-2000

IT1

500-

2000

CTZ

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-200

0

IT 2 500-2000

CAP500-2000

98°W 97°W 96°W 95°W 94°W 93°W 92°W18°N

19°N

20°N

21°N

22°N

Fig. 1. (A) Hourly drifter positions color coded by speed from September 2007

through September 2010. Monthly deployments of three–five drifters took place

south of 20.51N, most of them in the positions shown by the black circles. (B)

Histograms of speed (top) and time of the year (bottom) of the hourly drifter data

for the regions west and east of 941W, expressed in degrees of freedom for an

integral time scale of 8 days (see text). (C) Position of the current meter mooring

arrays. Isobaths are shown at 200, 1000, 2000 and 3000 m.

2. Data and methods

2.1. Surface drifters

Starting with an initial deployment of seven drifters in lateSeptember 2007, groups of three–five surface drifters aredeployed by air every month south of 20.51N, with 174 drifterssuccessfully launched by the end of August 2010. We use FarHorizon Drifters (FHD, Horizon Marine Inc.), which consist of acylindrical shaped buoy hull (96.5 cm long and 12.4 cm indiameter) with a 45 m nylon tether line attached to a 1.2 mdiameter ‘‘para-drogue’’, that serves both to protect the buoywhen air-deployed as well as a drogue to reduce slippage of thebuoy in the water. They measure hourly positions with a GPSreceiver, and transmit the data via Argos (Anderson and Sharma,2008). A central difference scheme estimates the velocity at eachdata point. The hourly data were subject to a double qualitycontrol process: an automatic quality control interpolated posi-tions between data gaps and removed locations over land, pointswith speeds exceeding 3 m/s and interpolated positions betweengaps larger than 6 h long. A visual examination of the first qualitycontrol data for each individual drifter removed points withconspicuous peaks in speed or evidently bad positions, resultingfrom bad fixes from the GPS receiver, tracks on aircraft previousto deployment, or data collected after buoys were evidentlypicked up by a vessel. The hourly drifter data are shown inFig. 1(A and B). We note that the drifters used have not the same

design as the Surface Velocity Program (SVP) drifters, the latterbeing the standard Lagrangian instruments used to measure thenear-surface velocity field in oceanography (see review of

Fig. 2. Seasonal statistics of the velocities at 40 m. Standard deviation ellipses

from the binned drifter (blue) and mooring data (cyan) are shown for (A) winter

and (B) summer. The mean velocities for the binned drifter data (black) and

current meter moorings (gray) are centered on the corresponding ellipses. Mean

currents significantly different from zero at the 5% significance level are shown in

red. Isobaths are shown at 200, 2000 and 3000 m.

P. Perez-Brunius et al. / Deep-Sea Research II 85 (2013) 182–194184

Lumpkin and Pazos, 2007). A discussion addressing the waterfollowing capabilities of the FHD drifters can be found in the(Appendix A).

The seasonal mean and standard deviation of the surfacevelocity field were estimated by binning the three-year longdrifter dataset on 0.51�0.51 bins with 0.251 overlap. Marchthrough August (September through February) is considered asthe summer (winter) season, which corresponds to the seasonalvariation in the direction of the winds (Zavala-Hidalgo et al.,2003). Following DiMarco et al. (2005), an integral time scale of 8days was used as a proxy for statistical independence of thebinned drifter data, and the degrees of freedom for each bin wereestimated as the number of observations within the bin dividedby the number of observations needed to equal the integral timescale. We excluded bins with less than 5 degrees of freedom (960hourly observations) from the analysis. The results are discussedin Sections 3.1 and 3.2.

2.2. Current meter moorings

An intensive current measurement program in the BOC wasstarted in November 2007 with an initial deployment of sixmoorings, complemented 8 months later with four additionalmoorings (Fig. 1C). The moorings are deployed in waters 500 and2000 m deep and are instrumented throughout the water column.All moorings are equipped with an upward looking 300 kHz ADCPat around 120 m that profiles from this depth to the surface, with8 m bins sampled every half hour. In addition, a downward-looking 75 kHz ADCP profiles currents from 120 m down to about700 m, with 16 m bins sampled every 30 min. The mooringsdeployed in 2000 m have an additional downward-looking75 kHz ADCP at 700 m profiling currents from this depth downto 1300 m. Below this depth, the deep moorings are equippedwith point measuring current meters deployed around 1300,1500 and 1750 m. The mooring array is currently in operation,serviced yearly, and it is to be maintained until the summerof 2013.

The velocity time series were low pass filtered (48 h cut-offfrequency) and linearly interpolated every 20 m from the surfaceto the maximum depth measured. The vertical coherency of theflow was addressed by two methods. First, complex correlationcoefficients between the velocity measured at 60 m and velocitiesat deeper depths were calculated following Kundu (1976). Thereference level is the shallowest depth at which all the mooringshad measurements during a minimum of 9 months. Second,Empirical Orthogonal Functions (EOFs) for the current profile atthe deep moorings were calculated using the low-pass filteredand vertically interpolated time series. In the first EOF analysis,the data were considered with their mean included, i.e. totalsubinertial currents. Hence, the EOF modes were obtained max-imizing the total kinetic energy of the flow. To address the mainmode of variability of the currents, a second EOF analysis wasperformed removing the mean velocity from the time series,maximizing the current’s variance. The results are discussed inSection 3.4.

