JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C08009, doi:10.1029/2011JC007007, 2011
Circulation over the continental shelf of the western
and southwestern Gulf of Mexico
Jean Dubranna,1 Paula Pérez‐Brunius,1 Manuel López,1 and Julio Candela1
Received 28 January 2011; revised 15 April 2011; accepted 21 April 2011; published 5 August 2011.
[1] The circulation over the continental shelf break of the western and southwestern Gulf
of Mexico is inferred from the analysis of drifter trajectories and 12–19 months of
continuous current measurements at seven different locations. The interpretation of the
data is backed up by satellite altimetry, coastal sea level from tide gauges and wind
model outputs. In accordance with previous numerical results, subinertial surface currents
are driven by the wind along the shelves of the states of Tamaulipas and Veracruz.
Northern wind regimes would force southward currents, whereas southern wind regimes
would force northward currents at the surface but southward near the bottom, through
a process involving Ekman drift and geostrophic balance. Our results show, however,
that alongshore current variations are not correlated with the wind over the Western
Campeche Bank. In addition, we identify other sources of current forcing. The transient
eddies that collapse along the continental shelf can force strong alongshore currents
and overwhelm the influence of established wind regimes. Their erratic occurrence is
likely to be a major factor of interannual variability of the alongshore currents. Also,
we point out the existence of coastally trapped waves generated by the wind in the
northern shelf of Tamaulipas and propagating down to the Western Campeche Bank.
The period of these waves ranges between 6 and 10 days, with phase speeds in the
4 m/s range.
Citation: Dubranna, J., P. Pérez‐Brunius, M. López, and J. Candela (2011), Circulation over the continental shelf of the western
and southwestern Gulf of Mexico, J. Geophys. Res., 116, C08009, doi:10.1029/2011JC007007.
1. Introduction
[2] Despite the continental shelf circulation has been well
documented in the American part of the Gulf of Mexico
(GoM), namely, north of 26°N, information about its Mexican counterpart is still underrepresented in the literature.
Most of the studies in the Mexican waters of the GoM are
actually dedicated to the characteristics of the offshore circulation. The northern continental shelf of the Mexican GoM
is approximately 100 km wide, thinning southward to about
30 km at the latitude of the southern Bay of Campeche
(BoC). From there, it widens again to reach about 200 km at
the level of the Western Campeche Bank (WCB, western part
of the Yucatan Peninsula).
[3] Very few measurements have been collected in the
continental shelves of Tamaulipas and Veracruz (hereafter
referred as TAVE shelves, between 18.2°N and 26°N), and
of the WCB. The ones that exist [Gutierrez de Velasco et al.,
1992, 1993] suggest a seasonal reversal of the shelf currents
30 km off Tuxpan (21.6°N, 97.1°W), with down‐coast (up‐
coast) flows in fall‐winter (spring‐summer). Here, down‐
coast (up‐coast) flow refers to alongshore displacement with
1
Centro de Investigación Científica y de Educación Superior de
Ensenada, Departamento de Oceanografía Física, Ensenada, México.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2011JC007007
the coast on the right (left). Boicourt et al. [1998] first suggested the possibility for the down‐coast current to extend
from the Texas shelf down to Tuxpan (20.6°N, 97.2°W)
during fall‐winter. Following the tracks of drifters released in
the Louisiana‐Texas shelf, Walker [2005] found that about
half of them entered the Louisiana‐Texas down‐coast coastal
current that extends to depths around 10 to 30 m during fall
and winter, and ended up in Mexican waters.
[4] Zavala‐Hidalgo et al. [2003] and Morey et al. [2005]
report the most comprehensive results about the circulation
and water properties over the continental shelf of the western
and southwestern GoM. They explore the salinity, temperature, sea level and current fluctuations using a 10 year simulation of the GoM by a very high resolution model and
hydrographic data. The model results corroborate the existence of a strong wind driven seasonal signal in the circulation of the surface waters over the shelf. During spring and
summer, the average wind flows toward the west and
northwest, driving up‐coast currents from the southern BoC
shelf up to the northern TAVE shelf. In fall and winter, the
direction shifts and the wind blows south to southwestward
forcing a down‐coast current from the northwestern GoM
down to the southern BoC. Most intense northerly winds are
associated with the coming of atmospheric cold fronts traveling from the northwest continental United States. This
reversal comes with the rise of the coastal water level which
reaches an annual peak in September and October from about
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Galveston (29.28°N–94.83°W) down to the north of the
Yucatan Peninsula. On the WCB, the model results show
an up‐coast surface current year‐round, due to the relative
orientation of the coast with the dominant winds. As a result,
the down‐coast flow of the TAVE shelf converges with the
up‐coast flow of the WCB between 93°W and 94°W during
fall and winter causing offshore currents in the area. Once the
waters have crossed the shelf break, they are likely to be
transported by the mesoscale eddy field of the BoC.
[5] Along with the wind stress, the circulation over the
shelf is locally affected by mesoscale eddies [Boicourt et al.,
1998], such as the permanent as well as time‐dependent
cyclonic circulation in the Bay of Campeche (BoC), south
of 22°N, mostly documented by Vazquez de la Cerda
et al. [2005] and DiMarco et al. [2005]. From the study
by Gutiérrez de Velasco and Winant [1996], this cyclone is
thought to be forced by the local positive wind stress curl
constrained by the presence of the high coastal Sierra Madre
mountain range that channels the wind toward the Isthmus
of Tehuantepec. Vazquez de la Cerda et al. [2005] confirm
that the seasonality of the cyclonic circulation consists in an
intensification in winter and a weakening in summer, in
accordance with the seasonality of the wind stress curl. They
also point out that the nonseasonal variations are related
with synoptic events such as smaller‐scale eddies passing
by. The energetic Loop Current Eddies (LCE) coming from
the eastern GoM are anticyclonic and can also influence the
shelf circulation, mostly north of 22°N. Walker [2005]
studied the effect of wind and eddies on the shelf and
slope circulation in the northwestern GoM and she mentions
the seaward entrainment of drifters located in the Mexican
continental shelf between 25°N and 26°N by cyclone/anticyclone pairs that influence the shelf circulation in the area.
Specifically, 60% of the drifters entering the Mexican slope
were exported seaward. Finally, the buoyancy gradients due
to numerous fresh water inputs by the river mouths along
the Mexican coast have a low impact on the shelf circulation
[Zavala‐Hidalgo et al., 2003].
[6] This paper compiles on‐site measurements in the
water column collected from November 2007 to July 2009,
satellite altimetry data and the trajectories of surface drifters
to study the circulation over the shelf of the western and
southern GoM, south of 26°N. After a description of the
data, we focus on the monthly circulation of the surface
waters in relation with the wind forcing in order to compare
our measurements with the model climatology presented by
Zavala‐Hidalgo et al. [2003] and Morey et al. [2005]. Then,
we describe the different circulation regimes observed during the measurement period, in relation to various forcing
factors. A variable alongshore wind component blowing
over a sloping and stratified continental shelf is a well‐
known generation factor of coastally trapped waves [Gill
and Clarke, 1974; Clarke, 1977]. Considering the variable
wind conditions as well as the bathymetric profile of the
continental shelf along the TAVE coast, such processes
could be forced in the area. Besides, Zavala‐Hidalgo et al.
