Decadal Warning Systems - Reducing Disaster: Early Warning Systems for Climate Change - page 300

16.2 Potential Sources of Skill

Individual weather events are generally not predictable more than a couple of weeks
in advance. This is because the atmosphere is chaotic, so that inevitable small errors
in the initial conditions grow over a few days into large-scale disturbances. However,
the tendency for the atmosphere to adopt particular weather patterns can also be
infl uenced by several potentially predictable factors. These provide sources of skill:
predicting them and their subsequent impact on the atmosphere is the goal of
decadal predictions. A trivial example is the annual cycle: for example, we expect
more mid-latitude storms during winter than summer, although the precise time and
location of each winter storm cannot be predicted more than a couple of weeks in
advance. However, there are other factors that can cause particular years or decades,
to be abnormal, as discussed below.

16.2.1 Natural Internal Variability

The El Niño Southern Oscillation (ENSO) is the most well-known example of natu-
ral internal variability and is the cornerstone of seasonal forecasting. ENSO is a
coupled mode of variability in the tropical Pacifi c that grows through positive feed-
backs between sea surface temperature (SST) and winds: a weakening of the east-
erly trade winds reduces the upwelling of cold water leading to a positive SST
anomaly in the eastern tropical Pacifi c, which in turn weakens the atmospheric
zonal (Walker) circulation to further reduce the easterly winds. The time between El
Niño events is typically about 2-7 years, but the mechanisms controlling the rever-
sal to the opposite La Niña phase are not understood completely (Kirtman 1997 ).
ENSO infl uences seasonal climate almost everywhere (Trenberth and Caron 2000 ;
Alexander et al. 2002 ; Smith et al. 2012a ), either by directly altering the tropical
Walker circulation or through Rossby wave trains that propagate to mid and high
latitudes (Hoskins and Karoly 1981 ). The strongest impacts occur in Indonesia,
North and South America, East and South Africa, India and Australia. There is also
a notable infl uence on the North Atlantic Oscillation (NAO), especially in late win-
ter (Brönnimann 2007 ). ENSO also modulates the vertical wind shear and stability
in the tropical Atlantic atmosphere, leading to fewer (more) hurricanes during El
Niño (La Niña) years (Goldenberg and Shapiro 1996 ; Tang and Neelin 2004 ). There
is evidence that ENSO may be predictable beyond the seasonal range, up to about 2
years ahead (Chen et al. 2004 ; Luo et al. 2008 ).
The Quasi-Biennial Oscillation (QBO) is a natural wave-driven reversal of tropi-
cal stratospheric winds between easterly and westerly with a mean period of about
28 months (see Baldwin et al. 2001 for a review). The QBO is predictable a couple
of years ahead and potentially provides moderate predictability of European winter
climate (Marshall and Scaife 2009 ).