Impact of Strong Tropical Volcanic Eruption on ENSO in a Coupled GCM

1. Impact of Strong Tropical Volcanic Eruption on ENSOin a Coupled GCM Masamichi Ohba(Central Research Institute of Electric Power Industry, Abiko, Japan) Hideo Shiogama, Tokuta Yokohata (National Institute for Environmental Studies, Tsukuba, Japan) Masahiro Watanabe(Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan) 1. Introduction 2. Coupled GCM: MIROC5(T42 ver.) b. El Niño& La Niña: Phase-dependency Experimental design • Explosive strong volcanic eruptions (SVE) acts to scatter and absorb incoming solar radiation in the stratosphere, reducing the amount of surface solar radiation (Robock 2000). Analysis of paleo-climate records suggests that the radiative effects of tropical SVE can lead to a more El Niño–like state and increase the probability of El Niño occurrences (Adams et al. 2003; McGregor et al. 2010). Identical twin forecast experiment 1. Normal year experiment 2. El Niño and La Niña phase experiment Duration of El Niño! • It is of considerable importance to determine how the tropical Pacific Ocean responds to volcanic forcing, since El Niño/Southern Oscillation (ENSO) is known to affect weather and climate around the globe. • Using an simple anomaly air-sea coupled model (Zebiak and Cane 1987), Mann et al. (2005) and Emile-Geay et al. (2008) show an central eastern equatorial Pacific (CEP) warming in response to a uniform reduction of surface heat fluxes. The response can be explained by the dynamical thermostat mechanism (Clement et al. 1996). This mechanism assumes that the mean upwelling of subsurface water in the CEP combined with the reduction of the ocean vertical temperature gradient act to reduce the volcanic cooling in the region, that can excite a zonal SST gradient and then El Niño. • The motivation for this study is to examine ENSO’s response to tropical SVE using a coupled general circulation model (CGCM). Solid: NoSVE Dash: SVE Volcanic eruption initially cause Red: SVE Black: NoSVE 5 case 5 mem. composite SVE: Forcing El Niño La Niña Obs. GCM Adams et al. 2003 McGregor & Timmerman 2011 Weakened duration! Handler1984 2. Duration La Niña 200-yr Ctrl run Handler & Andsager 1990 McGregor et al. 2010 No response La Nina Peak! Simple model Obs. Mann et al. 2005 • SVE during El Niño • ⇒ contribute to the duration of El Niño. • SVE during La Niña • ⇒ counteract to the La Niñaduration. • RMSE from Ctrl for El Niño is much lager than that for La Niña • ⇒ The warming effect in the year during El Nino is much robust (Stenchikov et al. 2007). Nicholls 1990 Clement et al. 1996 El Nino El Nino Peak! Robock 2000 Emile-Geay 2008 Derived from obs. lag-regression (1900-2000)to tropical optical thickness Normal Self et al. 1997 GCM Start from 1 July Initial:5 ensemble by LAF Stenchikov et al. 2007 La Nina 1. Normal year init 2. Transition El Niño 3. Impact of SVE on the ENSO a. Normal year experiment • Decrease of surface temperature over the land is earlier(stronger) than that over the ocean surface. Cooling of the MC ocean is also earlier. Most remarkable case (30S-30N) PDF 4 case 5 mem. composite SVE excites El Niño for the next year as revealed in the paleo-climate records. • Enhanced wind speed & reduced q2 • Also contribute to the cooling around the MC Red: SVE Black NoSVE As the SVE forcing increases, anomalous westerly (weakening of the trade winds) is found in the equatorial western Pacific (WP), that can potentially excite El Niño. Black: Anomaly in noSVE Green&Shd: Diff between SVE -noSVE Reduced Easterly anom. around SVE W/m2 Intermediate Coupled Model:How extent explain by the MC cooling ? Enhanced zonal gradient between WP & CEP by SVE El Niño-related anomalous westerly is strengthened around the SVE peak and then prevent the following transition SST Atm：Empirical atmospheric model Ocn：1.5-layer linear ocean model (ZC ocn model) The result is very similar to SVE experiment in GFDL-CM2.1. (Stenchikov ea. 2007, AGU) SVE amplitude-dependency Sfc wind response is derived from regression on GCM. Wind CEPWarming Anomalous westerly over the WP Air-sea coupled feedback may Significantly amplify the response. IO-WP Cooling! Input GCM surface condition • ○ Early stage • （around SVE peak） • Cooling (withdrying) around the Maritime Continent (MC). • The zonal temperature gradient result in anomalous WP westerly. (Cooling over the MC reduces the Walker circulation) • Cooling expand into the WP and Indian Ocean (IO). • ○ Middle stage • Onset of El Niño • Mature phase of El Niño • Reduced IO cooling and enhanced WP cooling TMC TPO • El Niño phase is sensitive to change in the SVE intensity. • The El Niño duration in response to the strong SVE is robust. Instead of Nino-34 index, zonal difference of Ts is used. CZ model SST GCM • The role of dynamical thermostat (Clement et al. 1996) is relatively minor compared with the MC cooling. 4. Summary • Super volcanic eruption (SVE) in MIROC5 ⇒ excites El Niño like condition by the anomalous WP westerly and then increase the probability of El Niño occurrences as revealed by observational data (Adams et al. 2003, McGregor et al. 2010). The response of MIROC5 is relatively similar to that of GFDL-CM2.1 (Stenchikov et al. 2007, AGU) • The anomalous westerly could be mainly attributed to the land-sea contrast in surface cooling and its time-scale for adjustment, i.e., namely rapid response over the maritime continent. • The role of dynamical thermostat (Clement et al. 1996) may be minor as shown in McGregor et al. (2011). • SVE during El Niño ⇒ contribute to the duration of El Niño. SVE during La Niña ⇒ counteract to the La Niñaduration. • Because of the amplification by the air-sea coupled feedback, the effect of SVE on El Niño is much stronger than that in La Niña, which is consistent with Stenchikov et al. (2007). • A rapid change in radiative forcing by geoengineering may constitute a risk to cause ENSO events. MC Downward positive