|
Annual and seasonal mean of daily minimum and maximum temperatures. Annual mean of diurnal temperature range. Annual and seasonal number of relatively warm and cool days and nights. Percentage of the State with annual average temperatures above decile nine or below decile one. Because of the potential for global warming due to the enhanced greenhouse effect, there is much interest in reporting the annual surface temperature across Australia. However, it is likely that the maximum and minimum temperatures will not necessarily change at the same rate as the average. Therefore, it is appropriate to report on both the maximum and over-night minimum temperatures. Because of the impact of frost on many crops, the interannual variability of the minimum temperature is of great interest to agriculture.
For Tasmania, the paucity of stations with good quality long-term records makes it difficult to generalise across the whole State. An approximate indication of the trends can be obtained using the best of the available data. The annual maximum and minimum temperature for Tasmania are shown in the graph below.
The use of such general values, however, often masks regional and seasonal variation of which there is a lot in Tasmania. Information from individual sites that have had their data records quality controlled are the best for determining trends in Tasmania. The following description of minimum and maximum temperatures is drawn from the detailed graphs available for downloading in the BoM technical report (Barnes-Keoghan 2002) that provides temperature and rainfall information from around Tasmania. One of the main features of the various minimum temperature graphs is the very large amount of missing data. The selection process requires that 80% of a month's daily observations are present, and then further requires that all months in a year (or season) are available. At many locations these criteria are not often met. The most "complete" sets are those from Launceston Airport, Hobart and Grove as shown in the graphs below.
The annual series from Launceston shows a fairly strong rising trend, although the last 10 years appear to be trending down again. Another interpretation is a sharp increase around 1965-70, separating an earlier relatively cool period from a later relatively warm one. Hobart also shows a general rising trend, although a decrease since around 1990 is also apparent in the smoothed series. Hobart's winter series exhibits little, if any, long-term trend, whereas the summer and autumn sets suggest a change around 1965-70. It is possible, however, that this is an artifact of the data processing. The Grove data are quite different again, with most seasons showing a peak around 1975-85, surrounded by relatively cooler periods. The other stations, which suffer badly from missing data, generally show little clear trend. The variability on annual, or even decadal timescales is generally greater than any long-term trend that could be inferred. The series of relatively cool and relatively warm nights are more or less in agreement with the mean values, suggesting that the change in mean has been due to a general shift in the distribution rather than an alteration at the extremes. A major exception to this is relatively warm nights at Hobart, which in the period 1970 to 1990 did not increase anywhere near as much as relatively cool ones declined. This indicates the rise in the mean over the period was more due to a decrease in cool nights than an increase in warm ones, which is consistent with an increasing "urban heat island" around the observing site. Much of the above description for the minimum temperatures also holds for the maxima, with problems of missing observations chief among them. Most sites with reasonable long-term records exhibit a slight rising trend over the period shown. The seasonal series are hard to interpret, at least in part because the variability is so much greater in summer than in winter. Stations at Launceston Airport, Hobart and Grove again provide the best available long term data sets with the maximum temperatures displayed below. Data from Low Head station is also considered.
There are subtle differences across the State. For example, Launceston Airport and Low Head both had their warmest autumn in 1988, but at Hobart and Grove this was exceeded in 1990. Such a difference could possibly be explained by a change in atmospheric circulation patterns. The frequency of relatively cool days is not always a mirror of the frequency of relatively warm days. One explanation of this is that warm days can be a consequence of northerly winds ahead of a cold front, but after the passage of such a front there can be a cool day. An increase in the frequency and strength of cold fronts could see a rise in both warm and cool days. For example, there is a sharp peak in the number of relatively cool days in the mid- to late-1990s, with the trough in the number of relatively warm days not being so pronounced. This may indicate a change in the frequency or strength of cold fronts. Based on National core indicator A3 (Manton and Jasper 1998) |
