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Antarctic sea ice and ocean convection in HiGEM run xbylr John King, Amna Jrrar,

Antarctic sea ice and ocean convection in HiGEM run xbylr John King, Amna Jrrar, William Connolley and HiGEM consortium members. Antarctic sea ice area – xbylr and xciel, years 21-70. Antarctic sea ice fraction – September, years 21-70. HadGEM xciel. HiGEM xbylr. Observations.

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Antarctic sea ice and ocean convection in HiGEM run xbylr John King, Amna Jrrar,

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  1. Antarctic sea ice and ocean convection in HiGEM run xbylr John King, Amna Jrrar, William Connolley and HiGEM consortium members

  2. Antarctic sea ice area – xbylr and xciel, years 21-70

  3. Antarctic sea ice fraction – September, years 21-70 HadGEM xciel HiGEM xbylr Observations

  4. September ice fraction – years 1 –12 xbylr xciel

  5. September ice fraction – years 13 –24 xbylr xciel

  6. Why does ice disappear in Weddell and Ross in xbylr ? • Why does ice recover in Weddell but not in Ross ? • Why don’t we see this behaviour in HadGEM or in other HiGEM runs? • Does this tell us anything about the behaviour of the Antarctic sea ice system in reality? • Does it matter? • Can we fix it?

  7. “Great Weddell Polynya”, 1974-76

  8. Potl. Temp. Ross Sea,195E, September xbylr xciel 1978 1990 2018

  9. Potl. Temp. 70S,195E, September xbylr xciel Note: Ocean-atmosphere heat flux of 60 W m-2 translates to 0.11 K/yr cooling of the 4000 m water column

  10. Potl. Temp. Weddell Sea,20W, September xbylr xciel 1978 1990 2018

  11. Month Upper ocean salinity in Ross and Weddell Seas (plot from James Harle)

  12. Annual mean thermodynamic sea ice thickness increment 2004-2011 xbylr xciel Difference

  13. Why does ice disappear in Weddell and Ross in xbylr ? • - Deep convection develops as a result of salinification. Could be caused by excess ice production, deficit in P-E, advective transport or excessive upwelling/mixing of warm, saline waters. Jeff Ridley suggests that sea ice salinity used in the model is much lower than Antarctic observations – but this would cause problems in all models. • Why does ice recover in Weddell but not in Ross ? • - James Harle: Weddell Sea circulation advects ice into polynya, eventually freshening upper ocean sufficiently to stabilise convection. • Why don’t we see this behaviour in HadGEM or in other HiGEM runs? • Not sure. Dave Stevens suggest that runs initialised form Levitus may be more prone to this kind of behaviour, as observations are biased towards the more saline conditions prevailing during the 1980s. Dave also suggests GM mixing parametrisation in HadGEM may prevent deep convection setting in. Is the higher-resolution ocean of HiGEM more prone to convection? • Does this tell us anything about the behaviour of the Antarctic sea ice system in reality? • -Maybe of some relevance to 1970s Weddell Polynya.

  14. Does it matter? • Open-ocean deep convection results in unrealistic SO density structure and causes the ACC to accelerate • Better representation of Antarctic sea ice will be crucial for future applications of the model (e.g. IPCC AR5 runs, coupling to biogeochemistry)

  15. Can we fix it? • Essentially an ocean model problem (but it would be interesting to see a run with a different sea ice model to confirm this) • Need to examine all terms in the upper-ocean salt budget for the Weddell and Ross seas to identify the source of the problem • There may be a fundamental problem – ocean model needs to form deep water somehow and the processes by which this occurs in the real world (continental shelf convection, sub ice-shelf interactions etc.) are not properly represented in the model, which is then forced to form deep water by unrealistic open ocean convection. (Why does HadGEM behave differently?)

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