2.3. Sea surface height

As a source of sea level height variability, use is made of theabsolute altimetric data and related geostrophic currents fromAVISO, which include all the altimetric observations from the endof 1992 to the middle of 2010. It is important to note that thespatial resolution of the altimetric fields is 1/31, approximately30 km, and the time interval between fields is 7 days (althoughinterpolated daily fields are also available), therefore thealtimetric data only show variability related to the meso- and

large-scales. This product from AVISO contains as reference amean dynamic topography (MDT CNES-CLS09) determined at thebeginning of 2010, which considers a new geoid based on4.5 years of gravimetric measurements from the GRACE project,drifter data from 1993 to 2008, Ekman drift corrections and CTDobservations from 1993 to 2008 (for further details please refer tothe AVISO web page http://www.aviso.oceanobs.com/).

3. Results

3.1. Pseudo-Eulerian statistics from drifters vs. current meter data

Fig. 2 shows the seasonal mean and standard deviation of thesurface velocity field estimated from the binned drifter data. Alsoshown are the corresponding statistics of the velocity obtainedfrom the half-hourly, unfiltered, current meter mooring data at40 m.

The statistics derived from both datasets generally agree bothin the direction and speed of the mean flow, as well as on themagnitude of the variability of the surface currents, particularly

Fig. 3. (A) 4-day trajectories of the drifter data color coded by start region. The start of the trajectory is marked with a filled circle. Isobaths are shown at 200, 1000, 2000

and 3000 m. (B) Mean and standard deviation ellipses for the current meter data at 1200 m, mean currents significantly different from zero at the 5% significance level are

shown in red. The filled square at the position of the mooring is color coded by the maximum depth at which the velocities remain highly correlated with those at the

surface. Also shown are f/Fo contours ([7.8:0.1:8.5 9.5]�10�8 m�1s�1) with f the local Coriolis parameter, Fo¼Ho(1�e�H/Ho), H the bottom depth and Ho¼650 m. See

details in Section 4.1.

P. Perez-Brunius et al. / Deep-Sea Research II 85 (2013) 182–194 185

south of 21.51N. In some cases, such as in the southwesternregion, even the orientation and eccentricity of the standarddeviation ellipses of both datasets match closely. A statisticalanalysis showed no difference between the mooring and drifterderived means and standard deviations along both principal axes(5% significance level, see Appendix B).

Given the uncertainties in the drifter velocities (see Appendix A),and that the drifter data are unevenly spaced in time and space andaveraged over a large geographical area, this result gives confidenceon the representativeness of the drifter derived pseudo-Eulerianstatistics. Hence, the drifter data seem to give a good approximationto the mean surface flow and its variability in the regions absent ofcurrent meter moorings.

3.2. Mean and seasonal variability

The main feature that stands out in both seasons in thestatistics of the surface currents in Fig. 2 is a well defined cyclonicgyre, located on the western BOC (center near 201N and 951W),which extends from the southwestern boundary to 21.51N to thenorth and 941W to the east. Note that the mean currents in bothseasons at all moorings are significantly different from zero at the5% significance level (see Appendix B for details). The exception isthe easternmost mooring, where the mean flow is zero in bothseasons. The binned drifter data show means significantly differ-ent from zero in the cyclonic gyre in summertime, and in thesouthwestern boundary in wintertime.

No significant differences in the mean currents were foundbetween winter and summer in either mooring or drifter data,although the mean values suggest a slight intensification in thewestern boundary during wintertime (maximum mean currentsof 29–34 cm/s). Otherwise, the mean currents around the gyrehave about the same magnitude during both seasons (11–26 cm/s).The eastern basin generally has weaker mean currents (o15 cm/s),particularly in the summer, when they do not exceed 10 cm/s.

The standard deviation ellipses of the surface currents aresignificantly different between winter and summertime at all themoorings at the 5% significance level. No significant differenceswere found for the drifter derived statistics, likely due to thesmall number of degrees of freedom compared to the mooringdata. Nevertheless, the size of the wintertime variance ellipses islarger than in the summer in 94% of the binned drifter data.

3.3. Topographic influences

The position and size of the cyclonic gyre are determined bythe particular configuration of the isobaths in the region, whichconsists of a deep western basin with a steep continental shelfbreak, and a shallower, rougher, and gentle sloping ocean floor inthe eastern BOC that results from the fanning out of the isobathseast of 94.51W (see bathymetry of the region in Fig. 1C).

The topographic confinement of the cyclonic gyre shows upmore clearly in the independent drifter trajectories shown inFig. 3(A). Drifters located south of 211N and west of 941Wcirculate around the cyclonic gyre, while east of 941W, the driftertrajectories are shorter and more erratic, imbedded in a broadnorthward drift. In the cyclonic gyre (red, blue and magentatrajectories in Fig. 3A), between 40% and 50% of the 4-daytrajectories are longer than 120 km (mean daily speeds435 cm/s),while only 20–30% of the drifters in the eastern region traveled thatfar (yellow and cyan trajectories in Fig. 3A). On the other hand, lessthan 20% of the trajectories in the cyclonic gyre were less than70 km long (mean daily speedso20 cm/s), while in the easternregion the percentage lies between 25% and 35%.

