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th
Proceedings of the 17 Physics of Estuaries and Coastal Seas (PECS) conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24 October 2014
Mechanisms Driving a Coastal Dynamic Upwelling
2
CECILIA E. ENRIQUEZ1, ISMAEL MARIÑO-TAPIA
1
Facultad de Ciencias, Universidad Nacional Autónoma de México, Sisal Yucatán, México (+52 988,
9311000 Ext: 7146).
Email: [email protected]
2
Procesos Costeros y Oceanografía Física. CINVESTAV-IPN, Unidad Mérida, Yucatán, México.
Email: [email protected]
Keywords: upwelling, Yucatan
SUMMARY
Seasonal dynamic coastal upwelling events in the northeastern coast of Yucatan, Mexico, nourish the
coastal sea enhancing fisheries and promoting the presence of large species like the whale shark, but also
initiate the development of algal blooms, which are commonly harmful. The forcing mechanisms driving
the upwelling process in that site obey to a combination of processes that are not always clearly
differentiated. This study aims to investigate the relative importance of various forces capable of
controlling this upwelling process through a combination of numerical modeling, using the DELFT3D
hydrodynamic model and observations. Preliminary numerical results show that the characteristics of the
Yucatan Current (e.g. intensity, trajectory and proximity to the continental shelf break) can determine the
development of positive vertical velocities in the curved features located along the shelf break in the
region; southeasterly winds might enhance a coastal upwelling due to the resulting offshore Ekman
transport whereas northerly winds can suppress the process.
1. INTRODUCTION
Coastal upwelling processes are extremely important nutrient sources transported with deep-water masses
to the surface layers, which are commonly oligotrophic and unproductive. The sudden nutrient abundance
developed generates high productivity at all trophic levels, from micro algae to large fish. In Cabo
Catoche (CC), at the northeastern coast of Yucatan, Mexico, upwelling events are known to occur during
the spring and summer seasons. These events enhance fisheries and promote the presence of large species
like the whale shark, but also initiate the development of algal blooms, which are commonly harmful.
The CC upwelling was first identified in 1966 [3] and since then, several studies have confirmed its
presence and some have described its thermohaline structure and extent, but its origin and behavior have
not yet been explained. The CC upwelling site is located between the Caribbean Sea and the Gulf of
Mexico (Figure 1a). It is located in the eastern border of the wide and shallow continental shelf at the
north of the Yucatan Peninsula, which breaks abruptly towards the east in the Yucatan Channel. Through
the Yucatan Channel flows the Yucatan Current (YC), a deep, intense western boundary current. The
Yucatan Current flows into the Gulf of Mexico adjacent to the Yucatan Shelf carrying different water
masses from the Caribbean and Atlantic. The trajectory of its transit varies before becoming the Loop
Current taking different shapes related to other processes such as the passing of eddies through the
Yucatan Channel [2, 1].
Recent in situ measurements show that other processes can influence the CC upwelling process,
specifically the wind. The wind in the region is dominated by easterly winds (Trades), which range from
NE to SE (the last being typically more intense). However, during the winter, the transit of cold fronts
from higher latitudes brings intense northern and northwestern wind events of short duration (3-5 days),
which are capable of temporarily reversing the current towards the east.
.
th
Proceedings of the 1
17 Physics of Estuaries
uaries and Coastal Seas (PECS) conference,
conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24
1
October 2014
Figure 1 Yucatan dynamic upwelling region. Cabo Catoche is marked with the black dot (a). Along-shore
temperature transect revealing an upwelling event during September 2005 (c); data selected from the coastal CTD
profiles (b) from
om a wider survey by the Mexican Navy.
This
his study investigate
investigates the relative importance of various forces capable of controlling the upwelling
process in CC, Yucatan through numerical modeling studies.
2. METHODS
The different contributions of the CC upwelling development are determined by numerical
modeling studies and observations. The 3D version of the DELFT3D hydrodynamic model
model,
developed
loped by WL/Delft Hydraulics in a curvilinear grid of 258 by 171 grid points with spatial
resolution of approx.
prox. 3
3.5 km covering the Gulf of Mexico.
Mexico For this work,, the model is barotropic and
does not include density gradients but 10 vertical sigma layers were set to analyze the vertical
currents at the shelf break.
The model has 2 open boundaries: at the Yuca
Yucatan
tan Channel, forced with currents velocities, and at
the Florida Strait, which allows the transit of outgoing waves and currents calculated by the model
to allow continuity and stability. The bottom boundary is the ETOPO1 bathymetry from NOAA with
1-minute spatial resolution.
The numerical experiments were designed to test the sensitivity of the vertical velocities in the CC
upwelling regions to the characteristics of the Yucatan Current, particularly its intensity, proximity
to the Yucatan shelf and path followed towards the Gulf of Mexico. The first two variables were set
directly within the forcing set at the boundary. But the path followed by the current responds to
different processes with scales and origins, which go beyond the limits of this
is study. IIt is known,
however
wever that the direction of the current may be in the range of NW-N-NE,
NW NE, and that the coastal
hydrodynamics in the northern Yucatan Sea are highly sensitive to this variations [4].
