1. Page 1© Crown copyright 2004 The Hadley Centre The forcing of sea ice characteristics by the NAO in HadGEM1 UK Sea Ice Workshop, 9 September 2005 Chris Durman 1,2 and Jonathan Bamber 2 1. Hadley Centre for Climate Prediction and Research, Met Office 2. Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol With thanks to: Tim Johns, Ann Keen, Alison McLaren and Jeff Ridley.

2. Page 2© Crown copyright 2004 The Hadley Centre Talk Outline 1.Introduction 2.Simulated NAO and climate response in comparison with observations 3.Mechanisms of ice response: Beaufort Sea / East Arctic Basin Labrador Sea 4.Conclusions

3. Page 3© Crown copyright 2004 The Hadley Centre The NAO and Arctic Sea Ice NAO dominant mode of atmospheric variability in the North Atlantic region (Hurrell, 1995). “See-saw” of intensification and weakening in PMSL between Azores High and Icelandic Low centres of action. NAO “high +ve state” when pressure gradient is at a maximum. Over the Arctic, modified circulation patterns result in perturbed geostrophic wind forcing → drives surface temperature changes. Correlation between geostrophic winds and ice motion can be as high as 0.8 in the central Arctic (e.g. Thomas, 1999). Perturbed surface temperatures influence the rate at which ice grows / melts thermodynamically.

4. Page 4© Crown copyright 2004 The Hadley Centre Experimental setup and data HadGEM1 IPCC AR4 Control run: Fixed 1860 forcing levels for greenhouse gases, ozone, sulphur, other precursor emissions and land surface conditions. Spinup integration of 85 years in length was carried out from which the control run was initialised. Spinup ocean state initialised from September climatology of Levitus. Spinup Northern Hemisphere sea ice volume initialised from HadISST. Data: First 300 years of control run analysed here.

5. Page 5© Crown copyright 2004 The Hadley Centre DJF PMSL

6. Page 6© Crown copyright 2004 The Hadley Centre DJF PMSL

7. Page 7© Crown copyright 2004 The Hadley Centre Leading order EOFs of DJF PMSL Variance: 44.8%Variance: 41.1%

8. Page 8© Crown copyright 2004 The Hadley Centre HadGEM1 NAO Index Index: Difference in PMSL between Azores and Iceland. Normalisation: subtract mean and divide by standard deviation. Define “High NAO Years” as those where normalised Index exceeds unity. 300 year control run yields 48 High NAO Years.

9. Page 9© Crown copyright 2004 The Hadley Centre HadGEM1 DJF PMSL

10. Page 10© Crown copyright 2004 The Hadley Centre DJF Surface Temperature Anomaly DJF Ice Concentration Anomaly

11. Page 11© Crown copyright 2004 The Hadley Centre HadGEM1 DJF Ice Property Anomalies

12. Page 12© Crown copyright 2004 The Hadley Centre HadGEM1 DJF ice velocity

13. Page 13© Crown copyright 2004 The Hadley Centre HadGEM1 DJF Wind Velocity

14. Page 14© Crown copyright 2004 The Hadley Centre JJA Ice Response Concentration Thickness

15. Page 15© Crown copyright 2004 The Hadley Centre Ice volume flux (m 3 per season) through longitude 180 o W averaged between 70 o N and 80 o N

16. Page 16© Crown copyright 2004 The Hadley Centre DJF Labrador Sea Ice Anomalies

17. Page 17© Crown copyright 2004 The Hadley Centre DJF Labrador Sea Anomalies

18. Page 18© Crown copyright 2004 The Hadley Centre Implied DJF ice thickness response in the Labrador Sea Average increase in rate of change of ice thickness due to thermodynamic processes = +2.07x10 -8 ms -1. => anomalous thickness of 0.16m compared with model climatology. But average thickness anomaly = 0.086m. Thermodynamics not the whole story?

19. Page 19© Crown copyright 2004 The Hadley Centre DJF ice thermodynamics and velocity

20. Page 20© Crown copyright 2004 The Hadley Centre HadGEM1 DJF PMSL

21. Page 21© Crown copyright 2004 The Hadley Centre Conclusions HadGEM1 realistically simulates the NAO dipole pressure pattern. Simulated HadGEM1 surface temperature and Arctic sea ice concentration anomaly patterns associated with the NAO are consistent with observations. The response of Arctic basin sea ice to high NAO events is a dipole with increased ice in the western Arctic and reduced ice in the east. This pattern is formed as a result of reduced ice transport due to perturbed wind forcing. Ice thickness anomalies in the Labrador Sea are largely initiated by thermodynamic processes but dynamic processes act to limit the anomalies.

22. Page 22© Crown copyright 2004 The Hadley Centre Extra slides

23. Page 23© Crown copyright 2004 The Hadley Centre Fram Strait ice export response

24. Page 24© Crown copyright 2004 The Hadley Centre Fram Strait export response? (Zhang et al., 2000, J. Climate)

25. Page 25© Crown copyright 2004 The Hadley Centre HadGEM1 normalised NAO index vs average Fram Strait ice export Corr = 0.03

26. Page 26© Crown copyright 2004 The Hadley Centre HadGEM1 DJF Wind Velocity

27. Page 27© Crown copyright 2004 The Hadley Centre Running 7-year means of simulated normalised NAO index

28. Page 28© Crown copyright 2004 The Hadley Centre (Ringer et al., 2005, J. Climate)