Oral exam

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• Oral exam
• Upwelling in SAB

Alfredo Lopez de Aretxabaleta 30-Jul-2004
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Outline

• Definition of upwelling
• Different mechanisms for coastal upwelling
• Wind-driven coastal upwelling
• Western Boundary Currents upwelling
• other
• Upwelling in SAB
• Summer wind-driven upwelling
• Upwelling associated with Gulf Stream meanders
• Connection between two processes
•  Upwelling in SAB in 2003
• Wind forcing
• Strength, proximity and meander activity of G-S
• Stratification (preconditioning)
• Upstream effects

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Working Definition

• The vertical advection of nutrient-rich
  subsurface waters of lower temperature and
  greater density into upper layers of the ocean.

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• Different mechanisms for coastal upwelling
• Wind-driven coastal upwelling
• Locally forced (Ekman)
• Remotely forced (Kevin and shelf waves)
• Western Boundary Currents
• (baroclinic instability ? meanders, filaments,
  frontal eddies)
• Topographic (canyons, shelf edge)

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Wind-driven Upwelling mechanisms
Equatorial Upwelling
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Wind-driven upwelling East versus West

• Wind pattern
• (extent, intensity, persistency)
• Width depth of shelf
• Depth of thermocline
• Stratification
• Presence of Western Boundary Current
• Differences between east and west coast
  biological provinces

NASA/GSF
Savidge et al, 1992
Martins Pelegri, submitted
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Coastal Wind-driven Upwelling
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Surface Ekman Layer
Cushman-Roisin, Fig 5-4
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Surface Ekman Layer Upwelling
The upwelling velocity is proportional to the
curl of the wind stress
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Ekman Transport

• The replacement of water is confined to coastal
  zone and the offshore scale is given by the
  baroclinic radius of deformation
• The resulting cross-shelf Ekman transport
  (confined to surface Ekman layer)
• And cross-shelf velocity

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Remotely forced wind-driven upwelling
Response to elevation change is not purely local.
Raised thermocline (or low elevation) might
propagate disturbances equatorward (polarward) in
EC (WC). The dynamics are the same as in normal
upwelling, but propagation happens as a Kelvin
wave.
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Remotely forced wind-driven upwelling
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Western Boundary Current Upwelling
Bane, 1994
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Western Boundary Current Upwelling

• As a parcel of fluid gets to shallower areas it
  squeezes and to conserve potential vorticity it
  gains anticyclonic spin
• As the parcel of water moves to deeper areas, it
  stretches and therefore gains cyclonic rotation
• Several factors (e.g., divergence of isobaths,
  displacement of isopycnals) might cause growth on
  the meanders

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Western Boundary Current Upwelling
Ito et al, 1995
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Western Boundary Current Upwelling
Cushman-Roisin, Fig 16-1
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Vorticity Balance
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Western Boundary Current Upwelling
Osgood et al, 1987
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Topographically induced upwelling

• Examples of several mechanisms
• Narrowing of continental shelf accelerating
  along-shelf flow through narrowing section
• Upwelling along continental slope induced by
  change in turbulence from shelf to slope under
  onshore wind conditions (Heaps, 1980)
• Upwelling in bottom boundary layer in stratified
  flow over sloping bottom
• Flow over
• coastal canyon

t
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Upwelling in SAB
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SAB
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SAB bathymetry
Phil Weinbach, http//oceanexplorer.noaa.gov/
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Gulf Stream structure
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Gulf Stream structure
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SAB processes
http//www.skio.peachnet.edu/
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SAB processes
FRED experiment, 1989
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Gulf Stream meanders
Brooks Bane, 1983
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Outer shelf characteristics
20-22 April 1979
Yoder et al, 1981
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Turbulent Fluxes
Lee et al, 1991
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Turbulent Fluxes
Lee et al, 1991
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Gulf Stream structure
Table 2 from Miller Lee, 1995
Vertical Velocity W lt0.2 cm
s-1 Along-shelf Rossby number Rv 0.08 Cross-shelf
Rossby number Ru 1.1 Planetary beta parameter
ßp 2 10-13 cm-1 s-1 Topographic beta
parameter ßt 3 10-11 cm-1 s-1 Relative
vorticity ?V/Lz 7 10-5 s-1
Parameter
Value
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Gulf Stream energetics
DKF EPW BTC EEC
Dewar Bane, 1989
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Turbulent Fluxes
Eddy decay region ? onshore transport
Eddy growth region ? offshore transport
Lee et al, 1991
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NO3 Flux
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Winter-spring NO3 Flux
Lee et al, 1991
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Summer wind conditions
Blanton et al, 2003
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Summer SAB upwelling
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Ekman Transport for SAB

• The replacement of water is confined to coastal
  zone and the offshore scale is given by the
  baroclinic radius of deformation
• The resulting cross-shelf Ekman transport
  (confined to surface Ekman layer)
• And cross-shelf velocity

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Summer NO3 Flux
Lee et al, 1991
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Fall NO3 Flux
Lee et al, 1991
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SAB nitrate flux and production
Lee et al, 1991 Menzel, ed, 1993
Total production Total SAB 35 1012gC
yr-1 Inner shelf 620gC m-2yr-1 Mid
shelf 248gC m-2yr-1 Outer shelf 360gC
m-2yr-1 Potential new production Nitrate
input 78gN m-2yr-1 Potential new production 7.4
1012gC yr-1 Realized new production 4.3 1012gC
yr-1
Primary production rates
Value
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GABEX II (1981)
Bottom Temperature and Currents
Atkinson et al, 1987
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GABEX II (1981)
Atkinson et al, 1987
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GABEX II (1981)
Atkinson et al, 1987
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1981 Modes of variability
Variance explained 37.4 16.0 13.5
Proximity of G-S
Frontal eddy activity
Local wind effect
Hamilton, 1987
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GABEX II (1981)
St Augustine section
NO3 is nitrate calculated from temperature
Atkinson et al, 1987
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SAB volume of cold water

