PowerPoint-Pr

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Western boundary circulation and the role of deep
eddies in the tropical South Atlantic
Marcus Dengler Friedrich Schott, Peter Brandt,
Jürgen Fischer, Lothar Stramma Carsten Eden,
Rainer Zantopp, Karina Affler Leibniz-Institut
für Meereswissenschaften, Kiel, Germany

• Overview
• Western Boundary Circulation (Schott et al.
  2004) - shipboard sections at 5S (8 sections)
  and 11S (4 sections) - moored observations at
  11S (2000-2003)
• Deep Western Boundary Current Eddies
• Summary

Tropical Atlantic Workshop, June 2004
2
Observational program (1990-2004)
3
Boundary Circulation in the tropical South
Atlantic
Mean salinity at 11S and transport layer
distinction
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Boundary Circulation at 5S (8 Sections,
1990-2003)
Along-shore LADCP/ADCP velocity sections from 5S

• NBUC always at shelf edge
• DWBC sometimes reversed at slope
• (new LADCP post-processing method)

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Boundary Circulation at 5S (8 Sections,
1990-2003)

• NBUC26.8 Sv
• 14.3 Sv in SACW layer
• offshore southward AAIW layer flow (5.2 Sv)
• net upper 21.5 Sv
• DWBC -28.1 Sv
• upper NADW -14.7Sv
• m/l NADW -5.3Sv
• net NADW -20.0 Sv
• AABW 1.3 Sv
• Section total almost closed (10)

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Boundary Circulation at 11S (4 Sections,
2000-2003)

• NBUC welldeveloped
• AAIW counter current
• Large NADW transports and counter currents

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Boundary Circulation at 11S (4 Sections,
2000-2003)

• NBUC 23.3 Sv
• 12.1 Sv in SACW layer
• southward offshore flow at AAIW level
• net upper 21.2 Sv
• DWBC 41.2 Sv
• 18.2 Sv return offshore
• net NADW -23.0 Sv
• AABW 1.4 Sv
• Section nearly balanced (lt1Sv).
• Theat 1.0 PW Tfresh - 0.2 Sv

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Budget of upper circulation
lt24.5 24.5-26.8 26.8-32.15 total
(Sv) 5S sectionboundary 3.7 13.3
8.8 26.8 Offshore -- .2
-5.1 -4.9 net 3.7 13.9
3.0 20.6
11S sectionboundary 1.5 12.1
9.7 23.3 Offshore -.4 1.1
-2.6 -1.9 net 1.1 11.0
7.1 19.2

• NBUC already fully established at 11S (i.e. is
  part of the STC)
• Box 5-11S requires westward flow in upper layer
  (but upper 20m uncertain due to extrapolation)
  and eastward flow in AAIW layer (300-1200m)
• EUC layer (main STC carrier) little inflow from
  east good site!!
• Offshore southward flow in AAIW layer!

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Budget of upper circulation
5S section and AAIW-layer circulation in 1/12
Miami model _ NBUC retroflection supplies _
recirculation
Courtesy of Z. Garaffo and E. Chassignet
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Western boundary circulation and the role of deep
eddies in the tropical South Atlantic
Marcus Dengler Friedrich Schott, Peter Brandt,
Jürgen Fischer, Lothar Stramma Carsten Eden,
Rainer Zantopp, Karina Affler Leibniz-Institut
für Meereswissenschaften, Kiel, Germany

• Overview
• Western Boundary Circulation (Schott et al.
  2004) - shipboard sections at 5S (8 sections)
  and 11S (4 sections) - moored observations at
  11S (2000-2003)
• Deep Western Boundary Current Eddies
• Summary

Tropical Atlantic Workshop, June 2004
11
Moored observations at 11S (2000-2003)
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Moored observations at 11S (2000-2003)
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Moored observations at 11S (2000-2003)
Mean section and boxes (1-6) for time series
Box transports (Sv) 1 12.52 2.83. 9.84
2.71-4 27.8 (NBUC)(Ship NBUC was
23.3) 5-3.0 (0.7) (AAIW recirculation,
-2.6 from ship obs. ) 6 -18.7 (DWBC)(ship
mean was 44 Sv)
1
2
4
3
5
6
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Moored observations at 11S (2000-2003)
NBUC transport variability (box 1-4)

• current records mapped using EOFs from all time
  series
• mode 12 explains NBUC variability

strong variability on different time scales
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Moored observations at 11S (2000-2003)
DWBC transport variability (Box 6)
strong intraseasonal variability
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Moored observations at 11S (2000-2003)
Annual and semi-annual cycle of transport time
series

• Significant seasonal (semiannual) cycle of
  NBUC and DWBC
• explains lower NBUC transport from ship-board
  observations

• NBUC variability at 11S may be (partially)
  caused by SEC bifurcation migration (pers. comm.
  S. Huettl)

