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Operated by Los Alamos National Security, LLC for NNSA Slide 1
Agulhas Leakage in the CCSM4
Wilbert Weijer (LANL)
Erik van Sebille (UNSW, Sydney)
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Agulhas Current Classical Western Boundary Current (like Gulf Stream, Kuroshio, etc.)
Closes subtropical wind-driven gyre of South Indian Ocean
Stramma & Lutjeharms (1997)
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Agulhas Current Classical Western Boundary Current (like Gulf Stream, Kuroshio, etc.)
Closes subtropical wind-driven gyre of South Indian Ocean
Stramma & Lutjeharms (1997)
Continental termination
Zero wind stress curl
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Agulhas Current: Retroflection Agulhas Current undergoes Retroflection
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Agulhas Current: Retroflection Agulhas Current undergoes Retroflection
Retroflection is unstable • Periodic shedding of Agulhas Rings (~ 6 rings per year)
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Agulhas Leakage: Ring Shedding Agulhas Rings filled with warm and salty water
• Drift into South Atlantic: Agulhas Leakage
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Agulhas Leakage: Super Gyre
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Agulhas Leakage: Global Impact Gordon (1985)
• “Such a warm water link between the Atlantic and Indian oceans would strongly influence global climate patterns”
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Agulhas Leakage: Global Impact Gordon (1985)
• “Such a warm water link between the Atlantic and Indian oceans would strongly influence global climate patterns”
Weijer et al. (1999, 2001, 2002) • Heat and salt injection through Agulhas Leakage
— Strengthens MOC — Stabilizes MOC
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Agulhas Leakage: Global Impact Biastoch et al. (2008)
• High-resolution model of Agulhas region, nested in global model • “Dynamical signals from Agulhas region contribute MOC signal of same order of
magnitude as those arising in the north”
Biastoch et al. (2008)
With Agulhas variability
Without Agulhas variability
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Agulhas Leakage: Implications How will Agulhas Leakage change in warmer climate?
• Poleward shift of wind belts
How will this affect the MOC?
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Agulhas Leakage: Implications How will Agulhas Leakage change in warmer climate?
• Poleward shift of wind belts
How will this affect the MOC?
How is Agulhas Leakage represented in state-of-the-art Climate Model?
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Agulhas Leakage in CCSM4
Lagrangian analysis • CCSM4 20th century runs • Monthly 3D velocity fields, 1980-2005 • Release 110,000 numerical floats in Agulhas Current
— How many make it into South Atlantic? — How many make it across 21°S?
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Agulhas Leakage in CCSM4
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Agulhas Leakage in CCSM4
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Agulhas Leakage in CCSM4
Agulhas Current transport
69 Sv
70 Sv (Bryden et al. 2005)
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Agulhas Leakage in CCSM4
Good Hope line transport
43 Sv
14-17 Sv (Doglioli et al. 2006; van Sebille et al. 2009; Richardson 2007)
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Agulhas Leakage in CCSM4
21ºS transport
10 ± 1 Sv
10 Sv (Donners and Drijfhout 2004)
4 Sv (Biastoch et al. 2009)
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Agulhas Leakage in CCSM4
3.6 ± 1.3 Sv/decade
-0.1 ± 0.6 Sv/decade
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Agulhas Leakage in CCSM4 Agulhas Current okay
Agulhas Leakage overestimated by factor 3
Recirculates in super-gyre
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Agulhas Leakage in CCSM4 Agulhas Retroflection inertial process
Not captured by low-resolution 1° models
Instead, leakage takes place in viscous boundary layer
De Ruijter (1982)
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Does Agulhas Leakage influence MOC in CCSM4?
Leakage influences MOC through salinity anomalies • Actual volume flux irrelevant
Reference time series: S34S • Salinity averaged over upper 1000 m • In southeastern South Atlantic
Look for • Coherence between S34S and MOC • Correlations between S34S and anywhere else
X
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Coherence of the AMOC with S(34S)
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Coherence of the AMOC with S(34S)
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Joint response to ENSO: AMOC
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Signal propagation: lagged correlation
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Signal propagation: lagged correlation
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Signal propagation: lagged correlation
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Signal propagation: lagged correlation
∆S = 0.24 psu
σ(S34S) = 0.11 psu
∆S = -0.04 psu
σ(S34S) = 0.02 psu
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Conclusions
Agulhas Leakage too strong in CCSM4 • Factor of 3 • Too strong coupling between South Atlantic and Indian Oceans (“super gyre”) • Salinity too homogeneous
No discernible impact of Agulhas Leakage variability on MOC • Salinity variability too weak • Study is inconclusive
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Meridional Coherence of the AMOC
T > 50 yr
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Good Metric of Agulhas Leakage Impact?
X
