Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview Western Boundary Circulation (Schott et al. 2004) - shipboard

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%)


• NBUC welldeveloped

• AAIW counter current

• Large NADW transports and counter currents


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


• 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


Overview




• Summary


Marcus Dengler



Moored observations at 11°S (2000-2003)

100m level (large gaps)

1900m level


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)


Mean section and boxes (1-6) for time series

1 2

34

5

6


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


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


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


Correlation of NBUC and DWBC transport with alongshore currents of array


Variability in the upper ocean is not correlated to deep ocean variability.


Overview




• Summary


Marcus Dengler



Deep Western Boundary Current Eddies

along-shore velocity

Structure of DWBC similar to counter current structure


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


50-90d band-pass filtered time series

• EKE maximum largest at 1900m (K3,K4) and 2400m (K5)

• Maxima away from continental slope


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

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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


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


)s/cm1U,V(Volume eddyeddy Eddy Volume

average Volumeeddy = (13.9 +/- 5.9) x 1013 m3

DWBC- Volumeeddy = (10.5 +/- 3.6) x 1013 m3 (depth range: 1200-3800m)

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.


FLAME Model eddy kinetic energy along 10°S

Deep EKE maximum similar to observations

Snapshot of along-shore and across-shore velocity


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

Summary

Documents

Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview Western Boundary Circulation (Schott et al. 2004) - shipboard