3.4. Vertical coherency of the flow

The magnitude of the complex correlation between the surfacevelocity and the velocity at a lower depth is considered as aqualitative measure of the vertical coherence of the flow. Fig. 3(B)shows the depth at which the magnitude of the complex correla-tion coefficient calculated from the mooring data exceeds 70%,which we used as a proxy for the maximum depth at whichvelocities are still correlated with the flow at 60 m. See Table 1 fora summary of the magnitude of the correlations, 95% confidencelimits and 5% significance levels. The moorings within thecyclonic circulation in the western basin consistently show adeeper coherent upper layer (700–1000 m thick). By contrast, themooring located in the eastern basin suggests a thickness of about260 m. Fig. 3(B) also shows the mean and standard deviationellipses of the velocity field at 1200 m, which are consistent witha cyclonic mean flow west of 941W, while the mean flow is notsignificantly different than zero in the eastern basin.

To further address the vertical coherency of the flow, EOFs forthe current profile at the deep moorings were calculated using thesubinertial velocity time series. Fig. 4 shows the eigenvector and

Table 1Results related to the magnitude of the vectorial correlation between the currents at 60 m and the currents at depth H: r( %u(60), %u(H)), including 95%

confidence intervals and 5% significant levels. Hbot—depth of the deepest measurement, Hr40 maximum depth for which r is significantly different than

zero, Hr40.7—maximum depth for which r4¼0.70.

Mooring Hbot (m) Hr40 (m) r( %u(60), %u(Hr40))

95% conf. interval

r( %u(60), %u(Hr40))

5% sig. level

Hr40.7 (m) r( %u(60), %u(Hr40.7))

95% conf. interval

r( %u(60), %u(Hr40.7))

5% sig. level

LNK 2000 2000 [0.39 0.40] 0.33 820 [0.70 0.71] 0.33

IT1 1780 1780 [0.47 0.49] 0.40 1020 [0.70 0.71] 0.63

CTZ 2000 1860 [0.18 0.21] 0.26 660 [0.70 0.72] 0.50

IT2 2000 2000 [0.38 0.41] 0.33 260 [0.70 0.71] 0.13

Fig. 4. Eigenvector (left panels) and principal component (right panels) of the first Empirical Orthogonal Function for three of the 2000 m deep current meter moorings. (A, D)

Mooring in the southwestern boundary, (B, E) central basin mooring and (C, F) eastern basin mooring. The percent of the total kinetic energy represented by the first EOF is shown

at the upper-right corner in panels (A–C).

P. Perez-Brunius et al. / Deep-Sea Research II 85 (2013) 182–194186

principal component of the first EOF mode of the total velocitymeasured at the 2000 m moorings located at the western, centraland eastern BOC. In the cyclonic gyre, the first mode explains66–80% of the total kinetic energy of the flow (Fig. 4A and B),while only 42% is explained in the eastern basin (Fig. 4C). More-over, the direction of the eigenvector in the western moorings(IT1 and CTZ) varies less than 51 on the top 800–1000 m, while atthe eastern mooring (CAP), the eigenvector rotates clockwisefrom top to bottom, achieving a 301 turn from the surface downto 1000 m. Below 1000 m, the eigenvector rotates clockwise 20–301 towards the bottom in both western moorings, although atthose depths, the magnitude of the mode has decreased below5 cm/s. Finally, the principal component of the first EOF of thewestern moorings remains positive 90% of the time (Fig. 4D–F),further supporting the persistence of the cyclonic circulation inthe western basin. Adjusting the function v(z)¼v(0)ez/Ho to thevertical profiles of the first EOF mode results in an e-folding scaleHo�650 m.

An additional EOF analysis removing the mean flow (notshown) resulted in a first mode explaining more than 40% ofthe variance. In the case of the western moorings (IT1 and CTZ),

the eigenvector is unidirectional and has the same orientation(within 201) as the mean flow and the corresponding EOFeigenvectors for the total velocity (Fig. 4A and B). This is not thecase for the eastern mooring (CAP), where the correspondingvertical profiles are not unidirectional and the mean direction ofthose fields differs up to 401.

The EOF analysis provides further evidence that the cyclonicgyre is a vertically coherent and nearly unidirectional flowthroughout the water column, whose main mode of variabilityoccurs along the same direction as the mean flow.

3.5. Intraseasonal variability

3.5.1. NGOM anticyclones

The statistics shown in Fig. 2 indicate that the fluctuating fieldis generally larger than the mean flow. Nevertheless, the cyclonicgyre is a persistent feature present in the western BOC, which canbe seen clearly from the trajectories of drifters and the sea levelmost of the time, and from the EOF analyses of Section 3.4. Wewill refer to this setup as the ‘‘unperturbed state’’ of the cyclonicgyre, that is when the drifter trajectories and sea level show a

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clear circulation in the entire deep western BOC basin, west of941W. The drifter observations combined with the altimetry datasuggest that the variability of the surface currents in the westernbasin are mainly due to changes in the position, size, form, andintensity of the cyclonic gyre. One of the most obvious processesthat give rise to this variability is the influence of energetic LoopCurrent Eddies following the southern route presented byVukovich (2007) and then impacting on the western boundary.The following sub sections analyze three examples of NGOManticyclones influencing the circulation of the BOC.

