Background about Yucatan upwelling
The passage connecting the Caribbean Sean and the Gulf of Mexico is known as the
Yucatan Channel or the Straits of Yucatan. Strong currents (170cm s-1 at 6.3m depth,
Pillsbury, 1890) flows on the western side of the channel and southerly flows on the
eastern side of the channel (Ochoa et al, 2001). The northward flow on the western side is
now known as the Yucatan Current, and the southward flow on the eastern side was
named the Cuban Countercurrent (Emilsson 1971; Ochoa et al. 2001). Underneath the
Yucatan Current there is a southward Yucatan Undercurrent that is an important part of
the upwelling mechanism at the Campeche Bank (Merino 1997; Ochoa et al. 2001).
US scientists (Cochrane 1966, 1968, 1969; Ruiz 1979) and Soviet-Cuban expeditions
(Belousov et al. 1966; Bogodanov et al. 1968; Bessonov et al. 1971; Bulanienkov and
Garcia 1973) reported upwelling along the eastern edge of the Yucatan Shelf years ago
(Merino 1997). However, most of their data were published in reports that are not readily
accessible in the scientific literature. International efforts to study the Yucatan Shelf were
scarce after 1973 because Mexico claimed this area during the International Conference
on the Law of the Sea as part of the Mexican Exclusive Economic zone (Merino 1997).
Because the current is on a western boundary and for other reasons, scientists thought
that wind-driven divergence might not be an important mechanism for upwelling in the
Yucatan region (Merino 1997). Cochrane (1968, 1969) suggested that bottom friction of
the strong Yucatan Current against the slope on the eastern edge of the Yucatan Shelf
caused the upwelling instead. Another possible mechanism was proposed by Garcia
(1990), who speculated that the upwelling is a result of interactions between the Yucatan
Current and the countercurrent that Bulaniekov and Garcia (1973) found (Merino 1997).
Merino (1997) used hydrographic data from four cruises to study upwelling on the
Yucatan Shelf and to describe its structure. Subsurface Caribbean water from depths of
220-250 m upwelled at about 10-2 cm s-1 along the eastern slope of the Yucatan Shelf into
the euphotic zone, but it rarely broke the sea surface. It was characterized by
temperatures of 16 to 20°C and salinity of 36.1 to 36.5. The upwelling appeared to have a
seasonal cycle. During spring and summer, the upwelled water created a two-layered
column over the Yucatan Shelf. Because there was strong stratification between the
Caribbean Surface Water and the upwelled water, the two layers did not mix easily until
the winter months, when there were periods of northern winds.
The strength of the upwelling process along the eastern portion of the Campeche Bank
seems to depend largely on changes in the intensity of the Yucatan Current (Cochrane,
1966; Merino, 1997). However, the local wind may play and indirect role on the Yucatan
upwelling intensity and variability by inducing high-frequency variations in the current
strength (Merino, 1997). Once the upwelled water intrudes over the shelf, the wind can
also induce longshore upwelling of this water along the north coast of Yucatan during
spring and summer, as proposed by Ruiz (1979). This process is likely to enhance the
fertilizing effect of the Yucatan upwelling, a possibility supported by the existence of
high surface chlorophyll bands along the north coast of the peninsula (Espinosa, 1989;
Merino,1992).
Zavala-Hidalgo et al (2006) had observed strong and persistent upwelling favorable
winds prevailed during the year on the Campeche Bank, but cooler coastal water along
the north of the Yucatan Peninsula is only observed from May to August, with a peak in
July. The cold water could be found at the surface along the coast of Yucatan. Chl-a
maximum was also observed in summer. A probable explanation is that most of the highnutrient content waters on the Campeche Bank come from the East, along the shelf break,
where relatively low-temperature 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 chla
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 chla concentration near the coastline and the positive anomaly off
the coast (Zavala-Hidalgo et al, 2006).
Luis et al (2004) studied the structural variations of phytoplankton in the coastal seas of
Yucatan in the northwind season (late fall to winter). They separated the study region into
three sub–zones, including the west (Campeche), north (Yucatan) and east (Quintana
Roo). The hydrographic results showed the Campeche zone as having the lowest salinity
(<35 psu) values, as well as the highest inorganic nutrient and chl a values, all of which
are related to continental water contributions. The Yucatan zone had the lowest
temperatures and the lowest inorganic nutrient values, indicating influence from the
Yucatan Current and the Gulf of Mexico. Quintana Roo had the highest temperature and
salinity and intermediate nutrient. Phytoplankton exhibited greater concentration
richness, equitability and diversity in Campeche, then Yucatan zone, while the lowest
community structure values were had in the Quintana Roo zone. The study also showed
that the Campeche was the most disturbed and heterogeneous zone and Quintana Roo
with the least heterogeneity.
