Does the AMOC possess a stable off-state in the coupled ocean-atmosphere system?

Record δ18O from the GRIP ice core. The glacial climate characterized by so

called Dansgaard-Oeschger events (numbered) and Heinrich events (dotted).

Younger Dryas

Jenny Mecking and Sybren Drijfhout University of Southampton & Utrecht

The Atlantic Meridional Overturning Circulation (AMOC)

Atlantic Meridional Overturning Circulation

AMOC = Atlantic Meridional Overturning Circulation

• The AMOC strength is typically measured in Sverdrups:

1 Sv = 106 m3/s

How can the AMOC be shutdown?

• Stommel (1961)

• Marotzke (1990)

• Rahmstorf (1996)

Bi-stability in Box Models

T2 S2 T1 S1

warm cold

P P

Meridional Transport:

mµr1 - r2

Bi-stability in Marotzke 1990 Box Model

Equation of State:

Assume ΔT = T2-T1 = constant

T2 S2 T1 S1

warm cold

P P

mµr1 - r2

Bi-stability in Marotzke 1990 Box Model

Combine the above equations at steady state (ΔS=S2-S1) and rearrange:

Bi-stability in Marotzke 1990 Box Model

Solutions (assuming ΔS > 0):

E can be used as an indicator for stability

A box model of the Atlantic MOC

Rahmstorf (1996)

* *

Mov ~ - F1 is: Advective Stommel feedback

Mov < 0:

Huisman et al. (2009)

The salt feedback and the

bifurcation diagram

Time series of the AMOC intensity evolution in the 1.0-Sv water-hosing experiments.

PMIP Water hosing experiment

Stouffer et al. 2006

Time series of the intensity of the THC (AMOC), RTHC, and the bottom circulation (BC). The THC intensity is defined as the maximum meridional streamfunction value in 30°S–90°N, 500–3000 m. The RTHC intensity is the minimum streamfunction value at 30°S and between 200 and 1000 m. The BC intensity is the minimum streamfunction value below 2500 m. In the hosing experiments, the freshwater perturbation is removed at the end of the 100th year.

The 2 GFDL models contrasted

Yin and Stouffer 2007

The RTHC

The AMOC patterns in the control run and 1.0 Sv hosing experiment

The RTHC seems to control the stability of the off-state. It is stable in R3.0 but unstable in CM2.1

The 30N-30S pressure difference seems instrumental for the development of the RTHC In R30 the 30N SSS is fresher (too fresh) In R30 the 30N SST is warmer (too warm)

• NA-ST SSS in CM2.1 is saltier because the southward ITCZ-shift signal is much stronger

• NA-ST SST in CM2.1 is colder because the WES feedback is stronger

• Atmospheric feedbacks destabilise the RTHC

Schematic illustration of how different systems in the Atlantic work together to stabilize the off state of the AMOC in R30. Process 1: the RTHC transports the fresh AAIW to the subtropical North Atlantic; process 2: the gyre circulation distributes the low salinity water in the subtropical region; process 3: the excess evaporation tends to enhance the salinity over the subtropical region; process 4: the northward mixing of salt across the frontal zone at about 40°N is much weaker in R30 (compared with that in CM2.1). The AMOC-off state gets to equilibrium when freshwater gain (process 1) balances freshwater loss (process 3).

The Yin and Stouffer schematic

The take home message

• The southward shift of the ITCZ after an AMOC collapse makes the NA subtropics saltier

• This atmospheric feedback destabilises the off-state and seems stronger than the oceanic advective salt feedback in climate models

• On a shorter timescale the imbalance between ΔMov (freshwater transport

divergence) over the NA-Subtropics and ΔEPR seems more important than the sign of ΔMov over the whole North Atlantic its between northern and southern boundary.

However, could salinity biases play a role?

However …

• CM2.1 features a small ΔMov over the NA-Subtropics that cannot balance ΔEPR. The weak salt transport divergence over the NA-Subtropics is associated with flow from the south not being salty enough.

• This bias is associated with a positive ΔMov over the North Atlantic (freshwater import instead of salt import) which excludes a stable off-state in coarse resolution ocean only models because the oceanic advective salt feedback is damping (wrong sign).

• The newest UK climate model, HadGEM3, has an eddy-permitting (turbulent) ocean and features a negative ΔMov over the North Atlantic. The salinity bias is much smaller. We anticipate equally strong or stronger atmospheric feedbacks as in CM2.1.

• Could this newer climate model possess ocean circulation feedbacks that are able to counteract the atmospheric feedbacks that prevent a stable off-state in older, coarser-resolution on-turbulent climate models?

Conclusions

• The turbulent climate model sustains an AMOC off-state for over 400 years

• Also in this model the atmosphere responses with a southward shift in ITCZ -> salinification of the subtropical North Atlantic = strong negative feedback

• The strong salt import in the control climate causes freshening of the whole Atlantic after the AMOC collapses. This freshening is large enough to counteract the damping atmospheric response.

• The strong salinification of the NA subtropics by the AMOC in the control climate is made possible by freshening due to turbulent motions (importing freshwater from subpolar latitudes and stronger boundary currents also freshening the subtropics.

The bifurcation diagram for turbulent coupled climate models is different from non-turbulent climate and ocean-only models and even more simple box models. The off-state seems stable, while the reversed THC does not seem stable.