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
expected for eastern Caribbean basin waters;
River Plume in Eastern Caribbean
core was particularly apparent. Near-surface
Salinity structure below -75 m was as might be shoaling of the sub-surface,high-salinity,sub tropical underwater mass within the eddy waters,however,showed influence of the low salinity ORP with a marked front separating
PAGES 197,201-202 "Mesoscale" eddies are large whirlpools in the ocean with diameters of hundreds of kilo meters.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 Reso lution Imaging Spectroradiometer-MODIS sensors aboard NASAS AQUA and TERRA platforms, and support for direct observation at sea, have now
section (Figure 2) of the eddy In situ charac
the high-salinity core from surrounding low
terization of Caribbean eddies was lacking
salinity ORP waters (Figure 2b).This pattern was reflected in the distribution of dissolved
prior to Ca VortEx l. 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 (Chi 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.
silicate,a nutrient that is abundant in the ORP
[Corredor and Morell, 2001],but scarce in sur face 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
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 (CD OM) [Blough et al., 1993; Morell and Corre dor, 200 1], 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 sur face 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'.
The
Caribbean Vorticity Experiment
The Caribbean Vorticity Experiment (CaVortEx l) was undertaken in August 2003 to characterize the physical, biogeochemical,and optical structure of the eddy and to assess the influ ence of the eddy,and the ORP on biological productivity Scientists from the University of Puerto Rico characterized surface and subsur face features of the eddy down to 1000 m.The cruise track aboard RN 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. I. K490MODIS data product for 5 August 2003 at I-km resolution. K490 is the diffuse attenua tion 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'WDuring 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. Original color image appears at back of this volume.
Eos, Vol. 85, No. 20, 18 May 2004 (b)
(a)
(c)
Latitude N 15
14
14
16
15
16
� 200 300 400
E .r:
Q.
500
Q)
0
600 700 800 900 nnn
Sigma-t
21.0 22.0 23.0 24.0 25.0 26.0 27.0
Practical Salinity 33.6
34.2
34.8
35.4
36.0 36.6 37.2
....
0.00 0.09 0.18 0.27 0.36 0.45 0.54
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-I) 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. Original color image appears at back of this volume. dominated by prasynophytes and diatoms was
and salinity at scales of 10-20 m and interme
and re-form within the island arc with little loss
present in the eddy periphery.
diate depths (400-600 m),common in the
of mass [Simmons and Nof, 2002]. Genesis of
Caribbean [Lambert and Sturges, 1977],are
Caribbean cyclonic eddies is less clear,but
fied high-salinity, low-chlorophyll waters such
related to double-diffusive processes obeying to
may follow a similar path, and the entrained
as the "Haulani" eddy off Hawaii [Vaillancourt et 01.,2003] show shoaling of the deep chloro
differences between diffusive rates of salt and
spiral filament may result as a consequence
heat. Staircasing was strongest in the mid-eddy
of eddy reformation.
phyll maximum (DCM) in concert with the
shear zone, but was substantially reduced in
vertical water mass displacement. Such "eddy
the eddy core and its periphery. Dynamics of
Cyclonic eddies propagating through strati
Caribbean Eddies
pumping" enhances primary production by
these features in the course of eddy propaga
The rich eddy field of the Caribbean is depicted
bringing nutrient-rich waters into the euphotic
tion may prove to be significant in the return
in data-assimilative model products available
zone.The DCM in the Caribbean eddy responds
transport of deepwater masses within the con
at http://www7300.nrlssc.navymil/global_
to both eddy pumping and the influence of
text of the global meridional overturning
nlom/globalnlom/ias.html and www7320.
the buoyant river plume. The DCM in the
circulation.
nrlssc.navymilIIASNFS_WWW/today/IASNFS_
region shoals considerably in waters influenced by the ORp,reaching depths under 30 m in the east-central Caribbean,in contrast to the greater
ias.htm!. Eddy trajectories in the Caribbean
A Spiral in the Eddy
depths (>90 m) prevailing in the absence of
In northern hemisphere cyclonic eddies,
plume waters and in the adjacent Atlantic
Coriolis forcing displaces surface water out
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 .
