The Late 20th Century Climate Shift in SE Australia

Author: Dr. Michael Chase

Introduction

Monthly average surface air temperature data in South-East Australia (and probably in other regions) show a relatively sudden increase in maximum temperatures at the end of the 20th century. Unfortunately, this was also the time when the BoM introduced Automatic Weather Stations (AWS) at many of its sites. This post presents some data on temperature and rainfall changes around this “climate shift” and shows graphically that the calibration of the AWS systems in the area examined had close matches to those of the systems they replaced, at least at the level of monthly Tmax averages. The seasonal differences in temperature and rainfall variations may provide clues to the cause(s) of the climate shift.

Regional Average Temperatures and Rainfall

The figure below shows the climate shift in a region of NSW/VIC bounded by lines joining Mildura, Hillston, Wagga Wagga, Rutherglen, Echuca, Nhill and back to Mildura:

DIY_P1_Tmax_6mthav

The data shown in the figure above represent estimates of the regional average temperature history, in this case for 6-monthly Tmax data. Details of how to estimate regional averages, detecting and correcting inhomogeneities, will be given in later posts.

I have examined the regional average temperature history for each separate month, and find that each month from September to February has a similar upward shift in Tmax near the end of the 20th century, so have averaged over this 6-month period to illustrate the phenomenon (red curves above). The other months all show a similar lack of anything special happening around that time (blue curves for the 6-month average).

There is normally a close association between Tmax fluctuations  and rainfall levels, but the following figure shows that there was no particular trend in rainfall around the time of the climate shift:

DIY_P1_Rain_6mthav

Are AWS Systems Involved?

Many stations in the region had AWS systems installed in the late 20th century, for example becoming the primary sensors in November 1996 at both Mildura Airport and Wagga Wagga AMO. Fortunately, many nearby stations retained their manual systems, and I have checked their temperature histories against those that switched to AWS.

The following figure shows the temperature history (12-month and 15-year moving averages, after subtraction of regional average temperature fluctuations) for 3 stations that switched to AWS, together with the regional average temperature history (black curves):

DIY_P1_AWS

Note that there are no substantial deviations from the regional average when the AWS systems became the primary sensors. For comparison, the following two figures show the same data for 6 stations that did not get converted to AWS:

DIY_P1_nonAWS1

DIY_P1_nonAWS2

Conclusions

There may be calibration differences of a tenth or two degrees C at the level of monthly Tmax averages between the AWS and manual systems employed in the region, but not more than that. This conclusion is consistent with the absence of corrections for AWS installations in ACORN-SAT, the early one at Cape Otway being the only one that has a correction.

Later posts will look at how the climate shift varied around Australia, which may shed some light on cause(s).

 

 

 

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