Fig. 5. Effect of LCE Cameron on the cyclonic circulation. Each frame shows 15-day

drifter trajectories, color coded by speed, superimposed on the mean absolute sea level

from AVISO of the last 15 days of the month. An overall spatial and temporal mean was

substracted from the sea level data. The sea level contours are shown every 4 cm, black

(gray) contours are positive (negative). The thick black contour corresponds to 0 cm.

Magenta dots show the last position of the drifter. Thin black lines connecting drifter

trajectories correspond to data gaps larger than 6 h. Drifter numbers are shown next to

the drifter’s last position. Isobaths are shown at 200, 1000, 2000 and 3000 m.

3.5.1.1. Cameron. Cameron followed the southern route (Vukovich,2007) and started influencing the northern boundary of the BOC byDecember 2008 (Fig. 5A). The sea level contours show a largesouthward penetration of Cameron in January 2009, whichreplaced the upper half of the area otherwise occupied by theunperturbed cyclonic gyre. This resulted in a reduction in size of thegyre from its unperturbed state, judging by the lack of closednegative sea level contours in the western basin. The water thatotherwise circulated around the gyre was displaced by the presenceof Cameron, some of it likely entrained into the northward flowbetween Cameron and the continental shelf, as suggested by thetrajectories of drifters 66 and 76 in Fig. 5(A). Cameron reached thesouthwestern continental shelf in February 2009 and then migratednorthward. By March, Cameron had lost intensity and movednorthwards along the western boundary, no longer influencing thecirculation in the BOC, and the cyclonic gyre reappeared, somewhatelongated in the northwest–southeast direction, as evidenced by theclosed sea level contours and the path of the drifters south of 21.51Nand west of 941W (Fig. 5B). The gyre recovered its unperturbed stateby April 2009 (Fig. 5D).

3.5.1.2. Darwin. Eddy Darwin split into two eddies in the NGOM.One of them, referred to as Darwin2, stayed in the NGOM, whilethe other one, which we will refer to as Darwin, followed thesouthern route proposed by Vukovich (2007) towards the westernboundary. Its southern rim started influencing the northernboundary of the BOC by August 2009 (Fig. 6A). As in the case ofCameron, the absence of closed sea level contours and the lack ofdrifters circulating cyclonically suggest that the cyclonic gyre wasdisrupted by the presence of the energetic Loop Current Eddy. Thewaters displaced from the northwestern BOC by the arrival ofDarwin seem to have been entrained by the flow between theanticyclone and the boundary, as shown by the trajectories ofdrifters 109, 111, 114, and 116 (Fig. 6A). We interpret this as areduction of size or even a complete disruption of the cyclonicgyre, given by a loss of mass as part of its waters were displacedand advected away from the BOC by the presence of Darwin.

Darwin stayed in the region several months, losing intensityand moving towards the west until reaching the continental shelfbreak in October 2009 (Fig. 6B), remaining centered around 221N,961W, until December 2009, when it was no longer discernible inthe altimetry data (Fig. 6C).

During December 2009, the cyclonic circulation reappeared elon-gated in the northwest/southeast direction and displaced towards thenorthwest from its unperturbed position, as evidenced by the low sealevel centered near 211N and 951W, and the trajectories of the driftersaround it (drifters 132, 134, 140 and 141, Fig. 6C). At that time, arelatively strong NGOM anticyclone (AC1) was moving towards thenortheastern boundary of the BOC, which may explain the elongatedform of the gyre, as well as the strong northwestward currentsexperienced by the drifters flowing along the boundary between thecyclonic gyre and AC1 (Fig. 6C). By February 2010, the cyclonic gyrereturned to its unperturbed state, although displaced southwards,likely due to the presence of AC1 near the western boundary (Fig. 7A).

3.5.1.3. AC1. AC1 influenced the northern boundary of the BOCsince its appearance in December 2009 (Fig. 7A–C). The effectson the cyclonic gyre differed from what was observed with

Fig. 7. Effect of NGOM anticylone AC1 on the cyclonic circulation. See description

of figure elements in Fig. 5.Fig. 6. Effect of LCE Darwin on the cyclonic circulation. See description of figure

elements in Fig. 5.

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Darwin and Cameron. The presence of the anticyclone eitherinfluenced its size and intensity, by squeezing and intensifyingthe cyclonic gyre (Fig. 7A and C, drifters recording hourlyspeeds480 cm/s) or deforming it and moving it from itsunperturbed position (Fig. 7B). At the end of our study period,AC1 continued to be present in the northern boundary of the BOC(Fig. 7C).

3.5.2. Jets in the central basin

Fig. 8(A–C) shows the time series of current speed measuredby three of the 2000 m bottom depth meter moorings located inthe BOC. We see a clear decay in the intensity of the currentstowards the east. During the period when all three moorings wereinstalled, currents exceeding 25 cm/s at 180 m occurred 30% ofthe time in the southwestern boundary (IT1-2000, Fig. 8A and D),

Fig. 8. Time series of speed for the 2000 m current meter moorings at the (A) southwestern boundary, (B) central BOC, and (C) eastern BOC. The data were low-pass filtered

with a cut-off frequency of 40 h. Horizontal white lines denote periods when (A) NGOM anticyclones or (B) locally generated anticyclones were present in the BOC. Arrows

show the presence of strong currents due to the interaction of the cyclonic gyre with (gray) NGOM anticyclones, (black) locally generated anticyclones, or (white) unknown

reasons. (D) Histogram of speed classes at 180 m for each mooring, expressed as the percentage of the total duration of the time series.

