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