[2003] first mentioned the possibility of the variability of
the current along the WCB being produced by remote forcing due to coastal waves associated with the circulation
variations on the TAVE shelf. We therefore examine the
existence, propagation and generation of coastally trapped
waves in the investigated area in the fifth section of this
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article. We then discuss and compare with previous studies
the different processes found in our measurements, followed
by general conclusions.
2. Data
2.1. Moorings
[7] A total of seven moorings were deployed between
November 2007 and July 2009 at seven different sites along
the 130 m isobath of the western and southwestern GoM,
corresponding approximately to the continental shelf break
(Figure 1). Two of them have collected current profiles
from the end of November 2007 to mid‐July 2009 (about
19 months) at stations LNK and CTZ, and the remaining
5 were deployed from about the end of July 2008 to mid‐
July 2009 (about 12 months).
[8] Each mooring was equipped with an Acoustic Doppler
Current Profiler (ADCP) looking upward (Workhorse 300 kHz
from RDI) with measurements averaged over 8 m bins, and
another looking downward (Workhorse 600 kHz from RDI)
with measurements averaged over 0.5 m bins, focusing on the
bottom boundary layer. Both ADCPs were positioned at 14 m
above the seafloor.
[9] Current values correspond to 1 h average of 45 evenly
spaced measurements over the sampling interval. Measurement bins located close to the boundaries (bottom and surface)
or to other instruments near the 600 kHz ADCPs were discarded due to evidence of signal contamination. The shallowest measurement bins consequently range between 12
and 20 m, and the deepest between 125 and 135 m. After
removing the contaminated bins, the U and V components of
the currents have been low‐pass filtered with a cutoff frequency of 48 h, in order to damp the tidal and inertial motions.
[10] To our knowledge, this is the most comprehensive
current data set collected in this area of the GoM. Several
authors have actually raised the lack of onsite measurements
in the Mexican waters of the GoM [Boicourt et al., 1998;
Zavala‐Hidalgo et al., 2003, 2006].
2.2. Wind
[11] The wind speed vectors at 10 m elevation were collected from the North American Regional Reanalysis model
(NARR) outputs, provided by the NOAA/OAR/ESRL PSD,
Boulder, Colorado, from their Web site (http://www.esrl.noaa.
gov/psd/). The data cover the period between 1 November
2007 and 31 July 2009, with a 3 h time step and an approximate resolution of 0.3 × 0.3 degrees. Each component was
low‐pass‐filtered with a cutoff frequency of 48 h to match
the filtering undergone by the current measurements.
[12] The wind components along the Mexican coastline of
the GoM were linearly interpolated from the 4 closest outputs of the NARR grid. The alongshore wind stresses were
then calculated using the formula by Large and Pond
[1981], and monthly averaged over the measurement period.
2.3. Drifters
[13] A set of trajectories of surface drifters flowing in the
continental shelf region during the period analyzed was
included in the analysis of the surface currents. The data set
forms part of an ongoing surface drifter program designed
to monitor the surface currents over the deep waters of the
Bay of Campeche. Monthly airborne deployments of three
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Figure 1. Position of the moorings along the 130 m isobath. Shown are the 500, 1000, 1500, 2000 and
3500 m isobaths. Moorings marked with a circle were deployed from November 2007 to July 2009.
Moorings marked with a square were deployed from July 2008 to July 2009.
to five surface drifters have taken place south of 21°N over
bottom depths greater than 1000 m since late September
2007. Of 154 drifters successfully launched by August 2010,
36 flowed into waters less than 200 m deep. Their trajectories
in shallow water (shallower than 200 m) were selected for
this study.
[14] The drifters consist of a cylindrical shaped buoy hull
(96.5 cm long and 12.4 cm in diameter) with a 45 m nylon
tether line attached to a 1.2 m diameter “paradrogue,” that
serves both to protect the buoy when water landing as well
as a drogue to reduce slippage of the buoy in the water
(Far Horizon Drifter, Horizon Marine Inc.). Hourly positions are recorded by a GPS receiver, and transmitted via
Argos [Anderson and Sharma, 2008].
[15] Data gaps less than 6 h long were interpolated and a
quality control process eliminated data of drifters on land,
stuck on the bottom or onboard vessels, speeds exceeding
3 m/s or showing conspicuous peaks, and erroneous positions that result from bad fixes from the GPS receiver.
Velocities were estimated from the hourly data using a
central difference scheme. The hourly data from the drifters
were separated in two groups depending on the time of the
year they were collected: spring‐summer (March to August)
and fall‐winter (September to February), and for each group
another selection divided the data set into up‐coast or down‐
coast flowing periods.
3. Monthly Dynamics
3.1. Wind Forcing
[16] The monthly mean along coast wind stress observed
in the two years analyzed can be divided into 3 regional
regimes (Figure 2).
[17] Along the State of Tamaulipas (22.5°N to 26°N,
moorings PER and LMP), the wind blew up coast from
February to August, and during November and December,
with greatest intensity from June to August. It was weakly
down coast in January and experienced a dramatic inversion
from up coast to down coast during September and October.
[18] For the State of Veracruz (18°N to 22.5°N, moorings
ARN, LNK, IT1 and CTZ), the monthly mean alongshore
wind stress was mostly weak from February to May,
although 2 northerly wind events could be spotted monthly
on average. From June to August, it increased toward the
north (moorings ARN and LNK) taking a steady up‐coast
direction, and remained weak near the BoC (moorings
IT1 and CTZ). The down‐coast reversal that occurred in
September and October was intense, taking greatest values
in September between LNK and IT1 due to favorable orientation of the coastline relative to the wind. These stronger
values correspond to the most intense monthly alongshore
wind stress experienced during the period analyzed over
the TAVE and WCB shelves. In November and December,
the intensity dropped back to weak values.
[19] In the area of the WCB (mooring IT2), the monthly mean
winds blew up coast year‐round with moderate intensities.
3.2. Surface Currents
[20] As stated before, the shallowest measurements correspond to the current averaged over a bin 8 m thick, centered between 12 and 20 m depth depending on the mooring.
Those measurements will be referred as “surface currents.”
Due to the planning of the deployment and recovery
operations, the current velocity statistics for July, August
and September are only partial for some moorings. PER and
LMP have either no or less than 5 days of measurement
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Figure 2. Alongshore wind stress component averaged monthly over the measurement period. Positive
(Negative) values indicate up‐coast (down‐coast) direction.
during these months. ARN, LNK, IT1, CTZ and IT2 had
between 14 and 28 days of measurement during July, IT2
had 20 days in August.
[21] The correlation coefficient between the monthly
alongshore wind stress and the averaged alongshore surface
current is significant at the 95% level with 12 degrees of
freedom at ARN (0.77) and LNK (0.78) along the TAVE
shelf. These values are lower than the ones obtained with
modeled currents by Zavala‐Hidalgo et al. [2003], which
were in the 0.90 range in the same area. Specifically, no
significant correlation was obtained at the northern locations
of the TAVE shelf, namely PER and LMP. In the southern
BoC, CTZ and IT2 have significant correlations of 0.61 and
0.67 respectively. At a location close to IT2 however,
Zavala‐Hidalgo et al. [2003] did not find a significant
correlation between the wind stress and the surface currents.
Also, no significant correlation was found at IT1 (0.47),
despite the fact that this station is located very close to CTZ,
with similar wind regime.