P. Perez-Brunius et al. / Deep-Sea Research II 85 (2013) 182–194 189

while the corresponding percentage for the central and easternmoorings were 10% and 3%, respectively (IT2-2000 and CAP-2000,Fig. 8B–D). Note that the drifter data also show a tendency forstronger flows west of 941W (Fig. 1B).

Although most of the events of strong currents in the mooringat the southwestern boundary (450 cm/s at 60 m) coincide withthe presence of NGOM eddies in the northern and northwesternBOC (Fig. 8A), not all are related to strong flows present at thecentral mooring (IT2-1600, Fig. 8B), which experienced six occa-sions when currents at 60 m exceeded 50 cm/s. These jets canhave various depths of influence, some of them reaching speedsabove 25 cm/s down to 500 m.

Examples of three events of strong flow in the central BOC thatdo not appear to be related to NGOM eddies are shown inFigs. 9–11. Fig. 9 shows the evolution of an anticyclone (AC2)that appeared west of Campeche Bank in mid-November 2007.AC2 migrated northwestwards and was still found within the BOCuntil March 2008, when it got entrained into the meso-scale fieldof the NGOM. The resulting convergences with the eastern rim ofthe cyclonic gyre resulted in strong flows between 951 and 941W(drifters recorded hourly speeds470 cm/s), evidenced by thecurrents experienced by the drifters and the IT2-2000 mooringand by the convergence of geostrophic streamlines from thealtimetry data (Fig. 9A–D).

The strong currents observed along 941W in February 2008(Fig. 10A–D), appear at a time when AC2 had migrated towardsthe northwestern BOC, displacing the cyclonic gyre towards theeast. The strong currents recorded by the mooring and driftersalong 941W (speeds exceeding 70 cm/s above 60 m) coincide witha convergence of the geostrophic streamlines, due to a relativelyhigh sea level at the southeastern boundary (Fig. 10A–C). In thiscase, the altimetry data does not suggest that the high sea levelevolved into an anticyclonic eddy.

Lastly, Fig. 11 shows the evolution of another anticyclone(AC3) generated in the southeastern boundary in mid-October2008, as can be seen from the trajectory of two drifters (A and B,Fig. 11A). This anticyclone appears well defined by the altimetrydata as it migrates northwestwards, reaching the northern BOC bymid-November (Fig. 11B). The convergence of flow between AC3and the cyclonic gyre results in the highest currents recorded inthe 2 years of data of the eastern mooring (CAP2000, speedsabove 50 cm/s at 100 m, Fig. 11D). Drifters recorded hourlyspeeds480 cm/s in the flow between the cyclonic gyre and AC3.

4. Discussion

4.1. Topographic control of the cyclonic gyre

Our results confirm the presence of the cyclonic gyre docu-mented by Vazquez de la Cerda et al. (2005) and DiMarco et al.(2005), but our higher resolution dataset shows that it is confinedto the deep western basin. In the absence of meso-scale eddies,the gyre appears as a symmetric cyclonic circulation, bound to thewest and south by a steep continental shelf break, and to the eastby the fanning out of the isobaths that results in the rougher andshallower eastern region. An in-depth analysis of the altimetryand surface drifter data for the year of 2010 by SandovalHernandez (2011) found that the center of the gyre was locatedeast of 94.81W 90% of the time. In addition, Sandoval Hernandez(2011) determined quarterly mean velocity fields for the year2010 on a coordinate system with the origin in the gyre’s center.Those fields showed that the size of the gyre is limited by thedimensions of the deep region west of 941W.

The circulation in the Bay of Campeche is an example of theimportance of topography in organizing upper layer flows, and it

Fig. 9. Effect of anticyclone AC2 on the currents in the central BOC. Panels (A–C) show the surface current fields for a given day using: sea level from AVISO (contours every

4 cm), low-pass velocities at the bin closest to the surface from the 2000 m moorings (magenta vectors), and the 7-day long trajectories of the drifters, ending at the day in

question (black dot). Isobaths are shown at 200, 1000, 2000 and 3000 m. Panels (D, E) show the subinertial time series of velocity at the shallowest bin (top) and speed as a

function of depth (bottom) for the central BOC moorings. The dashed black vertical lines show the time corresponding to each of the panels (A–C).

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is noticeable that evidence of this has been clearly provided byobservational programs that are Lagrangian in nature. Subsurfacefloats have shown that the path the North Atlantic Currentbranches follow towards the eastern North Atlantic are deter-mined by the Fracture Zones of the Mid-Atlantic Ridge (Boweret al., 2002; Bower and von Appen, 2008). Floats have also shownthat the spreading of warm waters from the North Atlantic intothe Nordic Seas occurs by topographically controlled routes,defined by underwater mountain ridges (Søiland et al., 2008;Rossby et al., 2009). These studies note that the paths followed bythe baroclinic fronts are not necessarily determined by sharpescarpments, but by gentler sloping bottoms, such as those alongthe Iceland-Faroe Ridge and the Vøring Plateau in the Nordic Seas.