[Corredor et 01.,2003] . Consequently, while
wards as deeper waters are transported toward
[1999] have remarked that Caribbean eddies
doming of the DCM is apparent in the eddy
the surface. Such an eddy intersecting a highly
are primarily anti-cyclonic, other analyses
core, a shallow DCM dominates its periphery.
colored river plume would displace the buoyant
[Carton and Chao, 1999] indicate that cyclonic
Remnants of the oceanic DCM,presumably
plume waters and traverse the plume intact.
and anti-cyclonic pairs are also formed through
It would appear from space as a coherent
interaction with the island masses of Trinidad
diminished by shading,underlie the river-related DCM (Figure 2c). Optical properties of the cyclonic eddy,
disk of clear water within the turbid surface
and Tobago. Caribbean eddies have been
plume. High-resolution MODIS imagery,however,
implicated in ventilation of the Cariaco basin
measured by profiling radiometry,spectropho
reveals a distinct filament of ORP water spiraling
[Astor et al., 2003], icthyoplankton transport,
tometry, and turbidimetry, were similarly mod
within the eddy core, a feature replicated in
and the safety of oil and gas exploration
ulated by both the eddy and the river plume.
surface transects appearing as 10calized,low
structures.
While the oceanic water of the eddy core
salinity and high-Chi a anomalies within the
exhibited low optical absorption and turbidity,
eddy (FIgure 3a). Anti-cyclonic (rotating clockwise)
tional information on their effects on biological
near-surface waters of the surrounding river
Caribbean eddies are thought to originate
processes , on their contribution to large-scale
plume were not only more turbid,they exhib
from Atlantic Ocean eddies known as North
ocean circulation,and on their variability in
ited sharply increased light absorption of the
Brazil Current Rings (NBCR),which are gener
response to global change. Spaceborne ocean
shorter wavelengths (blue and ultraviolet),a
ated by instabilities in the NBC retroflection
color imagery has proven to be a powerful
property characteristic of high CDOM waters.
[Johns et 01.,1990]. Following impingement
tool for characterizing such complex interac tions; it provides detail unattainable with
Continued investigation will provide addi
A final feature of interest is the distribution
on the island arc of the Antilles,some NBCRs,
of "staircase" patterns across the eddy (Figure
particularly the weaker,larger rings,appear to
traditional shipboard techniques or with other
3b).Abrupt discontinuities in temperature
be able to squeeze through the island passes
remote sensing products such as SSHT.
Eos,Vol. 85, No. 2 0 , 1 8 May 2004 Acknowledgments
Latitude N
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
(a)
Center for Earth and S p a c e Studies, which is dedicated to the assessment of biological pro ductivity in the eastern Caribbean basin.We thank the captain and crew of R/V
Chapman
and m e m b e r s of the CaVortEx S c i e n c e Team: Fernando Gilbes, Ernesto Otero, Alvaro Cabrera, Oswaldo Cardenas, Miguel Canals, Ana Lozada, Milton Murioz,Yaritza Rivera, Lumarie Perez, R a m o n Lopez, and David Pecora. References Astor,Y,FMuller-Karger,and M.Scranton ( 2 0 0 3 ) , S e a s o n a l and interannual variation in t h e hydrog raphy of the Cariaco Basin: Implications for basin ventilation, Cont. Shelf Res., 23,125-144. Blough,N.V,O.C.Zafiriou,and J.Bonilla (1993),Optical absorption spectra of waters from the O r i n o c o River outflow: Terrestrial input of c o l o r e d organic matter to the Caribbean, J Geophys. Res., 98,2271-2278. Carton,J.A.,andYChao (1999),Caribbean S e a eddies inferred from TOPEX/POSEIDON altimetry a n d a 1/6° Atlantic O c e a n simulation, J Geophys. Res., 704,7743-7752. Corredor, J., and J. Morell ( 2 0 0 1 ) , S e a s o n a l variation of physical and biogeochemical features in eastern Caribbean surface water, J Geophys. Res., 106,4517-4525. Corredor, J.,et al. ( 2 0 0 3 ) , Remote continental forcing of phytoplankton biogeochemistry: Observations across the "Caribbean-Atlantic front," Geophys. Res. Lett., 3 0 , 2 0 5 7 - 2 0 6 0 . J o h n s , W E.,T. N. Lee, FA. Schott, R. J. Zantopp, a n d R.H.Evans,(1990),The North Brazil Current retroflection: Seasonal structure and eddy variability,J. Geophys. Res., 7 0 4 , 2 5 , 8 0 5 - 2 5 , 8 2 0 . L a m b e r t , R . B . , a n d WSturges ( 1 9 7 7 ) , A thermohaline staircase a n d vertical mixing in the thermocline, Deep-Sea Res., 24,211-222.