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Moored observations at 11S (2000-2003)
Interannual differences and trends 2000-2003
Annual averages Time NBUC 3/00-3/01
23.6 3/01-3/02 23.7 3/02-3/03
26.4
NBUC Line fit 1.5 Sv/year
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Moored observations at 11S (2000-2003)
Correlation of NBUC and DWBC transport with
alongshore currents of array
Variability in the upper ocean is not correlated
to deep ocean variability.
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Western boundary circulation and the role of deep
eddies in the tropical South Atlantic
Marcus Dengler Friedrich Schott, Peter Brandt,
Jürgen Fischer, Lothar Stramma Carsten Eden,
Rainer Zantopp, Karina Affler Leibniz-Institut
für Meereswissenschaften, Kiel, Germany

• Overview
• Western Boundary Circulation (Schott et al.
  2004) - shipboard sections at 5S (8 sections)
  and 11S (4 sections) - moored observations at
  11S (2000-2003)
• Deep Western Boundary Current Eddies
• Summary

Tropical Atlantic Workshop, June 2004
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Deep Western Boundary Current Eddies
along-shore velocity
Structure of DWBC similar to counter current
structure
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Deep Western Boundary Current Eddies
Salinity distribution and contours of along-shore
velocity
DWBC and offshore counter currents carry same
water masses
Oxygen distribution and contours of along-shore
velocity
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Deep Western Boundary Current Eddies
50-90d band-pass filtered time series

• EKE maximum largest at 1900m (K3,K4) and 2400m
  (K5)
• Maxima away from continental slope

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Deep Western Boundary Current Eddies
50-90 day band pass filtered velocities at 1900m
(2400m) depth
along-shore velocity
cross-shore velocity
Sequence of anti-cyclonic sub-mesoscale eddies
(about 6 per year)
Dengler et al., DSR, 2004
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Eddy fit to mooring data
Eddy model based on Gaussian-shaped density
distribution
alongshore component
across-shore component
as array is one-dimensional
2000m
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Eddy fit to mooring data
Eddy model fit min

• Data
• 14 velocity time series
• 40h low-pass data
• 40-80 day ensembles

_ Explained Variance 46
fit failed for two ensembles
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Average eddy parameters
Eddy translation Along-shore 3.8 cm/s
Cross-shore 2.1 cm/s
3.9 cm/s westward drift gt CR(3. mode) 2.6
cm/s 1.8 cm/s southward drift
_
_ eddy translation not explainable by advection
only
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Average eddy parameters
Eddy scale Amplitude (gaussian) 0.50 m/s Radius
(e folding) 60 km Eddy offshore center 160
km Height (e folding) 1100 m Depth of maximum
2100 m
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Average eddy parameters
average Volumeeddy (13.9 /- 5.9) x 1013
m3 DWBC- Volumeeddy (10.5 /- 3.6) x 1013 m3
(depth range 1200-3800m)
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Eddy Transport
Box 6 transport from eddy sequence (-) and
moorings (--)
Transport Box 6 (moorings) -14.2
Sv Eddies Box 6 -15.2 Sv total
eddy DWBC transport (1200-3800m) -17.9 Sv
DWBC array seesrectified eddy mean!
- eddy scales could not be determined
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Deep Western Boundary Current Eddies in Models

• FLAME Model
• 1/12 resolution
• Domain 18S-70N, 100W-16E
• setup includes open boundary at 18S

EKE distribution similar to distribution from
1/6 OPA model. (Trequier et al., JPO,
2003) DWBC-EKE at 11S only slightly enhanced in
1/12 Miami model.
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Deep Western Boundary Current Eddies in Models
FLAME Model eddy kinetic energy along 10S
Deep EKE maximum similar to observations
Snapshot of along-shore and across-shore velocity
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Deep Western Boundary Current Eddies in Models
Velocity vectors and density perturbation

• Model suggests eddy generation near 8S.
• Continental slope turns sharply to the west.

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Summary

• Upper layers
• NBUC - average transport at 5S
  from 8 sections 26.8 Sv, at 11S from moorings
  27.8 Sv
  - 13 Sv as
  part of STC - SEC bifurcation well
  south of 11S - stable
  NBUC core at 5-11S - strong annual and
  semi-annual transport variability - significant
  interannual differences in NBUC (linear trend
  1.5 Sv/year) - relation to STC
  variability? - how much near boundary vs.
  interior? (in Pacific interior variability
  partially compensated by wbc)
• AAIW-Layer - offshore
  southward recirculation (5 Sv) - how
  does it connect? - Miami model NBUC
  retroflection
  - - 800m RAFOS show inflow from east

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Summary

• Deep circulation
• DWBC transports at 5-11S
  - at 5S
  (8 sections) net southward flow 20 Sv
  - at 11S (4 sections) section net 23 Sv -
  DWBC transport from eddies about 18 Sv
• DWBC Eddies - 5-6 DWBC eddies per year
  (similar to NBC rings)
  - carry water masses of the
  DWBC southward
  - westward translation
  agrees with eddy dynamics (southward
  translation?) - FLAME simulation
  indicate
  
  - generation at 7-8S
  - eddy size correlates with
  upstream DWBC transport - eddy
  existence implies no classical DWBC south of S