[22] The monthly averaged surface currents and corresponding standard deviation ellipses are presented in Figure 3.
As in most coastal environments, the mean of the currents
and the principal axis of the standard deviation ellipses are
mostly oriented in the alongshore direction for all months,
meaning that the water motions are essentially alongshore.
[23] The moorings situated in the outer continental shelf
of the BoC, namely CTZ, IT1 and LNK, mostly experience
low surface currents on average, with a high variability. The
mean velocities are typically in the 5 to 15 cm/s range from
November to May, with standard deviations about twice
as large. The alongshore current at LNK follows the wind
stress trends at the same position, increasing in January and
changing direction several times between November and
May. According to the correlation coefficient values mentioned earlier, the relation between the wind and the surface
currents is highest at this mooring. The currents at CTZ and
IT1 show no preferential direction or particular intensification, possibly due to low alongshore wind stresses, but have
a greater standard deviation in November. Current speeds
at CTZ, IT1 and LNK increase slightly during summer
to about 15 to 20 cm/s and are clearly pointing up coast,
in agreement with the increase of the up‐coast wind stress
observed from June to August. The corresponding standard
deviations decrease to an average of 10 cm/s.
[24] The currents between ARN and LMP are always in
phase regarding their alongshore direction, except in November when they are down coast at LMP and up coast at ARN.
They are flowing up coast from about December to August
with intensities and standard deviations greater than the ones
observed in the continental shelf of the BoC. In December and
April at LMP, or in February and March at ARN, the mean
current velocities increase to 30 to 40 cm/s, although the wind
stresses remain low or at most moderate during the same
months. An obvious discrepancy between the wind stress and
the current occurs in January, when the current is heading north
at about 26 cm/s on average at LMP while the wind stress
is down coast. The up‐coast current velocities stabilize around
30 to 40 cm/s with lower standard deviations, during late
spring and summer, as the summer wind regime develops.
[25] The mean currents at PER are lower than 20 cm/s and
mostly lower than 10 cm/s from November to May, flowing up coast from November to January and down coast
from February to May. The standard deviations are around
10 cm/s to 15 cm/s. Although the alongshore wind stress is
among the most intense at this mooring, the correlation
coefficient between the wind and the alongshore current is
not significant. The variations of the wind and the alongshore current are therefore disparate, and they are flowing
in opposite directions several times between November and
May (January, March, April, May). However, in June and
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Figure 3. Monthly averaged surface currents and corresponding standard deviation ellipses. Shown are
the 500, 1000, 1500, 2000 and 3500 m isobaths.
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July, the current turns up coast and accelerates to around
40 cm/s, in conjunction with the intensification of the
southerly wind stress.
[26] The currents in the southwestern Campeche Bank
were recorded by IT2. They are oriented up coast during
the entire year except in September and October. The mean
current varies between 4 cm/s and 13 cm/s between December
and June, with standard deviations between 8 cm/s and
20 cm/s. The low velocities do not reflect the intense alongshore wind forcing experienced at this mooring. The current
accelerates to about 20 cm/s in summer while its standard
deviation drops to 5 cm/s on average. A possible convergence
can be seen in November between IT2 (up coast, 18.9 cm/s)
and CTZ (down coast, 13.3 cm/s).
3.3. Drifters
[27] The trajectories of the surface drifters on the continental shelf are shown in Figure 4. During spring‐summer,
the drifters headed up coast 70% of the time. North of 21°N,
all nine drifters flowed up coast except for one, which for a
short period headed in the other direction. Speeds increased
from south to north, with values above 80 cm/s recorded by
four drifters off the Tamaulipas shelf. Most of the up‐coast
trajectories occurred between April and June. South of 21°N,
drifters moved both in the up‐coast and down‐coast directions, their speeds lower compared to their northern counterparts, with the exception of a down‐coast flowing drifter
that recorded speeds over 80 cm/s for a short time. Most of
the down‐coast data were collected between April and May.
[28] During fall and winter, a majority of drifters headed
down coast (74% of the drifter data), five of them recording
speeds over 80 cm/s at various locations all along the shelf,
reaching sustained speeds over 80 cm/s between 19°N and
22°N. The down‐coast data were nearly evenly distributed
among the months corresponding to this period. By contrast,
up‐coast flows occurred only 26% of the time, located
exclusively north of 22°N, with the majority of the data
collected between September and November.
4. Description of Specific Circulation Regimes
4.1. Fall Event
[29] On Figures 2 and 3 we can observe a dramatic reversal
of the alongshore wind stress and currents along the TAVE
and WCB shelves in September and October. This fall
directional shift is one of the most significant seasonal features of the circulation over the continental shelf of the GoM.
It distinguishes the spring‐summer conditions, with both
wind and currents flowing up coast, from the fall‐winter
conditions with wind and currents flowing down coast.
During this time period (fall and winter), the shelf circulation
is under the influence of strong northerly wind events forced
by cold atmospheric fronts traveling southward, inducing
important variability in the current and the atmospheric
forcing.
[30] The down‐coast component of the wind stress varied
little or slightly increased between PER and LNK, but it
increased notably farther south to reach a maximum between
LNK and IT1 due to the coastline orientating more parallel
to the wind. Then, it decreased slightly but remained high
in the region of IT1 and CTZ. The intensification of the
alongshore wind in the southern TAVE shelf is consistent
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with the highest alongshore current velocities recorded at
LNK in September, and IT1 and CTZ in October with values
around 40 to 60 cm/s. These are the strongest monthly averaged speeds reached at these locations over the measurement
period by at least a factor of 2. The current reversal is also
visible at IT2 with values between 13 and 20 cm/s on average,
although the wind kept an up‐coast orientation year‐round.
[31] Figure 5 describes the variations of the most significant physical parameters in the fall of 2008, during the
wind and current reversal. Although not all the parameters
have values during this period, the same timeline has been
kept for all of them to facilitate the tracking of the significant events, spatially and temporally.
[32] Displayed as bold solid lines is the daily alongshore
wind stress at the position of each mooring, with positive
(negative) values being up coast (down coast) (Figures 5a–
5g). The wind was blowing up coast at all the moorings until
the end of August, and shifted to a steady down‐coast
direction in mid‐September, except at IT2. It kept this
direction during the second half of September and most of
October, although some up‐coast wind events could be
found during this last month especially at the northernmost
moorings. Some episodic intensifications of the down‐coast
wind stress occurred on 18 and 27 September (W1 and W2
respectively) and around 17 and 29 October (W5 and W6
respectively). At the beginning of November, the wind
shifted again to a mostly up‐coast direction at all moorings.
[33] The variations of the water level anomaly at the coast
taken from the tide gauge of the City of Veracruz, as seen
from Figure 5h (solid line), show a very good correlation
with the low‐passed alongshore wind stress of IT1, also
presented in Figure 5h (dotted line). Specifically, the shift in
wind direction in September coincided within 5 days with
the water level anomaly shifting from negative to positive.
Then, the persistence of the down‐coast wind speed in
September led to a gradual increase of the water level at the
coast, caused by the accumulation of water by onshore
Ekman transport. It decreased by the end of September and
the beginning of October as the wind stress decreased,
before rising again in the second half of October following
the wind stress reinforcement.
