Cyclonic eddy entrains Orinoco River Plume in ... - Wiley Online Library

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pigments contribute strongly to light attenuation at this wavelength. The eddy ... waters of the eddy core (light blue). A filament of ... related to double-diffusive processes obeying to differences ... be able to squeeze through the island passes and re-form .... Arrows point to transient salinity and Chi a anomalies associated.
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

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600 700 800 900 nnn

Sigma-t

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Practical Salinity 33.6

34.2

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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

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1

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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

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15

16

34.2 34.8 35.4 36.0 36.6 37.2

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0

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0

2

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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