1. Atlantic Multidecadal Variability: Consequences, Causes & Prediction? Dan Hodson, Jon Robson & Rowan Sutton NCAS-Climate, University of Reading

2. Outline o Motivation o The Atlantic Ocean & The AMO o Consequences o Causes o A look back at the 1960s o Prediction o Conclusions

3. Motivation: Prediction? Weekly Seasonal Centennial Decadal ? If we want to predict future decadal variations, good to start by examining past decadal variability, and asking: Do we understand it?

4. The Atlantic Ocean

5. The Atlantic: Mean State

6. The Atlantic Ocean: Circulation Ocean Currents and Sea Ice from Atlas of World Maps Sub Polar Gyre Gulf Stream North Atlantic Drift Cooling HEATHEAT

7. The Atlantic Ocean: Stream Function HiGEM GCM Max ~ 20 Sv (10 6 m 3 /s) Hodson (2012), Climate Dynamics, 39 (12), 3057-3073 OBS 17.9 ± 3.2 Sv (10 6 m 3 /s) Mielke, Frajka-Williams & Baehr 2013

8. The Atlantic: Variability

9. Historical Sea Surface Temperature

10. Atlantic Multidecadal Oscillation  Identified by Schlesinger & Ramankutty in 1994  Noticed a 50-88 year spectral peak in North Atlantic Temperature records  “AMO” coined by Kerr in 2000  Not really an ‘Oscillation’

11. Atlantic Multidecadal Variability: Consequences?

12. Temperature Rainfall Sea-Level Pressure Model Observations Warm-Cold AMO Summer (JJA) Climate Impacts of a Warmer North Atlantic R. T. Sutton, D. L. R. Hodson (2005), Science, 309 (5731)

13. Impacts on Rainfall Sutton, R.T. ; Dong, B.-W. ; Nature Geoscience, 5, 788-792) Rainfall Differences Notable Rainfall differences between Warm and Cool Atlantic periods.

14. Impact on Sahel Rainfall Surface Temp MAM Precip JJA Precip MTM-SVD analysis of HadCM3 1400 year Control Knight et al (2006) GRL 33, 17, L17706

15. Impact on Hurricane Genesis August–October (ASO) 850–200 hPa shear HadCM3 Main Development Region Obs 1951:60 – 1971:80 Number of major hurricanes Goldenberg et al. Science 2001 ;

16. Atlantic Multidecadal Variability: Causes?

17. Causes of AMO variations? o Internal Variability o Changes to External Forcings ● External Forcings? ● AMOC?

18. Internal Variability

19. Internal Variability: AMOC Vellinga et al. (2008) Climatic Change Surface Temperature Change after AMOC shutdown Hodson (2012), Climate Dynamics, 39 (12), 3057-3073 Surface Temperature after AMOC peak

20. External Forcing

21. Changes in external forcings Aerosols Solar Flux Volcanic CO 2 CMIP5 Forcings

22. Anthropogenic Aerosol Forcings HadGEM2-ES Booth et al (2012) Nature 484, 228–232

23. Volcanic Forcing Temperature following a volcanic eruption. Robock (1995): J. Climate, 8, 1086–1103. Ottera et al Nature Geosci 2010 MSLP anomaly 2 years after Large Tropical eruption

24. Solar Forcing o Variations in Solar Output Small (~0.5 W/m 2, Pinatubo ~3 W/m 2 ) o But Impacts on dynamics may lead to regional heat flux convergences. Ineson et al (2011) Nature Geosci, doi:10.1038/ngeo1282 MSLP (Solar Min - Solar Max)

25. The 1960s

26. A look back at the 1960s o Can we understand causes by examining the past? o The Atlantic cooled from the 1960s o How did this cooling progress? Hodson et al (2014), Journal of Climate, 27 (21), 8229-8243

27. 1960s cooling in stages ON..J 1 2 3 4 Consistent Hodson et al (2014), Journal of Climate, 27 (21), 8229-8243 The Great Salinity Anomaly(s) TemperatureMean Sea Level Pressure

28. Extratropical Forcing Smirnov, (2012): J. Climate, 25, 20562076. NDJ FMA MJJ ASO

29. Aerosol Mechanism Hodson et al (2014), Journal of Climate, 27 (21), 8229-8243

30. Interactions? Hodson et al (2014), Journal of Climate, 27 (21), 8229-8243 Interaction?

31. Feedbacks? Dust Hodson et al (2014), Journal of Climate, 27 (21), 8229-8243 Feedbacks?

32. Atlantic Multidecadal Variability: Prediction?

33. Observing the Ocean: ARGO www.argo.ucsd.ed

34. Predictability Smith (2010) Nature Geoscience 3, 846–849

35. Predicting Decadal Cooling, and Warming Robson et al, 2012, GRL Robson (2012) GRL, 39, L19713 Robson (2014) Climate Dynamics, 42 Sub Polar Gyre Upper Ocean Temperatures Obs Predictions Transient

36. 1960s Cooling Robson et al Climate Dynamics (2014), 42, I9-10, pp 2353-2365, ModelObs Pressure Temperature DJF o Anomalies weaker? o Initial Ocean state could be result of external forcings?

37. Conclusions The Atlantic Ocean has historically displayed variability on multi-decadal timescales. Consequences: Circulation, Precip & Temp: especially over the US, EU. Sahel Rainfall Hurricane Genesis? Causes: The source of this variability is not entirely clear.. But we have some good hypotheses! Prediction: Given a good knowledge of the state of the ocean we may be able to predict some of this variability, and associated climate impacts, into the future. d.l.r.hodson@reading.ac.uk

40. 1960s cooling in stages JAS 1 2 3 4

41. Regional SST evolution AMO

42. Tropical Lag SST lags SPG by 2 yearsTrop Atl SST lags SPG

43. Regional MSLP evolution

44. Sahel Rainfall AMO Sahel PPT DT corr = 0.53 (p<0.01)

45. Volcanic Forcing Warm Stratosphere External forcing as a metronome for Atlantic multidecadal variability, Ottera et al Nature Geosci 2010 doi:10.1038/ngeo955 MSLP anomaly 2 years after Large Tropical eruption

46. Solar Forcing Cooler Stratosphere At Solar Min Solar forcing of winter climate variability in the Northern Hemisphere, Ineson et al Nature Geosci, 2011 doi:10.1038/ngeo1282 MSLP (Solar Min - Solar Max)