[34] Figures 5a–5g show the alongshore current velocity
profiles as measured by the moorings. Red stands for down‐
coast currents and blue stands for up‐coast currents; white
patches stand for contour speeds below 10 cm/s. The general
pattern of the alongshore current velocity is consistent with
the wind‐driven, low‐frequency variations of the coastal
water level. The current shifted from up coast to down coast
at the beginning of September, during which the overall
velocities were greatest. They decreased slightly between
the end of September and the first half of October, before
increasing again during the second half of October. The
current turned up coast again as well as the wind by the time
the water level anomaly became very low at the beginning
of November. This current pattern was observed at least
along the southern TAVE shelf (moorings ARN to CTZ),
possibly extending farther north considering that the
alongshore wind trends were similar along the western
coast. However, the absence of measurements during this
period at PER and LMP prevents us from having a clear idea
of the current behavior in the northern TAVE shelf during
the wind reversal.
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Figure 4. Drifter trajectories as functions of time of the year and along‐coast direction, color coded by
speed. Solid black circles show the last position of the drifter (end of trajectory) next to the corresponding
drifter number. Data collected during (left) March through August and (right) September through
February. Histograms show the proportion of data heading (top) up coast or (bottom) down coast for
the corresponding time of the year. Thin black lines correspond to portions of trajectories either on waters
deeper than 200 m or heading in the opposite along‐coast direction.
[35] Superimposed with the underlying variations of the
water level, alongshore wind stress and current velocity,
higher frequency variations of these parameters can also be
seen. The major ones are related with intense wind events
lasting about two or three days, traveling southward and
highlighted by the “W” marks on Figures 5a–5g. Those
northern winds blew down coast over the TAVE shelf but
up coast with globally lower stress in the western Campeche
Bank (IT2), due to different orientation of the coastline.
They systematically came with an increase of the water level
anomaly at the coast by about 50 to 150 mm recorded by the
tide gauge of the City of Veracruz (arrows on Figure 5h)
and stronger down‐coast currents.
[36] The first short event occurred the first time the wind
significantly shifted from up coast to down coast all along
the TAVE shelf around 7 September (W1). The down‐coast
current acceleration was coherent through the entire water
column. It was first detected at ARN, and about 72 h later at
IT2 which corresponds to an average down‐coast propaga-
tion speed of about 2.9 m/s between the two moorings. The
greatest velocities (in the 60 cm/s range) were reached at the
level of LNK, then weakening toward IT2. The sea level rise
at the City of Veracruz and the current acceleration measured at IT1 were simultaneous.
[37] Another example of such propagation and coherence
between the wind, sea level height and alongshore currents
can be observed at the end of October, with the current
signal propagating from LMP (possibly PER) to IT2 at
about 5.4 m/s on average (W6). At the northern moorings,
the current acceleration was sharp, with maximum speeds
increasing between LMP and LNK and steady down‐coast
wind stress. Farther down coast, the current acceleration was
more scattered in time, with decreasing maximum speed
while the wind stress faded. At the level of IT2, most of the
signal was already dissipated. Similar description can be
made for W5, although the maximum current speed was
reached at LNK and the intense down‐coast wind stress held
all along the TAVE shelf.
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Figure 5. Color maps of the alongshore current velocities measured by the moorings in cm/s: (a) PER,
(b) LMP, (c) ARN, (d) LNK, (e) IT1, (f) CTZ, and (g) IT2. Superimposed are the alongshore wind stresses at the position of the moorings. Positive (negative) values are up coast (down coast). The beginning of
events of interest are marked by a vertical solid line marked with a W. Arrows show the maximum mean
current speed following each W event. (h) The low‐passed (cutoff period is 30 days, dotted line) and
band‐passed (periods between 2 and 30 days, solid line) sea level anomaly (SLA) at the City of Veracruz
which is located between LNK and IT1. SLA corresponding to the W events are marked by an arrow.
Low‐passed (cutoff period is 30 days) alongshore wind stress at the City of Veracruz is represented
by a dashed line. Arrows show the positive SLA associated with each W event.
[38] Although the current was flowing down coast during
most of the time in fall, some periods of up‐coast currents and
baroclinic circulation can also be seen. Specifically, during
the weak W4 event, when a down‐coast current acceleration
propagated at least from ARN down to CTZ and came with
an elevation of the sea level at the coast, the signal was
interrupted by an up‐coast current at LNK. However, this up‐
coast current did not prevent the down‐coast signal from
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propagating farther south and was very likely forced by
a local anticyclonic eddy present in the area of LNK at
this time.
[39] Only one drifter entered the area shallower than
200 m during the current reversal in early September 2008.
It arrived in the vicinity of the 500 m isobath on 12 September, at the position of LMP. There, its speed increased
from about 0.2 m/s to as much as 0.8 m/s and its direction
changed dramatically from northwest to exclusively down
coast. The drifter entered the continental shelf (shoreward of
the 200 m isobath) at the latitude of ARN. There, its velocity
varied between about 0.2 and 1.05 m/s in a succession of
accelerations and decelerations. When passing by ARE on
16 September, the speed of the drifter was very similar to
the current velocity measured by the mooring at 50 m depth.
Both were about 0.7 m/s. The greatest speed was reached
when the drifter approached LNK on 20 September. Again,
very good consistency was obtained between the velocity
of the drifter and of the mooring, recording respectively 1.05
and about 0.8 m/s at 50 m depth and up to 1 m/s at the
surface. The drifter then spent 4 days traveling between
LNK and IT1, with speeds in the 0.3 m/s range, and experienced an intense acceleration in the vicinity of IT1 and
CTZ, consistently with the measurements at the two moorings. After entering the bay in front of the City of Coatzacoalcos (19.15°N–94.43°W), the drifter’s speed decreased
to less than 0.4 m/s.
4.2. The Spring/Summer Regime
[40] From a climatological point of view, upwelling
favorable winds on the TAVE shelf mostly blow during
summer conditions (about April through August) and have a
southerly to southeasterly orientation. The current velocity
profiles of the moorings LMP, ARN and LNK are shown in
Figures 6a, 6c and 6e, with the daily alongshore wind stress
superimposed, from mid‐March to July 2009. The color
code is the same as for Figure 5. Also plotted are the water
temperature variations at 14 m above the seafloor measured
at the same moorings.
[41] According to our data, southerly (up coast) winds
prevailed from mid‐April onward, occasionally interrupted
by intense down‐coast events. Most of the time at ARN and
LNK the currents flowed up coast near the surface and
reversed down coast near the bottom. This configuration
became more and more obvious southward, and as summer
approached, with the nodal point of the current reversal
gradually moving closer to the surface. Similar vertical
structure was also seen at LMP from about the end of May
onward. The most intense up‐coast currents were reached at
LMP with values up to about 90 cm/s, decreasing southward
to around 25 cm/s at LNK. Although the up‐coast currents
at ARN and LMP were very consistent with the wind forcings, they were likely to be enhanced by the presence of the
anticyclonic LCE Cameron, which collapsed along the
continental shelf at the latitude of ARN in February 2009.
From March to May, this eddy gradually traveled north,
impacting the circulation in the vicinity of LMP.
