1. Odd Helge Otterå1,3, Jerry Tjiputra1,3 and Tao Wang2
The role of volcanic forcing on northern hemisphere decadal to multidecadal climate variability and future carbon cycle
Odd Helge Otterå1,3, Jerry Tjiputra1,3 and Tao Wang2
Uni Bjerknes Centre, Uni Research, Bergen, Norway
Nansen-Zhu International Research Centre, Beijing, China
Bjerknes Centre for Climate Research, Bergen, Norway
IAVCEI conference: Forecasing volcanic eruptions – July 19-24, Kagoshima, Japan

2. Outline Motivation Model description
Decadal variability and volcanic eruptions
Pacific decadal oscillation (PDO)
Atlantic Multidecadal Oscillation (AMO)
North Atlantic Oscillation (NAO)
Atlantic merdional overturning circulation (AMOC)
Volcanic eruptions and future carbon cycle
Conclusions

3. Volcanic impacts on climate
Injects sulphur gases to the stratosphere  forms aerosols
Warms the lower stratosphere
Enhances pole-equator temperatur gradient
Increases diffusive light
Stratospheric ozone destruction
Net surface cooling
Lasting impact for a few years
Decadal or longer?

4. Possible volcanic aerosol influence on tropical Atlantic
Northern tropical Atlantic surface temperature sensitive to regional changes in aerosols
Simple physical model for estimating the temperature response of the ocean mixed layer to changes in aerosol loadings
Aerosols (dust+volcanoes) excert their strongest influence on ocean temperatures along the coast of West Africa and extending westwards between 10 to 20oN
Suggest that 69% of recent upward trend is due to changes in aerosols (volcanic=46%, dust=23%)
Observations and models demonstrate that northern tropical Atlantic surface temperatures are sensitive to regional changes in stratospheric volcanic and tropospheric mineral aerosols. However, it is unknown if the temporal variability of these aerosols is a key factor in the evolution of ocean temperature anomalies. Evan et al. used 26-years of satellite data to drive a simple physical model for estimating the temperature response of the ocean mixed layer to changes in aerosol loadings.
Evan et al. 2009, Science

5. Multidecadal variability and climate impacts
European and US summer climate
Atlantic hurricanes
East Asian and Indian summer monsoon
Sahel rainfall
Detrended Atlantic SSTs ?

6. Bergen Earth System Model (BCM-C)
Resolution
Atmosphere 2.8o L31
Ocean o L35
CTL600: Control run (pre-industrial)
NAT600: TSI + VA,
ALL150 (x5): GHG + TA + TSI + VA,
Scenarios: A1B, E1, future volcanic eruptions
BCM (Otterå et al. 2009,Tjiputra et al. 2010, Geosci. Mod. Dev.)

7. Natural forcings and NH temperature (A.D. 1400-1999)
Forcing based on Crowley et al. (2003)
NH temperature
Volcanic and solar forcings play an important role up until about 1950
Otterå et al. (2010), Nature Geosci.

8. Zoom in over the Pacific

9. Simulated Pacific climate response to volcanic forcing
PDO pattern
OBS
MODEL
The 162  SEA technique is a statistical method that is often used to resolve significant signal to noise problems. This is especially useful in cases where the responses to particular 164  events (in our case large radiative forcing from explosive volcanic eruptions) may beobscured by noise from other competing influences that operate on similar time scales, 166  such as internal El Niño variations.
A standard Monte Carlo randomization procedure 168  has been used to determine the statistical significance (a total of 1000 Monte Carlo simulations is used here). In addition a simple compositing analysis have been applied170  which involves sorting the data into different phases.
Superposed Epoch Analysis of the volcanic response based on 18 large tropical eruptions since 1400
Wang et al (2012), Clim. Dyn.

10. Simulated Pacific climate response to volcanic forcing
A delayed response in the PDO that is maintained for several years
The mechanism involves changes in the surface winds accross the central Pacific through stratosphere/troposphere interactions
Modifications in heat fluxes and ocean circulation further enhances the response
Volcanic forcing thus plays a key role for the phasing of PDO in BCM
The 162  SEA technique is a statistical method that is often used to resolve significant signal to noise problems. This is especially useful in cases where the responses to particular 164  events (in our case large radiative forcing from explosive volcanic eruptions) may beobscured by noise from other competing influences that operate on similar time scales, 166  such as internal El Niño variations.
A standard Monte Carlo randomization procedure 168  has been used to determine the statistical significance (a total of 1000 Monte Carlo simulations is used here). In addition a simple compositing analysis have been applied170  which involves sorting the data into different phases.
Superposed Epoch Analysis of the volcanic response based on 18 large tropical eruptions since 1400
Wang et al (2012), Clim. Dyn.

