Open Journal of Marine Science, 2012, 2, 141-149 Published Online October 2012 (
Hydraulic Jump in the Gulf of California
David Salas-Monreal1, David Alberto Salas-de-Leon2, María Adela Monreal-Gomez2,
Mayra Lorena Riverón-Enzástiga3, Erika Mojica-Ramírez3
1Instituto de Ciencias Marinas y Pesquerias, Universidad Veracruzana, Veracruz, Mexico
2Instituto de Ciencias del Mar y Limnología, Mexico City, Mexico
3Posgrado en Ciencias del Mar y Limnología, Mexico City, Mexico
Received June 8, 2012; revised July 3, 2012; accepted August 1, 2012
Acoustic Doppler current profiles and water density profiles were measured over the 280 m deep continental slope of
the Gulf of California to elucidate the bathymetric effect on zooplankton distribution. These measurements were com-
bined with water velocity and density simulations from the Regional Ocean Model System with and without the influ-
ence of Coriolis acceleration. The data revealed an acceleration of the near-bottom flow as it moved toward increasing
depths. This acceleration was produced by the adjustment of the isopycnals to bathymetry (hydraulic jump). Zooplank-
ton patches moved downward at the continental slope and then upward, thus exhibiting wave patterns. Model outputs
without the effect of Coriolis acceleration also suggested that vertical zooplankton concentration followed a wave pat-
tern. However, when Coriolis acceleration was added to the momentum equation, the horizontal zooplankton distribu-
tion was enhanced, which reduced the vertical zooplankton concentration observed over irregular bathymetries. Coriolis
acceleration was responsible for horizontal dispersal of up to 20% of the total zooplankton concentration located over
the wave trough.
Keywords: Hydraulic Jump; Acoustic Doppler Current Profiles; Internal Waves; Zooplank ton Biovolumes; Gulf of
1. Introduction
An understanding of the stratified flow dynamics over
irregular bathymetries is crucial for predicting flow pat-
terns [1], lee wave generation [2], and their interactions
with planktonic and organic matter concentration [3-7].
Such interactions depend on the degree of stratification
and on the strength of tidal forcing [8]. Internal waves
generated over irregular bathymetries explain part of the
temperature and salinity variations observed in the water
column [2]. Therefore, the vertical variation of the py-
cnocline, where most particulate and dissolved organic
matter are concentrated [9], could be related to irregular
bathymetries such as the continental slope [10], subma-
rine canyons [7], and seamounts [11].
Increased vertical and horizontal nutrient flux occurs
in areas with abrupt bathymetric changes where internal
waves break and produce mixing and increase primary
productivity. For example, in Mon terey Bay, one-third of
the planktonic production was associated with amplified
internal wave activity caused by the bathymetry of the
Monterey Canyon [12]. Vertical and horizontal water ex-
change in areas with abrupt bathymetric changes is also
attributed to shear instability [13]. The vertical displace-
ment of high-nutrient deep-water enhanced by internal
wave activity has a major ecological impact in adjacent
waters [14,15]. Therefore, an understanding of current
velocities and vertical water density variations induced
by internal wave activity is crucial to describe near-sur-
face concentration of planktonic organisms, nutrients,
and detritus.
The amplification of internal waves over the con tinen-
tal slope are not commonly observed, therefore numeri-
cal models have described the near-surface concentration
of organisms over internal wave troughs [16]. The near-
surface distribution of planktonic organism over bathy-
metric changes is usually assumed to be related to cyc-
lonic eddies, nutrients, light penetration, and amplified
internal wave activity [17]. However, the asymmetry of
the water mass exchange is attributed to Coriolis accel-
eration [18]. The goal of this study is to advance our un-
derstanding of the distribution of zooplankton and its
dependence on amplified internal wave activity induced
by bathymetric changes such as the continental slope,
under the influence of Coriolis acceleration.
