Decadal Warning Systems - Reducing Disaster: Early Warning Systems for Climate Change - page 302

Australia (Power et al. 1999 ; Deser et al. 2004 ). The combination of PDV, AMV
and climate change appears to explain nearly all of the multi-decadal US drought
frequency (McCabe et al. 2004 ) including key events like the American dustbowl
of the 1930s (Schubert et al. 2004 ). However, mechanisms underlying PDV are
less clearly understood than for AMV. Some argue that the broad ENSO-like pat-
tern of PDV is simply a residual pattern that results from the spatial asymmetries
of ENSO and skewness in ENSO statistics (Rodgers et al. 2004 ; Schopf and
Burgman 2006 ). Others argue that decadal changes in the tropical Pacifi c mean
state are forced by separate mechanisms, and may in fact infl uence the amplitude,
frequency and teleconnections of ENSO (Power et al. 1999 ; Meehl and Hu 2006 ;
Meehl et al. 2010 ). There is also evidence that PDV may be partially driven by
AMV (e.g. Chikamoto et al. 2012 ). However, predictability studies show much
less potential skill for PDV than AMV (Collins 2002 ; Boer 2004 ; Pohlmann et al.
2004 ; Branstator and Teng 2010 ).

16.2.2 External Factors

External factors, both anthropogenic (changes in greenhouse gases, tropospheric
aerosols, ozone and land use) and natural (volcanic eruptions and variations in solar
irradiance), signifi cantly infl uence climate on decadal timescales (see Box 16.1) and
are therefore potentially important sources of predictability.
Large volcanic eruptions, although relatively rare (typically less than one per
decade), have a signifi cant impact on climate (Robock 2000 ). Aerosol injected into
the stratosphere during an eruption cools global mean temperature for a couple of
years. The hydrological cycle and atmospheric circulation are also affected. Globally,
precipitation is reduced due to a cooler and therefore dryer atmosphere but winters
in Northern Europe and Central Asia tend to be milder and wetter due to additional
changes in the NAO. Volcanic eruptions are not predictable in advance, but once
they have occurred they are a potentially important source of forecast skill (Marshall
et al. 2009 ). Furthermore, volcanoes impact ocean heat content and circulation for
many years, even decades (Stenchikov et al. 2009 ). In particular, the Atlantic merid-
ional overturning circulation (AMOC) tends to be strengthened by volcanic erup-
tions (Stenchikov et al. 2009 ), although the response may depend on the underlying
climate state (Iwi et al. 2012 ; Zanchettin et al. 2013 ). Volcanoes could therefore be
a crucial source of decadal prediction skill (Otterå et al. 2010 ), although further
research is needed to establish robust atmospheric signals on these timescales.
Although much progress has been made recently, solar infl uences on climate
remain uncertain (Gray et al. 2010 ). The most predictable component of solar activ-
ity is the Schwabe solar cycle in which both the number of sunspots and the solar
radiative output vary with an average period of approximately 11 years. Observations
suggest a warming of about 0.1°C in global temperature between the minimum and
maximum phases (Lean and Rind 2009 ), with small changes in tropical atmospheric
circulation. Furthermore, stratospheric temperatures are infl uenced by the solar
cycle through absorption of UV radiation by ozone. Associated changes in