1. Ice-ocean interactions and the role of freshwater input Didier Swingedouw, Adele Morisson, Hugues Goosse

3. Impact of Antarctic ice sheet melting LOVECLIM 4xCO2 for 3000 years with interactive ice sheet or fixed version Large warming over the Antarctic leads to large melting Large impact in the Southern Ocean = negative feedback Swingedouw et al., GRL, 2008 Interactive minus fixed ice sheet: surface temperature Surface temperature Sea ice concentration Year 2900-3000 Fixed Interactive AIS Sv Stouffer et al. 2007

4. Can it explain recent trend in sea ice? Negative trend in salinity over the recent period Comparable with an EC-EARTH model simulation with 250 Gt/yr over 40 years Model simulation differences in salinity with and without FW release Salinity Observations: Trend in April-September trend for 1985-2010 Bintaja et al., Nat. Geosc., 2013

5. End of the story? Overestimation of the FW flux in Bintaja numerical simulations : less than 200 Gt/yr even nowadays and on longer time frame Important role for internal variability Swart and Fyfe, GRL, 2013 “The null hypothesis that model mean and observed trends are equal is accepted at the 10% significance”

6. Periods of large increase in sea ice extent in a preindsustrial experiment performed with the LOVECLIM = internal variability Role of ice-ocean feedback in sea ice extent trend Goosse and Zunz., The Cryosphere 2013 Time series of the anomaly of annual mean sea ice extent in a long control run performed with LOVECLIM. The 11 periods characterised by an increase at a rate larger than 10 5 km 2 per decade during 30 years in each month of the year are in red.

7. Trends for periods of increase in sea ice extent in control experiment 1)The increase in ice extent is associated with a larger heat content at intermediate depth 2)The larger heat content at intermediate depth is associated with a lower upward heat flux in the ocean and shallower mixed layer depths Role of ice-ocean feedback in sea ice extent trend Goosse and Zunz., The Cryosphere 2013

8. Role of ice-ocean feedback in sea ice extent trend

9. Polynya variability in climate models Observations 1974-1976 Climate model There has only been one observed Weddell Polynya (1974-1976). Was this a freak occurrence, or multi-decadal / centennial variability?

10. Slow buildup of subsurface heat content. Triggered by eddies at Maud Rise, SAM variability, wind-driven ice divergence… Intense multi-year convection occurs until heat reservoir is depleted, FW cap. Large impact on sea-ice, SH temperatures, AABW transport and properties… Hirabara et al. 2012 GFDL model CM2.6 Modelled polynya dynamics

11. Internal or forced? De Lavergne et al. (2014): disappearance of polynya over recent period may be related with freshening of SO due to anthropogenic forcing Latif et al. (2013): internal variability in a model show large variations in SO similar to what is observed over last century

12. Conclusions Antarctic ice sheet melting may have contributed to recent increased trend in sea ice cover But internal variability can also be sufficient Indeed, models do show large internal variability in this sector in link with sea-ice cover and polynya activity But they do not agree in the exact mechanism and characteristics of this variability And miss important processes: coastal dense water formation (issues with katabatic winds, downslope flows, entrainment), eddies

13. Thank you for your attention!