Melo Conzalez et al (2000) studied the near-surface phytoplankton distribution in the
western Intra-Americas Sea by CZCS. They found that the Campeche bank off the
Yucatan Peninsula, shows high pigment concentrations year-round. This upwelling zone
frequently spans over 300km along the Yucatan Peninsula and reaches 100km offshore.
The CZCS show shows a gradient ranging from the low values of offshore surface waters
typical of the Caribbean Sea of <0.12mg m-3(Muller-Karger et al, 1991), to more than 2
mg m-3 off Cabo Catoche.
North of Cabo Catoche, a cyclonic gyre formation marks the area of most intensive
upwelling (Perez et al ,1999). There was a quasipermanent frontal zone existing northeast
of Cabo Catoche, parallel to the eastern shelf break of the Yucatan Peninsula (Perez et al,
1996). Northeast of Cabo Catoche were found elevated concentrations of Chla, organic
particulate matter, and phytoplankton around the forntal zone in summer. The most
abundant phytoplankton species in July 1994 was the dinoflagellate Scrippsiella
trochoidea. While dinoflagellates dominated at all stations, their abundance was greatest
at the convergence zone. In March 1995, even higher suspended organic matter
concentration was detected near the front. Diatoms, particularly Thalassiosira subtilis,
dominated at the forntal zone relative to adjacent waters. [Perez et al, 1996]
The cause of the upwelling phenomenon along the eastern Yucatan shelf may be the
interaction of the geostrophic flow of the Yucatan Current with the bottom, resulting in
bottom Ekman boundary layer of the sea with nutrients from the strata below the
thermocline (Perez, 1999). Another forcing factor on the upper layer is the wind (Perez,
1999). Candela[1997] notes that measurements carried out in the Yucatan channel show
that the average wind stress in the area has a component from the south. That would
cause the surface Ekman transport to move water away from the shelf, and the waters
from lower layers would be forced toward the surface as a consequence of the resulting
surface divergence. (Perez, 1999)
The composition, abundance and distribution of Siphonopore fauna off the Yucan
Peninsula and in the Mexican Caribbean Sea during summer 1984 and spring 1985 was
described by Gascal and Suarezl (1991).
The local distribution and abundance of
siphonophores were found to be strongly influenced by upwellings, especially during the
spring survey. Community analysis suggests that siphonophore populations around the
upwellings are relatively homogeneous.
The composition, abundance and distribution of fish larvae and the role of seasonal
upwelling in determining the distribution pattern of assemblages at the Yucatan shelf and
in the Mexican Caribbean Sea were analyzed during the spring of 1985 (Sanchez and
Flres, 1994). Under the dissimilarity index of Bray-Curtis, four assemblages were
defined: East-Neritic, West-Neritic, south0Oceanic and North group. The composition
and the distribution of these assemblages clearly correspond to the linking of the spawing
strategies of the dominant fish species of each assemblage (location, timing and intensity)
with the variaition of the mian physical factors in the area such as the Yucatan Current,
the seasonal upwelling and the cyclonic eddy, which occurred during the study period.
The great larval abundance of just a few species in the neritic assemblages over the
Yucatan shelf seems to be a consequence of the upwelling fertilizing effect in this area.
References
Belousov, I.M., Y.A. Ivanov, S.A. Pasternak, T.S. Rass, and V.V. Rossov, 1966:
Oceanographic research by the Soviet-Cuban Marine Expedition. Oceanology, 6,
312-320.
Bessonov, N., O. Gonzalez, and A. Elizarov, 1971: Resultados de las investigaciones
Cubano-Sovieticas en el Banco de Campeche. In: Coloquio sobre Investigaciones y
Recursos del Mar Caribe y Regiones Adyacentes. UNESCO, pp. 317-323.
Bogodanov, D.V., V.A. Solokov, and N.S. Kromov, 1968: Regions of high biological and
commercial productivity in the Gulf of Mexico and Caribbean Sea. Oceanology, 8,
371-381.
