Introduction - The Global Climate System: Patterns, Processes, and Teleconnections - page 13

Figure 1.6 A climatic
classification of Australia
illustrates an example of
so-called normal climatic
regimes.
Normal climate is more than just average conditions. It includes extremes
as well. There is a general view that Australia in the time of El Ni˜o is under
severe drought conditions. The 1997-8 El Ni˜o was the most intense in the
twentieth century. The map in Figure 1.7 shows the wide range of weather
and climate conditions that can occur on the same continent, Australia, during
an El Ni˜o year. This is shown to reinforce the view that while climatological
averages are useful for some purposes, by no means do they tell the whole
climate story for a given country.
Climate can be defined either statistically or perceptually (Tribbia 2002 ).
Statistical definition
The International Research Institute for climate prediction defines normal
rainfall for use in forecasting, for example, as follows:
''Normal'' rainfall is defined as the average rainfall for 30 years for the period 1961
to 1990.
''Above Normal'' corresponds to one-third of the observations of which cumula-
tive totals of rainfall were the highest (33%).
''Below Normal'' corresponds to one-third of the observations of which the
cumulative rainfall totals were least (33%).
''Near Normal'' corresponds to the group of remaining years.
(from: iri.columbia.edu/climate/forecast/sup/May01_Afr/index_eng.html)
Officially, normal climate is designated by the UN WMO as the most
recent three-decade averages of temperature, rainfall, etc. (1961-90; in May
2001 this normal was replaced by statistics for the 1971-2000 period).
Others, however, tend to view normal as based on the statistics of the entire
period (i.e. time-series) on record for a given location. Both approaches are