WHOI, 20-Dec-
2006
Cold Event in the South Atlantic Bight during Summer 2003: Observations and Model
Simulations
Alfredo Aretxabaleta
Applied Ocean Physics & Engineering
WHOI
J. Nelson, J. Blanton (SkIO)
B. Blanton, H. Seim, F. Werner, J. Bane (UNC-CH)
E. Chassignet (FSU)
R. Weisberg (USF)
WHOI, 20-Dec-
2006
• Introduction
• Observations
• Model simulations
• Implications
• Conclusions
Outline
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2006
South Atlantic Bight (SAB) Region
Bane et al., 2001
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2006
Summer 2003 beach report
http://www.washingtonpost.com/ac2/wp-dyn/A25865-2003Aug6?
Shivering in the Surf Atlantic's Sudden Temperature Dive A Midsummer Mystery for Scientists By John F. KellyWashington Post, Thursday, August 7, 2003
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2006
Anomalous hydrographic and atmospheric conditions
A. Aretxabaleta, J. Nelson, J. Blanton, H. Seim, F. Werner, J. Bane, R. Weisberg
Published JGR, 2006
WHOI, 20-Dec-
2006
2003 observations
Cross-shelf hydrographic transectsWind
observationsBottom temperatureNOS coastal water level
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2006
Surface Temperature Anomaly
NDBC (NOAA)
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2006
R2 Bottom Temperature
Year 2003
Tem
pera
ture
(oC
)
6 oC temperature
anomaly
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2006
SAB processes
http://www.skio.peachnet.edu/
Inner-shelf:coast-20m
isobath
Mid-shelf:20-40m isobaths
Outer-shelf:40-75m isobath
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2006
Wind temporal variability
2003 Upwelling favorable
wind stronger than in
previous years
Apr May Jun Jul Aug Sep Oct
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2006
Wind Persistence
Direction (degrees)
July 2003
July previous years (1988-2002)
2003 wind persistentl
y in upwelling favorable direction (red line)
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2006
River Discharge
Higher than average river discharge during the study period
(USGS)
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2006
May Salinity
Large surface pool
of fresher water
Strong salinity
stratification
07-May-2003 observed salinity
May climatological salinity
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2006
June Temperature
By June
Transformed into
thermal stratificatio
nJune climatological temperature
25-Jun-2003 observed temperature
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2006
August Temperature
August climatological temperature
27-Aug-2003 observed temperature
By August
Strong stratification with water
8°C colder!!
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2006
Salinity
Tem
pera
ture
(oC
)T-S characteristics
MAY
Surface water greatly
influenced by river
discharge
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2006
Salinity
Tem
pera
ture
(oC
)T-S characteristics
JUNE
“Deep” related to Gulf Stream water
“Surface” still influenced by fresh water
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2006
Salinity
Tem
pera
ture
(oC
)T-S characteristics
AUGUST
Closely linked to Gulf
Stream water
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2006
~10-15 cm lower than climatology
During Jul + Aug
Virginia Key
Cape Hatteras
Water Level anomaly (2003 – climatology)
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2006
• Anomalous upwelling favorable winds
• Increased river discharge produced stronger stratification
• Salinity and thermal stratification enhanced upwelling process
• The cold water upwelled onto the shelf and the Gulf Stream water had similar characteristics
• No single mechanism explains the cold anomaly
Conclusions from observations
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2006
Model Simulations and Implications
A. Aretxabaleta, B. Blanton, H. Seim, F. Werner , J. Nelson, E. Chassignet
Submitted JGR
WHOI, 20-Dec-
2006
GOALS
• Relative importance of different forcings• Anomalous wind • River discharge and increased
stratification • Gulf Stream intrusion
• Comparison with observations
• Estimate volume of anomalous water on the shelf
• Possible biological implications
Summer 2003 cold event model study
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2006
Model
QUODDY
• Nonlinear prognostic, finite element model
(Lynch & Werner, 1991)
• Uses higher order advection scheme (Kliem, 2004)
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2006
Model Procedure
HYCOM/CLIM
Post-processing
Comparison with obs.
River discharge
USGS
EDAS
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2006
• UNC-SAB model nested regional-scale QUODDY (right).
• Within the 1/12 deg operational HYCOM-GODAE model (left).
SAB Nested Regional Model
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2006
HYCOM as IC and BC estimator
HYCOM (NRL) QUODDYResolution ~1/12 deg O(10 km) 50 m to 5 km
BathymetryETOPO5 +
local improvements
NGDC
Coastal Relief Model
Coastal Wall ~10-m isobath 0-m contour line
in NGDC CRM
Rivers NONE in SAB 7
Tides NONE Tides (Blanton et al, 2004.)
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2006
Surface Salinity
Inclusion of River Discharge
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2006
Summer 2003 hindcast model results
Tem
pera
ture
(oC
)
2003 Surface Temperature @ Gray’s Reef
Gray’sReef
Year 2003
RMS difference
1.81 oC
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2006
2003 Bottom Temperature @ R2
R2
Tem
pera
ture
(oC
)
Summer 2003 hindcast model results
RMS diff.
0.90 oC
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2006
Model results
Heat flux errors affect the surface
temperature field and the
total stratification
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2006
Model results (river effect)
Effect of river
discharge (river -no
river simulations
)
~0.5 oC in subsurface
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2006
Model results
Effect of atmospheric
forcing
(2003 simulation
minus simulation with 2002
wind)
1-2 oC in subsurface
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2006
Bottom Temperature Animation
Tem
pera
ture
(oC
)
Bottom Temperature < 23o C
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2006
Remote upwelling dynamics
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2006
Vertically-averaged temperature anomaly
Influx of anomalously cold water
(2oC colder than normal)
along shelf-break in
central and southern
areas
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2006
Volume of anomalous water
Volume (km3)Percentage of shelf volume
16-May-2003 73 5.3
01-Jun-2003 602 43.8
16-Jun-2003 512 37.2
01-Jul-2003 687 50.0
16-Jul-2003 788 57.3
01-Aug-2003 846 61.6
16-Aug-2003 655 47.8
01-Sep-2003 477 34.9
16-Sep-2003 69 5.0
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2006
Implications
Bottom nitrate
estimate from
temperature
Large pool of extra
nitrate in mid-shelf
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2006
• Model provides realistic description of event
• Anomalous wind stress conditions• Main driver (inner- and mid-shelf)
• High river discharge • Effect on stratification• Controlled inner-shelf structure
• Strong stratification• Enhanced upwelling process
• Gulf Stream • Source of cold water
• Anomalous cold water >50% shelf
• Biological implications (to be determined)
Event Conclusions
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2006
• Improve atmospheric forcing• Improve initial conditions• More elaborate nesting between
basin-scale (HYCOM) and shelf-scale
• Evaluation of nitrate fluxes
Next steps
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2006
Acknowledgments
Funding
• SEACOOS (ONR)
• SABLAM (NOPP)
Institutions• UNC-CH • Skidaway Institute of Oceanography• Dartmouth College
People• Dennis McGillicuddy, Keston Smith, J.L.
Pelegri, Rick Luettich