Impacts of Atlantic Multidecadal Variability: seasonal mean climate and ENSO

1. Impacts of Atlantic Multidecadal Variability: seasonal mean climate and ENSO Rowan Sutton Dan Hodson, Buwen Dong 1. Walker Institute for Climate System Research, University of Reading 2. National Centre for Atmospheric Science – Climate

2. Outline • Proximal impacts on seasonal mean climate inferred from atmospheric GCM experiments – mainly JJA • Remote coupled responses in Indo-Pacific region impacts on ENSO • Outstanding issues fast A little slower

3. Atlantic Ocean Forcing of North American and European Summer Climate Sutton & Hodson, Science, 309, 115-118, 1 July 2005

4. Enfield et al, 2001: Correlation between North Atlantic Sea Surface Temperatures and U.S. summer rainfall • Also significant changes in river flow and drought frequency (Mccabe et al, 2004), including potential contribution to 1930s dustbowl (Schubert et al, 2004)

5. Our study • Previous evidence linking Atlantic Ocean to variations in N. American summer rainfall mainly circumstantial Questions: • Could we identify/clarify causality? • Might there be other impacts, possibly in other regions? Methodology: • Analysis of simulations with an atmosphere model forced with realistic and idealised variations in sea surface temperature & comparison with observations

6. CoolWarm Cool AMO Index: North Atlantic SST (0-60N; 75-7.5W), low pass filtered Regression of annual mean SST on AMO index Analyse epoch differences between warm and cool phases

7. ~0.6hPA 1.5hPa Observed JJA Anomalies (1931-60)-(1961-90) sea level pressure precipitation (mm/day) land surf. air temp (oC) • SLP and SAT white where not significant at 90% level • Negative precipitation anomalies over Mexico & central US ; also South America • Positive temperature anomalies over US 0.5-0.75oC

8. Atmosphere Model Simulations Model: • Hadley Centre HadAM3 general circulation model • 2.5 x 3.75 degrees, 19 levels • Observed Global SST: • Surface boundary conditions: observational HadISST SST and sea ice data 1871-1999 • No variations in external forcings such as greenhouse gases (to isolate role of oceans) • Ensemble of 6 simulations differing only with respect to atmospheric initial conditions B. Idealised North Atlantic SST Simulations:

9. ~0.6hPA 0.5hPa JJA Anomalies (1931-60)-(1961-90) sea level pressure precipitation (mm/day) land surf. air temp (oC) Ensemble mean anomalies from HadAM3 simulations forced with HadISST • Comparison suggests changes in the oceans are responsible for main atmospheric signals

10. Isolating the role of the North Atlantic Idealised North Atlantic SST patterns NA TNA XNA • Patterns from regression on AMO index (x4) • HadAM3 forced with +/- anomaly patterns • 20 yr expts for NA, 10 yr for TNA, XNA Additional experiments with the HadAM3 model:

11. ~0.6hPa ~0.5hPa JJA anomalies NA+ - NA- C20C Observations

12. SLP anomalies for western European region (30W-25E; 40-60N) obs 0 SLP anomalies for North American region (130W-70W; 15-45N) obs 0

13. Response to tropical and midlatitude SST anomalies • TNA response Gill-like? (note also a tropic-wide signal) • XNA response baroclinic “downstream low” XNA+ - XNA- TNA+ - TNA-

14. 200 hPa streamfunction Contours: vertical velocity Response of Gill Model to diabatic heatingVectors: low level horizontal flow Contours: pressure JJA

15. r=-0.69 Eur. Obs slp Importance of the Atlantic over a longer period r=-0.52 • AMO index accounts for significant fraction of variance in all cases (28-66%) • Other influences (e.g. Pacific) relevant to U.S. especially U.S. Obs slp r=-0.67 Eur. Model slp r=-0.81 U.S. model slp

16. Response to actual change in Atlantic SST JJA (1951-60)-(1961-90) Obs Response to global SST Response to observed Atlantic SST change (30S-80N) Impact of SST anoms outside Atlantic larger Stronger impacts on South American precipitation

17. Conclusions 1 • During the twentieth century, interdecadal variability of North Atlantic SST had an important role in modulating boreal summerclimate – certainly in North America and possibly in Europe too. • Focussed here on time mean anomalies, but changes in frequency of extreme events (droughts, heat waves) likely to be most important for impacts – see Mccabe et al, 2004; Cassou et al, 2005 • Implications for interpretation of past climate records – e.g. results suggest that change in North Atlantic in 1960s caused a cooling of U.S. & European summer climate; a further change may have contributed to recent warming.