2. Data Collection and Processing
2.1. Data Collection
Current profiles and backscatter intensity data from an
opyright © 2012 SciRes. OJMS
acoustic Doppler current profiler (ADCP) were obtained
over the 280 m deep continental slope of the Gulf of
California to elucidate the effect of bathymetric depres-
sion on zooplankton concentration. The along-transect
bathymetry over the continental slope was measured with
an EA600 echo-sounder, which was also used to observe
zooplankton concentration [19]. Those data come from
an 11 days cruise performed on the western Gulf of
California. However, this paper is mainly focus on the
data collected on February 27, 2006 near a canyon lo-
cated in the Gulf of California close to Carmen Island
(Figure 1) when a hydraulic jump was observed. The
hydraulic jump was detected using the ratio of the hori-
zontal advective acceleration term versus the bottom
friction term and the Froude number [20]. The transect
was oriented with the axis of maximum standard devia-
tion (principal axis) of tidal cu rrents [18] in order to em-
phasize the velocity structure over the continental slope.
During transect sampling, current velocity profiles and
backscatter were recorded with a 150-kHz vessel-mounted
ADCP. Th e ADCP ping rate of 1 Hz was avera ged every
5 s, yielding a horizontal resolution of approximately 25
m and a vertical resolution of 5 m. The ADCP compass
was calibrated using a global positioning system data set
following Trump and Marmorino [21].
The density profiles were obtained from a conductivity-
temperature-depth (CTD of the Niel Brown company,
version IV) along transect A-B (Figure 1). The concen-
tration of zooplankton within the water column was
estimated using an echo-sounder operating at a single
frequency (32 kHz) combined with backscatter outputs
from the 150-kHz ADCP. Both the echo-sounder and the
ADCP were mounted on the Research Vessel “El Puma”
from the National Autonomous University of Mexico.
In addition, a time series of velocity data was recorded
on February 27, 2006 to observe the tidal phase when lee
waves were observed over the continental slope. The
station was located over the continental slope (@ station
in Figure 1) (111.038˚W; 25.846˚N). To emphasize the
velocity structure over the bathymetric slope, the time-
series velocity data were modified by rotating the cali-
brated velocities to the angle of their maximum standard
deviatio n [ 2 0].
To determine the relative abundance of the major zoo-
plankton groups and to calibrate the acoustic-scattering
data, double oblique tows of paired 60 cm mouth diame-
ter Bongo nets fitted with 333 and 505 µm mesh and
calibrated flow meters were made for 15 minutes. Tows
were conducted from a maximum depth of 200 m or
from near the bottom in shallower zones to the surface at
a speed of 2 knots (1 m·s–1). Tows were conducted at 48
zooplankton data point along the 11 days cruise. The
zooplankton samples were fixed in a 4% formaldehyde in
seawater solution and preserved with 70% alcohol. The
Gulf of California
108 W
114 W
25.8 26.1
Carmen Island
Figure 1. EA600 echo-sounder and 150-kHz ADCP mea-
surements were taken along the A–B transect in the Car-
men Basin in the Gulf of California. The ADCP data series
was conducted at @ and the CTD profile station is indicated
by +.
samples were fractioned and analyzed in 1/32 and 1/8
subsamples for samples with high and low content of
zooplankton, respectively [22]. The organisms were
identified following Tregouboff and Rose [23].
2.2. Model Setup
Robertson [24] used the Regional Ocean Model System
(ROMS) to model internal tides and to estimate tidal
fields for studying circulation and mixing. This model
showed good agreement with observations of semidiur-
nal baroclinic tides. However, the diurnal K1 baroclinic
tides were poorly simulated. Robertson [24] found that a
resolution of 4 km was sufficient for a qualitative esti-
mate; whereas a resolution of 1 km reproduced most ac-
curately the major axes and mean velocities of the semi-
diurnal baroclinic tides. These results illustrate that the
ROMS model can reproduce the major features of baro-
tropic and baroclinic tidal currents.
