Eos,Vol. 85, No. 20, 18 May 2004
VOLUME 85
NUMBER 20
18 MAY 2004
PAGES 197–208
EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
Cyclonic Eddy Entrains Orinoco
River Plume in Eastern Caribbean
PAGES 197, 201–202
“Mesoscale” eddies are large whirlpools in
the ocean with diameters of hundreds of kilometers.Their influence can extend to depths
of 1000 m or greater. Oceanographers are only
now beginning to document the prevalence,
extent, and influence of such features in the
world ocean.The availability of third-generation
ocean color imagery from the Moderate Resolution Imaging Spectroradiometer-MODIS sensors
aboard NASA’s AQUA and TERRA platforms, and
support for direct observation at sea, have now
allowed characterization of such an eddy
interacting with the Orinoco River plume (ORP)
while traversing the eastern Caribbean basin.
The ORP extends seasonally across the basin
from August through November,3 to 4 months
after the peak of the seasonal rains across
northeastern South America.At this time, a
thin plume of relatively low-salinity water, rich
in phytoplankton and bearing significant
amounts of colored dissolved organic matter
(CDOM) [Blough et al., 1993; Morell and Corredor, 2001], covers a large swath of the basin,
offering a striking contrast to the intensely
blue oceanic waters of the adjacent northwest
Atlantic Ocean.
Ocean color imagery (Figure 1) and sea surface height topography (SSHT) in August 2003
revealed a large circular structure extending 230 km
across the eastern Caribbean basin embedded
within the ORP. SSHT indicated that the feature
was a cyclonic eddy, rotating counterclockwise
about its axis,drifting westward at about 7 cm/s .
section (Figure 2) of the eddy. In situ characterization of Caribbean eddies was lacking
prior to CaVortEx I.
Density distribution across the eddy (Figure 2a)
showed displacements within the eddy core
extending to ~700 m, conforming well to
observations of cyclonic eddies elsewhere.
Shipboard monitoring of near-surface salinity
and chlorophyll a (Chl a) concentrations confirmed
the horizontal extent of the eddy, as visualized
in the ocean color imagery (Figure 3a).
Several features particular to the interaction
of this eddy with the massive ORP are novel.
Salinity structure below ~75 m was as might be
expected for eastern Caribbean basin waters;
shoaling of the sub-surface, high-salinity, subtropical underwater mass within the eddy
core was particularly apparent. Near-surface
waters, however, showed influence of the lowsalinity ORP with a marked front separating
the high-salinity core from surrounding lowsalinity ORP waters (Figure 2b).This pattern
was reflected in the distribution of dissolved
silicate, a nutrient that is abundant in the ORP
[Corredor and Morell, 2001], but scarce in surface oceanic waters. High silicate content of
near-surface waters in the buoyant plume
contrasted with the silicate-poor waters of the
eddy core.High-pressure liquid chromatography
of surface phytoplankton extracts revealed a
phytoplankton community typical of oceanic
waters, where cyanophytes are dominant in
the eddy core, but a distinct community
-1
The Caribbean Vorticity Experiment
The Caribbean Vorticity Experiment (CaVortEx I)
was undertaken in August 2003 to characterize
the physical, biogeochemical, and optical
structure of the eddy and to assess the influence of the eddy, and the ORP on biological
productivity. Scientists from the University of
Puerto Rico characterized surface and subsurface features of the eddy down to 1000 m.The
cruise track aboard R/V Chapman transited the
eddy core,providing a diametric north-south
BY JORGE E. CORREDOR, JULIO M. MORELL, JOSE
M. LOPEZ, JORGE E. CAPELLA, AND ROY A.ARMSTRONG
Fig. 1. K490MODIS data product for 5 August 2003 at 1-km resolution. K490 is the diffuse attenuation coefficient for light penetration at a wavelength of 490 nm. Both CDOM and phytoplankton
pigments contribute strongly to light attenuation at this wavelength.The eddy core in the image
is located at approximately 15°N, 67°00’W. During CaVortEx I (14–16 August), the eddy core had
reached 67°50’W, a westward displacement of approximately 50 nautical miles. High-chlorophyll,
high-CDOM, near-surface river plume water (green) surrounds the low-chlorophyll, low-CDOM
waters of the eddy core (light blue).A filament of plume water spirals within the eddy core.
Eos,Vol. 85, No. 20, 18 May 2004
Fig. 2. North-south oceanographic sections across the cyclonic eddy are shown. (a) Baroclinic structure of the eddy is apparent in the sea water
density (sigma-t) section as a dome extending throughout the top of the water column to ~700 m. (b) Oceanic waters of the eddy core displace the
surface plume water, resulting in a high-salinity core surrounded by a buoyant, low-salinity ring. (c) High chlorophyll concentrations in the buoyant
plume create a halo around the doming deep chlorophyll maximum (DCM) of the eddy. Light attenuation in the buoyant plume waters north and
south of the eddy core limits phytoplankton abundance of waters directly below, resulting in decay of the oceanic DCM.
dominated by prasynophytes and diatoms was
present in the eddy periphery.
Cyclonic eddies propagating through stratified high-salinity, low-chlorophyll waters such
as the “Haulani” eddy off Hawaii [Vaillancourt
et al., 2003] show shoaling of the deep chlorophyll maximum (DCM) in concert with the
vertical water mass displacement. Such “eddy
pumping” enhances primary production by
bringing nutrient-rich waters into the euphotic
zone.The DCM in the Caribbean eddy responds
to both eddy pumping and the influence of
the buoyant river plume.The DCM in the
region shoals considerably in waters influenced
by the ORP, reaching depths under 30 m in the
east-central Caribbean, in contrast to the greater
depths (>90 m) prevailing in the absence of
plume waters and in the adjacent Atlantic
[Corredor et al., 2003]. Consequently,while
doming of the DCM is apparent in the eddy
core, a shallow DCM dominates its periphery.
