Ocean Dynamics
DOI 10.1007/s10236-006-0072-3
Jorge Zavala-Hidalgo . Artemio Gallegos-García .
Benjamín Martínez-López . Steven L. Morey .
James J. O’Brien
Seasonal upwelling on the Western and Southern Shelves
of the Gulf of Mexico
Received: 15 June 2005 / Accepted: 23 February 2006
# Springer-Verlag 2006
Abstract An 8-year database of sea surface temperature
(SST), 7 years of Sea-viewing Wide Field-of-view Sensor
(SeaWiFS) ocean color images, wind fields, and numerical
model results are analyzed to identify regions and periods
of coastal upwelling on the western and southern shelves of
the Gulf of Mexico. On the seasonal scale, it is found that
on the Tamaulipas, Veracruz, and southwestern Texas–
Louisiana shelves there are upwelling favorable winds
from April to August, when southeasterly winds are
dominant and cold SST anomalies associated with upwelling are observed along their coasts. However, during
summer, values of chlorophyll-a concentration are lower
than those in autumn and winter, which are high due to
advection of old bloom biological material from upstream.
During winter, there is a cold front on the Tamaulipas shelf
produced by advection of cold water from the Texas–
Louisiana shelf and not due to upwelling. On the eastern
Campeche Bank, persistent upwelling is observed due to
favorable winds throughout the year with cold SST and large
Responsible editor: Paulo Salles
J. Zavala-Hidalgo (*) . B. Martínez-López
Centro de Ciencias de la Atmósfera,
Universidad Nacional Autónoma de México,
Circuito exterior s/n, Cd. Universitaria,
04510 Coyoacán, México, D.F., Mexico
e-mail: [email protected]
Tel.: +52-55-56224096
Fax: +52-5616-0789
e-mail: [email protected]
A. Gallegos-García
Instituto de Ciencias del Mar y Limnología,
Universidad Nacional Autónoma de México,
Cd. Universitaria,
04510 Coyoacán, México, D.F., Mexico
e-mail: [email protected]
S. L. Morey . J. J. O’Brien
Center for Ocean-Atmospheric Prediction Studies,
Florida State University,
Tallahassee, FL 32306-2840, USA
e-mail: [email protected]
e-mail: [email protected]
chlorophyll-a content along the inner shelf from May to
September. On the Tamaulipas shelf, the summer upwelling
delays the annual SST peak until September, while in most
of the Gulf SST peaks in August. This difference is due to
the end of the upwelling favorable wind conditions and the
September seasonal current reversal.
Keywords Gulf of Mexico . Coastal upwelling .
Coastal circulation . AVHRR SST
1 Introduction
The western and southern shelves of the Gulf of Mexico
(GoM) include the southwestern Texas–Louisiana Shelf
(hereafter referred to as LATEX Shelf) and the shelves of the
Mexican states of Tamaulipas, Veracruz, Tabasco, Campeche, and the Campeche Bank. These shelves change from
200 km wide, as in the Campeche Bank, to ∼30 km, as in
Veracruz (Fig. 1). The surface circulation on these shelves is
influenced by atmospheric cold fronts coming from the
northwest continental United States during autumn–winter,
and by southerly and southeasterly winds during summer. It
is also affected by mesoscale eddies, coming from the
eastern Gulf, when they interact with the slope (Elliot 1982;
Smith 1986; Vidal et al. 1992; Zavala-Hidalgo et al. 2003).