The main numerical experiments are described in Table 1. Within
hin the experiments, high intensity
current means a linear distribution of cero at the coasts of Yucatan and Cuba increasing to 2 m/s at
the core of the current. The low intensity current increased to 0.8 m/s at the core. Current
proximity is "middle" when the core is in the middle of the Yucatan Channel and "adjacent" when
the core is stretched towards Yucatan. The two options for current direction are towards the NE or
NW and to modify its path within this study, some numerical experiments were designed with a
section of grid cells set as impermeable to prevent the Yucatan Current to flow to the NE and
forcing it to flow to the North or NW follow the procedures used by Enriquez et al [4].
th
Proceedings of the 1
17 Physics of Estuaries
uaries and Coastal Seas (PECS) conference,
conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24
1
October 2014
Scenario
01
02
03
04
05
06
07
10
11
Current
dir.
NE
NW
NE
NW
NW
NW
NW
NE
NE
Current
Intensity
high
high
high
high
low
low
low
low
low
Current
Proximity
middle
middle
adjacent
adjacent
adjacent
adjacent
adjacent
adjacent
adjacent
Wind
SE
NW
SE
NW
Table 1 Forcing conditions in the numerical experiments.
3. RESULTS
The characteristics of the Yucatan Current highly influence the upwelling in the CC region. Numerical
results show that the direction of the trajectory of the YC towards the north is very important in the
development of an upwelling. When the current is directed to the northeast as it detaches from the western
boundary in the CC curved bathymetric feature, positive vertical velocities develop (Figure 2a). When the
current trajectory is to the northwest,, flowing very close to the continental shelf, the region of upwelling
at is less extent and further offshore (to the north) (Figure 2b). The proximity of the core of the YC to the
Yucatan continental margin did not aappear
ppear to have an important influence in the extent nor in the intensity
of the upwelling. The intensity of the YC has a direct impact in the intensity of the upwelling when its
trajectory promotes the development.
Preliminary results with wind forcing rev
revealed
ealed that the wind direction does not influence the distribution
or the intensity
ity of the upwelling but it modifies the horizontal currents. Within the continental shelf, the
wind has an important influence even in the deeper layer currents. Strong southeasterly
southeasterly winds promote the
transport of upwelled bottom water towards the shelf to the west (Figure 2c). Strong northwesterly winds
tend to generate inshore transport and promote transport to the east (Figure 2d).
Additionally, analytical scaling of the different forcing terms in the Preliminary numerical results show
that strong southeasterly winds might enhance a coastal upwelling due to the resulting offshore Ekman
transport together with the characteristics of the Yucatan Current (intensity and proxim
proximity to the
continental break), whereas northerly winds can suppress the process.
th
Proceedings of the 17 Physics of Estuaries and Coastal Seas (PECS) conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24 October 2014
Figure 2: Vertical velocities (m/s) developed along the shelf break in the Cabo Catoche region showing upwelling
(red) and downwelling (blue). The figure includes bathymetric contours for reference. Vectors of the horizontal
currents in the deeper layer are plotted for the experiments with wind forcing (scaled by 3 and plotted every 3).
4. DISCUSSIONS
The pressure gradient generated by the Yucatan Current through its transit from de Caribbean Sea into the
Gulf of Mexico may promote the development of positive vertical currents in a region where the upwelled
water may be transported over the Yucatan continental shelf. If the vertical currents are enough to raise
the Caribbean Subtropical UnderWater (CSUW) water mass, the whole Yucatan Sea is nourished with
nutrients. It lies below 150 m depth and is characterized by its salinity maximum (>=37). It is known to
upwell during spring and summer [6, 5]. Recent in situ observations have revealed that after a strong wind
event from the north, the upwelling ceased [7]. Our results confirm the observation and suggest that while
the vertical positive velocities are not significantly modified by a northerly wind, the horizontal currents
promote transport to the east. This transport would return the upwelled water, which is significantly
denser than the surrounding water at those depths to a region where it would sink fast and not be able to
reach the surface layers.
5. REFERENCES
[1]
Badan, A., Candela, J., Sheinbaum, J., Ochoa, J. (2005). Upper-layer circulation in the approaches to Yucatan Channel.
In: Sturges, W., Lugo-Fernandez, A. (Eds.). Circulation in the Gulf of Mexico. Observations and Models. AGU Books Board,
Geophysical monograph 161, Washington, DC, pp. 57–69
[2]
Candela, J., Sheinbaum, J., Ochoa, J., Badan, A., Leben, R. (2002). The potential vorticity flux through the Yucatan
Channel and the Loop Current in the Gulf of Mexico. Geophys. Res. Lett. 29 (22), 4.
[3]
Cochrane, J.D., (1966). The Yucatan current, upwelling off Northeastern Yucatan, and currents and waters of Western
Equatorial Atlantic. Oceanography of the Gulf of Mexico. Progress Report TAMU Ref. no. 66-23T, pp. 14-32.
[4]
Enriquez, C., Marino-Tapia,I.J., Herrera-Silveira,J.A. (2010). Dispersion in the Yucatan coastal zone: implications for
red tide events. Continental Shelf Research. 30, 127–137.
[5]
Enriquez, C., Marino-Tapa, I., Jeronimo, G., Capurro-Filograsso, L. (2013). Thermohaline processes in a tropical
coastal zone. Continental Shelf Research. 69, 101-109.
[6]
Merino, M. (1997). Upwelling on the Yucatan Shelf: hydrographic evidence. Journal of Marine Systems. 13, 101-121.
[7]
Reyes-Mendoza, O., Mariño-Tapia, I., Herrera-Silveira, J., Ruiz Martínez, G. Enriquez, C. (2014). Observations and
modelling of plankton dynamcs in a tropical upwelling region, cabo catoche, mexico. Book of Abstracts of the Ocean Science
Meeting, American Geophysical Union. Hawaii.