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
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