Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview • Western Boundary Circulation (Schott et al. 2004) - shipboard sections at 5°S (8 sections) and 11°S (4 sections) - moored observations at 11°S (2000-2003) • Deep Western Boundary Current Eddies • Summary Tropical Atlantic Workshop, June 200 Marcus Dengler Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler Leibniz-Institut für Meereswissenschaften, Kiel, Germany
Transcript
Western boundary circulation and the role of deep eddies in the tropical South Atlantic
Overview
• Western Boundary Circulation (Schott et al. 2004)- shipboard sections at 5°S (8 sections) and 11°S
(4 sections) - moored observations at 11°S (2000-2003)
• Deep Western Boundary Current Eddies
• Summary
Tropical Atlantic Workshop, June 2004
Marcus Dengler
Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler
Leibniz-Institut für Meereswissenschaften, Kiel, Germany
Observational program (1990-2004)
8 sections
4 sections
Array 2000-2004 (gaps!)
Mean salinity at 11°S and transport layer distinction
Boundary Circulation in the tropical South Atlantic
10
• NBUC always at shelf edge
• DWBC sometimes reversed at slope
(new LADCP post-processing method)
Boundary Circulation at 5°S (8 Sections, 1990-2003)
Along-shore LADCP/ADCP velocity sections from 5°S
NBUC=26.8 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%)
Boundary Circulation at 5°S (8 Sections, 1990-2003)
• NBUC welldeveloped
• AAIW counter current
• Large NADW transports and counter currents
Boundary Circulation at 11°S (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 (<1Sv).
Theat = 1.0 PW Tfresh = - 0.2 Sv
Boundary Circulation at 11°S (4 Sections, 2000-2003)
• NBUC already fully established at 11°S (i.e. is part of the STC)
• Box 5-11°S 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!
<24.5 24.5-26.8 26.8-32.15 total (Sv)
5°S sectionboundary 3.7 13.3 8.8 26.8
Offshore -- .2 -5.1 -4.9
net 3.7 13.9 3.0 20.6
Budget of upper circulation
11°S 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
5°S section and AAIW-layer circulation in 1/12° Miami model
NBUC retroflection supplies recirculation
Budget of upper circulation
Courtesy of Z. Garaffo and E. Chassignet
Western boundary circulation and the role of deep eddies in the tropical South Atlantic
Overview
• Western Boundary Circulation (Schott et al. 2004)- shipboard sections at 5°S (8 sections) and 11°S
(4 sections) - moored observations at 11°S (2000-2003)
• Deep Western Boundary Current Eddies
• Summary
Tropical Atlantic Workshop, June 2004
Marcus Dengler
Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler
Leibniz-Institut für Meereswissenschaften, Kiel, Germany
Moored observations at 11°S (2000-2003)
100m level (large gaps)
1900m level
Moored observations at 11°S (2000-2003)
Southward „DWBC“ aliasing during ship section times
Box transports (Sv)
1: 12.52: 2.83. 9.84: 2.7
1-4: 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)
Moored observations at 11°S (2000-2003)
Mean section and boxes (1-6) for time series
1 2
34
5
6
Moored observations at 11°S (2000-2003)
NBUC transport variability (box 1-4)
• current records mapped using EOFs from all time series
• mode 1+2 explains NBUC variability
strong variability on different time scales
Moored observations at 11°S (2000-2003)
DWBC transport variability (Box 6)
strong intraseasonal variability
• Significant seasonal (semi+annual) cycle of NBUC and DWBC
• explains lower NBUC transport from ship-board observations
Moored observations at 11°S (2000-2003)
Annual and semi-annual cycle of transport time series
• NBUC variability at 11°S may be (partially) caused by SEC bifurcation migration (pers. comm. S. Huettl)
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
Moored observations at 11°S (2000-2003)
Correlation of NBUC and DWBC transport with alongshore currents of array
Moored observations at 11°S (2000-2003)
Variability in the upper ocean is not correlated to deep ocean variability.
