ATS2545 Lecture Notes - Lecture 9: Westerlies, Descriptive Statistics
25 May 2018
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Lecture 9 - Patterns and Variability
Climate Averages - what are they, and what they conceal
• #1
o Typically based on 30 years of contemporary observational data
o Conceal variability on timescales less than this interval
▪ Multi year cycles of ENSO phenomena
o Also fail to report any time variation related to ongoing environmental
changes
• #2
o Use of climate averages assumes stationarily in the data - a mean that
does not change with time
o Rainfalls over Australia are not stationary - both sharp and gradual
changes
• The Kraus Data
o Smoothed data using 30 year moving average and processed the data
separately for each of the 4 seasons
o Spring and autumn rainfall declined from a peak around 1880
o Summer rainfall was high in the 1870s, fell to a low in 1895 and then
increased again
o Winter rainfall did just the opposite
▪ Low in 1870s
▪ Rose to a peak in early 20th century → began to fall steadily
o Kraus showed that there was a major rainfall changes in 1895
o Concluded that in mid 1890s, the belt of westerlies became stronger
and moved equatorwards
▪ Northern monsoons became weaker
o Wet winters in southern Australia were caused by a higher frequency
of frontal systems crossing the continent
o Rising summer rainfalls in the 1940s may reflect the westerlies moving
South once more
• Pittock’s analysis
o Performed similar analysis using data from mainly rainfall stations in
interval 1913-1978
▪ Compared 1913-1945 with 1946-1978
o Found that large area of E Australia experienced a >10% increase in
rainfall post 1945
o In Spring, Summer and Autumn, large areas of E Australia show
increase of >20% and up to 30-40% in some areas
o Winter showed the opposite trend
▪ Declining notably over SW and WA
Brief Review of Australian Rainfall, evaporation and runoff
• Very low runoff in southern Murray Darling Basin
o Due to: less autumn and winter rainfall
▪ Fewer rainfall years
▪ Increased temperatures
• Rainfall distribution is uneven
o Decreases towards central Australia
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