Volcanism, the atmosphere and climate through time - Volcanism and Global Environmental Change - page 200

(Labitzke and McCormick, 1992 ). About 18 months following the June
1991 Mount Pinatubo eruption, the peak reduction in globally averaged lower-
tropospheric temperatures reached 0.5 K, with climate models predicting a
cooling of the same magnitude when including water-vapour feedbacks and
removing the El Niño - Southern Oscillation signal (Soden et al ., 2002 ,and
references therein).
Kirchner et al .( 1999 ) showed that the aerosol formed after tropical eruptions
results in an enhanced pole-to-equator heating gradient in the lower stratosphere
that creates a stronger polar vortex and associated positive mode of the Arctic
Oscillation in tropospheric circulation. This results in a warming over northern
America, northern Europe and Russia, and a cooling over the Middle East, during
the first winter, and sometimes the second winter, following a tropical eruption

(Robock and Mao, 1992 ; Graf et al ., 1993 ). This indirect advective effect on
temperature is stronger than the direct radiative effect that dominates at lower
latitudes and during summer. Figure 13.2 highlights that while observations
con response the majority of current climate models

fail to reproduce the magnitude of this effect (Driscoll et al ., 2012 ).
Research on tropical and high-latitude eruptions revealed that feedbacks
between the additional aerosol loading and atmospheric dynamics can weaken
the Asian and African summer monsoon systems (Oman et al ., 2006b ). For high-
latitude eruptions in particular, the season of eruption is important for assessing
the magnitude of the climatic response (Schmidt et al ., 2010 ; Kravitzand
Robock, 2011 ). Trenberth and Dai ( 2007 ) showed that after the 1991 Mount
Pinatubo eruption precipitation over land and continental freshwater discharge
decreased signi

rm this ' winter warming '

cantly between October 1991 and September 1992. Following
Robock and Liu ( 1994 ), Haywood et al .( 2013 ) showed that the asymmetric
aerosol loading between the northern and southern hemispheres after eruptions
can in

uence the sea surface temperature gradient in the Atlantic, which in turn
affects Sahelian precipitation rates (three of the four driest Sahelian summers
between 1900 to 2010 were preceded by an eruption in the northern
hemisphere).
Figure 13.1 also shows that aside from the aerosol direct effect, aerosol particles
can also alter cloud amount and albedo by acting as cloud condensation nuclei
or ice nuclei, thus modifying the optical properties and lifetime of clouds ( ' aerosol

indirect effects ). Whether volcanic particles alter cirrus cloud properties has been

the subject of several studies with contradicting results (e.g. Sassen, 1992 ;
Luo et al ., 2002 ; see Robock et al ., 2013 for a review). In contrast, the ability
of volcanic sulfates to act as cloud condensation nuclei and their indirect
effect on low-level warm clouds has been con

'

rmed by means of measurements