Remnants of the oceanic DCM, presumably
diminished by shading,underlie the river-related
DCM (Figure 2c).
Optical properties of the cyclonic eddy,
measured by profiling radiometry,spectrophotometry, and turbidimetry, were similarly modulated by both the eddy and the river plume.
While the oceanic water of the eddy core
exhibited low optical absorption and turbidity,
near-surface waters of the surrounding river
plume were not only more turbid, they exhibited sharply increased light absorption of the
shorter wavelengths (blue and ultraviolet), a
property characteristic of high CDOM waters.
A final feature of interest is the distribution
of “staircase” patterns across the eddy (Figure
3b).Abrupt discontinuities in temperature
and salinity at scales of 10–20 m and intermediate depths (400–600 m), common in the
Caribbean [Lambert and Sturges, 1977], are
related to double-diffusive processes obeying to
differences between diffusive rates of salt and
heat. Staircasing was strongest in the mid-eddy
shear zone, but was substantially reduced in
the eddy core and its periphery. Dynamics of
these features in the course of eddy propagation may prove to be significant in the return
transport of deepwater masses within the context of the global meridional overturning
circulation.
A Spiral in the Eddy
In northern hemisphere cyclonic eddies,
Coriolis forcing displaces surface water outwards as deeper waters are transported toward
the surface. Such an eddy intersecting a highly
colored river plume would displace the buoyant
plume waters and traverse the plume intact.
It would appear from space as a coherent
disk of clear water within the turbid surface
plume. High-resolution MODIS imagery,however,
reveals a distinct filament of ORP water spiraling
within the eddy core, a feature replicated in
surface transects appearing as localized, lowsalinity and high-Chl a anomalies within the
eddy (Figure 3a). Anti-cyclonic (rotating clockwise)
Caribbean eddies are thought to originate
from Atlantic Ocean eddies known as North
Brazil Current Rings (NBCR), which are generated by instabilities in the NBC retroflection
[Johns et al., 1990]. Following impingement
on the island arc of the Antilles, some NBCRs,
particularly the weaker, larger rings, appear to
be able to squeeze through the island passes
and re-form within the island arc with little loss
of mass [Simmons and Nof, 2002]. Genesis of
Caribbean cyclonic eddies is less clear, but
may follow a similar path, and the entrained
spiral filament may result as a consequence
of eddy reformation.
Caribbean Eddies
The rich eddy field of the Caribbean is depicted
in data-assimilative model products available
at http://www7300.nrlssc.navy.mil/global_
nlom/globalnlom/ias.html and www7320.
nrlssc.navy.mil/IASNFS_WWW/today/IASNFS_
ias.html. Eddy trajectories in the Caribbean
have been directly observed by deployment
of Lagrangian drifters, and details of their
apparent height and periodicity derived from
SSHT are available.Although Murphy et al.
[1999] have remarked that Caribbean eddies
are primarily anti-cyclonic, other analyses
[Carton and Chao,1999] indicate that cyclonic
and anti-cyclonic pairs are also formed through
interaction with the island masses of Trinidad
and Tobago. Caribbean eddies have been
implicated in ventilation of the Cariaco basin
[Astor et al., 2003], icthyoplankton transport,
and the safety of oil and gas exploration
structures.
Continued investigation will provide additional information on their effects on biological
processes, on their contribution to large-scale
ocean circulation, and on their variability in
response to global change. Spaceborne ocean
color imagery has proven to be a powerful
tool for characterizing such complex interactions; it provides detail unattainable with
traditional shipboard techniques or with other
remote sensing products such as SSHT.
Eos,Vol. 85, No. 20, 18 May 2004
Acknowledgments
CaVortEx I was supported by a grant from
the Office of Naval Research for the study of
Caribbean eddies and by the NASA-UPR Tropical
Center for Earth and Space Studies, which is
dedicated to the assessment of biological productivity in the eastern Caribbean basin.We
thank the captain and crew of R/V Chapman
and members of the CaVortEx Science Team:
Fernando Gilbes,Ernesto Otero,Alvaro Cabrera,
Oswaldo Cárdenas, Miguel Canals,Ana Lozada,
Milton Muñoz,Yaritza Rivera, Lumarie Perez,
Ramon Lopez, and David Pecora.
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Author Information
Jorge E. Corredor, Julio M. Morell, Jose M. Lopez,
Jorge E. Capella, and Roy A.Armstrong
For additional information,contact Jorge E.Corredor,
Department of Marine Sciences, University of Puerto
Rico, P.O. Box 908, Lajas, PR 00667-0908, USA; E-mail:
[email protected]
Fig. 3. (a) The horizontal distribution of near-surface salinity and Chl a across the eddy is shown
(14–15 August 2003). GPS-referenced data were obtained by means of a shipboard continuous
sea water pumping system,a thermosalinograph,and a chlorophyll fluorometer.Real-time shipboard
salinity and chlorophyll a measurements across the track closely reflect the pattern observed in
the satellite imagery.Arrows point to transient salinity and Chl a anomalies associated with the
spiral of ORP water entrained in the eddy. (b) Temperature “staircasing” in the eddy; the wellformed staircase structure is apparent at 15°20’N in the eddy shear zone, but is absent in the
eddy core at 14°50’N.