• Transport of cold water onshore

v
Volume of water on mid and outer shelf (from DoE
report)
Time needed to fill out lower part of mid and
outer shelf
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Upwelling in SAB during summer 2003
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Upwelling in SAB during summer 2003
Shivering in the Surf Atlantic's Sudden
Temperature Dive A Midsummer Mystery for
Scientists By John F. KellyWashington Post
Staff WriterThursday, August 7, 2003
http//www.washingtonpost.com/ac2/wp-dyn/A25865-20
03Aug6?languageprinter
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Region

• Main inputs of water
• river discharge
• interaction with Gulf Stream through meanders
  and filaments
• exchange with the Mid Atlantic Bight through the
  Cape Hatteras region.

Bane et al., 2001
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SAB 2003
Szekielda, 2004
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SST anomaly July 2003
MODIS, NASA Earth Observatory
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Wind temporal variability
Mean along-shelf wind statistical test comparison
for Buoy 41008 (Grays Reef)
Signif. greater
Prev. years
2003
No
0.42
-0.01
Apr
Yes
0.66
1.42
May
Yes
1.20
3.47
Jun
Yes
2.51
3.83
Jul
Yes
0.92
2.31
Aug
No
-1.52
-3.29
Sep
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Wind persistence
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Wind anomaly spatial variability
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Rainfall
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May Salinity
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June Temperature
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August Temperature
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August Density
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2004 Temperature
26 July 2004
Jim Nelson
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T-S characteristics
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Origin of upwelled water
where a is the thermal expansion, ß is the saline
contraction, TGS is the Gulf Stream temperature
and Tj is the observed temperature
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Origin of upwelled water
remote origin
local origin
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Origin of upwelled water
No barrier
warm barrier
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Sea Level
15-20 cm lower than clim. Values During Aug?
Weisberg Liu, pers. comm.
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Gulf Stream transport
Adapted from Baringer Larsen, 2001
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Lower Mean Sea Level
Increased Transport Increased Cross-stream
Slope LOWER Coastal Sea level
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Higher Mean Sea Level
Decreased Transport Decreased Cross-stream
Slope HIGHER Coastal Sea level
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Modeling results
Barotropic clim wind Vs Barotropic 2003 wind
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Modeling results
Barotropic reduced mixing
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Stratification
maxN20.01s-1
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Stratification 1981
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Stratification effect
Low stratification
High stratification

• Strong velocity shear (even reversed flow ?)
• Wind forcing controls upper layer processes
• Lower layer controlled by pressure forcing

• Some velocity shear
• Wind forcing controls mid and inner shelf
  processes

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Summer 03 dynamics
X
X
X
X
X
X
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Position of G-S
Noble Gelfenbaum (1992) found no correlation
between G-S position and coastal water level, but
what is the effect on frontal eddies and
upwelling (Hamilton, 1987) ?
Olson et al, 1983
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Position of G-S
Savidge et al, 1992
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2003 Data limitations
Hydrographic observations
Atmospheric observations
SST, SSH
QuickScat
SAV cruises
Towers
Cable ?? Calibrat. cruises
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2003 Remaining questions

• Gulf Stream effects
• Gulf Stream position
• Look at SSH and SST
• Meander activity anomalous
• Modeling activities
• Relative importance of different forcings
• Anomalous wind alone
• Effect of river discharge and increased
  stratification in spring
• Effect of Gulf Stream
• Estimate volume of anomalous water on the shelf
• Effects on circulation and biological
  implications

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2003 Conclusions

• Anomalous upwelling favorable winds (strength and
  persistence) were a principal driver of the event
• Increased river discharge could have
  preconditioned the shelf through late spring
  salinity stratification that by early summer
  became thermal stratification.
• The cold water upwelled onto the shelf and the
  deep Gulf Stream water presented similar
  characteristics suggesting that the anomalous
  water came from the deep part of the Stream.
• Further analysis of observations and model
  simulations are needed to quantify the effect of
  the different forcing mechanisms.

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Acknowledgements

• Funding
• SEACOOS (ONR)
• SABLAM (NOPP)
• People
• -My committee Cisco Werner, Harvey Seim, John
  Bane, Jim Nelson, Rick Luettich, Jose Luis
  Pelegri.
• -Brian Blanton, Karen Edwards, Bob Weisberg,
  Chris Meinen, Ed Kearns

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Additional slides
An indicator of relative importance of the
barotropic and baroclinic instabilities is the
ratio of the total available potential energy
(TAPE) to the total kinetic energy (TKE) i. e. b
TAPE/TKE   Luther and Bane, 1985. The TAPE and
TKE are given by                               
                                                  
                            where D is the volume
of the flow field and lt gt is the mean density
bgtgt1 for predominantly baroclinic flow, while
bltlt1 for predominantly barotropic flow.
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Climatology
Bottom temperature and salinity
Blanton et al, 2003
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Climatology
T, S, Rho transects at 32N
Blanton et al, 2003
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Climatology
Velocity solutions (wind mass- field forced)
Blanton et al, 2003
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Connectivity
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Additional slides
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Governing equations
Momentum
Continuity
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Governing equations
Momentum
Continuity
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Governing equations
2-D Continuity
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Governing equations