The last post showed that there was little change in the timing of, or the seasonal maximum of temperatures in Australia. Here I look at the long term trends in 3 different locations Yongala in SA, Alice Springs in central Australia and Townsville near the coast in Queensland.
The top signal is the daily maximum temperature recorded for over a 73 year period (73 summers). There are some very hot days but overall the signal is remarkably stable with no evident warming trend. The bottom trace shows the daily range in temperatures between night and day. There is a clear tendency for higher daily temperature swings in winter but there is no long term trend evident.
Alice Springs located in central Australia shows a much stronger seasonal change in maximum temperatures
Townsville is located in Northern Queensland next to the sea and the Great Barrier reef . It has a semi tropical climate and shows a much smaller seasonal cycle, with a smaller diurnal temperature range, inversely proportional to the seasons. Presumably this is because the ocean is cooler in winter reducing overnight temperatures.
None of these stations show any significant warming signal in this data. So why does ACORN show a warming trend ? The main reason is that they all use a selected set of “anomalies” based on the monthly differences to their 30 year temperature average. These values are called station “normals”. However before all this another bit of black magic called “homogenisation” is applied. Finally just 112 stations are selected for this warming trend analysis
Homogenisation is based on the assumption that nearby stations all follow the same warming trend, So there is an inbuilt tendency to correct or even reject any negative trends. Systematic differences can indeed be due to a site relocation, inducing a positive trend. When I looked at this earlier I found that this increased the average temperature anomaly trend by about 0.3C
One more problem is that anomalies don’t tell you whether it is maximum temperatures that are rising or instead minimum temperatures that are rising.
I found previously that in Australia it is nights (Tmin) that are warming rather than days (Tmax) !