High chlorophyll values are observed along the inner
coast throughout the year in the GoM. Upwelling conditions are not the only cause of these phenomena. Summer
coastal upwelling has been identified on the western shelf
through the analysis of historical hydrographic data and
numerical modeling (Nowlin et al. 1998; Zavala-Hidalgo
et al. 2003). Walker et al. (2003) observed upwelling in
summer Geostationary Operational Environmental Satellite images and Walker (2005), through the analysis of
mooring temperature data and SST satellite images, identified coastal upwelling along the coast of the western Gulf,
from 23° N to 29.2° N, during the summers of 1992, 1993,
and 1994. On the Campeche Bank, upwelling along its
eastern side (Merino 1997) is produced by a dynamic uplift
associated with the Yucatan Current. Most of the cold water
Fig. 1 Location and bathymetry of the western and southern Gulf of
Mexico. Sites mentioned in the main text are indicated. Diamonds
indicate the sites in the along-coast wind stress component plot
(Fig. 3), and thick lines show the location of vertical sections in
Fig. 6
that is upwelled onto the shelf does not reach the surface
and remains along the bottom of the shelf; however, as
discussed in this manuscript, it may be found at the surface
along the coast of Yucatan.
Based on a 10-year simulation of the circulation of the
Gulf of Mexico, and considering the circulation patterns on
a seasonal scale, Zavala-Hidalgo et al. (2003) identified
three regions: the Tamaulipas–Veracruz (TAVE) shelf, the
LATEX Shelf, and the Campeche Bank. On the TAVE
shelf, they identified a swift current reversal, being northward from May to August and southward from late
September to March. The monthly mean circulation on the
Campeche Bank is upcoast (in the opposite direction of the
propagation of a coastal Kelvin wave) throughout the year,
and on the inner LATEX Shelf there is a downcoast
circulation, except during summer months when the flow is
upcoast. During spring–summer, the upcoast current on the
TAVE Shelf reaches the southern LATEX Shelf where it
encounters a downcoast coastal current favoring offshore
transports. The southward current in autumn–winter
reaches the southernmost part of the Gulf, at the Bay of
Campeche, where it meets an opposing along-shelf current,
generating seasonal offshore transports in this region
(Fig. 2).
Seasonal currents on the western and southern shelves of
the GoM are mainly wind-driven (Cochrane and Kelly
1986; Cho et al. 1998; Zavala-Hidalgo et al. 2003). Their
directions are determined by the concavity of the eastern
Gulf coastline and the orientation of the wind relative to
this littoral. Correlations greater than 0.90 between the
along-coast wind stress component and the along-coast
currents on the TAVE shelf were computed by ZavalaHidalgo et al. (2003). Seasonally, the winds on the GoM
have strong variability, changing the sign of the along-coast
wind stress component with extreme conditions in January
and July (Fig. 3). The largest variance is observed on the
TAVE shelf with upwelling favorable winds from April to
August, peaking in July. On the Campeche Bank, favorable
upwelling winds prevail throughout the year. The regions
of strong convergence of the along-coast wind stress
component are associated with cross-shelf transports,
mainly in the southern Bay of Campeche during autumn–
winter and on the southwestern LATEX Shelf in spring and
summer.
The conditions, periodicity, and location of coastal
upwelling on the western and southern GoM, at seasonal
scales, are reviewed in this manuscript by the analysis of
7 years of SST from AVHRR satellite imagery and 8 years
of SeaWiFS chlorophyll-a satellite images, climatological
winds, and a high-resolution numerical simulation of the
Gulf of Mexico using the Navy Coastal Ocean Model
(NCOM) (Morey et al. 2003; 2005).
2 Results
Fig. 2 January and July monthly mean surface currents from a 10year simulation of the circulation of the shelves of the Gulf of
Mexico. The 25-, 50-, and 200-m isobaths are shown (adapted from
Zavala-Hidalgo et al. 2003)
Colder water along the TAVE and southwestern LATEX
inner shelves from May to August, with a peak in July
(Fig. 4), is revealed by an 8-year monthly mean SST time
series, in concordance with the current and wind analysis.