(b)
8
9
T°C 10
1
\
i
1
i
i
i
distribution
of near-surface
I
11
12 r
-
Morell,J.M.,and J . E . C o r r e d o r ( 2 0 0 1 ) , Photomineralization of fluorescent dissolved organic matter in the Orinoco River plume: Estimation of a m m o n i u m release,./ Geophys. Res., 106,16,807-16,813. Murphy,S. J . , e t al. ( 1 9 9 9 ) , T h e connectivity of eddy variability in the Caribbean Sea, the Gulf of Mexico and the Atlantic O c e a n , i Geophys. Res., 104,1431-1453. S i m m o n s , H . L . , a n d D.Nof ( 2 0 0 2 ) , T h e Squeezing of eddies through gaps, J Phys. Oceanogr., 32,314-335. Vaillancourt, R. D., J. Marra, M. PSeki, M. L. Parsons and R. R. Bidigare ( 2 0 0 3 ) , Impact of a c y c l o n i c eddy o n phytoplankton community structure a n d photosynthetic c o m p e t e n c y in the subtropical North Pacific O c e a n , D e e p Sea Res. I, 5(9,829-847.
j
Fig. 3. (a) The horizontal (14-15
August 2003).
sea water pumping
Information
spiral formed
J o r g e E.Corredor,Julio M . M o r e l l , J o s e M.Lopez, Jorge E . C a p e l l a , a n d Roy A.Armstrong For additional information, contact Jorge E. Corredor, Department of Marine S c i e n c e s , University of Puerto Rico, PO. B o x 908, Lajas, PR 00667-0908, USA; E-mail:
[email protected]
staircase
eddy core at
a measurements
imagery. Arrows
of ORP water
data
were
system, a thermosalinograph,
salinity and chlorophyll the satellite
Author
GPS-referenced
point
entrained
structure
14°50'N.
across
to transient
salinity and Chi a across
obtained
fluorometer.
the track closely
reflect
,
"staircasing"in
in the eddy shear
zone,
shown
continuous
Real-time
the pattern
salinity and Chi a anomalies
at 15°20 N
the eddy is
of a shipboard
and a chlorophyll
in the eddy, (b) Temperature
is apparent
by means
shipboard
observed
associated the eddy; the but is absent
in
with the wellin the
Eos, Vol. 8 5 , No. 2 0 , 18 May 2 0 0 4
Fig. 1. K490MODIS data product for 5 August 2003 at 1-km resolution. K490 is the diffuse attenua tion 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. 8 5 , No. 2 0 , 1 8 May 2 0 0 4 (a) 14
15
16
21.0 22.0 23.0 24.0 25.0 26.0 27.0
c
Latitude N
(b) 14
33.6
( )
15
16
34.2 34.8 35.4 36.0 36.6 37.2
14
0
0
0
0
0
9
15
0
1 8
0
2
7
0
3
6
0
16
4
5
0
5
4
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.
Page 201
Fig. I. Hydrographers measure stream the Kankakee River at Dunns Bridge, Page 197
flow using an Acoustic Indiana.
Doppler
Current Profiler (ADCP)
on
