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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
On the dynamics of upwelling events in the Yucatan Shelf using in situ
observations.
2
ISMAEL MARIÑO-TAPIA1,|CECILIA E. ENRIQUEZ , OSCAR REYES-MENDOZA1, AND JORGE
HERRERA SILVEIRA, 1
1
CINVESTAV-IPN, Unidad Mérida, Yucatán, México. (+52 999 9429458).
Email: [email protected], [email protected], [email protected]
2
Facultad de Ciencias, Universidad Nacional Autónoma de México, Sisal Yucatán, México
Email: [email protected]
Keywords: upwelling pulses, upwelling seasonality
SUMMARY
The NE corner of the Yucatan shelf is known to experience important upwelling events characterized by
the presence of nutrient rich, relatively cold water (T < 22.5 ⁰C) which fertilizes the ocean and attracts
species including the whale shark and the giant manta ray. This generates an important income to the
tourism industry of the region. Furthermore, this water mass propagates into the Yucatan continental shelf
boosting the local fisheries. The forcing mechanisms that generate this upwelling are not as yet entirely
understood, and it is usually explained through dynamic uplift processes, associated with the very strong
Yucatan current, rather than the usual Ekman (wind generated) dynamics. Observations from in situ
measurements suggest that the coastal expression of this upwelling event is most evident during springsummer when sea level is below average and the low frequency currents are to the west. During this time,
cold water pulses of a 13 day period are observed at the coast and are de-correlated to atmospheric
temperature variability. During autumn-winter, the water temperature is governed by atmospheric
temperature variability, showing no influence of the upwelling. This coincides with a positive sea level
anomaly, which is greatest during October, and is accompanied by low frequency currents flowing
towards the East. It is hypothesized that the otherwise permanent upwelling is stopped during autumn and
winter due to an increased sea level over the Yucatan shelf generated by basin scale processes and local
effects of northerly storms.
1. INTRODUCTION
It is well established that the Ekman transport induced by wind stress parallel to the coast is the
mechanism that generates coastal upwelling where eastern boundary currents occur. There are a large
number of studies indicating how these ecosystems play a key role in the socio-economic and ecological
factors of the population (Pauly and Christensen, 1995). However the upwelling ecosystems that are
outside eastern boundary currents are less understood and the physical mechanisms that generate them are
still under discussion. The mechanisms that generate the upwelling in the NE corner of the Yucatan shelf
follow different theoretical approaches. Cochrane (1966) proposed a mechanism of topographic friction
associated with the Yucatan current and the slope of the platform. Bulaniekov and García (1973) propose
that the upwelling is a result of the interaction between the Yucatan current and the undercurrent of Cuba.
There are other theoretical mechanisms including the effect of southeast wind stresses that could generate
Ekman offshore transport (Perez-Santos et al.2010) and the effects of local pressure gradients that could
raise the thermocline [Furnas and Smayda, 1987; Zavala-Hidalgo, 2003; Enriquez et al. 2010].
Merino (1992) characterized the upwelled waters (Yucatan Upwelled Water, YUW) as having a
temperature range between 16 and 20°C, with the isotherm of 22.5 established as the limiting boundary of
upwelling influence; salinity is reported between 36.1 to 36.5, and Chl-a values are greater than 1 mg/m3.
The upwelling is seasonal with peak values between spring and summer and it is absent from October to
February (Salmerón-García, 2010). Figure 1 shows the study site, Holbox-Cabo Catoche.
th
Proceedings of the 17 Physics of Estuaries and Coastal Seas (PECS) conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24 October 2014
2. METHODS
An intensive field campaign was performed during 12 days of April 2012, in which every day a transect
perpendicular to the coast with 20 stations separated by 1 km was made. At each station profiles of
temperature and electrical conductivity were measured with a CTD (SBE 19 plus), with a sampling
frequency of 4 Hz. The CTD also had a WetLabs Fluorometer (ECO - FL-NTU) installed to measure
Chlorophyll-a. Data was treated with the Sea Bird Data Processing software to correct for phase lags, and
a Gaussian interpolation method was performed to generate maps of vertical distribution of variables. The
upwelling intensity was set based on the position of the 22.5oC isotherm on the water column relative to
the surface, the closer to the surface the greater the intensity. Alongside with these profiling
measurements, two acoustic Doppler profilers (Nortek-AWAC) were installed at 8 and 14 m depth (8 and
16 km from the shore respectively) in order to better understand the dynamics and coastal influence of the
upwelling events.