These results are also in agreement with the analysis of
historical hydrographic data that show colder water during
summer along the entire TAVE shelf. The signal is stronger
at subsurface, as can be observed in the 30-m historical data
(Zavala-Hidalgo et al. 2003, their Fig. 5). Low coastal SST
is also observed during autumn–winter but not due to
coastal upwelling because winds during this period
generate downwelling (Fig. 3). This cool buoyant coastal
water is advected from the LATEX shelf. It has low salinity
predominantly due to the influence of the Mississippi–
Atchafalaya river system as well as other Texas rivers and
low temperature due to the strong cooling that affects the
Fig. 3 Monthly mean alongcoast wind stress component,
computed over a smoothed
25-m isobath. Positive values
indicate downcoast direction.
Abbreviations indicate positions
as specified in Fig. 1
LATEX shelf in autumn and winter (Nowlin and McLellan
1967). The summer cold water stripe along the coast of
Tamaulipas and southern Texas is significantly narrower
than the autumn–winter one.
On the Campeche Bank, strong and persistent upwelling
favorable winds are observed during the year, but cooler
coastal water north of the Yucatan Peninsula is only observed from May to August, with a peak in July (Fig. 4).
Another region with a SST signature indicative of strong
upwelling is the southernmost part of the Bay of
Campeche, with favorable upwelling winds most of the
year (Fig. 4). Notably, the coastal orientation of the western
side of the Campeche Bank does not favor upwelling
during summer, where the SST of the inner shelf is warmer
than the offshore SST.
Fig. 4 Mean sea surface temperature in the Gulf of Mexico
for January (left) and July (right)
computed from an 8-year
AVHRR database
Along the coasts of the GoM, images of chlorophyll-a
concentration derived from SeaWiFS show large values
and strong offshore gradients throughout the year, making
it difficult to identify upwelling signatures. However, the
chlorophyll-a anomalies from the annual mean unveil
processes on the shelf. On the inner shelf north of the
Yucatan Peninsula, higher values are observed between
July and November, with an East-to-West propagation of
the positive anomaly during this period (not shown). Along
the southern coast of Veracruz, positive anomalies are
observed from July to September, which may be associated
with upwelling due to the southerly winds, locally known
as sures (Fig. 5), as well as the increased discharge from
local rivers in this season. The positive anomalies last
until December, but from September to December, they
may be associated to other processes, mainly vertical
Fig. 5 SeaWiFS chlorophyll-a concentration monthly anomaly with respect to the annual mean for the period 1997-2004 for January (left)
and July (right)
mixing due to the strong winds associated with passage of
the cold fronts over the GoM. On the southwestern
LATEX and Tamaulipas shelves, contrary to what may
be expected, positive chlorophyll-a anomalies are observed
from October to March, when winds do not favor upwelling, indicating that processes different from upwelling,
mainly advection of old bloom biological material from
upstream, become dominant.
Temperature sections of the July monthly mean in three
sites, computed from the third year of a NCOM simulation,
where there are favorable wind upwelling conditions and
relatively low SST signature, show the vertical structure of
the summer upwelling (Fig. 6). On the Campeche Bank,
the low-temperature water on the bottom moves inshore
while it upwells. A meridional section along 93.2° W in
front of the state of Tabasco, and a zonal section along
Fig. 6 July monthly mean
temperature sections in three
Gulf of Mexico sites, from a
numerical simulation. a Along
89 W, on the Campeche Bank,
b along 93.2 W, in front of
Tabasco State, and c along 23 N,
in front of Tamaulipas State (see
Fig. 1)
23° N, also show the upwelling tilt of the isotherms, being
stronger in the latter probably because of the influence of
the current over the slope, which is mainly produced by the
summer anticyclonic circulation in this region (Sturges,
1993).