[42] Sharp periods (about 2 days) of down‐coast currents
through the entire water column came with the intense
down‐coast wind events. Maximum speeds were often
reached at middepth or below and varied between about
40 and 60 cm/s, being highest to the south of the area. The
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water temperature systematically warmed up 2 to 5 degrees
during those northerly winds as revealed by the Figures 6b,
6d and 6f. The most significant warming events were
observed at ARN at the end of March and the beginning of
April, when the near bottom temperature at 130 m reached
about 24°C, which is similar to the mean summer temperature at 40 m depth according to the numerical results of
Zavala‐Hidalgo et al. [2003]. For the other warming events,
the temperatures increased from less than 20°C, to values
between 21 and 23°C. On the contrary, the up‐coast winds
tended to maintain water temperatures in the 17 to 20°C
range, which is about 1 to 4 degrees less than the climatology of Zavala‐Hidalgo et al. [2003].
4.3. Influence of Small‐ and Medium‐Scale Eddies
[43] From February to May 2009, the Loop Current Eddy
(LCE) “Cameron” collapsed against the continental shelf of
the GoM at the latitude of ARN and gradually traveled up
coast while decaying. The mean Sea Surface Height
Anomaly (SSHA) in February and May 2009 is shown in
Figure 7 and illustrates the existence of the anticyclone
paired with a cyclonic eddy located farther north, at the level
of PER. Considering the position of the two eddies, they are
expected to have influenced the circulation at PER, ARN
and LMP. During the same period, the monthly wind stress
headed up coast at LMP and was very low at ARN as seen
in Figure 2.
[44] In February and March, the alongshore surface currents at ARN and LMP were qualitatively consistent with
the alongshore wind stress, running up coast. However,
from a quantitative point of view, the current at ARN was
faster than at LMP by at least a factor of 2, when the LCE
was located in its vicinity. The influence of Cameron on the
intensity of the up‐coast flow is strongly confirmed by the
trajectory and speed of the drifter that entered the continental slope and shelf in February (Figure 7). According to
the altimetry, the westward displacement of the drifter was
likely driven by Cameron’s circulation as it approached the
northern BoC. Once entering the shelf, the drifter experienced an up‐coast acceleration from about 0.5 m/s when
drifting shoreward, to 1.07 m/s upon arrival to the vicinity of
ARN. Speeds near 1 m/s were maintained until it reached the
convergence area between the anticyclone and the cyclone.
There, the velocity dropped to 0.4 m/s in conformity with the
lower current recorded by LMP at the same time.
[45] In April and May, the decaying LCE traveled north,
closer to LMP. The monthly surface currents at this mooring
increased to about 0.5 m/s under the influence of the anticyclone. Again, the motion of the drifter that entered the
continental slope and shelf in May readily illustrates the
influence of the local eddies on the slope and shelf circulation. It flowed exclusively up coast with increasing speed
from LNK up to the convergence area between the two
eddies, located south of PER. The highest speeds (in the
1 m/s range, up to 1.18 m/s) of the drifter were reached
while traveling along the western edge of the decaying LCE.
The speed dropped upon arrival to the convergence area, as
it was advected seaward by the circulation associated with
the anticyclone and the adjacent cyclone. There, the motion
of the drifter was dominated by the eddy current as it circled
cyclonically and took a down‐coast/shoreward direction
along the western edge of the cyclone, in accordance with
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Figure 6. (a, c, and e) Color maps of the alongshore current velocities measured by the moorings LMP,
ARN and LNK, respectively, in cm/s; the color scale is the same as for Figure 5. Superimposed are the
alongshore wind stresses at the position of the moorings. Positive (negative) values are up coast (down
coast). (b, d, and f) Temperature at 14 m above the seafloor at the position of the moorings.
the direction of the currents measured at PER. The drifter
was then expelled a second time from the outer shelf after
entering the convergence zone.
[46] From February to May, the cyclonic eddy paired with
the LCE remained in the vicinity of PER. The currents measured by the mooring kept flowing down coast during that
period, in accordance with the cyclonic circulation forced by the
eddy, but against the high to moderate alongshore wind stress.
5. Vertical Structure of the Currents
5.1. Empirical Orthogonal Function Decomposition
[47] Empirical orthogonal function (EOF) analysis in the
time domain has been performed on the alongshore current
profiles following the method described in Emery and
Thomson [1998]. The results of the decomposition are presented in Figure 8, and Table 1 gives the value and lag times
of the maximum correlation coefficient between every pair
of first principal components.
[48] The first EOFs account for 85 to 91% of the total
variance of the alongshore velocity profiles. The spatial
modes outline is very similar from one mooring to another,
showing a quasi‐barotropic profile with little variations
within the upper 60 to 80 m of the water column, then a
gradual decay bottomward.
[49] The first principal components represent the variations with time of the first EOFs. Positive values account for
up‐coast currents, and negative values account for down‐
coast currents over the entire water column. Most of the
general features already reported for the surface currents are
also observed in the variations of the first principal component, some of which are (1) the down‐coast currents
observed almost all along the coast in September–October,
with the principal components taking important negative
values, (2) the effect of the LCE collapsing on the continental break in late February 2009, driving up‐coast currents
at ARN, and (3) the increase of the up‐coast currents during
spring‐summer in the TAVE shelf.
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Figure 7. Average sea surface height anomalies in (left) February and (right) May 2009. Solid (dashed)
lines are positive (negative) anomalies; thick line is the isoanomaly 0. Superimposed is the path of the two
drifters that entered the continental shelf in the same period. They entered the continental shelf to the
south of the TAVE shelf and traveled up coast. The color scale stands for the velocity of the drifters,
in m/s.
[50] Very little variability of the principal component was
experienced to the north of the TAVE shelf, in the area of
PER, as can be seen from the variance preserving spectra of
Figure 8, confirming the trend already observed at the surface. Also, almost no significant correlation has been found
between this mooring and the others located down coast as
can be seen on Table 1.
[51] Low but significant correlation is found between
LMP and the moorings farther south. There the variability
rises, most of which is being captured by periods greater
Figure 8. (left) First empirical orthogonal function at moorings PER to IT2. (middle) First principal
components and percent of variance explained by mode 1. (right) Variance preserving spectra of first principal components.
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Table 1. Maximum Correlation Coefficients (rmax), Lags and Corresponding Propagation Speeds Between the First Principal Componentsa
PER
LMP
ARN
LNK
IT1
CTZ
IT2
Mooring
rmax
Lag
rmax
Lag
rmax
Lag
rmax
lag
rmax
Lag
rmax
Lag
rmax
Lag
PER
LMP
ARN
LNK
IT1
CTZ
IT2
C
1
×
×
×
0.30
×
×
×
0
×
×
×
79
×
×
×
1
0.48
0.36
0.44
0.43
0.38
3.40
×
0
12
27
35
51
80
×
0.48
1
0.73
0.62
0.63
0.54
3.42
×
−12
0
15
28
39
66
×
0.36
0.73
1
0.67
0.69
0.68
3.97
×
−27
−15
0
10
19
40
0.30
0.44
0.62
0.67
1
0.92
0.68
4.01
−79
−35
−28
−10
0
12
28
×
0.43
0.63
0.68
0.92
1
0.71
3.64
×
−51
−39
−19
−12
0
22
×
0.38
0.54
0.68
0.68
0.71
1
3.24
×
−80
−66
−40
−28
−22
0
a
Lags are in hours. Correlation coefficients and lags below the 95% significant level were discarded. Positive lag means that the mooring under the
column heading is leading the mooring under the row heading. C values correspond to the experimental propagation speed (m/s), computed through
linear regression between the alongshore distance separating 2 moorings, and the lag of maximum correlation. Only the 6 last moorings had a
sufficient number of experimental points for a linear regression to make sense.
than 5 days. Specifically, the 5 to 8 day and 15 to 20 day
bands seem to take most of the variance from LMP down to
IT2, with greatest values at ARN, decreasing down coast.
