Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview...

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

• 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

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

)tvtv())tt(ux(x(

0000eddy ee

))tt(ux(xAvV

)tvtv())tt(ux(x(

0000eddy ee

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

Eddy fit to mooring data

Eddy model fit: min ])UU()VV[( 2eddyobs

2eddyobs

• 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

Western boundary circulation and the role of deep eddies in the tropical South Atlantic Overview...

Documents