Issued

The Bureau of Meteorology maintains an extensive network of environmental monitoring systems across Australia. An important part of the climate monitoring system is the Australian Climate Observation Reference Network; homogenised datasets from this network are used to monitor climate variability and change in Australia and Australian territories.

To monitor long-term temperature and rainfall trends away from mainland Australia, the Bureau maintains homogenised monthly ground-based temperature records—maximum and minimum temperatures—as well as rainfall records at Australian remote island stations. These locations are—the sub-antarctic Macquarie Island, Lord Howe Island in the Tasman Sea, Norfolk Island in the southwest Pacific Ocean, Willis Island in the Coral Sea and Cocos Island in the eastern Indian Ocean. In addition, homogenised monthly temperature series are maintained for three Australian Antarctic Territory stations in east Antarctica—Davis, Mawson and Casey (Figure 1).

These unique sites are representative of large geographical areas and are located in remote, pristine and unique environments. The datasets they provide allow scientists to better analyse long-term trends and changes in the climate of Australia and the surrounding region, and provide important information to fill in key gaps in our knowledge.

Trends in mean temperature and rainfall for the period common for all sites, 1970–2018
Figure 1: Locations of the remote Australian islands and Australian Antarctic Territory stations. Also shown are trends in mean temperature and rainfall for the period common for all sites, 1970–2018. Units are °C/century (°C/c) for temperature and mm/decade (mm/d) for rainfall.

Annual mean surface temperature at Australia's remote island stations is characterised by a small inter-annual variability, with all sites showing a warming trend. The total annual mean temperature increase since 1950 is 0.6 °C at Norfolk, Willis and Macquarie islands (since 1970 it is 0.5 °C, 0.6 °C and 0.1 °C respectively) and 0.8 °C at Lord Howe Island (0.4 °C since 1970). At Cocos Island mean temperature has increased 0.4 °C since 1960 (0.2 °C since 1970). The slightly lower rate of increase at Cocos Island is consistent with changes in the ocean which have seen the tropical western Indian Ocean warm faster than the eastern Indian Ocean (where Cocos Island is located).

While temperatures have risen, rainfall has declined at all subtropical and tropical sites: 23 mm/decade (since 1915) at Norfolk Island; 17 mm/decade (since 1950) at Lord Howe Island and 7 mm/decade for both Willis Island (since 1924) and Cocos Island (since 1916). In percent of the annual mean for the indicated periods, rainfall declined about 18 percent at Norfolk Island, 8 percent at Lord Howe Island, 5 percent at Willis Island and 4 percent at Cocos Island. Over the shorter period of time, since 1970, rainfall declined 36 mm/decade at Norfolk and Willis Islands, 32 mm/decade at Lord Howe Island and 51 mm/decade at Cocos Island.  Norfolk and Lord Howe islands have recently experienced significant drought events which are consistent with the overall decline in rainfall. It is worth noting that much of mainland eastern Australia has also seen rainfall declines since the 1950s, particularly during autumn, with these data suggesting the rainfall declines extend to Australia's east.

Norfolk Island annual rainfall totals (mm) for the period 1915 to 2018
Figure 2: Norfolk Island annual rainfall totals (mm) for the period 1915 to 2018. The green line represents timeseries of the annual rainfall, with the black line being the 5-year running mean. Annual rainfall exhibits strong year-to-year variability with fluctuations of more than 500 mm. Even more noticeable are the large fluctuations between exceptionally wet and dry years.
Norfolk Island decadal seasonal rainfall changes (mm) for the period 1915 to 2018
Figure 3: Norfolk Island decadal seasonal rainfall changes (mm/10 years) for the period 1915 to 2018.

At the sub-antarctic Macquarie Island the opposite has happened—total annual rainfall has increased, 37 mm/decade, amounting to a total rise of more than 250 mm since measurements begun in 1949, or about 26 percent of the annual mean for the whole period of record. The rate of increase in annual rainfall is even higher since 1970, 62 mm/decade. This increase has occurred in all seasons, with the largest increase in winter. Over the same period, rainfall in southeast and southwest Australia has declined. In effect, those weather systems which bring rainfall to southern Australia have contracted poleward over recent decades as the climate has warmed, resulting in more rainfall over the Southern Ocean including Macquarie Island, and reduced rainfall for parts of mainland Australia.

 Increasing annual rainfall and mean temperature for Macquarie Island
Figure 4: Timeseries of annual rainfall (mm) and mean temperature ( °C) for Macquarie Island. In 2018 mean temperature was highest on record (5.6 °C). The rate of increase in annual rainfall is higher since 1970 (62 mm/decade), compared to the whole period of record (37 mm/decade).
 
Decreasing mean temperature at Casey
Figure 5: Annual mean temperatures ( °C) for Casey, Antarctica for the period 1970 to 2018. The red line represents timeseries of the annual mean temperature, with the wider grey line being the 1981–2010 average (-9.02 °C).

Annual mean surface temperature at Australian Antarctic stations is characterised by high inter-annual and inter-decadal variability. Homogenous temperature series for the period 1958 to 2018, show a relatively small increase in mean annual temperature of 0.1 °C at Davis and 0.2 °C at Mawson (0.02 °C and 0.05 °C respectively since 1970), in accordance with rates of warming estimated for East Antarctica. At Casey the mean annual temperature has decreased by 0.7 °C since 1970. The decrease in maximum and minimum temperatures at Casey is of the same magnitude (0.7 °C), and occurred in most months during the year, with the strongest decrease in temperatures observed during June. Casey's temperature record also shows strong decadal variability; since 2010 mean temperature at Casey is persistently below average, resulting in a negative temperature trend at this site. More observations are needed, given the very large variability at this site, to determine whether the trend reflects a long-term pattern of change or variability.

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