Atlantic Multidecadal Oscillation as seen in models, observations and paleo data

Transcription

1 Atlantic Multidecadal Oscillation as seen in models, observations and paleo data Odd Helge Otterå, et al., G.C. Rieber Climate Institute, NERSC, Bergen Bjerknes Centre for Climate Research, Bergen

2 Overview AMO definition Observations and paleo data Some climate impacts Mechanisms Ocean circulation Coupled modes Radiative forcing AMO in Bergen Climate Model Summary

3 Atlantic Multidecadal Osccilation (AMO) Definition: The Atlantic multidecadal oscillation (AMO) is a mode of natural variability occurring in the North Atlantic Ocean, and which has its principle expression in the sea surface temperature (SST) field. I. Linear detrending (Sutton and Hodson 2005) II. Global SST removed (Trenberth and Shea 2006) III. Difference between observed area-averaged Atlantic SST and a multimodel ensemble mean (IPCC AR4) (Knight 2009) Sutton and Hodson 2005 I Trenberth and Shea 2006 II III Knight 2009

4 Observed AMO Warm phases in late 19th century and Cold phases during and Most of the Atlantic between the equator and Greenland changes in unison. Some area of the North Pacific also seem to be affected. Sutton and Hodson 2005

5 Reconstruction of Atlantic Multidecadal Oscillation (AMO) Tree-ring based Gray et al., 2004

6 Spatial pattern of multidecadal signal in multiproxy records Approximate time scale of 70 years Delworth and Mann 2000

7 Faroe Island section see Lie/Dokken talk Courtesy of Trond Dokken et al

8 Reconstructions from Gardar Drift AMO vs Sorted silt (proxy for overflow) Courtesy Tor Mjell and U. Ninnemann

9 Climate impact on boreal summer Composite: minus Sutton and Hodson 2005 Low-pressure anomaly over US and UK Precipitation reduction of up to 20% over southern US Enhanced precipitation over western Europe (5-15%) Warming over central Europe Enhanced Sahel precipitation

10 Atlantic Hurricanes and AMO Goldenberg et al. (2001) claim a link between the frequency of major Atlantic hurricane formation and AMO variations in North Atlantic SST. Major Hurricanes Suggest AMO affects vertical shear in the hurricane formation region via circulation changes Emanuel (2005) suggests a more direct link between SST and the integrated intensity of storms. Goldenberg et al (2001)

11 Potential mechanisms for Ocean dynamics interdecadal variability Climate shift between late 1890 s and the mid-1920 s in the North Atlantic explained by an enhanced sub-tropical gyre (Bjerknes 1964) Hopf bifurcation in box models (Greatbatch and Zhang 1995; teraa and Dijkstra 2002) Noise driven ocean oscillator (Delworth et al 1993; Delworth and Greatbatch 2000; Dong and Sutton 2005; Jungclaus et al 2005) Coupled air-sea mode (Timmermann 1998; Vellinga and Vu 2004) Surface radiative forcing Total solar irradiance variations The role of volcanic and tropospheric aerosols (Shindell and Faluvegi 2009; Evan et al 2009) A combination of the above

12 Multidecadal oscillations in simplified models Streamf Zon temp Prescribed heat flux The mechanism is associated with the balance between the strength of the poleward heat transport and local heat storage Thermally driven 50 year period Greatbatch and Zhang 1995

13 Oscillations due to Hopf bifurcation teraa and Dijkstra 2002 Steady state under restoring temperature condition Mechanism described as an out-of-phase response of zonal and merdional overturning anomalies to westward propagating temperature (or more generally, density) anomalies Time scale: ~65 yr

14 Evidence from GCMs Driven by the low-frequency variability of the atmospheric fluxes The ocean is setting the dominant time scale (model dependant) Low-pass filter (<20 yr) Jungclaus et al 2005 Little influence from the ocean on the atmosphere Arctic freshwater export modulate the fluctuations (Jungclaus et al 2005) High-pass filter (>20 yr) Weak MOC Cooling of the NA ocean Acceleration of sub-polar gyre Increased advection of salt to the sinking regions enhanced deep water formation Intensified MOC Delworth and Greatbatch 2000

15 A coupled air-sea mode Intensified THC and polw. heat transport Positive SSTA Negative SSTA Ekman trans. Period ~35 years Negative SLPA Weakened NAO Strength. convection in sinking region Negative FW anomaly, Ekman transport into North West Atlantic Positive SSS anomaly in North West Atl. Timmermann et al 1998

16 Second EOF In the IPSL coupled model the East Atlantic pattern (EAP) plays a leading role in the multidecadal fluctuations Msadek and Frankignoul 2009

17 Simulated AMO in HadCM3 Maximum overturning at 30 o N Persistant band of variability between yrs Knight et al 2005

18 The role of the ITCZ Vellinga and Vu 2004 Precipitation change associated with an ITCZ shift caused by SST anomalies Supplies the tropical fresh water flux forcing Coupled mechanism involving a delayed oceanic salinity feedback

19 Radiative forcing Simulated NH temperature in IPCC AR4

20 Ocean variability or radiative forcing? Atlantic Constrained Experiment (ACE): Atlantic replaced by a slab ocean. Prescribed anomalous heat flux ACE RFE Radiatively Forced Experiment (RFE): Forced by raditaive forcing agents since 1861 Illustrates the difficulty in separating effects of Atlantic variability and radiative forcing from NH temperature records Zhang et al. 2007, GRL

