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