Our model was set up following Shchepetkin and
McWilliams [25] and Moore et al. [26]. The outputs
were used to describe the dynamics of an idealized ge-
neric system similar to the one located near Carmen Is-
land. The three-dimensional primitive equations ocean
model [26] uses σ coordinates to increase the vertical
resolution at the depth of the internal wave. The model
simulates a 250 m depth slope with a horizontal domain
of 2 × 2 km alongshore and offshore, with 10 vertical
levels and a horizontal resolution of 125 m. The free sur-
face elevation, which uses a non-gradient open boundary
condition and the salinity, temperature, and water velo ci-
Copyright © 2012 SciRes. OJMS
ties at each grid point were recorded over a 6 day period
after the model reached stability. Bottom stress was as-
sumed to be a quadratic function of the bottom velocity
with a drag coefficient of 2.5 × 10–3 [27]. The model
started from a steady state with a uniform horizontal sa-
linity and temperature field and a vertical step stratifica-
tion of 1024 kg·m–3 at surface and 1027 kg·m–3 at the
bottom. The boundary conditions were obtained using
data from the ADCP (square transect performed around
the study area) and with tidal amplitude and phase ob-
tained from Salas-de-Leon et al. [18] and Carbajal and
Backhaus [28]. The potential and kinetic energy were
calculated for each grid point. The stability of the model
was analyzed using the potential and kinetic energy.
Once differences in energies from successive iterations
were on the order of 10–3 or lower, the model was con-
sidered to be stable; this occurred after 4 days of simula-
tions. The simulation s were run with and without consid-
eration of the Earth’s rotational effects in the momentum
equation in order to elucidate the relevance of Coriolis
acceleration in the continental slope dynamics and zoo-
plankton concentration. Although the ROMS is a hydro-
static model [29], the velocities obtained here were used
to compare the inertial versus gravitational forces (Frou-
de number).
The model does not account directly for zooplankton
or detritus concentration because it does not contain any
type of ecosystem model. However, it is assumed that
over irregular bathymetries, planktonic organisms and
detritus are concentrated at the pycnocline depth [3].
Although planktonic organisms are biologically active,
there are similarities between the diffusion of salinity and
the plankton advection [11]. Therefore, an analysis of the
salinity field was used as an approximation to describe
zooplankton distribution at the pycnocline depth. This
analysis was validated with observations from the EA
600 echo-sounder and the ADCP (Figure 2(a)), where
zooplankton concentrations were related to high values
of backscatter intensity of the ADCP and the EA 600
echo-sounder signal within the area studied.
The Reynods averaged Navier-Stokes equations used
in the model are:
Zu uZuvZu wZu
tx yZv
Zgu w
xx Z
 
 
 
 
 
 
Zv uZvvZv wZv
tx y
Backscatter (dB)
Depth (m)
60 80
Distance (km)
Depth (m)
Depth (m)
Figure 2. (a) The intensity contours from the backscatter
(zooplankton biovolumes) in dB and current velocities ar-
rows during ebb; (b) The transversal velocities in cm·s1 ob-
tained from the 150 kHz ADCP during ebb; and (c) The
echo-sounder observations. The heavy black line in a and b
represents the bottom and the grey zone represent zoo-
where u, v and w are water velocities in the x, y and σ co-
ordinate, respectively. Z is the vertical stretching factor. f
is the Coriolis parameter. P is the pressure. ρ and ρ0 are
the total density and the mean average density (reference
density). g is the acceleration due to gravity. υ is the mo-
lecular viscosity. The bars represent the time average of
the turbulent fluctuations. The continuity equation used
in the model is:
uZ vZ wZ
tx y
 
 
  (4)
where ξ is the averaged free surface elevation. Finally the
transport of salt or any tracer is given by:
source/sin k
tx y
sw S
 
 
 
Zgu w
yy Z
 
 
 
 
 
 
(2) where S is the tracer used in this study such as salt or
suspended material.