The correlations within the pool of the 6 southernmost
moorings are systematically significant with largest values
encountered between the moorings of the Tamaulipas and
BoC shelves. They take values between 0.36 and 0.92, with
0.60 on average (0.69 when only the moorings of Tamaulipas and BoC shelves are considered). The associated lags
are consistently positive in the down‐coast direction and
negative in the up‐coast direction, characterizing a down‐
coast propagation rate in the variations of the alongshore
current. Also, for the six down‐coast moorings, the lag of
maximum correlation depends linearly on the alongshore
distance, with an average slope of 3.59 m/s.
5.2. Coherence of the Barotropic Signal Along
the Coast
[52] Considering the low correlation of all the moorings
with PER, the latter has been discarded from the present
analysis. From the 6 time series of principal components
determined independently at the 6 remaining moorings, we
can look at the cross coherency between the principal
component time series at LMP and any other mooring
(Figures 9a–9e).
[53] The cross coherency of the alongshore current at
LMP with all the other moorings is significant at the 95%
level for periods ranging from 6.1 to 10.7 days. A second
band, around the 3 day period, also shows some significant
coherency except at LNK. This band however seems of
secondary importance compared to the 6.1 to 10.7 day band
which shows the greatest and most consistent values from
one mooring to another. The coherence in this band remains
high all along the TAVE shelf, with highest values at LNK.
For further analysis, the periods marked by the solid circles
in Figures 9a–9e were investigated. They are 8.5, 7.1 and
6.1 days and were picked off because they have the highest
squared coherency on average, over the entire set of moorings.
[54] Figures 9f–9h show the lag observed between two
time series of principal components at a specific frequency
(or period), plotted against the alongshore distance separating the moorings corresponding to those time series. The
lags were determined from the phase difference between
the 2 moorings considered, at the appropriate frequency.
With 6 series to analyze and accepting all of the positive
phase differences, there are 15 different pairs of time series.
Some degree of redundancy is introduced in the process as,
for example, propagation from ARN to LNK and from LNK
to IT1 requires propagation from ARN to IT1 provided that
the coherences are large. However, including all the displacement/phase points allows to form an unbiased estimate
of the alongshore propagation rate. Phase differences corresponding to squared coherency values below the 95%
confidence level were discarded.
[55] At a period of 6.1 days, all the displacement/lag
pairs are tightly aligned along a propagation rate of about
4.35 m/s, calculated from the orientation of the principal
axis of variance of the point cloud. Its slope implies a down‐
coast propagation of the up‐coast signal. At 7.1 and 8.5 day
periods, the scatter of points around the fitted line is slightly
larger than at 6.1 days, and the phase propagation speeds
decrease significantly to 3.88 and 3.82 m/s respectively with
a down‐coast direction. A striking result is the low scattering of the experimental points reported on the dispersion
diagram, Figure 9i. The three wave numbers were computed
for each frequency as ki = w/Ci, Ci being the phase speed
estimated and reported on Figure 9f–9h. The phase speed
deduced from this diagram, 4.00 m/s, is the harmonic mean
of the Ci.
[56] The squared coherency between the alongshore wind
stress and the first principal components is shown in Figure
10. It is significant in the 6.1 to 8.5 day band whether the
local wind (dotted lines) or the wind at LMP (solid line) is
considered. Only at IT2 is the squared coherency between
the local wind stress and the principal component not significant at 6.1 days. At all moorings, the wind stress at LMP
appears to have better coherence with the alongshore currents than the local wind. The coherence is particularly high
at ARN, LNK, IT1 and CTZ in the 6.1 to 7.1 day band when
considering the wind at LMP. From the phases of the
coherence analysis, the wind signal at LMP would lead the
current signal by about 12 h at the same mooring. Then,
the lags with the moorings farther down coast increase with
the distance from LMP at a mean rate of about 4.2, 4.3 and
4.6 m/s for 6.1, 7.1 and 8.5 day period, respectively.
[57] The squared coherency spectra between the alongshore wind stress at LMP and the sea surface height
anomaly at Port Isabel (located at the U.S.–Mexico border,
at about 26°N) and the City of Veracruz are plotted on
Figure 11a. When compared with the sea level in Veracruz
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However, no significant coherence is found between the
sea level height anomalies at both locations in the 6 to
10 day band.
6. Discussion
Figure 9. (a–e) Cross coherencies of the first principal
components at LMP with the other moorings. The dotted
lines stand for the 95% confidence level for 16 degrees of
freedom. The solid circles identify the frequencies used in
Figures 9f–9i. (f–h) Phase difference of first principal components plotted against alongshore displacement. Every possible pairs of displacement/phase between the moorings
south of LMP with coherence over the 95% confidence level
are reported for each frequency. The solid line stands for the
phase speed computed from the major axis of the variance
ellipse. (i) Experimental dispersion diagram, with bold line
having a slope equal to the harmonic mean of the individual
phase speed estimates, listed on Figures 9f–9h.
(solid line), the coherence values are significant in the
20 day, the 6 to 10 day and about 3 to 4 day bands. The 6 to
10 day band is barely significant, but it takes values much
higher than when the analysis is carried out between the
same wind and the sea level at Port Isabel (dotted line), for
which the coherence is not significant in that band. At both
locations the highest coherences are reached in the 20 day
and 3 to 4 day bands, taking similar values at the City of
Veracruz and Port Isabel. The cross correlation function
between the sea level height at Veracruz and Port Isabel
shows two maxima taking values of 0.76 and 0.74 with Port
Isabel leading Veracruz by 72 and 11 h respectively. Most
of the correlation value is captured by coherent variations of
the sea level heights in periods longer than about 20 days or
in the 3 to 4 day band as can be seen in Figure 11b.
6.1. Comparison With Climatological Currents
[58] Our observations generally agree with the seasonal
trends of up‐coast and down‐coast flows shown by the
numerical results of Zavala‐Hidalgo et al. [2003], particularly along the Tamaulipas and Veracruz shelves. In their
case, the surface currents were driven by the climatological
wind stress, which changes from blowing up coast to down
coast in September, returning to the up‐coast conditions in
May. Drifter trajectories suggest that the flow is continuous
all along the shelf in both seasons. In addition, the measured
sea level changes along the TAVE shelf were in phase with
the low‐frequency variations of the observed down‐coast
currents during fall‐winter. Both results are consistent with
the idea of shelf flow driven by cross‐shelf pressure gradients. Considering the high correlation we observed
between the wind stress and the sea level variations in fall,
we suggest that the along coast wind stress would cause the
variations of the coastal sea level through Ekman transport.
The cross‐shelf pressure gradient would, in turn, force the
low‐frequency variations of the alongshore current.
[59] The period covered by the observations also shows
significant differences with the modeled climatological
currents. Along the Mexican/U.S. border (mooring PER),
for example, surface current variations show weak dependence with the local winds, and were mostly driven by the
presence of eddies interacting with the shelf during most of
the measurement period. The next two paragraphs address
further differences.