18. A few words on other seasons DJF MAM • Large seasonal cycle in climate impacts • In all seasons response is strongest (highest signal-to-noise) in the tropics [but recall no forcing north of 70oN] • Important for interpretaion of proxy records • Comparison with twentieth century obs suggests Atlantic influence most important in JJA and SON Surface air temperature JJA SON • Reduction in ASO vertical shear consistent with Gill response For more information: Climate response to basin-scale warming and cooling of the North Atlatnic Ocean, R. Sutton & D. Hodson, J. Climate, 2006, in press

19. Atlantic Impacts on the Pacific Basin

20. Atlantic impacts on the Pacific • Changes in the Atlantic Ocean can influence remote basins: • Oceanic teleconnections (mediated by Kelvin and Rossby waves) • Atmospheric teleconnections (Have seen that Atlantic SST anomalies can induce remote responses in Indo-Pacific) • Potential impacts on seasonal mean climate and interannual variability – including ENSO

21. Remote impacts of changes in the Thermohaline Circulation: oceanic versus atmospheric teleconnections • Extensive literature on oceanic adjustment to changes in the Atlantic THC (e.g. Kawase, 1987; Goodman, 2001; Johnson and Marshall, 2002) • Much more limited literature on the adjustment of the coupled ocean-atmosphere system (Dong and Sutton, 2002; Zhang and Delworth, 2005) • Timescale for oceanic teleconnections is much longer (many decades – set by Rossby wave propagation). • Dong and Sutton argued that atmospheric teleconnections dominate initial response to a rapid change in the THC. May also dominate equilibrium response.

22. Enhancement of ENSO variability by a weakened Atlantic THC in a coupled GCM (B. Dong & R. Sutton) THC hosing experiment: 1 Sv (or 0.1Sv) applied for 100 years uniformly over the North Atlantic 50-70oN in HadCM3 model

23. Change in Annual Mean State • response over eastern and central Pacific broadly consistent between uncoupled and coupled simulations – in particular equatorial zonal wind anomalies • response over Indian Ocean and west Pacific strongly affected by ocean-atmosphere coupling AGCM response in JJA to warm North Atlantic surface winds Note: major changes in Pacific mean state develop in first decade of hosing – strong evidence that atmospheric teleconnections dominate precipitation

24. Warming of SST in boreal autumn and winter – reduced upwelling? Westerly anomalies in boreal summer & autumn (consistent with AGCM) Change in Annual Cycle on Equator Precip anomalies suggest ocean-atmos coupling Thermocl-ine depth responds to winds Weakening of annual cycle in SST on Equator

25. Changes in ENSO variance Weaker THC => enhanced ENSO variance & enhanced skewness Peak change 0.6oC

26. Composite SST anomalies (+/-1.5s) Control 1.0 Sv experiment

27. - zonal stress anomalies stronger & shifted eastward - OHC precursor in west => “thermocline mode” events Change in structure of El Nino events

28. Little change in the structure of La Nina events

29. Understanding the changes • Mean warming of central Pacific SST in boreal summer and autumn implies an eastward extension of west Pacific warm pool in the seasons when El Nino events grow to largest amplitude • Likely cause of eastward displacement of westerly wind anomalies during El Nino events • Also favours larger amplitude El Nino events: • Higher absolute SST => higher rates of evaporation, precipitation and latent heating => stronger zonal wind anomalies • Upward slope of mean thermocline to the east implies that eastward displacement of wind anomalies will enhance importance of “upwelling feedback” (impact of anomalous upwelling on SST). • Anomalies in thermocline depth (shallower in March-June) could also favour growth of larger SST anomalies (“thermocline feedback”) Mechanism based on ideas from Wang and An (2002); Codron et al (2001); Wu and Hsieh (2003); Kang and Kug (2002)

30. Rossby wave propagation Mean westerly anomalies in boreal summer and autumn Shoaling signal propagates into central/east Pacific in boreal spring/summer

31. Relevance to Multidecadal Variability? • No evidence of THC shutdown in twentieth century, but – as we have seen – there is evidence of THC variability. • Could this variability have affected ENSO? • B. Dong, R. Sutton, A. Scaife, Modulation of ENSO variance by Atlantic Sea Surface Temperatures, GRL 2006.

32. The amplitude of ENSO varies on interdecadal timescales Possible association with AMO: AMO- phase (1930-60):Nino 3 st dev: 0.63oC AMO+ phase (1965-95): Nino 3 st dev: 0.81oC Coincidence or causality? Nino3 amplitude based on standard deviation in a 13 year running window

33. Testing the hypothesis that the Atlantic could drive variations in ENSO amplitude: regional coupling experiments • 2 x 150 year experiments with HadCM3 coupled model (Atm: 2.5oX3.75o with 19 levels. Ocean: 1.25oX1.25o with 20 levels) • Initial state from a 1700y control simulation • +/- AMO: Atlantic SSTs relaxed to seasonally varying climatology from the coupled model +/- 3 x EOF pattern in Atlantic. • Relaxation timescale = 2.5 days