Copyright © 2012 SciRes. OJMS
3. Results and Discussion
3.1. Underway Transects
The 150-kHz ADCP and EA 600 echo-sounder observa-
tions showed a clear wave signal pattern during ebb tide
at a distance < 1.5 km from the starting point A of the
transect (Figure 2), as indicated by the backscatter inten-
sity (contours in Figure 2(a)) and echo-sounder (Figure
2(c)) data. The contours of the backscatter intensity and
the echo-sounder signal suggest an adjustment of the
isopycnal to bathymetry on the leeward side of the slope
[30]. The depth of the backscatter intensity from the
ADCP and the depth of the maximum values of the
backscatter intensity from the echo-sounder were highly
correlated among them during transect samplings (corre-
lation value of 0.87). The isopycnal adjustment to bathy-
metry increased the near-bo ttom flow at the slope, as de-
picted by the current velocities (arrows) during ebbs
(Figure 2(a)). The along-transect flow showed a dece-
leration of the upper-layer flow (at 200 m depth near km
0.7 and at 175 m depth at ~4.4 km in Figure 2(a), the
velocity changed from 0.5 to 0.25 m·s–1), where the
back-scatter intensity suggested a vertical increment of
the water surface with the same density; this scenario is
also consistent with higher transversal velocities (Figure
2(b)). In turn, the near-bottom flow showed an accelera-
tion, where a decrement of the water level with the same
density was observed (140 - 190 m depth). The back-
scatter intensity signal from the ADCP and the echo-
sounder also provide evidence of internal waves at the
pycnocline interface and over the sills.
As the flow moved from the first sill toward the sec-
ond sill (from km 4.5 to 2 in Figure 2(a)), the near-bot-
tom flow (at 225 m depth) decelerated (changes in ve-
locity from 0.6 to 0.2 m·s–1) due to mass conservation.
However, the mid-depth flow (~200 m depth) exhibited
areas where the flow accelerated and other areas where it
decelerated at the same depth, suggesting wave patterns.
Such patterns were previously suggested by the echo-
sounder and backscatter intensity data. Therefore, lee
waves generated over the continental slope near Carmen
Island should enhance nutrient exchange within the water
column when internal waves break, thus producing mix-
ing and making this an area of high biological producti-
Previous studies related zooplankton abundance to
high echo-sounder and backscatter intensity signals [31].
The observed zooplankton distribution over the water
column and the water accelerations (changes from low to
high velocities at the same depth) were further evidence
of the presence of lee waves over the slope during ebb
(Figure 2). Near the beginning of the slope transect, the
flow decelerated at mid-depths and near the bottom (Fi-
gure 2(a)) when compared to the end of the transect
(point B in Figure 2). This was due to bottom friction
and perhaps to tidal ph ase, as the two ends of the transect
were out of phase by approximately 0.41 h. However, the
mid-depth flow at the end of the transect, where the slope
starts to increase sharply, accelerated. This was due to
the hydraulic jump and the subsequent formation of lee
waves at this location (Figure 2), which constrained the
bottom flow to a smaller area, thereby increasing its ve-
locity because of mass conservation. At mid-transect
between the sills, the more homogeneous flow (between
km 2.5 to 4.5 in Figure 2) (i.e., relatively low vertical
shear of the horizontal velocity (uz)) when com-
pared to the transect ends diminished lee wave formation.
The Brunt-Väisälä frequency, calculated using the den-
sity values obtained from the CTD (Figure 3) at station +
(Figure 1), showed a maximum value near 90 m depth,
while at station A (Figure 3) the maximum value was
observed at 160 m depth.
The relaxation of the flow and the lee waves produced
a large zooplankton concentration over the water column
(at km 1.8 and 4.2 from the starting point in Figure 2)
where the internal waves broke. The pycnocline sug-
gested by the backscatter intensity at the slope (from km
2 to 3.5 in Figure 2) showed a more stable pattern, and
the wave pattern observed at the transect ends was lack-
ing since they were no longer under the influence of the
sills. Therefore, the areas of high biological productivity
observed near Carmen Island [32] (Figure 1) could be
related to lee wave activity that mainly was produced
Figure 3. CTD profiles and the Brunt-Väisälä frequency at
station + on February 27, 2006.
Copyright © 2012 SciRes. OJMS
when internal waves broke. The vertical displacement of
zooplankton, nutrients, and detritus from close to 170 m
depth to near the sea surface has a major ecological im-
pact within the area. Turbulence is usually related to nu-
trient flux and to nutrient suspension within the water
column [32]. When internal wave break the turbulence of
the water increase, therefore turbulence could be one of
the reason for the high surface biological productivity
observed near Carmen Island, owing to wave activity.