[60] The fall‐winter wind reversal was 4 months shorter
than the climatological one, with winds blowing down coast
only in September and October. During that period, the
surface currents flowed down coast all along the shelf from
the Mexican/U.S. border to the WCB. The strongest down‐
coast winds and currents were observed along the shelf of
Veracruz, specifically in the western and southern BoC area.
This contrasts with the weak mean currents observed in the
southern shelf during the rest of the year and showing no
preferred direction of flow. Both the wind and current
reversals were in phase within about 5 days according to
Figure 5h. Similar swift reversal along the Louisiana‐Texas
shelf in fall were reported by Nowlin et al. [1998], suggesting a continuity of this phenomenon all along the
shelves of the northern and western GoM.
[61] The currents measured in the WCB flowed down
coast between September and October, while the local
alongshore wind was blowing in the opposite direction. This
contrasts with the modeling results of Zavala‐Hidalgo et al.
[2003], which show both currents and winds flowing up
coast throughout the climatological year. Hence, our measurements do not suggest an obvious convergence of the
shelf flow caused by opposing currents in the southern Gulf of
Mexico during the fall‐winter. Instead, we observe convergence of the mean flow in the southern shelf during the strong
wind reversals of September and October, caused by differences in strength rather than in direction of the mean currents.
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Figure 10. Cross coherencies of the alongshore wind stress at LMP with the first principal component of
the moorings LMP to IT2 (solid line). Cross coherencies of the local alongshore wind stress with the first
principal component of the moorings ARN to IT2 (dotted line). The dashed lines show the 95% confidence level for 16 degrees of freedom.
6.2. Vertical Structure
[62] The first EOF modes at all the moorings show a
quasi‐barotropic structure and account for at least 86% of
the variance of the alongshore current. The most significant
variations of the current were therefore coherent over the
water column. Specifically, during September and October
2008, when the current patterns were likely to be typical of
the climatologic fall‐winter regime, the flow was mostly
down coast at all depths and the strongest accelerations were
usually experienced throughout the entire water column.
This quasi‐barotropic behavior is consistent with the low
stratification of the water column during the fall‐winter
regime, when the first principal components are mostly high
in absolute value. The homogenization of the water column
is supported by wind induced vertical mixing and possibly
by water accumulation at the coast which leads the thermocline to deepen [Gill and Clarke, 1974]. Indeed, although
not represented, the temperature along the TAVE shelf at
14 m above the seafloor tends to increase from about 20°C
or below at the beginning of September to mostly more than
22°C when the fall‐winter regime develops. The thermo-
cline would therefore be maintained deeper and seaward of
the position of the moorings. In addition, the fresh water
inputs from the Mississippi and Atchafalaya rivers brought
by the down‐coast currents remain in the area shallower
than the 50 m isobath [Zavala‐Hidalgo et al., 2003] and do
not impact the stratification close to the shelf break.
[63] On the contrary, when the spring‐summer regime
develops from May onward, vertical stratification in the
upper layers increases due to warm and low‐salinity waters
at the surface but cooler waters at depth due to upwelling
development [Zavala‐Hidalgo et al., 2006]. This would
cause the pycnocline to migrate shoreward and rise above
the depth of the mooring. Higher stratification favors a
baroclinic‐like structure of the water column. As a consequence, the current shows a directional inversion in the
vertical, shifting gradually from up coast at the surface to
down coast at the bottom. Also, its temporal variability
decreases probably because the wind regime is more stable
in spring‐summer than in fall‐winter. The current direction
is coherent in the vertical only when short northern wind
events force the current to flow down coast at all depths.
Figure 11. (a) Cross coherencies of the alongshore wind stress at LMP with the sea surface height anomaly at the City of Veracruz (solid line) and Puerto Isabel (dotted line), etc. (b) Cross coherencies of the sea
surface height anomalies at the City of Veracruz and Port Isabel. The dashed lines show the 95% confidence level for 16 degrees of freedom.
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The increase of the water temperature near the bottom
demonstrates the deepening of the thermocline during these
events and suggests a temporary destratification of the water
column near the shelf break.
6.3. Influence of Small‐ and Medium‐Scale Eddies
[64] Comments on the influence of slope eddies on the
circulation over the Mexican shelf of the GoM are mostly
brought by Walker [2005]. The anticyclone/cyclone pair she
describes is very similar to the one commented in this
article. The trajectory of drifters entering the continental
shelf confirms that the convergence area between the two
eddies favors seaward cross‐shelf transport. On the contrary,
the northern and southern edges of the dipole would favor
shoreward motions, as inferred from the trajectory of the
same drifters before they enter the continental slope or shelf.
Also, the purely shoreward mean surface current obtained at
PER in March, while the mooring was located at the northern
edge of the cyclone paired with the LCE, supports the possibility of eddy driven shoreward cross‐shelf transport.
[65] The slope eddies have been shown to influence the
alongshore circulation as well. For example, the circulation
at PER is found to be independent from the currents measured at the moorings farther down coast and is not significantly correlated with the wind forcing. On the contrary, the
local current is very consistent with the existence of small to
mesoscale cyclonic eddies along the shelf break during most
of the measurement period, which is probably the reason
for the specificity of the current variations in this area. The
strong up‐coast current reaching more than 1 m/s according
to drifters and mooring measurements along the TAVE shelf
closely fit with the presence of the anticyclonic LCE
“Cameron” in the area as well. In February 2009, the drifter
we focused on flowed mostly between 100 and 200 m of
bathymetry and exceptionally shallower than 100 m along
the western edge of LCE Cameron (Figure 7). The slight
drop in the drifting velocity observed between 22.5 and 23°
N occurred precisely while the drifter was in water shallower than 100 m. This suggests that the influence of the
LCE was still strong between the 50 and 100 m isobaths but
started decaying shoreward of about 100 m depth.
[66] The lower values of the correlation coefficient
between the alongshore wind stress and the alongshore currents obtained from our measurements compared with the
ones obtained by Zavala‐Hidalgo et al. [2003] give less
credit to the wind driven circulation, and more credit to the
forcing by slope eddies. Indeed, the wind driven circulation,
considered as being the major forcing from a climatologic
point of view, can be dominated by the erratic influence of
transient eddies locally and temporarily.
[67] However, it is still not clear how far onshore this
influence can be felt, and how much the onshore excursion
of the eddy is related with its size and strength. For example,
although Cameron reached the outer slope in early February,
it is not until it started to decay that its influence at 130 m
could be recorded at ARN and LMP (March through May).
[68] Finally, the circulation along the outer shelf of the
BoC is likely to be influenced by the variations of the intensity
of the semipermanent cyclonic gyre present in this area,
reported by Vazquez de la Cerda et al. [2005] and DiMarco
et al. [2005]. More research on the influence of small‐ to
medium‐scale eddies on the TAVE and WCB shelves circu-
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lation should help to evaluate how much and how far into the
shelf those structures impact the current variability.