LaCasce (2000) showed that floats in high and low latitudes inthe Atlantic and Pacific Oceans preferably spread along f/Hcontours, and that there is a statistical tendency of flows tospread parallel to steep, large-scale topographic features. In theSouthern Ocean, the analysis of ALACE floats by Gille (2003)confirmed results from previous studies showing that the Ant-arctic Circumpolar Current is steered by bottom topography,explained by the conservation of potential vorticity for anequivalent barotropic flow (e.g. Krupitsky et al., 1996; Gille,2003; Gille et al., 2004).

Baroclinic flows in stratified regions may be steered bytopography if they are equivalent barotropic, meaning that the

geostrophic flow is unidirectional at all depths (e.g. Gille et al.,2004). In that case, the vertical structure of the flow can beapproximated by a simple function, for example, a velocity profilethat decays exponentially with depth with a fixed e-folding scaleHo. Then, conservation of potential vorticity for large scale flowsin the absence of forcing or dissipation restricts the current tofollow geostrophic contours, which are defined by f/Fo, whereFo¼Ho(1�e�H/Ho), f is the Coriolis parameter and H is the bottomdepth (Krupitsky et al., 1996; Gille, 2003; Gille et al., 2004).

The particular topography of the BOC results in two regionswith distinct dynamical regimes. In the deep western basin, thedynamics is dominated by the persistent cyclonic circulation.By contrast, the currents in the shallower eastern basin aregenerally weak, ill defined, with vertical coherences less than300 m. We conclude that the cyclonic gyre results from conserva-tion of potential vorticity in an equivalent barotropic flow. Ourresults show that the surface currents remain highly correlatedwith the currents at 700–1000 m in the western basin of the Bayof Campeche. The EOF analysis shows that the western mooringshave a vertical structure consistent with a unidirectional flow inthe entire water column, that can be approximated by a decayingexponential with an e-folding scale Ho�650 m. That is, thevelocity at one depth is proportional and parallel to the velocityat another depth. In addition, over 40% of the variability of thecurrents is related to changes in intensity of the mean flow, and

Fig. 10. Evolution of a strong northward current in the central BOC. See description of figure elements in Fig. 9.

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not to changes in flow direction. The above results support thehypothesis that the cyclonic flow has a vertical structure consis-tent with an equivalent barotropic flow.

The corresponding f/Fo contours (Fig. 3B) are closed west of941W, and fluid parcels are constrained to move along them inorder to conserve potential vorticity. In that sense, the submarinefan of the ocean floor that starts at 951W acts as a dynamical wall,dividing the upper layer flow into two different regions: awestern (deeper) region with closed geostrophic contours, domi-nated by the cyclonic circulation, and an eastern (shallower)region with geostrophic contours intersecting the eastern con-tinental shelf, where the flow is weaker and less well defined.

4.2. What drives the circulation in the BOC?

For large scale equivalent barotropic flows within a basin,potential vorticity balance for a flat seafloor results in geostrophiccontours which are blocked by the meridional boundaries. Hence,flow can only exist across blocked contours in the presence ofwind-stress curl and/or dissipation (Sverdrup balance betweenvorticity input by the wind-stress curl and changes in planetaryvorticity due to a meridional flow). By contrast, if the topographyof the basin is such that it results in closed geostrophic contours,any flow can exist along those contours in the absence of forcingand dissipation. In that case, a net torque around the contouraccelerates the fluid along that path until dissipation limits theintensity of the flow. Finally, forcing drives much stronger flowsaround closed geostrophic contours than it does across blockedgeostrophic contours (Young, 1987).

Vazquez de la Cerda et al. (2005) suggest the cyclonic gyre isdriven by the positive wind-stress curl, via Sverdrup dynamicsapplied to a flat seafloor, given the evidence of a westernboundary current whose intensity apparently varies with theseasonal variation of the wind-stress curl. In addition, ‘‘back ofthe envelope’’ calculations resulted in consistent results betweenthe orders of magnitude of the Sverdrup transport estimated fromthe wind-stress curl, and the flow estimated from hydrography.Our higher resolution velocity fields do not find concludingevidence of a western intensified flow in either seasons. Notethat if topography is taken into account in the potential vorticitybalance, there is no need for a western boundary current, sincethe streamlines are given by the geometry of the closed geos-trophic contours in the western basin. In conclusion, the wind-stress curl and the topography of the basin results in a nearlysymmetric cyclonic gyre, located west of 941W.

We find no clear evidence of an influx of positive vorticity intothe BOC by cyclones generated by LCE collisions with the westernboundary, as was observed by Vidal et al. (1992). If this was thecase, we would expect an intensification of the cyclonic gyreduring or right after a collision of a LCE against the boundary.In two of the three cases analyzed, we found that the presence ofLCEs against the northwestern boundary nearly disrupted thecyclonic gyre rather than intensified it, draining it from its watersas they squeezed it towards the southwestern shelf. Once theLCEs migrated northwards or dissipated, the cyclonic gyrereturned to its unperturbed state.

We have insisted that the vertical stiffness of the flow in thecyclonic gyre puts a strong topographic constraint on it.