The backscatter intensity data suggest that surface wa-
ter was constricted to a smaller area over the sills during
ebb tide periods due to internal wave activity, and this
would have pumped zooplankton and nutrients to the
near-surface waters. However, the high productivity area
concentrated near the surface during maximum ebb
should have moved back to its original depth at the end
of the ebb once the flow relaxed due to buoyancy stabi-
3.2. Velocity Time Series, the Absolute Acoustic
Intensity, and Zooplankton
Figure 4 shows the ebb tide period depicted with a time
series of velocity contours (Figure 4). The time-series
velocity measurements were taken where the slope starts
to increase sharply at a distance of 4 km from the transect
observations (@ in Figure 1). At this location, the sinu-
soidal shape in Figure 4 represents the ebb tide period
when the lee waves were observed.
The absolute aco ustic intensity (dB) obtained from the
backscatter intensity of the 150-kHz ADCP was signifi-
cantly correlated with zooplankton biomass dry weight
(mg·m–3) (correlation of 0.64, p < 0.01) (Figure 5), the
linear regression explained ~70% of the variability. Thus,
the backscatter of the ADCP can be almost directly re-
lated with the zooplankton dry weight b iomass. The zoo-
plankton community was represented by 24 major groups;
the five main groups in terms of percent abundance were
cladocera (44.05%), copepoda (23.13%), siphonophora
(8.42%), chaetognatha (7.69%), and crustacean larvae
(3.86%) (Table 1). This is important since more than
86% of zooplankton organisms had a hard structure. The
backscatter intensity signal obtained with the ADCP in
areas with hard zooplankton structure organisms pro-
vides accurate and confinable data. If the same percent-
age were formed by jelly organisms the backscatter in-
tensity error will be greater and it would be hard to cal-
culate the biovolumes owing to the attenuation of the
signal produced by jelly organisms.
Finally a spectral analysis using a Fourier transform
method combined with the boundary conditions obtained
from the ADCP and with tidal amplitude and phase ob-
tained from Salas-de-Leon et al. [18] and Carbajal and
Backhaus [28] were used to run a model in order to si-
mulate the hydraulic jump. Accordingly to the Fourier
9 108
Depth (m)
Figure 4. Velocity contours (cm·s–1) at station @ on Febru-
ary 27, 2006. Positive values indicate seaward (southwest-
ward) currents.
Log [DW(mg/m)]= 5.685 + 0.053
3AAI (dB)
-90 -86-82 -78-74 -70
Log [DW(mg/m)]
AbsoluteAcoustic Intensity (dB)
Figure 5. Correlation between the absolute acoustic inten-
sity (dB) and log of the dry weight of zooplankton biomass
(mg·m–3) for the “El Puma” R/V 150 kHz ADCP.
Table 1. Percentage abundance of zooplankton groups col-
lected with the 505 µm net.
1. Cladocera (44.05%) 13. Mysidacea (0.64%)
2. Copepoda (23 .13%) 14. Polychaeta (0.57%)
3. Siphonopho ra (8.42%) 15. Icthyop l ankton (0.41%)
4. Chaetognata (7.69%) 16. Cirripedia (0.39%)
5. Larvaecrustacean (3.86%)17. Hydrozoa (0.34%)
6. Salpida (2.23%) 18. Heteropoda (0.22%)
7. Euphausiacea (2.12%) 19. Asteroidea (0.17%)
8. Pteropoda (2.61%) 20. Decapoda: Penaeoidea ( 0. 17%)
9. Ostrcoda (1%) 21. Amphipoda (0.14%)
10. Foraminifera (0.93%) 22. Ctenophora (0.04%)
11. Appendicularia (0.78%) 23. Doliolida (0.03%)
12. Scyphomedusae (0.67%)24. Cepha lopoda (0.01%)
analysis (Figure 6) and the in-situ observations the hy-
draulic jump was observed during ebb tide period under
specific conditions such as the intensification of the
along slope current. The intensification of the current
was attributed to advective processes such as remote
winds [30] and possibly to the cyclonic circulation ob-
served along the Gulf of California [33].