21 Variations in total solar irradiance Reconstructed TSI for the last 6000 years Gleissberg cycle Wanner et al 2008

22 The role of aerosols Dust Simple ocean temperature model Evan et al Increased Sahel precipitaton reduced African dust 2. Positve phase of NAO increased African dust Aerosols exert their strongest influence on ocean temperatures along the coast of West Africa and extending westwards between 10 to 20 o N Evan et al 2009

23 ML response to aerosol Observed SST anomalies Averaging region: 0-30 o N,15-65 o W Residual (Obs SST Aerosol comp.) Suggest that 69% of recent upward trend is due to changes in aerosols (volcanic=46%, dust=23%) Evan et al 2009

24 Dynamical feedback to (tropical) volcanic aerosol forcing SAT DJF Stratosphere-troposphere coupling SLP Simulated winter warming after Pinatubo in ARPEGE (Otterå 2008) After Robock 2000

25 AMO in forced IPCC AR4 models (Knight 2009) Atlantic SST is inconsistent with the forced response for much of the last 150 years Implications AMO is an internal mode Models are inadequate to represent the effects of known forcings on climate The forcings used are incorrect or incomplete

26 Summary (1) Evidence (instrumental, paleo) that something like the AMO exists Lack adequate theoretical understanding Suggested climate impacts include: Atlantic Hurricanes Sahel precipitation European and American summer climate Potential mechanisms Forced mode (solar, aerosols) Ensamble mean of IPCC models do not support a forced AMO Internal mode Ocean circulation (noise driven oscillator) Air-sea coupling (ITCZ, NAO, EAP) Time scale (50-120) - model dependant

27 Bergen Climate Model (version 2) ARPEGE Resolution: T42, ~2.8x2.8, 31 layers Volcanic aerosols implemented MICOM Resolution: ~2.4x2.4, 35 isopycnic layers New pressure gradient formulation Reference pressure at 2000 m Incremental remapping for tracer advection (better conservation) Thermodynamic and dynamic seaice module (GELATO) Multi-ice categories Based on a 600 yr long quasi-stable pre-industrial simulation (1000 yr including spin-up) No carbon cycle or vegetation! ARPEGE MICOM

28 Performed simulations with BCM CONTROL600: All forcings kept constant at pre-industrial (1850) level NATURAL600: Same as CONTROL600, but with historic total solar irradiance (TSI) and volcanic aerosol variations for the last 600 years All150: Same as NATURAL600, but with variations in well-mixed greenhouse gases and tropospheric sulfate aerosols. Total of 5 ensemble members performed.

29 Ocean surface biases in the control run Error compared to Levitus SST SSS Otterå et al 2009

30 NH sea-ice extent in the control run Otterå et al 2009

31 Simulated Northern Annular Mode in the control run Otterå et al 2009

32 Simulated Atlantic overturning circulation in the control run 16.6 Sv * Maximum AMOC Obs. estimate: ± 1.6 Otterå et al 2009

33 Forcing and NH temperature response Forcing: Crowley (Crowley et al. 2003) Otterå et al, in prog

34 Atlantic Multidecadal Oscillation (AMO): Observed and simulated Observation BCM ALL Member 2 Otterå et al, in prog

35 Simulated Early-warming in the Arctic The ALL run (5 member) compares wey well with observations o N Suo et al, in prog

36 AMO for the last 600 years Power spectrum CONTROL600: Thin lines NATURAL600: Thick lines Reconstructed AMO (Gray et al 2004) from tree-rings shown in green. Otterå et al, in prog

37 Lag-correlations: AMO vs AMOC

38 Patterns of temperature and Atlantic streamfunction variability associated with the AMO-index in the natural forced run Otterå et al, in prog

39 AMOC linked to the derivative of the AMO (AMO ROC): NAO mechanism? AMOC SLP regressed onto the AMO ROC index

40 Reconstructions from Gardar Drift G. inflata Sortable silt strong O v e r f l o w weak warm cold W i n t e r t e m p Courtesy of Tor Mjell and U. Ninnemann

41 Upper ocean (300 m) temperature regressed on AMO-index (lag 0) The Gardar Drift region anti-correlates to the AMO-index in the simulation

42 Simulated winter temperature Gardar drift vs AMO-index

43 Summary (2) Simulated NH temperature for the last 600 years in general agreement with paleo reconstructions The simulated AMO resembles the observed AMO for last 150 years, and have many similarities with the reconstructed AMO over the last 600 years Early warming for the Arctic reproduced in a 5 member ensemble The simulated AMOC in BCM is out of phase with AMO strong AMOC in cold times and vice versa Preliminary comparison to paleo records AMO and winter temperatures at the Gardar drift out-of-phase in both the data and the model AMOC linked to the rate of change of AMO (AMO ROC) atmosphere link? Overall it seems like the natural forcing act as a pacemaker for the multidecadal variability in BCM

44 Some key questions Is the AMO persistent back in time? yes, at least a few centuries back in time Can changes in the radiative forcings explain the AMO? yes according to BCM; no according to IPCC AR4 models Is the AMO an internal mode? yes is the consensus view What is the key mechanism? ocean circulation (THC) variations Does the AMO impact large-scale atmospheric climate? maybe, model dependant What is the role of aerosols? poorly known, could be important (i.e. Evan et al 2009) Can we predict AMO? potentially yes, but much work remains