Odd Helge Otterå1,3, Jerry Tjiputra1,3 and Tao Wang2 WWW.BJERKNES.UIB.NO 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 E-mail: Odd.Ottera@uni.no IAVCEI conference: Forecasing volcanic eruptions – July 19-24, Kagoshima, Japan

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

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?

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

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

Bergen Earth System Model (BCM-C) Resolution Atmosphere 2.8o L31 Ocean 2.4o L35 CTL600: Control run (pre-industrial) NAT600: TSI + VA, 1400-1999 ALL150 (x5): GHG + TA + TSI + VA, 1850-1999 Scenarios: A1B, E1, future volcanic eruptions BCM (Otterå et al. 2009,Tjiputra et al. 2010, Geosci. Mod. Dev.)

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.

Zoom in over the Pacific

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.

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.

Zoom in over the Atlantic

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

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

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)

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.

Simulated response to volcanic forcing Composite analysis for the 18 largest tropical eruptions in the period 1400-1999 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.

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

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

Volcanic impact on future carbon cycle

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

Experimental design

Experimental design Tjiputra and Otterå (2011), ESD

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

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

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

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

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, 213-226 (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, 197-212 (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, 123-141 (2010) Otterå OH, Bentsen M, Drange H and Suo L: External forcing as a metronome for Atlantic multidecadal variability, Nature Geosci., 3, 688-694 (2010) Tjiputra JF and Otterå OH: Role of volcanic forcing on future global carbon cycle, Earth Syst. Dynam., 2, 53-67 (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:10.1007/s00382-012-1373-5 (2012)