Figure 1 Map of the Cabo Catoche region in SE Mexico, and a schematic representation of instrument arrays.
The instruments stayed at the site from May 2010 until February 2013. The time series of sea level,
zonal (u) and meridional (v) currents, were divided into upwelling (March-September) and not
upwelling months (October-February) based on the behavior of the water temperature. Spectral
analysis (Welch Periodrogram method) was applied in order to establish the magnitude and
frequency of upwelling pulses. The time series were of size 2n, treated with a Hanning window of
14.2 days with 50% overlapping, which gave 27.3 degrees of freedom at 95 % confidence interval.
3. RESULTS
CTD data gathered during the intensive field campaign clearly shows the presence of an upwelling pulse
during the first 4 days, where the water temperatures are below 22.5 and primary productivity is high. The
fifth day an intense northerly wind event halted the upwelling process, and both, temperature and primary
productivity returned to background levels. The results of this intensive field campaign clearly
demonstrate that northerly storms are capable of stopping the upwelling process.
The long term ADCP data shows, during the upwelling season (March to September), a periodic decrease
in temperature every 13 days, at 8 m depth, which is interpreted as the footprint of the upwelling near the
coast, which behaves as pulses of cooler water. The ocean temperature variability shows clear signals of
stratification, and the ocean temperature variability is independent of atmospheric temperature forcing,
which is an indication of clear upwelling influence. During autumn and winter (from October to March)
the 13 day periodicity disappears, no clear signs of stratification exist, and the low frequency atmospheric
th
Proceedings of the 17 Physics of Estuaries and Coastal Seas (PECS) conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24 October 2014
temperature variability is highly correlated to the ocean temperature, which is a signal of atmospheric
dominance in heat fluxes (Figure 2). The ocean and the atmosphere have a very similar temperature.
Figure 2: Ocean temperature (blue) at 8 m depth and low pass atmospheric temperature (red)
On the other hand, the subinertial velocity fluctuations are positively correlated with the corresponding
sea level fluctuations, and at lower frequencies (f < 1/40 days) there is a clear link between the upwelling
season (March – September), evaluated as active 13 day pulses (Figure 2), lower sea levels and westward
velocities, which favors the upwelling process and can carry the upwelled water into the Yucatan shelf
(Figure 3). On the other hand, a local high sea level, and eastward velocities seem to halt the upwelling
process during autumn and winter. Northerly storms are known to dominate the dynamics of the Yucatan
shelf during this time of the year, and are the most likely explanation for the observed behavior.
Figure 3: Subinetial low frequency (f < 1/40 days) variation in sea level (black) and East-West currents (blue),
measured at 8 m depth in Cabo Catoche region.
4. CONCLUSIONS
Results from intensive field campaigns and longer term mooring data, which include 15 days of CTD
profiling, and ADCP moorings at different depths, corroborate that the upwelling process occurs in pulses
and can be stopped by northerly winds. Since the Yucatan current does not have a clear seasonality
coincident with the upwelling behavior, northerly winds and density variations related to winter cooling
could be the responsible mechanisms for stopping the otherwise constant dynamic uplift that could be
driven by the Yucatan current.
5. REFERENCES
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Proceedings of the 17 Physics of Estuaries and Coastal Seas (PECS) conference,
Porto de Galinhas, Pernambuco, Brazil, 19–24 October 2014
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