Climate Variability and Basin Scale Forcing over the North Atlantic

1. Climate Variability and Basin Scale Forcing over the North Atlantic Jim Hurrell Climate and Global Dynamics Division National Center for Atmospheric Research (NCAR) jhurrell@ucar.edu BASIN Science Meeting 12-14 March 2005 Reykjavik, Iceland

2. Climate Phenomena in the Atlantic North Atlantic Oscillation (NAO) Tropical Atlantic Variability (TAV) Meridional Overturning Circulation (MOC) • MOC, TAV and NAO interact, but in ways that are not well understood • Understanding Atlantic climate variability/change requires a global view

3. The Mean State and Stationary Waves (DJF) Sea Level Pressure H Mid Tropospheric Height L H L H HIGH LOW

4. Change in Winter Surface Temperature since 1980 Cold Warm Human activities are superimposed on the background “noise” of natural variability

5. (related to) El Niño/Southern Oscillation (ENSO) North Atlantic Oscillation (NAO) Change in Winter Sea Level Pressure since 1980 Dec-Mar (hPa) Pressure Falls Pressure Rises

6. The North Atlantic Oscillation Spatial Structure and Seasonal Variability DJF MAM JJA SON

7. Alternative Definition: Cluster Analysis (Winter Only) NAO- 29% NAO+ 20% Ridge 30% Scand 21%

8. Time History of Occurrence NAO+ NAO- Scand Ridge

9. The North Atlantic Oscillation: Winter Temporal Evolution Dec-Mar r = 0.92 • • Spatial Structure Dec-Mar 39% SLP (hPa) Stronger Westerlies hPa

10. NAO influence (1 ) °C °C Cold Warm Cold Warm Winter Surface Temperature Change since 1980

11. NAO Influence on Winter Precipitation DRY WET Energy Supply and Demand (Example: Norwegian Energy Trade) Positive NAO Export Negative NAO Import Visbeck et al. 2002 (Positive Index Phase)

12. Ocean Response to NAO Variability

13. Ocean Response to NAO Variability Leading patterns of North Atlantic climate variability Sea Surface Temperature 500 hPa Geopotential Height r = 0.72

14. Ocean Response to NAO Variability SST SST SST Winter Index (1864-2002) Stronger Westerlies Weaker Westerlies

15. Ocean Response to NAO Variability Visbeck et al. (2003) Courtesy of Igor Yashayaev Changes in Water Masses

16. Ocean Response to NAO Variability Circulation Changes Oceanic NAO analogue: Eastward Transport Index Curry and McCartney (2001)

17. Sea Ice Response to NAO Variability - + Deser et al. (2000)

18. What Climate Processes Govern NAO Variability? • 200 years of CCM3 without variations • in “external” forcings • Basic structure & time scale arises • from internal nonlinear atmospheric • dynamics EOF1 SLP (Dec-Mar) Simulated (Dec-Mar) Random and Unpredictable Variations

19. What Climate Processes Govern NAO Variability? Observed Consistent with Observations (Climate Noise Paradigm) Except for the latter half of the 20th Century Observed r (1yr) = 0.4 • 200 years of CCM3 without variations • in “external” forcings EOF1 SLP (Dec-Mar) • Basic structure & time scale arises • from internal nonlinear atmospheric • dynamics Simulated NAO Index Random and Unpredictable Variations r (1yr) = -0.07 r (1yr) = -0.06 A role for external forcing?

20. The Role of Ocean Forcing Observed Global SST Tropical SST JFM 500 hPa Height Trend (1950-1999) m Falls Rises

21. Tropics (15°S-15°N) 0.46 ± 0.25°C Warm Pool (60°-170°E; 15°S-15°N) 0.62 ± 0.13°C JFM Trend in Tropical SST (1950-1999) Cold Warm

22. Precipitation (mm day-1) DRY WET 500 hPa Height (m) r = 0.80 FALLS RISES JFM Response to Indo-W. Pacific SST Trend

23. Change in Tropical SST (1950-1999) Observed Indian Ocean Temperature 0.62 ± 0.13°C Simulated Indian Ocean Temperature Climate Model Forced with Observed Changes in Greenhouse Gas Concentrations 0.59 ± 0.14°C

24. Concerning observed winter North Atlantic climate change • NAO is most prominent and recurrent pattern of atmospheric variability, driving upper-ocean and sea ice variations • Nonlinear approaches reveal spatial asymmetries between the two NAO phases • Basic structure and time evolution results from internal, nonlinear atmospheric dynamics • Climate noise paradigm does not explain behavior in recent decades • Model experiments suggest North Atlantic climate change has been driven, at least in part, by warming tropical SSTs, with the Indo-Pacific region key

25. Future Change ? Numerous modeling studies indicate two Atlantic phenomena that might change NAO and MOC

26. Future Change NAO • Most climate models simulate an increasing trend, with pressure decreases over the far North Atlantic and pressure increases in middle latitudes • Details vary considerably from model-to-model, and the simulated trends are smaller than observed

27. Future Change Observed (low pass filtered) NAO Simulated (7 models) Gillett et al. 2003 Concerning the discrepancy with observations • Inability to capture mechanisms of by which stratospheric flow anomalies affect the tropospheric evolution • Details of the simulated SST field are important

28. Future Change Courtesy Jonathan Gregory MOC • Most, but not all, climate models project some weakening due to warming and freshening

29. Future Change MOC • Most, but not all, climate models project some weakening due to warming and freshening • Nature of changes and mechanisms vary considerably from model-to-model • Ongoing CMIP subproject to investigate model differences • Effect of weakening is to moderate regional warming