Western boundary circulation in the tropical South Atlantic and its relation to Tropical Atlantic Variability

Rebecca Hummels1, Peter Brandt1, Marcus Dengler1, Jürgen Fischer1

1GEOMAR Helmholtz Zentrum für Ozeanforschung, Kiel, Germany

Workshop Brazil-Germany, Fortaleza, Brazil, 27.05.2014

Tropical Atlantic Variability

Kushnir et al. 2006

Two modes of climate variability

a) Meridional gradient mode

b) Zonal mode

interannual SST anomalies associated with these patterns are related to rainfall anomalies over the adjacent continents

Circulation in the tropical Atlantic is a superposition of

1) Meridional Overturning Circulation (MOC)

Kuhlbrodt et al. 2007

  

Subduction (blue)

Upwelling (green)

Equatorward transport (circles and numbers in [Sv])

Poleward Ekman transport (red arrows)

Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC)

2) Shallow Subtropical-tropical Overturning Circulation

Schott et al. 2004

Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC)

2) Shallow Subtropical-tropical Overturning Circulation3) Wind driven circulation

Circulation in the tropical Atlantic is a superposition of1) Meridional Overturning Circulation (MOC)

2) Shallow Subtropical-tropical Overturning Circulation3) Wind driven circulation

Interaction between the hemispheres is focused on the western boundary

Observations at 5°S and 11°S between 1990-2004:

• 9 research cruises: repeatedly occupied the 5°S and 11°S section

• Mooring array at 11°S

Schott et al. 2005

Observations at 5°S and 11°S between 2000-2004: Mean state

cm/s cm/s

Average transports at 5°S:

NBUC 22.1 +/- 5.3 Sv [=1 x106 m3s-1]

NADW 20.3 +/-10.1 Sv

Average transports at 11°S:

NBUC 21.7 +/-5.3 Sv

NADW 24.6 +/-5 Sv

Observations at 5°S and 11°S between 2000-2004: Variability

NBUC:• Average transport is similar to

ship sections (23.3 Sv)

• Seasonal variability : amplitude of annual and semi-annual harmonics < 2 Sv

• Interannual variability: for the 4 years is estimated to +/- 1.2 Sv

NADW:• Average transport similar to

ship sections

• Extremely large variability

Schott et al. 2005

Observations at 5°S and 11°S between 2000-2004:

Break up of DWBC in to eddies at around 8°S

Dengler et al. 2004

Other studies after observational period:

extremely close correspondence between AMOC strength at 6°S (red) and NBUC transport (black) in a model study

Biastoch et al. 2008

Other studies after observational period:

Biastoch et al. 2009

Salinity anomalies within the NBUC are related to the variability of the Agulhas leakage and might have implications for further evolution of MOC

Other studies after observational period:

Zhang et al. 2011

Strong interannual NBUC transport variability when constructing the geostrophic transport timeseries based on historical hydrographic observations in the NBUC region

Other studies after observational period:

Biastoch et al. 2009

Biastoch et al. 2008

2000 2005 2010 2015

? Zhang et al. 2011

New observations : a) velocities5°S

11°SNew observations : a) velocities

5°S

New observations : b) water mass characteristics

5°S

New observations : b) water mass characteristics

Average differences (5°S and 11°S)

ΔO1 (27.7<γn<24.5) = 3 μmol/kg/decade

ΔO2 (28.135<γn<27.7) = 1.36 μmol/kg/decade

Average differences (5°S and 11°S)

ΔS1 (27.7<γn<24.5) = 0.136 / decade

ΔS2 (28.135<γn<27.7) = -0.005 /decade

5°S and 11°S

Biastoch et al., 2009

°C

New observations : b) water mass characteristics

Mooring array deployed in July 2013

Recovered and redeployed in April/May 2014

New observations : c) mooring array

cm/sNew observations: mooring array

K1

K2

K3

K4

further processing necessary before providing the continuation of the transport timeseries

Further aims

• estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC

Further aims

• estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC

• monitor the transport variability of the NBUC on intraseasonal to interannual timescales

Further aims

• estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC

• monitor the transport variability of the NBUC on intraseasonal to interannual timescales

• analyse the connection between transport variations in the western boundary current system of the tropical South Atlantic (warm and cold water route) and the variability of the subpolar North Atlantic with respect to the signal propagation within the AMOC

Further aims

• estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC

• monitor the transport variability of the NBUC on intraseasonal to interannual timescales

• analyse the connection between transport variations in the western boundary current system of the tropical South Atlantic (warm and cold water route) and the variability of the subpolar North Atlantic with respect to the signal propagation within the AMOC

• analyse the propagation of water mass anomalies in the AMOC, which can e.g. be caused by the variability in the Agulhas leakage

Further aims

• estimate the northward transport of central and intermediate water within the NBUC as part of the AMOC and STC

• monitor the transport variability of the NBUC on intraseasonal to interannual timescales

• analyse the connection between transport variations in the western boundary current system of the tropical South Atlantic (warm and cold water route) and the variability of the subpolar North Atlantic with respect to the signal propagation within the AMOC

• analyse the propagation of water mass anomalies in the AMOC, which can e.g. be caused by the variability in the Agulhas leakage

• analyse the connection between NBUC variability at 11° S and EUC variability at 23°W on the equator and its relevance for climate variability

m

New observations: b) T/S characteristics

5°S