AUSTRALIA-NZ Tmax

Author: Dr Michael Chase

INTRODUCTION

This article shows reconstructions of how daily maximum surface air temperature (Tmax) has varied since the mid to late 19th century in Australia and New Zealand. Results have been obtained for each of the 12 separate monthly averages, but only 12-month moving averages are shown here.

The reconstruction of changes in surface air temperature from instrumental data is an interesting and important problem. In Australia and New Zealand many diligent observers have produced a high density of quality data, and researchers there have digitised much of this data, and have produced summaries of documented changes in the weather stations. We must also thank the Australian BoM and the New Zealand NIWA for making the data freely and easily available to the public.

Some people believe that temperature reconstruction is partly or wholly impossible or unreliable, because of issues such as urban heating and non-standard thermometer enclosures or siting. Those issues can be difficult to deal with, but primarily only when they change. It is CHANGE in the measurement system or its environment that can lead to errors in temperature reconstruction. The methodology used to suppress errors caused by such changes is as follows.

Typically around 40 RAW monthly average temperature records in a region are decomposed, separately for each month, into a moving average (MAV) time series, typically of 13 or 15 consecutive years, and a series of deviations from the MAV series. The deviations are averaged across stations to obtain their regional average. The regional average deviations are subtracted from each station’s raw temperature data, reducing the size of the “noise” caused by the always fluctuating weather. The resulting weather-corrected data are moving-averaged again, and a regional moving average obtained as follows: separately for each month, the year-to-year MAV temperature changes are averaged democratically across stations, and those average temperature changes are simply integrated forwards and backwards in time from an arbitrary reference year, 2015 in the examples shown in this article.

The processing described above gives results that are distorted by non-climatic perturbations, such as those that result from station moves, equipment changes, and sudden or gradual changes in the local thermal environment. This distortion is removed simply by excluding the periods of data deemed to be suffering from time-varying non-climatic influences. Such periods are detected visually (i.e. not automatically), by comparing station data with the latest version of the regional average, which is recomputed after each period of perturbed data is marked for exclusion.

Details of the reconstruction algorithms and procedure can be found in the pages above, starting with OUTLINE.

Results for 6 large areas of Australia, plus New Zealand, are presented below. The moving average plots are all presented together, to facilitate comparisons, followed by the complete set of temperature deviations from the moving averages. The post ends with a sample station analysis, for the town of Boulia in inland Queensland.

NORTH AUSTRALIA

The following figure shows the moving averages of Tmax variations for the following regions in the North of Australia:

• Port Hedland (Onslow to Broome, inland to Newman, Nullagine, Wittenoom)
• Darwin (Broome to Burketown, inland to Halls Creek, Victoria River Downs)
• North-West Queensland (Camooweal-Burketown-Hughenden-Barcaldine-Birdsville)

Note the similarity of the temperature variations, and the East-West trend in the size of the overall change in temperature, greatest in the West, least in the East.

WEST AUSTRALIA

The following figure shows the moving averages from the following regions in the West of Australia:

• Port Hedland (as above)
• Kalgoorlie – Merredin – Cue – Meekatharra – Wiluna
• Perth (Albany to Geraldton, inland to York, Northam, Dalwallinu, Morawa)

Note the similarity between Perth and Kalgoorlie in the South, and that they differ in shape from Port Hedland in the North, but with a similar overall change in temperature.

EAST AUSTRALIA

The following figure shows the moving averages from the following regions in the East of Australia:

• Cairns (Cooktown to Mackay, inland to Palmerville, Charters Towers)
• Brisbane (Rockhampton to Yamba)
• Sydney (Newcastle to Moruya Heads)

Note the similarity of the plot above with the previous one for the West, suggesting a consistent difference in shape between North and South.

CENTRAL-EAST AUSTRALIA

The following figure shows the moving averages from the following regions in Australia:

• Northern Territory (Alice Springs, Tennant Creek, Barrow Creek, plus near neighbours in WA, QLD, NSW, SA).
• QLD South (Thargomindah, Longreach, Emerald, Miles, Goondiwindi)
• NSW North East (Wilcannia, Bourke, Walgett, Goondiwindi, Dubbo)
• SA/NSW border (Woomera, Oodnadatta, Tibooburra, Ivanhoe, Mildura)

Note the temporary dips in temperature around 1950/60 in QLD-SOUTH and NSW-NE, possibly related to relatively high rainfall in those areas in that period.

ADELAIDE and INLAND VICTORIA/NSW BORDER

The following figure shows the moving averages from the following regions in South-East Australia:

• Adelaide (Port Lincoln, Adelaide area, Cape Borda)
• Inland Victoria/NSW border (Swan Hill, Hay, Wagga Wagga, Albury, Boort)

SOUTH-EAST AUSTRALIA and NEW ZEALAND

The following figure shows the moving averages from the following regions in Southern Australia and New Zealand:

• Melbourne (plus Cape Otway, Wilsons Prom, Sale)
• Tasmania (the whole island)
• New Zealand (excluding the far North and South)

Note the strong similarity between Melbourne and Tasmania, both of which have the mid 20th century dips seen in the previous examples further North.

TEMPERATURE DEVIATIONS

The following set of plots show the associated temperature deviations, as 12-month moving averages, from the moving averages shown above. In some cases early peaks, when added to the moving average, give temperatures similar to those of recent years, but note the statistical fact that there is more data to choose from in the early “cool” years, compared with the recent “warm” years.

SAMPLE STATION ANALYSIS

The following plot shows some of the analysis performed for one temperature record, from the town of Boulia in inland Queensland. The QLD-NORTH temperature deviations shown above were subtracted from various versions of the data for Boulia.

The raw data in red maintains close alignment with the regional average, apart from an anomalous warm period around 1914, probably due to a broken screen, and an anomalous cool period starting around 1980, probably caused by the onset of watering of the lawn on which the thermometers were sited, and ending in 1999 when the station moved to the airport. The metadata for these changes can be found in TOROK-1997 and ACORN-SAT documentation, see the DATA page above for links.

Berkeley Earth (BEST) (2013) data in blue for nearby Mount Isa maintains close alignment with the regional average throughout. GHCNMv3-adjusted appears to have been fooled by a transient warming around 1940. That 1940 transient was correctly ignored by ACORN-SATv2, but it appears that the onset of lawn watering around 1980 is badly over-corrected in that version of events, leading to over-cooling of early data by around 1C.

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