The effect of wind on the estimated plume extension

of the La Plata River

Erica Darken

Summer 2004

Purpose

• To study the relationship between the direction and speed of the wind and the extension of the high chlorophyll signal associated with the plume of the La Plata River in the Southwestern Atlantic Ocean, as estimated by the Oc4 version 4 chlorophyll algorithm.

Outline

• The basics of Ocean Color.

• The La Plata River- its importance and previous research.

• My project.

• Future work.

The significance of ocean color • Remote sensors mounted on satellites can gather water-leaving radiance data that

change according to varying concentrations of dissolved and suspended constituents.

IOCCG (2000)

Applying remotely sensed ocean-color data

Raw radiances

Normalized water-leaving

radiances

Standardized projection

Atmospheric correction

Geo-location

Bio-optical algorithmMax(490, 443, 510nm) 555nm

Chlorophyll concentration (phytoplankton pigments)

Oc4v4 Algorithm

Examples of remotely sensed images

Data from SeaWiFS,

April 28, 2002.

True color

Chlorophyll map

Where is the

La Plata River?

Source: American Association for the Advancement of Science, Plata Basin Initiative.

Importance of the La Plata River• Second in importance in South America.

• The Plata Basin covers 3,100,000 km2 (1,200,000 mi2), or about 20% of South America (AAAS, Plata Basin Initiative).

• Much larger than the Mississippi River. Historical discharge (Berbery and Mechoso, 2000):

• Forms the world’s largest estuary where it meets the Southwestern Atlantic Ocean.

• Provides the nutrients for many coastal fisheries.

Mississippi River La Plata River

Minimum 8,000 18,000

Maximum 55,000 72,000

Mean 17,000 21,000 m3/s

Influences on the plume extension of the La Plata River

Wind fields

Ocean CurrentsVolume of river

discharge

PLUME EXTENSION

Other forcing mechanisms

Previous Research: Alberto Piola• Alberto Piola has studied the plume of the La Plata River, using sea surface salinity as a tracer, comparing it with along-shore wind stress.

• Using decades of averaged data, Piola found that periods of high wind stress do not necessarily correspond to periods of high river discharge, and that the two in fact tend to be out of phase, limiting plume extension.

Along-shore wind stress

= [air density] * [drag coefficient] * vy2

y

x

Previous research, continued• Focusing on a small area, Piola found that the values of sea

surface salinity and wind stress follow opposite annual trends.

• Piola concluded that the seasonal variation of the wind is responsible for the seasonal variation of the plume extension.

Graph and data source:Piola, 2003.

Jan Mar May Ju l Sep NovM on th

2 8

3 0

3 2

3 4

3 6

3 8

Sa

linity

-0 .02

-0.01

0

0.01

0.02

0.03

0.04

Along-shore w

ind stress (Pa)

S

The data for my investigation

River plume

Area wind fields

SeaWiFS chlorophyll images (9 km resolution)

National Centers for Environmental Prediction wind vectors (~1.9º

resolution)

Monthly averages for austral summer

(January, February, March,) and austral

winter (July, August, September,) for six years (1998-2003)

The area of interest

60ºW 45ºW

25ºS

40ºS

Cape Santa Marta

Patos Lagoon

La Plata River

Estuary

True color image: MODIS Aqua, May 4, 2002

February August

1998 2001 1998 2001

1999 2002 1999 2002

2000 2003 2000 2003

Austral summer 2003: wind influence

January February March

Austral Winter 2003:wind influence

July August September

Measuring the plume

Measuring the surface area of the waters that have a chlorophyll concentration greater than 2 mg/m3 with MATLAB, we can get a quantitative representation of the annual cycle.

Plume area, river discharge, and wind stress• Anomaly[month’s area] =

[month’s area] - Mean[month’s area]

• Anomalous small plume area in January 2000 corresponds with anomalous strong, negative along-shore wind stress.

Ene-60 Ene-65 Ene-70 Ene-75 Ene-80 Ene-85 Ene-90 Ene-95 E ne-00

-0.02

-0.01

00.01

0.02

Alo

ng

-sho

re w

ind

stress (P

a)

3000

040

000

5000

060

000

Pla

ta o

utf

low

(m

3/s

)

Graph:Piola

Blue = outflow Red = wind stress

Conclusions

• While there are seasonal/monthly trends to the wind fields and to the plume extension, there is no visually convincing argument that the wind as a monthly mean is responsible for the extension of the plume of the La Plata River.

• Numerical arguments suggest that both wind stress and river discharge can greatly influence the extension of the plume.

Future work

• Compare chlorophyll fields with salinity fields for the area to fine-tune the plume definition.

• Use another optical property besides chlorophyll to examine the plume.

• Relate the plume extension to the direction and speed of the ocean currents.

• Calculate up-to-date wind stress data.

• Divide time more finely.

Collaborators

• Virginia Garcia• Carlos Garcia• Chuck McClain• Sergio Signorini• Research and Discover

Program, University of New Hampshire and NASA GSFC