1. The midlatitude influence of the AMOC onto the atmosphere Guillaume Gastineau 1, Blandine L’Hévéder 2, Francis Codron 2, Claude Frankignoul 1 1 LOCEAN/IPSL 2 LMD/IPSL

2. 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%

3. Influence of AMOC onto SSTs and 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

4. Atmospheric response to AMOC in IPSL-CM5A Z500 JFM (in m) regressed onto AMOC-PC1, AMOC-PC1 leads by 9 yr Gastineau and Frankignoul, Clim. Dyn., 2012 σ BI JFM (in day -1 ) at 850hPa regressed onto AMOC-PC1, AMOC-PC1 leads by 9 yr

5. 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?

6. Experimental set-up Experiment Name Ensemble size SST conditionsSea-ice CTRL250Climatology N-ATL250Anomalies 9 yr after AMOC-PC1 x3 in Atlantic North of 20°N Climatology N-ATL+SIC250Anomalies 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-ATL N 250Same as N-ATL but removing anomalies North of 45°N Climatology N-ATL S 250Same as N-ATL but removing anomalies North of 45°N Climatology ALL250Anomalies 9 yr after AMOC-PC1 x3 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.

7. SST boundary condition Oct Nov DecJan FebMar Apr SST and Sea ice anomaly JFM Mean SST (in K) Sea-ice Extent (10 6 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)

8. Influence of Atlantic SST and Sea ice JFM NDJ Z500 Regression IPSL-CM5A Z500 N-ATL - CTRL Z500 N-ATL+SIC - CTRL

9. Influence of Atlantic SST and Sea ice JFM NDJ Regression IPSL-CM5A N-ATL - CTRL N-ATL+SIC - CTRL

10. 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. Mean projection Atl-N Atl-N+SIC IPSL-CM5A Atl-N Atl-N+SIC IPSL-CM5A Projection Z500 anomalies onto the IPSL-CM5A AMOC-PC1 regression

11. Effect of North Atlantic SST HF JFM (W m -2 ) σ BI JFM (10 2 day -1 ) at 850-hPa IPSL-CM5A-LR regression Difference N-Atl - CTRL

12. Zonal wind and storm-track Zonal wind JFM (m s -1 ) (Z’ 2 ) 1/2 at 500hPa JFM IPSL-CM5A-LR regressionDifference N-Atl - CTRL Latitude (degree N) Pressure (Pa)

13. Effect of sea-ice cover Z500 (m) NDJ JFM σ BI JFM (10 2 day -1 ) at 850-hPa HF JFM (W m -2 ) Difference (N-Atl+SIC) - (N-Atl)

14. Effect of sea-ice cover Z500 (m) NDJ JFM σ BI JFM (10 2 day -1 ) at 850-hPa HF JFM (W m -2 ) Difference (N-Atl+SIC) - (N-Atl)

15. Which region is the most important? SSTA (in K) Z500 NDJ (in m) Z500 JFM (in m) N-ATL N - CTRL N-ATL S - CTRL

16. Which region is the most important? SSTA (in K) Z500 NDJ (in m) Z500 JFM (in m) N-ATL N - CTRL N-ATL S - CTRL

17. 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 ALL Z500 (m) ALL - CTRL NDJ JFM

18. 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 ALL Z500 (m) ALL - CTRL NDJ JFM

19. 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)

20. Response in NDJ IPSL-CM5A regression zonal wind (m s -1 ) onto AMOC-PC1 Difference SSTA-CTRL Pressure Latitude (degree N)

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