Impact of Strong Tropical Volcanic Eruption on ENSO

Impact of Strong Tropical Volcanic Eruption on ENSO

Impact of Strong Tropical Volcanic Eruption on ENSO in 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)
2.
Coupled
GCM:
MIROC5(T42
ver.)
b. El Kashiwa,
Nio & La
Nia: Phase-dependency
Masahiro Watanabe (Atmosphere and Ocean Research Institute, The University of Tokyo,
Japan)
incoming solar radiation in the Experimental design

1. Introduction

Explosive strong volcanic eruptions (SVE) acts to scatter and absorb
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 Niolike state and increase the
probability of El Nio occurrences (Adams et al. 2003; McGregor et al. 2010).
It is of considerable importance to determine how the
tropical Pacific Ocean responds to volcanic forcing, since
El Nio/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 EmileGeay 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 Nio.
The motivation for this study is to examine ENSOs
response to tropical SVE using a coupled general
circulation model (CGCM).

Volcanic eruption initially cause
El Nio
La Nia

Obs.

Adams et al. 2003
Handler 1984
Handler & Andsager 1990
McGregor et al. 2010
Simple model

Mann et al. 2005
Clement et al. 1996
Emile-Geay 2008
GCM

GCM

McGregor &
Timmerman 2011

No response
Obs.

Nicholls 1990
Robock 2000
Self et al. 1997

Stenchikov et al. 2007

Identical twin forecast experiment
1. Normal year experiment
2. El Nio and La Nia phase experiment
Solid: NoSVE
Dash: SVE

SVE: Forcing

200-yr Ctrl run

El Nino Peak!
Derived from obs. lag-regression (19002000) to tropical optical thickness

Start from 1 July
Initial:5 ensemble by LAF

1. Normal year init

4 case 5 mem. composite

SVE excites El Nio for the next
year as revealed in the paleoclimate records.

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 Nio.

CEP Warming
IO-WP Cooling!

Anomalous westerly
Air-sea coupled feedback may
over the WP
Significantly amplify the response.

Early stage
around SVE peak
Cooling (with drying) around the Maritime
Continent (MC).

Mature phase of El Nio
Reduced IO cooling and enhanced WP cooling

W/m

2

Intermediate Coupled Model: How extent explain by the MC
coolingAtm
? Empirical atmospheric

SST

Sfc wind response is derived from regression on GCM.
x C d W0u0 W0u0

y

Wind

vt fu g hy /( 0 h) v

y Cd W0 v0 W0v0

ht h(u x v y ) 0

Qh C p Ch W0 (Ts T0) W0(Ts T0 )

Tt umTx vmTy wsTsub / H m Q /( 0C p H m )

Qe LCe W0 ( qs (Ts) q0 ) W0( q s (Ts ) q0 )

1
W0
(u0u0 v0 v0 )
W0

4. Summary

Enhanced zonal gradient between WP & CEP by
SVE

Ocn 1.5-layer linear ocean model (ZC ocn
model)
x

ut fv g hx /( 0 h) u

GCM

MC

El Nino
Normal
La Nina

Most remarkable case

Downward positive

SVE amplitude-dependency

Reduced Easterly anom. around
SVE

El Nio-related anomalous
westerly is strengthened around
the SVE peak and then prevent the
following transition

Input GCM
surface condition

TPO

Instead of Nino-34 index, zonal difference of Ts is used.

Middle stage
Onset of El Nio

SVE during El Nio
contribute to the duration of El Nio.
SVE during La Nia
counteract to the La Nia duration.
RMSE from Ctrl for El Nio is much lager
than that for La Nia
The warming effect in the year during El
Nino is much robust (Stenchikov et al.
2007).

Black: Anomaly in noSVE
Green&Shd: Diff between SVE -noSVE

TMC

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).

Weakened duration!

Enhanced wind speed & reduced q2
Also contribute to the cooling around the
MC

model

The result is very similar to
SVE experiment in GFDL-CM2.1.
(Stenchikov ea. 2007, AGU)

2. Transition El Nio

Decrease of surface temperature over the
land is earlier (stronger) than that over the
ocean surface. Cooling of the MC ocean is also
earlier.

(30S30N)

PDF

2. Duration La Nia

5 case 5 mem. composite

Red: SVE
Black: NoSVE

La Nina Peak!

3. Impact of SVE on the ENSO
a. Normal year experiment

Duration of El Nio!

CZ model
SST

The role of dynamical thermostat (Clement et al.
1996) is relatively minor compared with the MC
cooling.

El Nio phase is sensitive to change
in the SVE intensity.
The El Nio duration in response to
the strong SVE is robust.

Super volcanic eruption (SVE) in MIROC5 excites El Nio like condition by the anomalous WP westerly and then increase the probability of El Nio 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 Nio contribute to the duration of El Nio. SVE during La Nia counteract to the La Nia duration.
Because of the amplification by the air-sea coupled feedback, the effect of SVE on El Nio is much stronger than that in La Nia, which is consistent with

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