Fig. 11. Effect of anticyclone AC3 on the currents in the central BOC. Figure elements as in Fig. 9, in this case panel (D) shows the time series of speed of the 2000 m

mooring located in the eastern BOC.

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Nevertheless, we have also noted that strong anticyclones arriv-ing from the NGOM can completely disrupt the cyclonic circula-tion. The way we interpret this apparent discrepancy is, in theabsence of meso-scale eddies, the potential vorticity balanceresults in a preferred state, which is the flow along geostrophiccontours, whose geometry largely depends on the topography andwhose intensity depends on the strength of the wind-stress alongthose contours. This could be interpreted as the stationary state.That state is not as energetic as some of the large meso-scaleeddies of the NGOM that drift towards the Bay of Campeche: thecyclonic gyre has mean surface currents �30 cm/s and sea levelanomalies �12–14 cm (Figs. 2 and 5–7) while strong LCEs canhave speeds well over 50 cm/s and sea level anomalies424 cm(Figs. 5 and 6). Hence, the interaction of the gyre with the strongNGOM eddies may result in a complete disruption of the sta-tionary state. For NGOM eddies that are not as energetic (sea levelanomalieso18 cm, Fig. 7), the interaction results in relativelysmall perturbations in size, form, intensity and position of thegyre, and the cyclonic gyre remains topographically confined tothe western basin.

4.3. Factors influencing intraseasonal variability

The EOF analysis of the sea level anomaly altimetry data byVazquez de la Cerda et al. (2005) suggests that the nonseasonalvariability in the BOC is mostly due to an influx of eddies from theNGOM. Our findings suggest that the sources of the nonseasonalvariability differ between the western and eastern BOC. In the

western basin, the variability seems to result from changes in theposition, shape, size and intensity of the cyclonic gyre as it interactswith NGOM eddies near its boundaries. In particular, LCEs whosesouthern rims penetrate into the northwestern BOC as they migratetowards the western boundary, following the southern route pro-posed by Vukovich (2007). On the other hand, the variability in theeastern basin seems to be mostly due to locally produced distur-bances, among them anticyclones generated near the southeasternboundary, with no evidence of NGOM eddies directly influencing theeastern basin of the BOC. In the central BOC, strong northward flowsappear due to intensification of the currents of the eastern rim of thecyclonic gyre. We showed that some of these events are due to theinteraction of locally produced anticyclones with the cyclonic gyre.

5. Concluding remarks

In this paper we have given evidence of the topographiccontrol of the cyclonic gyre and the role that LCEs play in itsvariability. An in-depth analysis between the wind-stress curl andthe current meter data is needed to determine the role the windsplay in the seasonal modulation of the cyclonic circulation.Our results suggest that the equivalent barotropic modelappropriately describes the BOC gyre, and that topography needsto be taken into account to properly model this flow. Theequivalent barotropic model has proven to be an elegant andsimple explanation for the large-scale topographic control of theAntarctic Circumpolar Current in the Southern Ocean. We consider

P. Perez-Brunius et al. / Deep-Sea Research II 85 (2013) 182–194 193

it is likely to be a good candidate to explain the steering by bottomtopography in baroclinic flows elsewhere, including the Bay ofCampeche.

Acknowledgments

Jose Luis Ochoa (CICESE) provided valuable insights and sugges-tions to this study. The Eddy Watch group (Horizon Marine Inc.),Juan Ignacio Gonzalez and Argelia Ronquillo (CICESE), have beencrucial for the planning, execution, data acquisition and qualitycontrol of the ongoing drifter program. The comments and sugges-tions by Luis Octavio Avila and the rest of the group at InstitutoMexicano del Petroleo helped improve the quality and managementof the drifter data. Altimetry data produced by Ssalto/Duacs anddistributed by Aviso with support of Center National d’EtudesSpatiales. JLAB software (Lilly, 2011) was used to produce the vectortime series plots in this manuscript. We thank everyone involved inthe planning, deployment, recovery, and data processing of themooring ADCP data: the CANEK group (CICESE) and the crew of R/VJusto Sierra (UNAM). We greatly appreciate the constructive com-ments from two anonymous reviewers. This project has been fundedby PEMEX Exploracion y Produccion under contracts SAP nos.428217896, 428218855, and 428229851. P. Perez-Brunius wishesto express her deepest gratitude to Tom Rossby as a generous,inspiring, and passionate mentor and advisor. In particular, Prof.Rossby helped her appreciate the scientific value of Lagrangianobservations and their role in the development of an intuitiveunderstanding of oceanic flows.

Appendix A

FHD drifters compared to standard velocity drifters

The Far Horizon Drifter is a low cost, easily deployable drifter foroperational monitoring of meso-scale oceanographic features, and isthe standard drifter used by the Gulf of Mexico oil industry. Theydiffer from the Surface Velocity Program (SVP) drifters used inphysical oceanography research, which are the product of extensivetesting and redesigning in the 80s by Peter Niiler (Scripps Oceano-graphic Institution) and collaborators to improve their water follow-ing capabilities (see review in Lumpkin and Pazos, 2007).