Bulanienkov, S.K., and C. Garcia, 1973: Influencia de los procesos atmosfericos en el
afloramiento del Banco del Campeche. Curata Reunion de Balance de Trabajo,
Centro de Investigaciones Pesqueras/Instituto Nacional de la Pesca, Cuba, Informe
de Investigacion, 2, 28 pp.
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 Rep. TAMU Ref. No. 66-23T, pp. 14-32.
Cochrane, J.D., 1968: Currents and waters of the eastern Gulf of Mexico and western
Caribbean, of the western tropical Atlantic Ocean, and of the eastern tropical
Pacific Ocean. Dep. Oceanogr. Meteorol., Texas A&M Univ. Ref. 68-8T, pp. 19-28
(unpubl. rep.)
Cochrane, J.D., 1969: Water and circulation on Campeche Bank in May. Bull. Jpn. Soc.
Fish. Oceanogr. Spec. (Prof. Uda's Commemorative Papers), pp. 123-128.
Emilsson, I., 1971: Note on the countercurrent in the Yucatan Channel and the western
Cayman Sea. Geofis. Int., 11, 139-149.
Espinosa, L. T., 1989. Biomasa fitoplanctonica y afloramiento en el Caribe Mexicano y la
porcion oriental del Banco de Campeche. Prof. Dissert., ENEP-Iztacala, UNAM,
Mexico, 121pp
Gascal R., and E. Suarezl, 1991. Siphonophores of upwelling areas of the Campeche
Bank and the Mexican Caribbean Sea. Journal of Hydrobiologica, 216-217(1), 497502.
Ghinaglia L. T., J. A. Herrera-Silveira, and F. A. Comin, 2004. Structural variations of
phytoplankton in the coastal seas of Yucatan, Mexico. Hydrobiologia, 519: 85-102.
Merino M., 1992. Afloramiento en la Plataforma de Yucatan: estructura y fertilizacion.
Doct. Dissert., Unidad Acad. Ciclos Prof. Posgrado Colegio Cienc. Humanidades,
Univ. Nac. Auton. Mexico, Mexico, D. F., 285pp
Merino M., 1997. Upwelling on the Yucatan shelf: hydrographic evidence. Journal of
marine systems, 13, 101-121.
Muller-Karger, F. E., J. J. Walsh, R. H. Evans, and M. B. Meyers, 1991. On the seasonal
phytoplankton concentration and sea surface temperature cycles of the Gulf of
Mexico as determined by satellites, J. Geophys. Res., 96, 12645-12665.
Melo Gonzalez N., F. E. Muller-Karger, S. Cerdeira Estrada, R. Perez, I. Victoria, P.
Cardennas Perez, and I. M. Arenal, 2000. Near-surface phytoplankton distribution
in the western Intra-Americas Sea: the influence of El Nino and weather events.
Journal of Geophysical Research, 105 (C6), 14029-14043.
Ochoa, J., H. Sheinbaum, A. Badan, J. Candela, and D. Wilson, 2001: Geostrophy via
potential vorticity inversion in the Yucatan Channel. Journal of Marine Research,
59, 725-747.
Perez, R., F. E. Muller-Karger, I. Victoria, and N. Melo, 1999. Cuban, Mexican, U. S.
Reserachers probing mysteries of Yucatan currents. Eos Trans. AGU, 80, 153, 158159.
Pillsbury, J.E., 1890: The Gulf Stream - A description of the methods employed in the
investigation, and the results of the research, USCC-Geodetic Survey, Silver
Spring, MD, 461-620.
Ruiz, E.G., 1979: Upwelling north of the Yucatan Peninsula. MS Dissert. Texas A&M
Univ., Dep. Oceanogr., College Station, TX, 85 pp.
Sanchez V. L. , and Flores C. C., 1994. Larval fish assemblages at the Yucatan shelf and
in the Mexican Caribbean Sea during the upwelling period (Spring, 1985). Scientia
Marina (Barcelona), 58(4), 289-297
Zavala-Hidalgo J., A. Gallegos-Garcia, B. Martinez-Lopez, S. L. Morey, and J. J.
O’Brien, 2006. Seasonal upwelling on the western and southern shelves of the gulf
of Mexica. Ocean dynamics, 56: 333-338.