1 Hadley Centre 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 Hadley Centre 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 20 th century THC from historical observations of climate? Forecast of natural THC evolution over the next few decades.

3 Hadley Centre Variations in historical instrumental SSTs HadISST Low-pass (> 13.3y) EOFs º S - 70 º N Projections After Folland et al., 1999 Variations, but are they modes?

4 Hadley Centre Model THC in the Frequency Domain

5 Hadley Centre THC-Climate mode: model vs observations 0° 60° 120° 180° Year band Year band

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

7 Hadley Centre NE Brazilian Rainfall Station data from Todd Mitchell ’ s site

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

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

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

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

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

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

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

15 Hadley Centre 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   

16 Hadley Centre 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 Hadley Centre 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 Hadley Centre Conclusions (2) Shows how coupled models can be used to study 20 th 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.