Impacts of atmospheric circulation on Arctic ice cover and possible feedbacks An introduction

1. Impacts of atmospheric circulation on Arctic ice coverand possible feedbacksAn introduction

2. Air pressure and ice motion • aaaa SLP and sea ice motion (SIM, arrows), data from Arctic Buoy programme 1979-1998 On long time scales, wind and ocean forcing on SIM is about equal. On short time scales, SIM follows the wind.

3. Sea ice response to wind forcing from AMIP models • Thickness pattern depends on clim. mean annual cycle of wind • Daily wind variations are necessary to create sufficient deformation and open water. Yet the ice response is not sensitive to the direction and amplitude of daily anomalies. Bitz et al. (2002) April sea ice thickness pattern

4. Interannual variability Consensus of observational studies • “Interannual variability in sea ice conditions is caused by the variability in the large scale-scale atmospheric circulation which locally manifests itself as surface air temperature and wind anomalies” (Prinsenberg, 1997) • “Feedbacks and other influences have been more difficult to detect due to the dominance of atmospheric forcing” (Deser et al. 2000)

5. Ice motion – wind vorticity correlation Zhang and Hunke (2001) Summer atmosphere circulation is more important than winter for inducing interannual variability in ice dynamics

6. Summer atmosphere circulation is more important than winter for inducing interannual variability • a Zhang and Hunke, 2001)

7. Interannual and decadal sea ice winter variability 1st EOF of winter ice concentration anomalies 1958-97: out of phase fluctuations between ice east and west of Greenland PC time series of the leading EOF Standard deviation of winter ice concentration anomalies 1958-97 (Chapman and Walsh data, 1993) • One-year-lag correlation = 0.69 • Predominance of decadal and longer timescale variations. • Trend towards reduced ice cover east of Greenland Deser et al. 2000

8. MSLP (mean sea level pressure) variability MSLP variability associated with the 1st ice EOF Relations between SLP and ice anomaly fields are qualitatively consistent with the notion that atmospheric circulation anomalies force the sea ice variations. Deser et al. 2000 NCEP atmosphere data

9. Standard deviation of summer ice concentration anomalies 1958-91 Summer ice concentration variability To what extent is the summer ice reduction • due to the general NH warming ? • due to changed atmospheric circulation ? Deser et al. 2000

10. Spring SLP variability associated with the summer ice extent time series SLP is above normal (high Arctic and North Atlantic) when sea ice extent (Barents, Kara, Laptev Sea) is below normal SLP patterns change sign during 1970s “Relations between SLP and ice extent are physically consistent with the notion that spring atmospheric circulation anomalies force sea ice variations.” SLP is below normal (Arctic) when sea ice extent (Siberian Sea) is below normal 1958-78 Deser et al. 2000 1979-96

11. Ice flow and AO Composites of ice motion during high and low AO index years (for all months of the year, data from IABP 1979-1998) Rigor et al. 2002 During positive AO/NAO winters, anomalous south-easterly and cyclonic winds forces the ice away from the Eurasian. -> thin ice and coastal leads. This preconditions the system to promote summer ice extent losses

12. Arctic ice extent NSIDC, 2005 NSIDC: This graph depicts the decline in sea ice extent from 1978-2005. The September trend from 1979 to 2005, now showing a decline of more than 8 percent per decade, is shown with a straight blue line.

13. Ice extent and NAO • During positive AO/NAO winters, anomalous south-easterly and cyclonic winds forces the ice away from the Eurasian coast. -> thin ice and coastal leads. This preconditions the system to promote summer ice extent losses • Warmer ocean currents • After 1996, the NAO is not any longer that positive, but the ice extent still reaches new minima WHY? Polyakov et al., 2003 Correlation of NAO with SAT

14. Possible Reasons for current changes Link to AO/NAO • After 1996, the NAO is not any longer that positive, but the ice extent still reaches new minima WHY? • Strong changes in surface conditions during the high NAO phase might be sufficient to help maintain persistence of current trends • Strong SLP anomalies maintain the warming trend by temperature advection • Delayed thermodynamic response of ice-ocean system (Rigor and Wallace, 2004) • Pacific influence (Maslowski, 2006) Overland and Wang, 2005

15. Arctic Ice extent NSIDC, 2005 Sept 1990 NSIDC: This graph depicts the decline in sea ice extent from 1978-2005. The September trend from 1979 to 2005, now showing a decline of more than 8 percent per decade, is shown with a straight blue line.

16. Sea ice anomaly 1990 • Ice dynamics, forced by anomalous wind conditions is the main factor for the generation of the ice anomaly • Ocean heat transport through Bering Strait is unlikely to contribute to the ice anomaly (from ocean standalone run) • Small variations in sea ice concentration (as between ECMWF and SSM/I) can strongly affect the atmospheric circulation (from atmosphere standalone run) • Feedback to sea ice • Coupled model is less successful in representing the Siberian Arctic anomaly due to deviations in simulated atmospheric circulation Rinke et al. 2003

17. Feedback of ice anomalies on NAO Reduced ice • Local response • increased O-A heat fluxes • Low pressure anomaly Alexander et al. 2006

18. Feedback of ice anomalies on NAO Reduced ice • Local response • increased O-A heat fluxes • Low pressure anomaly • Large scale response • Pos. SLP anomaly over the central Arctic • Neg. SLP anomaly over the northern NA • Resembles the negative phase of the NAO, i.e. constitutes a negative feedback of ice on the NAO Alexander et al. (2006) CCSM3.6 AGCM

19. Gerdes (2006) Effect of ice thickness on SLP SLP difference for AGCM composites based on ice concentrations in the 90s and 60s • Gerdes concentr. difference 90’s – 60’s SLP difference for AGCM composites based on ice concentration and thickness in the 90s and 60s Thickness difference 90’s – 60’s Feedbacks with NAO under discussion

20. = SAT anomaly = SAT anomaly = NAO index = pressure index SST-atm. circulation feedback Bengtsson et al. (2004) => feedbacks might play their roles only temporary

21. Summary • On long time scales, wind and ocean forcing on SIM is about equal. • Thickness pattern depends on clim. mean annual cycle of wind • Interannual variability in sea ice conditions is caused by thevariability in the large scale-scale atmospheric circulation • Feedbacks exist • Summer atm circulation is more important than winter for inducing interannual variability • Atmospheric circulation anomalies force sea ice variations. However, there is a local response to changing sea ice cover east of Greenland • Reasons for the current ice decrease: NAO, thresholds passed, thermal delay, Pacific water inflow, … • Small variations in sea ice concentration can strongly affect the atmospheric circulation • Atmosphere response to reduction of ice concentration/thickness differs

22. Open questions • What is the atmosphere response on changing ice • Are the positive or negative feedback mechanisms dominating • How do different feedbacks interact • Ice - atm.circulation feedbacks • Ice - albedo feedbacks • What is the oceans role in these ice – atm.circulation feedbacks? • Quantify the role of the Pacific water / North Atlantic water for Arctic sea ice reduction. • What is the relation between AO and global warming?

23. The End