11. Zoom in over the Atlantic

12. Simulated AMO and AMOC relationship
Otterå et al. 2010, Nature Geosci.
Leading pattern of Atlantic merdional streamfunction variability

13. Simulated AMO and AMOC relationship
Otterå et al. 2010, Nature Geosci.
Causal link between the forcing and AMO
Weak AMOC associated with a positive phase of AMO
Some similarities to reconstructed AMO indices, especially for the Maunder Minimum and 19th century

14. Simulated AMO and AMOC relationship
SST regression
48-60N
All data have been low-passed filtered (21 yrs)
Tropical SST controlled by the radiative forcing; subpolar SST controlled by MOC (lag ~10 yrs)

15. Volcanic forcing and atmospheric circulation: Stratosphere/troposphere interaction
Superposed epoch analysis NAT600
Wang et al. (2012), Clim. Dyn.
Positive NAO simulated after large tropical volcanic eruptions
Closely related to stratospheric and tropospheric circulation
Volcanically induced modulation of the polar vortex
Robock 2000, Rev. Geophys.

16. Simulated response to volcanic forcing
Composite analysis for the 18 largest tropical eruptions in the period
An important result from this study is therefore that explosive volcanism have a strong influence on NH climate, not only for short-term changes, but also for multidecadal AMO-type changes. Volcanic eruptions tend to strengthen the MOC through their direct radiative cooling, and in BCM also through a tendency for inducing positive phases of the NAO.
Positive NAO in post-volcano winters
NH winter warming
Strong heat flux response in the Labrador Sea
Increased local buoyancy forcing
Otterå et al. 2010, Nature Geosci.

17. Labrador Sea Water flow and volcanoes
Volcanic eruptions have a strong impact on LSW flow across Newfoundland (NF) section through NAO heat flux response
5-years after the eruption there is increased LSW across NF section and associated increase in AMOC
According to BCM the highest LSW flow during the last 600 year occurred after the Mt. Parker eruption in 1641!
Volcanic forcing

18. Simulated MOC response
Otterå et al. 2010, Nature Geosci.
Strengthened MOC in post-volcano years (pvy)

19. Volcanic impact on future carbon cycle

20. Simulating the 1991 Mt. Pinatubo eruption
Tjiputra and Otterå (2011), ESD
Global surface cooling of 0.5 degrees
Winter warming and summer cooling in the NH
Decrease in atmospheric CO2, dominated by reduced NH heterotropic respiration

21. Experimental design

22. Experimental design
Tjiputra and Otterå (2011), ESD

23. Change in global climate projection
REF = A2 scenario
Tjiputra and Otterå (2011), ESD

24. Change in global climate projection
REF = A2 scenario
Increased carbon uptake over land
Tjiputra and Otterå (2011), ESD

25. Change in global climate projection
NEP = NPP – respiration – fire emissions
Cooler climate reduces the terrestrial soil respiration
Tjiputra and Otterå (2011), ESD

26. Conclusions
Volcanic eruptions can influence climate on longer (decadal) time scales
BCM results suggests volcanic eruptions modifies key atmospheric and oceanic variability modes
Large-scale atmospheric circulation modes (NAO, Aleutian low)
Phasing of the PDO
Direct forcing on tropical North Atlantic SST (i.e. AMO)
Indirectly affects large-scale ocean circulation through modification of Labrador Sea water formation (i.e. AMOC)
Results likely model dependent  model intercomparison needed
Future large volcanic eruptions, if they occur frequent enough, can induce positive feedback to the climate (more carbon uptake) and mitigate climate change to a certain degree (e.g. climate engineering)
The feedback is only temporary and warming due to increasing anthropogenic CO2 will likely dominate in the long term
The other ” CO2-problem” (ocean acidification) remains

27. Relevant papers
Otterå OH: Simulating the effects of the 1991 Mount Pinatubo volcanic eruption using the ARPEGE atmosphere general circulation model, Adv. Atm. Sci., 25, (2008)
Otterå OH, Bentsen M, Bethke I and Kvamstø NG: Simulated pre-industrial climate in Bergen Climate Model (version 2): model description and large-scale circulation features, Geosci. Mod. Dev., 2, (2009)
Tjiputra JF, Assmann K, Bentsen M, Bethke I, Otterå OH, Sturm C and Heinze C: Bergen Earth System model (BCM-C): model description and regional climate-carbon cycle feedbacks assesment, Geosci. Mod. Dev., 3, (2010)
Otterå OH, Bentsen M, Drange H and Suo L: External forcing as a metronome for Atlantic multidecadal variability, Nature Geosci., 3, (2010)
Tjiputra JF and Otterå OH: Role of volcanic forcing on future global carbon cycle, Earth Syst. Dynam., 2, (2011)
Wang T, Otterå OH, Gao Y and Wang H: The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions, Clim. Dyn., doi: /s (2012)