Understanding the Atlantic Multidecadal Oscillation

1. The Atlantic Multidecadal Oscillation: A signature of persistent natural thermohaline circulation cycles in observed climate Jeff Knight, Rob Allan, Chris Folland, Michael Vellinga and Michael Mann* Hadley Centre for Climate Prediction and Research *Dept of Environmental Sciences, University of Virginia, USA

2. Introduction • Observed multidecadal variations in surface climate • HadCM3 Control Simulation: THC variability • Simulated THC links with climate • What do instrumental surface temperature records show with regard to the modelled multidecadal THC-climate mode? • Relationship of global and hemispheric mean temperatures with the modelled THC. • Links to regional climate phenomena, e.g. NE Brazil and Sahel rainfall. • Multidecadal modulation of North Atlantic hurricanes. • Reconstruction the 20th century THC from historical observations of climate? • Forecast of natural THC evolution over the next few decades.

3. Variations in historical instrumental SSTs HadISST Low-pass (> 13.3y) EOFs 1911-2002 40ºS - 70ºN Projections 1870-2002 After Folland et al., 1999 Variations, but are they modes?

4. Model THC in the Frequency Domain

5. THC-Climate mode: model vs observations 70-180 Year band 25-125 Year band 0° 60° 120° 180°

6. THC-temperature cross-correlation functions Global Southern Hemisphere Northern Hemisphere 0.09°C Sv-1 (0.55) 0.01°C Sv-1 (0.13) 0.05°C Sv-1 (0.59)

7. NE Brazilian Rainfall Station data from Todd Mitchell’s site http://jisao.washington.edu/data/brazil/

8. MAM Temperature, Winds and Precipitation 0° 60° 120° 180°

9. Sahel Rainfall 10°N-20°N 15°W-30°E Data from Climatic Research Unit, U. East Anglia

10. JJA Temperature, Winds and Precipitation 0° 60° 120° 180°

11. Atlantic HurricanesGoldenberg et al., Science, 2001  Major Atlantic Hurricanes (>50 m s-1) 1944-2000 1st Rotated EOF of SST ENSO Removed. 1870-2000  SST Reconstruction (Northern Box) 

12. Peak to peak - model: ~6 ms-1 NCEP: ~3 ms-1 Decadal Mean Shear: Model - black, NCEP - Red 200-850 hPa Shear in the Atlantic Hurricane Development Region

13. Significance level ±0.3 Significance level ±0.04 Regression of HadISST SST with Mean Shear in Development Region K (ms-1)-1 IPO-AMO correlation 0.06

14. North Atlantic SST Index N Atlantic mean 35°-80°N Running decadal mean from HadISST, detrended using a quadratic fit.

15. THC Reconstruction from Atlantic SST  North Atlantic region with local signal to noise ratio THC model based on area and s/n ratio weighted model SST  Running decadal THC reconstruction 1870-2002 

16. THC Forecast - model analogues Look for episodes where model THC rises through the reconstructed level of modern day THC. Take the 50 years after each intersection to form an ensemble of outcomes for the next 33 years.

17. Conclusions (1) • The model shows observed Atlantic Multidecadal Oscillation (AMO) variability is likely to be a long-lived climate mode linked to natural THC variability. • There is a strong influence on Global and Northern Hemisphere mean temperatures. • The quasi-periodic nature of the model AMO implies some predictability of climate for up to ~50 years. • Influences on regional climate e.g. NE Brazil, Sahel and Atlantic Hurricanes are similar to those observed. • A reconstruction of the THC based on the model and observed SSTs shows likely significant THC changes, including THC strengthening over the past 25 years. • Forecast of downturn in natural component of THC in coming decades.

18. Conclusions (2) • Shows how coupled models can be used to study 20th century climate phenomena – long simulations can be a surrogate for the ‘observations we would like to have’. • Highlights need for a range of C20C sensitivity experiments with imposed AMO anomalies eg. AMO- (like 1970s), AMO+ (like 1940s) to simulate climate anomalies across the range of seasons.