The most significant difference between the SVP and FHDdrifters is the design of the drogue. SVPs have typically a 1.2 mlong holey sock, centered at 15 m. Hence, the FHDs parachutedrogue is more likely to ‘‘kite’’ in vertically sheared flows. Anestimate of this effect using over 4000 km current profiles fromship mounted ADCPs in the Loop Current region in the summer-time suggests the parachute depth may reach 15 m in highlysheared flows, but oscillates mainly between 25 and 45 m depth,with an average depth of 36 m (Steve Anderson, Horizon MarineInc., personal communication). Given that the vertical shear in thesouthern Gulf of Mexico is likely to be significantly smaller thanin the Loop Current region, we expect the FHD data to representthe currents within the 25 and 45 m layer.

The largest source of error in the drifter velocity data is likelyto be wind produced slip with respect to the currents at thedrogue depth due to a small drag area ratio (Niiler et al., 1995).SVP drifters have a 40:1 drag area ratio, while our estimates forthe corresponding FHD drag area ratio range between 2:1 (para-chute collapsed, represented by a sphere) and 11:1 (parachutefully inflated under water). Qualitative tests and observations bythe manufacturer suggest the parachute stays inflated with theslightest tension on the tether line (Steve Anderson, HorizonMarine Inc., personal communication).

Correlations with winds and geostrophic currents

To get an estimate of the error due to wind slippage in thedrifter data, we correlated the drifter velocities to the winds, inthe same way as Poulain et al. (2009) did for SVP and CODE(Coastal Ocean Dynamics Experiment, 1 m depth drogue: Davis,1985) drifter data in the Mediterranean Sea. We use the NorthAmerican Regional Reanalysis model (NARR) winds, provided bythe NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Website (http://www.esrl.noaa.gov/psd/). The data cover the periodbetween September 1, 2007 and December 31, 2009, with a 3 htime step and an approximate resolution of 0.31�0.31. The hourlydrifter data velocities are low pass filtered (38 h cut-off fre-quency) and sub sampled at the same time steps of the NARRwinds. The winds were also low pass filtered and interpolatedonto the 3 hourly drifter positions, resulting in 14,419 simulta-neous wind and drifter data points. The complex correlationcoefficient between the wind and drifter velocities was thenestimated following Kundu (1976). The magnitude of the complexcorrelation coefficient gives the overall measure of the linearcorrelation between the two datasets.

We obtained a magnitude of the complex correlation coeffi-cient of 0.175 (with [0.170 0.180] corresponding 95% confidenceinterval and 0.140 the 5% significant level). The correspondingvalues calculated by Poulain et al. (2009) for the CODE (drogue at1 m depth), SVPs (drogue at 15 m) and undrogued SVPs (driftersthat had lost their drogue) were 0.28, 0.18, and 0.47, respectively.Hence, our drifter data shows the same magnitude of thecorrelation with the winds as the drogued SVP drifters.

We also estimated the complex correlation coefficientsbetween the absolute geostrophic currents from the AVISOaltimetry data and the drifter velocities for the same period asfor the winds. The low pass filtered hourly drifter positions weresub sampled daily. The geostrophic velocities were interpolatedonto the drifter positions and correlated with the mean dailyvelocities estimated by central differences from the daily drifterpositions, resulting in 4921 simultaneous data points. The corre-sponding complex correlation coefficient has a magnitude of 0.70([0.69 0.71] corresponding 95% confidence interval and 0.05 the5% significant level).

In conclusion, the FHD drifter data have the same correlationwith the winds as the 15-m drogued SVP drifters from the Poulainet al. (2002) study, and are highly correlated with the geostrophiccurrents derived from altimetry. Both results show that the FHDdrifter data represent well the meso- and large-scale features ofthe velocity field in the upper layer of the Bay of Campeche.

Appendix B

Statistical analysis considerations

The statistical analyses of the current meter time series andthe binned drifter data were performed in the following manner.The velocity was projected onto principal axes components.The mean and standard deviations were obtained for eachcomponent, as well as their 95% confidence intervals. The degreesof freedom were determined from the integral time scale of 8days (Emery and Thomson, 2001; DiMarco et al., 2005). This is aconservative value, since it is the upper bound for the Lagrangiantime scales in the Gulf of Mexico (see DiMarco et al., 2005) andthe decorrelation times of the mooring data vary between 1 and6 days. Hence, we are likely underestimating the degrees offreedom.

A t-Student test at the 5% significance level was performed tocompare differences between the means of (a) the mooring and

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drifter data at the bins containing the moorings, (b) the fall–winter and spring–summer for each dataset. In addition, the twocomponents of the standard deviation were analyzed between(a) the two datasets and (b) the two seasons for each dataset. Thestandard deviations were considered significantly different iftheir corresponding 95% confidence intervals did not intersecteach other.

Vectorial correlations considerations

Bootstrapping was used to obtain the 95% confidence interval,and permutation tests were performed to determine the 5%significance level of the vectorial correlations (Hesterberg et al.,2005). The confidence interval was determined from the boot-strap distribution of 1000 correlations, each of which was calcu-lated by resampling simultaneous pairs of vectors of the originaltime series. The pairs were selected randomly with replacement,and each sample had the same number of pairs as the originaltime series. Since in all cases the distributions were symmetricand bias was small, the interval from the 2.5 to 97.5 percentilescorresponds to the 95% confidence interval.

The 5% significance levels were obtained from the cumulativedistribution of the correlations between the first vector timeseries and 1000 permutations of the second vector time series.

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