Agulhas Leakage: The Neglected Player in the Variability of the Atlantic Meridional Overturning...
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Transcript of Agulhas Leakage: The Neglected Player in the Variability of the Atlantic Meridional Overturning...
Agulhas Leakage: The Neglected Player in the
Variability of the Atlantic Meridional Overturning
Circulation
Arne Biastoch
In collaboration with Claus Böning and Johann Lutjeharms (Univ. Cape Town)
Thanks to: Lisa Beal (RSMAS), Erik van Sebille (RSMAS), Franziska Schwarzkopf,
Markus Scheinert, Gerold Siedler, Erik Behrens, DRAKKAR, NEMO, AGRIF and ARIANE
System Teams
North Atlantic Circulation [schematic by G. Holloway]
Meridional Overturning Circulation (MOC)as an important
measure of poleward transport, e.g. of heat
in the subtropical North Atlantic
Motivation
Influences from the Sub-arctic Atlantic
deep-water formation
areas
North Atlantic Circulation [schematic by G. Holloway]
Effect of Atmospheric Forcing on MOC in the North Atlantic
MOC anomalies at 1000m in different experiments
Labrador Sea Water formation events (high NAO)
HEAT+SALT
REF
WIND
[Biastoch, Böning, Getzlaff, Molines, Madec; J. Clim, 2008]
Time-mean Atlantic MOC
Northern vs. Southern Influences
Upper Branch
North Atlantic Circulation [schematic by G. Holloway]
deep-water formation
areas
Gordon (2003)
Warm water return path of the thermohaline
circulation key role in global climate (change)
The Agulhas System as a Key Region of the Global Oceanic Circulation
DRAKKAR Hierarchy, based on NEMO v2.3 [Madec, 2006]
• Tripolar grid, coupled configuration with 2-way interaction between
Global coarse-resolution model (1/2°)
Regional high-resolution model (1/10° )
• 46 vertical levels, partial cell topography
• State-of-the-art physics / parameterizations
• Thermodynamic-dynamic sea-ice model
• 20-yr spinup with global model
• O(50-yr) atmospheric forcing:
NCEP/NCAR-derived (“CORE”),
applied via Bulk formulae,
6h/1d-resolution, inter-annual
variability (1958-2004)
ORCA05
AG01
Nested Agulhas Model
[Biastoch, Böning, Lutjeharms; Nature, 2008]
I. Dynamical effect of the Agulhas mesoscale on the
MOC:
Wave Process
Goal: to isolate the net effect of the mesoscale Agulhas variability on the large-scale circulation need two-way nesting
Global Model without Nest Global Model with two-way Nest
Agulhas-Induced MOC Variability
MOC shows decadal variability ±1.5 Sv rapid communication to the North Atlantic
Interannually filtered
MOC-difference at
1000m: Exp. with
minus without Agulhas
nest
Difference in MOC and North Brazil Current at 6°S
[Biastoch, Böning, Lutjeharms; Nature, 2008]
Signal Propagation
Topographic Shelf Waves
Rossby Waves / Agulhas Rings
Large-scale horizontal circulation and eddy kinetic energy
[Biastoch, Böning, Lutjeharms; Nature, 2008]
Agulhas Variability vs. Subpolar Deepwater Formation
Complete Forcing
Effect of Agulhas mesoscale variability
Effect of heat flux variability
Standard deviation of interannual MOC strength
Agulhas influence reaches into northern hemisphere
[Biastoch, Böning, Lutjeharms; Nature, 2008]
II. Changes in Agulhas leakage and consequences for
the MOC: Advective Process
Large-scale Circulation Changes South of Africa
Latitude of zero SSH in model and Aviso satellite altimetry
Time-mean horizontal
streamfunction
Zonal averages of streamfunction and wind stress in the 1970s (dashed) and 2000s (solid)
Super-gyre extended poleward due to the shift of the westerlies
[Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]
Increase in Agulhas Leakage
Fractional Agulhas transport crossing GoodHope section
trend : 1.2 Sv/decadeGoodHope
The Agulhas leakage has increased due to the poleward shift of the westerlies
[Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]
[Van Sebille et al; GRL, 2009]
Pathways of Agulhas Leakage
Example trajectories of virtual floats released along the GoodHope section
Analysis of historic profiles in NBC core off South
America
Increased Agulhas leakage leads to a 25% increase of the salt export towards the North Atlantic
[Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]
Nested Agulhas model
High-resolution nest realistically simulates the Agulhas Current system
Two-way nesting scheme feedback of the Agulhas region on the global circulation
Wave process
Mesoscale Agulhas leakage dynamics introduces decadal MOC variations of ±1.5 Sv
… quickly reaches into northern hemisphere, with similar magnitude as sub-arctic deepwater formation events
Advective process
Super-gyre has extended due to poleward shift of the westerlies
The Agulhas leakage has increased
… leading to a 25% increase of the salt export towards the North Atlantic
References
• Biastoch, Böning, Schwarzkopf & Lutjeharms, 2009: Increase in
Agulhas leakage due poleward shift in the southern hemisphere
westerlies, Nature, 462, 495-498.
• Biastoch, Böning & Lutjeharms, 2008: Agulhas leakage dynamics
affects decadal variability in Atlantic overturning circulation, Nature ,
456, 489-492.
• Biastoch, Beal, Lutjeharms & Casal, 2009: Variability and coherence of
the Agulhas Undercurrent in a High-resolution Ocean General
Circulation Model,
J. Phys. Oceanogr., 39, 2417-2435.
• Biastoch, Lutjeharms, Böning & Scheinert, 2008: Mesoscale
perturbations control inter-ocean exchange south of Africa, Geophys.
Res. Lett. 35, L20602.
• Biastoch, Böning, Getzlaff, Molines & Madec, 2008: Causes of
interannual – decadal variability in the meridional overturning
circulation of the mid-latitude North Atlantic Ocean, J. Clim., 21, 6599–
6615.
PO33E. The Greater Agulhas Current System and Its Key Role in Past, Present, and Future Climate 1-3 p.m. today The Climatic Importance of the Greater
Agulhas System (SCOR WG 136) Townhall meeting tomorrow 11:45-12:45