The midlatitude influence of the AMOC onto the atmosphere

Guillaume Gastineau1, Blandine L’Hévéder2,Francis Codron2, Claude Frankignoul1

1LOCEAN/IPSL2LMD/IPSL

The AMOC in IPSL-CM5A

• IPSL-CM5A-LR (low resolution), 500 yr preindustrial control simulation.

• AMOC has a 20yr multidecadal variability in IPSL-CM5A-LR,

• Decadal variability of AMOC linked to subpolar gyre currents (Escudier et al., 2013).

Mean AMOC IPSL-CM5

EOF1 yearly AMOC (49%) Power Spectrum of AMOC-PC1

99%

95%90%

Influence of AMOC onto SSTsand SIC (Sea Ice Cover)

• The AMOC leads the SST and SIC in Atlantic by 9 yr (model dependent)

SST (in K) regression onto AMOC-PC1 AMOC leads by 9 yrs

SIC (in %) regression onto AMOC-PC1 AMOC leads by 9 yrs

Atmospheric response to AMOC in IPSL-CM5A

Z500 JFM (in m) regressed ontoAMOC-PC1, AMOC-PC1 leads by 9 yr

Gastineau and Frankignoul, Clim. Dyn., 2012

σBI JFM (in day-1) at 850hPa regressed ontoAMOC-PC1, AMOC-PC1 leads by 9 yr

Question

• What is the cause of the atmospheric response? • Do sea-ice anomalies play a role?• Does some remote influence from tropics also play a role?

Experimental set-up

Experiment Name

Ensemble size

SST conditions Sea-ice

CTRL 250 Climatology Climatology

N-ATL 250 Anomalies 9 yr after AMOC-PC1 x3 in Atlantic North of 20°N

Climatology

N-ATL+SIC 250 Anomalies 9 yr after AMOC-PC1 x3 in Atlantic North of 20°N

Anomalies following AMOC-PC1 by 9 yr x3 in Atlantic and Arctic

N-ATLN 250 Same as N-ATL but removing anomalies North of 45°N

Climatology

N-ATLS 250 Same as N-ATL but removing anomalies North of 45°N

Climatology

ALL 250 Anomalies 9 yr after AMOC-PC1x3 everywhere but Atlantic North

Climatology

-> Ensemble simulation using LMDZ (atmospheric component of IPSL-CM5A-LR) forced by SST, sea-ice and surface sea-ice temperature from the coupled model.

SST boundary condition

Oct Nov Dec Jan Feb Mar Apr

SST and Sea ice anomaly JFM M

ean

SST

(in K

)

Sea-

ice

Exte

nt (1

06 km

2 )

8

16

24

32

SST and Sea ice anomaly NDJ

SST anomaly = time evolving SST anomaly given by the regression onto AMOC-PC1 (lag 9 yr)

Influence of Atlantic SST and Sea ice JF

MN

DJ

Z500 Regression IPSL-CM5A Z500 N-ATL - CTRL Z500 N-ATL+SIC - CTRL

Influence of Atlantic SST and Sea iceJF

MN

DJ

Regression IPSL-CM5A N-ATL - CTRL N-ATL+SIC - CTRL

Atmospheric response in simulations

JFMNDJ

Summary:- early winter (NDJ) EAP-like signal is well simulated using SST and SIC. - late winter (JFM) NA-like signal is lacking, seems to be due to SST

-> Non-linearity : atmospheric response much weaker.

Mea

n pr

ojec

tion

Atl-N

Atl-N

+SIC

IPSL

-CM

5A

Atl-N

Atl-N

+SIC

IPSL

-CM

5A

Projection Z500 anomalies onto the IPSL-CM5A AMOC-PC1 regression

Effect of North Atlantic SSTH

F JF

M (W

m-2

)σ BI

JFM

(102 d

ay-1

)at

850

-hPa

IPSL-CM5A-LR regression Difference N-Atl - CTRL

Zonal wind and storm-trackZo

nal w

ind

JFM

(m s

-1)

(Z’2 )1/

2 at

500

hPa

JFM

IPSL-CM5A-LR regression Difference N-Atl - CTRL

Latitude (degree N) Latitude (degree N)

Pres

sure

(Pa)

Pres

sure

(Pa)

Effect of sea-ice cover Z500 (m)

NDJ

JFM

σBI JFM (102 day-1) at 850-hPa

HF JFM (W m-2)Difference (N-Atl+SIC) - (N-Atl)

Effect of sea-ice cover Z500 (m)

NDJ

JFM

σBI JFM (102 day-1) at 850-hPa

HF JFM (W m-2)Difference (N-Atl+SIC) - (N-Atl)

Which region is the most important?SSTA (in K) Z500 NDJ (in m) Z500 JFM (in m)

N-A

TLN

- CT

RLN

-ATL

S - C

TRL

Which region is the most important?SSTA (in K) Z500 NDJ (in m) Z500 JFM (in m)

N-A

TLN

- CT

RLN

-ATL

S - C

TRL

Effect from tropics and North Pacific

• Does not project onto coupled model NAO-like response in JFM.

• Tropics and North Pacific induce the formation of an anticyclone over North Atlantic.

• Response over North Pacific well captured.

SST (K) anomalies JFM in simulation ALLZ500 (m) ALL - CTRL

NDJ

JFM

Effect from tropics and North Pacific

• Does not project onto coupled model NAO-like response in JFM.

• Tropics and North Pacific induce the formation of an anticyclone over North Atlantic.

• Response over North Pacific well captured.

SST (K) anomalies JFM in simulation ALLZ500 (m) ALL - CTRL

NDJ

JFM

Conclusion and discussion• The SSTA north of 20°N :

- form an anticyclone (EAP-like response) over the warm anomalies, especially over the subpolar regions,- contribute only weakly to the NAO-like response in late winter.

• The Northern Hemisphere SIC:- force a positive NAO-like pattern, thereby act to damp the late winter response to AMOC in the model

• Hypothesis to explain the discrepancy between the couple model NAO-like response and sensitivity experiments:

- As AMOC-PC1 show a 20-yr periodicity, the regression onto AMOC-PC1 may not solely reflect the influence of the AMOC onto the atmosphere and are affected by the atmospheric forcing of the AMOC.

- The NAO response is need also to be forced simultaneously in midlatitude and tropics -> important for stratosphere – troposphere interactions.

- There are non-linearity in the AMOC atmospheric response :-> response AMOC+ have differences with response AMOC- ,-> response to 3x SSTA is different from 3x response to SSTA .

• The atmospheric response to the AMOC is weak and is therefore difficult to simulate with sensitivity experiment (without strong hypothesis on linearity)

Response in NDJIPSL-CM5A regression zonal wind (m s-1) onto AMOC-PC1 Difference SSTA-CTRL

Pres

sure

Latitude (degree N) Latitude (degree N)

The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299NACLIM www.naclim.eu