Western boundary circulation and the role of deep eddies in the tropical South Atlantic
Overview
• Western Boundary Circulation (Schott et al. 2004)- shipboard sections at 5°S (8 sections) and 11°S
(4 sections) - moored observations at 11°S (2000-2003)
• Deep Western Boundary Current Eddies
• Summary
Tropical Atlantic Workshop, June 2004
Marcus Dengler
Friedrich Schott, Peter Brandt, Jürgen Fischer, Lothar Stramma Carsten Eden, Rainer Zantopp, Karina Affler
Leibniz-Institut für Meereswissenschaften, Kiel, Germany
Deep Western Boundary Current Eddies
along-shore velocity
Structure of DWBC similar to counter current structure
Deep Western Boundary Current Eddies
Salinity distribution and contours of along-shore velocity
Oxygen distribution and contours of along-shore velocity
DWBC and offshore counter currents carry same water masses
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
Deep Western Boundary Current Eddies
50-90 day band pass filtered velocities at 1900m (2400m) depth
Dengler et al., DSR, 2004
Sequence of anti-cyclonic sub-mesoscale eddies (about 6 per year)
cross-shore velocity
along-shore velocity
20
20
20
2000
2000
H
)zz(
R
)tvtv())tt(ux(x(
0
0000eddy ee
R
))tt(ux(xAvV
20
20
20
2000
2000
H
)zz(
R
)tvtv())tt(ux(x(
0
0000eddy ee
R
tvtvAuU
alongshore component:
across-shore component:
Eddy fit to mooring data
Eddy model based on Gaussian-shaped density distribution
0000 tvy,tvy as array is one-dimensional
00 u,v
heightandradiusfoldingeH,R 00
eddyofcenterz,x 00
amplitudeA
- eddy translation
LACDP data Eddy fit
2000m
Eddy fit to mooring data
Eddy model fit: min ])UU()VV[( 2eddyobs
2eddyobs
Data:
• 14 velocity time series
• 40h low-pass data
• 40-80 day ensembles
Explained Variance 46%
fit failed for two ensembles
Average eddy parameters
Eddy translation
Along-shore = 3.8 cm/s
Cross-shore = 2.1 cm/s
3.9 cm/s westward drift > CR(3. mode)= 2.6 cm/s
1.8 cm/s southward drift
eddy translation not explainable by advection only
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
Box 6 transport from eddy sequence (-) and moorings (--)
- eddy scales could not be determined
Transport:
Box 6 (moorings) = -14.2 Sv
Eddies Box 6 = -15.2 Sv
total eddy DWBC transport (1200-3800m) = -17.9 Sv
Eddy Transport
DWBC array seesrectified eddy mean!
Deep Western Boundary Current Eddies in Models
Eddy kinetic energy and mean kinetic energy at 1900m from FLAME model
FLAME Model
• 1/12° resolution
• Domain: 18°S-70°N, 100°W-16°E
• setup includes open boundary at 18°S
EKE distribution similar to distribution from 1/6° OPA model. (Trequier et al., JPO, 2003)
DWBC-EKE at 11°S only slightly enhanced in 1/12° Miami model.
Deep Western Boundary Current Eddies in Models
FLAME Model eddy kinetic energy along 10°S
Deep EKE maximum similar to observations
Snapshot of along-shore and across-shore velocity
Deep Western Boundary Current Eddies in Models
Velocity vectors and density perturbation
• Model suggests eddy generation near 8°S.
• Continental slope turns sharply to the west.
Upper layers
• NBUC - average transport: at 5°S from 8 sections: 26.8 Sv, at 11°S from moorings: 27.8 Sv - 13 Sv as part of STC
- SEC bifurcation well south of 11°S - stable NBUC core at 5-11°S
- 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
Summary
Deep circulation
• DWBC transports at 5-11S - at 5°S (8 sections): net southward flow –20 Sv
- at 11°S (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 ~7-8°S
- eddy size correlates with upstream DWBC transport - eddy existence implies no „classical DWBC“ south of °S
