The effect of wind on the estimated plume extension of the La Plata River Erica Darken Summer 2004.
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Transcript of The effect of wind on the estimated plume extension of the La Plata River Erica Darken Summer 2004.
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