Trends in Arctic sea ice drift and role of wind forcing: 1992– 2009

1. Trends in Arctic sea ice drift and role of wind forcing: 1992–2009 Gunnar Spreen, Ron Kwok,and Dimitris Menemenlis Jet Propulsion Laboratory, California Institute of Technology

2. Outline • Trends in sea ice drift speed from satellite radiometry and 10m wind speed from atmospheric reanalyses • Motivation • SSM/I sea ice drift data and wind from 4 reanalyses • Trend in sea ice drift and wind speed • Spatial pattern of trends • Arctic Basin wide time series • Connection between wind and sea ice speed trends • Conclusions

3. Introduction Sea Ice Thickness Sea Ice Extent • Arctic sea ice underwent profound changes in recent years: • Sea ice extent decreased by 4.3%/decade (1979–2010). • Sea ice thickness decreased by 1.6 m or 53% for 2003–2008 compared to 1958–1976. • These changes are accom-panied by an increase in drift speed of sea ice buoys of about 9%/decade (1979–2007). 1995 2010 1979 1970 1990 2010 Sea ice drift speed from buoys Rampal et al. (2009)

4. Motivation • Is there a trendin satellite sea ice drift observations between 1992 to 2009 and what is its spatial distribution? • What drove the observed sea ice drift speed increase? • Sea ice drift speed depends on • surface wind stress • internal ice stress, e.g., thickness and compactness • bottom ocean stress • Rampal et al. (2009): change in sea ice kinematics caused drift speed trend, as no wind speed increase was observed • Are there trends in surface wind speed over sea ice during 1992–2009 and can we exclude wind as a contributor to the observed increase in ice drift speed?

5. Data and Reanalyses • Sea Ice Drift (Kwok et al., 1998) • daily SSM/I 85VGHz data during winter (Oct. –May) • cross-correlation with Δt = 1day, 83km grid • October 1992 – May 2009 • Reanalyses 10m Wind • ERAinterim – ECMWF reanalysis • 1989 – present, ≈79km grid • JRA – Japanese Reanalysis • 1979 – present, ≈110km grid • NCEP/NCAR Reanalysis I • 1948 – present, ≈180km grid • NCEP/DOE Reanalysis II • 1979 – 2010, ≈180km grid

6. Sea Ice Speed Trend from Satellite Data • winter: Oct-May • 1992 - 2009 JPL • Positive trend in sea ice drift speed in most of the Arctic Basin IFREMER merged QuikSCAT/SSMI sea ice speed trend Spreen et al., GRL, 2011

7. Wind Trend: All Reanalyses ERAinterim JRA JPL • winter: Oct-May • 1992 - 2009 NCEP NCEP-2 all 4 reanalyses: only small trend in wind speed (92–09 and 79–09)

8. Positive trend of 0.9±0.1cm/s/decade or 11% per decade in sea ice speed Only small positive trends (1 to 2%/decade) in wind speed in all 4 reanalyses Satellite ice drift trend is similar to buoy trend for 1979–2007 Strongest ice drift trend after 2003 (+46%/decade) Wind and Sea Ice Speed in the Arctic Basin JRA wind speed (for ice drift region) SSM/I satellite sea ice speed Sea Ice Drift Speed NCEP Wind Speed trend: +1%/decade trend: +7%/decade Spreen et al., 2011

9. 92–00 and 00–09 Wind and Ice Drift Trends winter sea ice drift trend summer wind trend correlation wind – ice winter wind trend • Wind and ice trend show similar pattern and are correlated (≈0.5) • first half (92-00): no or negativetrend; second half (00-09): positivetrend 92–00 92–00 92–00 92–00 00–09 00–09 01–09 00–09

10. Sea Ice Concentration Trend Sea Ice Drift Speed Trend Ice Concentration Trend 1992–2009 • Strongest ice concentration trends in marginal seas, where ice drift and wind trends differ. • Less compact ice  higher drift speed NCEP Wind Speed Trend

11. AO Ice Drift Trend AO (blue) and PNA (green) • Winter AO shows small positive trend, but no shift. • AO+  strong Transpolar Drift, weak Beaufort High • Stronger (double) trend during AO+ than AO- 2000 2009 1992

12. Conclusions • Strong trend of 11±1%/decade in winter sea ice drift speed during 1992–2009 obtained from daily satellite data. Strongest trend after 2004 (46%/decade). • Satellite ice drift trend is comparable to buoy ice drift trend. • Onlysmall trend in surface (10m) wind speed obtained from four atmospheric reanalyses (≈1.5%/decade). • In the Central Arctic, colocated with the strongest ice drift trends, wind trends are positive (up to 20%/decade). • Wind trend pattern explains about 22% of the sea ice drift trend spatial variance. • Changes in wind speed explain a fraction of the observed increase in drift speeds in the Central Arctic but not over the entire basin. In other regions thinning of the ice cover is a more likely cause of the increase in ice drift speed. Thank you!