3.3. Model Outputs
Model outputs from the ROMS were also used to de-
Copyright © 2012 SciRes. OJMS
Power Spectra
[(cm/s) cph
Period (h)
10 32 28 24 20 16 1284
Figure 6. Period (1/f) spectra of the current velocity
at station @ of Figure 1.
scribe the dynamics of an idealized generic system simi-
lar to the one located near Carmen Island (Figure 7). The
model outputs showed lee wave formation where the
slope starts to increase sharp ly (down-slope). Model out-
puts during ebb tide periods without the influence of
Coriolis acceleration (f = 0) (Figures 7(a) and (c)) sho w ed
a hydraulic jump over the slope (i.e., the flow changed
from supercritical to subcritical). The pycnocline (repre-
sented by contours in Figure 7) showed an adjustment to
bathymetry, the near-bottom flow was constrained to a
smaller area. The seaward flow near the bottom acceler-
ated due to mass conservation. The near-surface flow, in
turn, decelerated because the same amount of water had
to pass through a bigger area per width unit. Without
considering Coriolis acceleration, lateral displacement of
any tracer located at the pycnocline depth was displaced
toward the sides from the slope location (Figure 8) due
to horizontal velocity gradients
0vxuy : u
and v are water velocities in the x and y direction, aligned
in the along and across-slope direction, respectively.
According to this equation, a simulation with (Figures
8(b) and (d)) and without (Figures 8(a) and (c)) Coriolis
acceleration was used to estimate the depth (in sigma
coordinates) at which a given particle released at sigma
equal 1 over the right boundary of the domain would be
found after 26 h of simulation (Figures 8(a) and (b)),
after the model reached stability. In the simulations in
which 0fv, the particles were displaced toward the
right side of the modelled region, which is equivalent to
being displaced toward Carmen Island in the real case.
This result suggests that an area of high productivity ex-
ists near Carmen Island. The number of times those par-
ticles will pass by each grid point during the 26 h simula-
tion (Figures 8(c) and (d)) also showed asymmetry for
the simulated case where 0fv. Which implies that
a given particle released at sigma equal 1 over the right
boundary of the domain would pass a higher number of
times toward the right side of the domain, after 26 h of
simulation, when 0fv
(Figures 8(d) and (c)).
0.4 m·s
0.4 m·s
0.4 m·s
0.4 m·s
Figure 7. Model outputs during ebb tide periods without the
influence of Coriolis acceleration (f = 0) at (a) t = 4.2 and (c)
t = 4.4 days and during ebb tide periods with the influence
of Coriolis acceleration at (b) t = 4.2 and (d) t = 4.4 days.
00.5 1.0 1.5 2.0
00.5 1.0 1.5 2.0
Distance (km)
Distance (km)
a) b)
c) d)
Figure 8. Depth in sigma coordinates of a given particle
released at sigma = 1 over the right boundary of the domain
after 26 h of simulation with (b) and without (a) Coriolis
acceleration and the number of times those particles will
pass by each grid point during the 26 h with (d) and without
(c) Coriolis acceleration.
Copyright © 2012 SciRes. OJMS
At the pycnocline interface (Figure 7) a clear wave
signal was observed, as previously described based on
observations. Because the observed zooplankton patches
were assumed to be located at the pycnocline depth due
to buoyancy stability, they also should follow a wave
pattern. The pycnocline strength from the model outputs
was determined by salinity. The temporal variability of
salinity produced a density variation of 4 kg
m–3, whereas the temporal variability of temperature
produced a density variation of 0.6 kg·m–3.
Under such simulations, zooplank ton should mostly con-
centrate at the halocline depth, which is located at the
same depth as the pycnocline and thermocline (Figure 3).
Therefore, zooplankton were assumed to move d ownw ard
following the slope and then upward, following a wave
pattern as the flow moved seaward (Figure 7).