34. Impact on annual mean climate • Opposite sign to hosing experiments but patterns very consistent • Seasonal evolution also very consistent

35. Impact on ENSO Relative to the AMO- run, Nino 3 standard deviation decreases by 23% in the AMO+ run. Taking into account the amplitude of forcing this suggests Atlantic might account for about half observed change (assuming perfect model)

36. Comparison between HadCM3 response to weakened THC and observed change in ENSO properties associated with 1976 “climate shift” • Wang and An (2002) suggested that the key decadal change in equatorial Pacific winds was a remote response to changes in the North Pacific. • Alternative hypothesis is that Atlantic changes were a key driver. • However, could be a generic response of the Pacific climate system to many potential triggers

37. Conclusions 2 • In the HadCM3 model changes in the THC, or related Atlantic SST anomalies, influence the amplitude of ENSO. Atmospheric teleconnections are responsible. • Weakened THC => enhanced ENSO variance and increased asymmetry: larger amplitude El Nino events. • Atlantic influence could help to explain interdecadal variability of ENSO characteristics in twentieth century and paleoclimate evidence linking North Atlantic cooling to more frequent or persistent El Nino conditions (e.g. Cobb et al, 2003). • Other coupled models appear to show similar ENSO response to THC weakening (Timmermann et al, 2006); extent of common mechanism to be determined / discussed. • Mechanism might also (help to) explain observed inverse association between annual cycle amplitude and ENSO amplitude (Fedorov and Philander, 2001; Guilyardi, 2006) – warming of central Pacific in boreal autumn is key to both.

38. Some Outstanding Issues • Model uncertainty in responses to AMO – e.g. Sahel rainfall, U.S. rainfall • More detailed understanding of mechanisms – e.g. impact of AMO on North and South American precipitation and – of course – on ENSO.

41. Observational Datasets(all monthly mean gridded products) • HadISST – 1x1 degree sea surface temperature and sea ice data (Rayner et al, 2003) 1871-2003 • HadSLP1 – 5x5 degree SLP 1871-1998 (update of Basnett and Parker, 1997) • HadCRUT2 – 5x5 degree land surface temperatures 1856-2003 (Jones and Moberg, 2003) • CRU precipitation – 3.75x2.5 degree 1900-1998 (Hulme, 1992)

42. Timmermann et al, 2006 Obs fig by Buwen Dong, after Fedorov and Philander (2001) GFDL CM2.1 HadCM3 MPI ECHAM5-OM1 CCSM3 CCSM2

43. Consistent with Gill-type response, vertical shear is reduced vertical shear in August-September-October Vertical shear of zonal wind See also: Vitart and Anderson (2001); Shapiro and Goldenberg (1998) Shear anomaly in MDR ~ 2m/s cf 8m/s in control experiment • reduced shear favours hurricane formation & hence numbers (distinct from direct effect of SST on intensity)

44. JJA Anomalies (1951-60)-(1961-90) Obs Response to global SST Response to Atlantic SST

45. Climate Response to a basin-scale warming/cooling of the North Atlantic ocean Aims • Study the climate impacts of a warming or cooling of the N. Atlantic ocean in a carefully controlled way • Focus here: • Seasonal evolution of the climate response • [ paper also discusses other issues: Roles of tropical vs higher latitude SST; Nonlinearities with respect to the sign of SST anomalies ] (R. Sutton & D. Hodson, J. Climate, 2006, in press)

46. The “Atlantic Multidecadal Oscillation” or AMO North Atlantic SST Index (0-60N; 75-7.5W), annual mean, low pass filtered Pattern derived by regression of annual mean SST on SST index [Seasonal variation of the time series is very small; seasonal variation of the pattern is modest: 10-30%]

47. Experimental Design • HadAM3 model (2.5 lat * 3.75 lon, 19 levels) • Control experiment (40 years) • Anomaly experiments (20 or 10 years, of which use last 19 or 9) • North Atlantic SST anomaly pattern derived from observations Cautionary note: There is evidence of considerable inter-model variation in the response to Atlantic SST anomalies

48. SST anomalies Experiments: NA+/- TNA+/- XNA+/- NA+ TNA+ XNA+ • Anomalies set to zero north of 70N where large uncertainties • Applied anomalies are 4s but for comparison with observations results are scaled by ¼ to give a fair linear comparison

49. Analysis • Examine differences between time-means of pairs of experiments • Statistical significance based on t-test assuming years are independent • Signal/noise measure: SST forced response Internally generated variability (Time mean anomaly between expts) (interannual standard deviation within an experiment) NB: S/N is independent of ensemble size

50. Response to (NA+)-(NA-) in surface temperature cooling DJF MAM JJA SON Anomalies plotted are significant at 95% level