3 Discussion and conclusions
The annual regularity of SST, winds, and ocean color
cycles on the western and southern shelves of the GoM
shows that a combination of processes determines the
presence of low-temperature and high-chlorophyll-a stripes
along the coasts. On the Campeche Bank, although there
are favorable upwelling winds throughout the year, cold
SST stripes and chlorophyll-a maximum are observed
Fig. 7 Phase in months of the maximum sea surface temperature
over the Gulf of Mexico, from an 8-year AVHRR database
during summer. A probable explanation is that most of the
high-nutrient content waters on the Campeche Bank come
from the East, along the shelf break, where relatively lowtemperature and nutrient-rich water is lifted due to the
Yucatan Current (Merino 1997). Once on the shelf, the cold
water near the bottom moves westward and southward due
to the wind stress and upwelling circulation until it reaches
the surface, just offshore. In summer, the water column on
the shelf is stratified, maintaining the cold nutrient-rich
water below the relatively thin-surface mixed layer, except
near the coastline, where it is upwelled. During autumn and
winter, the atmospheric cold fronts mix the entire water
column over the shelf, increasing the chlorophyll-a concentration in the middle and outer shelf. This process may
consume a fraction of the nutrients in the water column,
generating a deficit in the bottom water that moves onshore
and eventually reaches the photic zone. This scenario
explains the winter negative anomaly of chlorophyll-a
concentration near the coastline and the positive anomaly
off the coast (Fig. 5).
On the Tamaulipas and southwestern LATEX shelves, a
positive chlorophyll-a anomaly is observed during the
period when there is no upwelling, which reveals that other
processes are important in the region. During autumn–
winter, the circulation on these shelves is downcoast
(Cochrane and Kelly 1986; Zavala-Hidalgo et al. 2003),
advecting old bloom biological material from upstream
onto the Tamaulipas shelf through the LATEX shelf. This
water mass has lower temperature and salinity than the
water offshore the Tamaulipas shelf and produces a SST
cross-shelf gradient that is clearly identifiable in satellite
images. In summer, there is upwelling that produces a band
of cold water along the coast, but SeaWiFS chlorophyll-a
values are lower than those in autumn–winter.
In most of the GoM, the maximum SST is reached in
August (Fig. 7), which is mainly determined by the heat
fluxes annual cycle (Zavala-Hidalgo et al. 2002; Mendoza
et al. 2005), but in some regions the maximum is reached in
a different month as a consequence of regional upwelling
and dynamical processes. Along the coast of Tamaulipas,
upwelling during August avoids higher SSTs delaying the
peak to September when the upwelling regime switches to
downwelling and a swift reversal of the currents takes
place. On the southwestern LATEX shelf, the SST
maximum is reached in July, although upwelling is welldeveloped during this month. The process that makes the
surface temperature slightly lower in August than in July
may be the reversal of the circulation in this region, which
takes place in July, allowing the advection of cool water
from the Tamaulipas shelf, but more research has to be
done on this issue. Along the coast of the state of Tabasco
and a strip parallel to the shoreline of the state of
Campeche, the SST peak is reached in June. This shift is
undoubtedly due to upwelling, which is well-developed
during July and August in this region. Offshore, north of
the Yucatan peninsula, the peak is reached in September.
This is probably related with the downwelling coastwiseattached Kelvin waves that may be associated with the
change of the wind pattern and coastal circulation that takes
place in September. Again, more research has to be done in
this region.
Acknowledgements This project was funded by CONACYT Grant
SEP-2003-C02-45634, UNAM Grants PAPIIT IN122005-3 and
PAPIIT IN-1158042004. The authors would like to thank the
SeaWiFS Project and the Distributed Active Archive Center (Code
902) at the Goddard Space Flight Center, Greenbelt, MD 20771, for
the production and distribution of the data. These activities are
sponsored by NASA’s Mission to Planet Earth Program. We would
like to thank Rosario Romero-Centeno, who made important
suggestions to improve this manuscript, and Paul Martin and Alan
Wallcraft for their assistance with the NCOM; Olivia Salmerón, Erik
Márquez, and Ranulfo Rodriguez processed the SeaWiFS and SST
data. The numerical modeling work was sponsored by the ONR
Secretary of the Navy grant to J. J. O’Brien, and by NASA Physical
Oceanography.
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