Once the Coriolis parameter (f) was set to
, where
is the angular velocity of
the Earth (7.27 × 10–5 s–1), the Coriolis acceleration value
(0.73 × 10–4 m·s–2) was close to the along-slope advec-
tive acceleration term (1.2 × 10–4 m·s–2) (Figures 7(b)
and (d)), calculated at the pycnocline depth where zoo-
plankton should concentrate. Therefore, vertical zooplan-
kton concentration over the continental slope should not
be as marked as for the case in which 0fv because
some of the zooplankton located over the main slope axis
were dispersed horizontally (y direction) (Figures 6(a)
and (c)). Zooplankton displacement was not only attrib-
ute d to Coriolis acceleration. Horizontal displacement was
mainly produced by horizontal velocity shear
i.e. the cross-advective acceleration term (80.8% of
horizontal zooplankton displacement). However, Coriolis
acceleration deflects zooplankton concentration toward
Carmen Island (19.2% of horizontal zooplankton dis-
placement), making it an area of high biological produc-
tivity compared to the surrounding waters [32]. Data ob-
servations and satellite images also showed a high bio-
logical productivity area near Carmen Island [34], which
confirmed model simulations. Thus, when internal waves
break, zooplankton concentration is displaced within the
water column and toward the sides due to horizontal ve-
locity shear and Coriolis acceleration. Assuming zoo-
plankton to be a passive tracer and assuming it moves
with the water, the only mechanisms that will move it
toward the sides will be by advection and Coriolis accel-
eration, as both terms include lateral water flow.
When Coriolis acceleration was added to the simula-
tions uu xfv (Figure 9), the pycnocline also
adjusted to bathymetry, accelerating the constrained
along-slope flow near the bottom (Figure 7). In the
along-slope momentum equation, the cross-slope advec-
tive term
vu y
Depth (m)
Distance (km)
Figure 9. Contours of the horizontal advective acceleration
term versus Coriolis acceleration uu x
v  on Feb-
ruary 27, 2006.
rection) by more than 15% of the total concentration,
reducing the vertical concentration of zooplankton usu-
ally observed over the main axis of irregular bathym-
etries where 0uy
 The lateral displacement of zoo-
plankton was mainly assumed to be related to the hori-
zontal advective acceleration term, whereas the asym-
metrical, high productivity area n ear Carmen Island (nor-
thward) compared to surrounding waters (southward)
was attributed to Coriolis acceleration.
4. Conclusions
Rapid changes in zooplankton concentration can be due
to rapid population growth or to the redistribution or re-
dispersion of a stable population. This paper deals with
internal waves as a physical factor implicated in redistri-
bution or redispersion of zooplankton population.
ADCP and water density profiles were measured over
a 280 m deep slope to elucidate the effect of the irregular
bathymetry on zooplankton distribution. These measure-
ments were combined with velocity and density simu-
lations from the ROMS with and without the influence of
Coriolis acceleration. Measurements showed an accelera-
tion of the near-bottom flow and a deceleration of the
near-surface flow as it moved toward increasing depths.
The acceleration of the near-bottom flow as it moved
toward increasing depths was produced by the adjust-
ment of the isopycnals to bathymetry (hydraulic jump).
Zooplankton concentrations were located near the pycno-
cline interface due to buoyancy stability, moved down-
ward at the slope due to the hydraulic jump, and then
moved upward, thereby exhibiting wave patterns. The
high productivity area observed near Carmen Island was
caused by nutrient pumping created by internal waves
features produced by the slope.
Model outputs without the influence of Coriolis accel-
eration described a scenario similar to that based on ob-
servations over the slope. The cross-slope advective term
in the along-momentum equation dispersed zooplankton
across the slope. However, when Coriolis acceleration
was added to the along-slope momentum equation, the
increased when compared to the
previous case without Coriolis acceleration. This condi-
tion dispersed zooplankton across the slope (in the y di-
Copyright © 2012 SciRes. OJMS
lateral flow increased by 19.2%. Therefore, Coriolis ac-
celeration deflected zooplankton concentration toward
Carmen Island. Thus, the vertical zooplankton concentra-
tion usually observed over the main axis of irregular
bathymetries, such as slopes, was reduced for relatively
high values of the Coriolis acceleration term
in the momentum equation, as was the case for the sys-
tem in the Gulf of California.
5. Acknowledgements
This study was supported financially by the “Instituto de
Ciencias del Mar y Limnología” of the Universidad
Nacional Autónoma de México (UNAM). We would like
to thank the “El Puma” crew and officers who helped
during different stages of data collection. Jorge Castro
improved the figures in this manuscript. The anonymous
reviewers improved the quality of the manuscript with
their suggestions and comments.
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