6.4. Evidence of Coastally Trapped Waves
[69] Variable alongshore winds blowing over a sloping
and stratified continental shelf are a well known generation
factor of coastally trapped waves (CTW), whose properties
and governing equations are extensively described by Gill
and Clarke [1974] and Clarke [1977]. The free propagation of those waves away from the generation area is
exclusively equatorward (poleward) in the western (eastern)
boundaries of the oceans. Evidences of propagation will be
observed as periodic water movements farther along the
coast, where the wind or other generation factors are no
longer related to them. Those movements partly consist of
alongshore current fluctuations that are in phase and in near
geostrophic balance with rather modest sea level variations.
Typical oscillation periods of such waves range from a few
days to 2 or 3 weeks for wavelengths in the order of 2000 km.
[70] The cross coherencies of the first principal components of the alongshore current profile between LMP and the
other moorings located farther down the coast are very
significant for periods ranging between 6.1 to 10.7 days.
The wavelengths computed from the phase lags between the
different moorings in this band range from about 2300 km to
2700 km for an average phase speed of 4.00 m/s. These
numbers are within the range expected for CTW and to our
knowledge, no phenomenon other than CTW propagating
along the coast would present such characteristics of direction
of propagation, frequencies, phase speeds and wavelengths.
[71] If the computed wave phase speed of 4.00 m/s is
considered to be associated with the first baroclinic mode of
the coastally trapped waves, then its offshore length scale is
the Rossby radius of deformation given by a = c/f, where
c is the phase speed of the wave, and f is the Coriolis
parameter. The Rossby radius takes values of about 65 km
in northern Tamaulipas, increasing southward to about 85 km
in the southern BoC and WCB. Due to relatively narrow
shelves in the State of Veracruz and the southern BoC area,
the coastally trapped waves are therefore expected to influence the circulation over the shelf and slope, up to about the
2000 m isobath. Wider shelves of the WCB and the State of
Tamaulipas limit their influence to, at most, the shelf region,
and place the mooring PER offshore of this area. Along with
the local eddies influence mentioned previously, this characteristic may participate in the peculiar current regime
measured at this mooring compared to the others.
[72] The wind stress variance in the 6 to 10 day band
as plotted in Figure 12 is much higher at the moorings PER
and LMP than at the other moorings. Wind induced CTW in
this frequency band are therefore more likely to be generated in the northern TAVE shelf than farther down the coast.
Additional results suggest that the CTW would be generated
in the area of LMP rather than PER.
[73] First, a very meaningful result giving credit to a
generation area located near LMP is that the cross coherency
between remote winds and the first principal components
in the 6 to 10 day band is significant and highest when
the wind is taken at LMP than when it is taken locally
(Figure 10) or at any other mooring (not shown).
[74] Second, Figure 11b shows that the sea surface height
anomalies in Port Isabel and Veracruz are not coherent in
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Figure 12. Mean variance of the alongshore wind stress in the 6–10 day band at the location of the
moorings.
the 6 to 10 day band, suggesting that CTW are unlikely to
propagate between these 2 sites. The existence of wind
forced CTW at Port Isabel is therefore doubtful, bringing
their generation area farther south.
[75] These observations are similar to those expected for
CTW generated in the area of LMP and propagating freely
down the coast. The most striking evidence of this free
propagation is the very significant cross coherency between
the wind fluctuations at LMP and the alongshore current at
IT2, located about 950 km down the coast, whereas the
cross coherency between the current and the local wind at
IT2 is barely significant in the same band. Also, current
accelerations experienced at IT1 and CTZ while the local
alongshore wind is low and no transient eddy is present in
the vicinity (Figure 5, W6 event) would suggest a remotely
forced phenomenon.
[76] Finally, very strong squared coherency is obtained in
the 3 day band. Considering the time lag of about 6 h for this
band, with Port Isabel leading Veracruz, the down‐coast
phase speed of the signal is about C = 30 m/s. If one supposes that this value is associated with the propagation of 1a
barotropic Kelvin wave, then it is given by: C = (gH)2 ,
where g is the acceleration due to gravity. A depth H = 86 m
is obtained, which could be interpreted as the average depth
of the continental shelf. From the very high squared
coherency obtained in Figure 11a, we would suggest that
such barotropic Kelvin wave is wind induced. Furthermore,
a visual analysis of the band passed series of sea surface
height anomalies at both locations around the 3 day frequency band (not shown), shows that these events appeared
simultaneously with hurricanes and tropical storms hitting
the southern Texas coast during 2008 and to winter fronts
propagating rapidly from the north during the 2007–2008
winter. Therefore, these events appear to be related to storm
surge phenomena propagating down coast.
7. Conclusion
[77] This article describes the circulation over the continental shelf break of the western and southwestern Gulf of
Mexico using a combination of onsite measurements of
current velocities from moorings, drifter trajectories, satellite altimetry and wind model outputs. Along the Tamaulipas‐Veracruz shelves (TAVE), the differences between the
climatologic circulation from numerical models outputs and
the current patterns presented in this article are partly
attributed to the quite unusual wind regime experienced
during the period studied. This is consistent with the dominance of the wind driven circulation already proposed by a
number of authors. Specifically, the down‐coast current
settling along the TAVE shelf, which is characteristic of the
fall/winter regime, only lasted about 2 months. Accordingly,
the steady down‐coast winds that usually blow during the
entire fall and winter seasons were active only during those
two months. In addition, northerly wind forcing is shown to
generate coastal accumulation of waters brought by onshore
Ekman transport. The cross‐shore pressure gradient would
force a down‐coast jet with current velocities between 0.5 to
1 m/s. Along the shelf of the Western Campeche Bank
(WCB), the circulation is found to be conditioned by remote
rather than by local wind forcing. It was found that the
current is poorly correlated with the local wind in this area.
We also emphasized the influence of small‐ to medium‐
scale transient eddies as a cause of the differences between
the climatologic circulation and the current measurements.
Their presence looks erratic and they are very likely to play
a major role in the interannual variability of the alongshore
currents as they can overwhelm the wind driven circulation.
[78] In order to investigate the existence and generation of
coastally trapped waves propagating along the western GoM,
we have carried out cross‐coherency analysis involving
(1) the principal component of the first EOF mode of the
alongshore current profile, (2) the alongshore wind stress at
the position of the moorings, and (3) the sea level height
anomalies at Port Isabel and the City of Veracruz. The
direction of propagation, frequencies, phase speeds and
wavelengths of the coherent signal led us to conclude the
existence of coastally trapped waves generated by the wind in
the northern TAVE shelf, and propagating down coast down
to the WCB.
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[79] Acknowledgments. Data gathering and analysis are financed by
Convenio PEMEX Exploración y Producción‐CICESE 428229851, Medición
y análisis metoceánico del Golfo de México, etapa 2009 2013. We thank
everyone involved in the planning, deployment, recovery, and data processing
of the moorings: the CANEK group (Centro de Investigación Científica y
de Educación Superior de Ensenada) and the crew of R/V Justo Sierra
(Universidad Nacional Autónoma de México). We show our gratitude to Paula
García Carillo for the processing of the drifter trajectories. We appreciate the
continuing support with the drifter program by Eddy Watch group at Horizon
Marine Inc.
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J. Candela, J. Dubranna, M. López, and P. Pérez‐Brunius, Centro de
Investigación Científica y de Educación Superior de Ensenada,
Departamento de Oceanografía Física, Carretera Ensenada‐Tijuana 3918,
Zona Playitas,C.P. 22860, Ensenada, B. C. México. (dubranna@gmail.
com)
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