Patterns of climate change across Scotland: technical report - Sniffer

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SNIFFER Project CC03: Patterns of Climate Change across Scotland April 2006 Figure 22 - Gridded change for a) winter half-year cold-wave duration (days) and b) summer half-year heat-wave duration (days), based on a linear trend from 1961 to 2003. 2.3. Rainfall As with the temperature records presented in section 2.2 there is a long record of rainfall observations for Scotland. Where longer-term data records are available, the changes are presented alongside those for the 1961 to 2004 period. This is the case in Table 8. Here trends in total precipitation amount (i.e. rain and snow) over each period are expressed as a percentage change since the start of each period. What is most striking is the statistically significant increase in winter precipitation over the 1961 to 2004 period. In each region, and nationally, the winter change is statistically significant at the 1% level, based on the Mann Kendall test. In this analysis, over this specific period, an increase of almost seventy percent in winter precipitation since 1961 has been identified in North Scotland. This is equivalent to an average increase of approximately three millimetres a day throughout the winter season (December to February). Annually averaged precipitation has also increased significantly over the same period. As a whole, Scotland has become twenty percent wetter during the period 1961 to 2004, equivalent to an average increase of approximately 240 millimetres of rainfall a year. Conversely, there has been little or no change in regionally averaged summer rainfall totals, although a slight decrease (seven percent) can be identified in the northern region. Over the same period summer rainfall has increased by a similar amount in West Scotland. Changes in summer rainfall are not however significant for the 1961 to 2004 period. 24
SNIFFER Project CC03: Patterns of Climate Change across Scotland April 2006 Table 8 - Changes in total precipitation amount (percentage), 1961 to 2004 and 1914 to 2004. Statistically significant trends are shown in bold (significant at the 1% level) or italic (significant at the 5% level) type. 1961-2004 1914-2004 North East West North East West Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Spring 16.2 9.4 17.3 14.8 13.9 6.1 22.0 14.3 Summer -7.0 0.2 7.3 -0.6 -12.7 -18.9 -7.5 -12.7 Autumn 5.3 22.2 5.9 9.1 13.6 0.7 15.6 11.1 Winter 68.9 36.5 61.3 58.3 20.9 -0.8 9.0 11.6 Annual 21.0 18.4 23.3 21.1 9.6 -3.5 9.5 6.2 When the time-series is extended to consider the full period 1914 to 2004 the trend is less well established. Over the longer period the statistically significant trends identified are significant are at the five, rather than one, percent level. However, there is a statistically significant reduction in summer rainfall in all regions. East Scotland has experienced a significant nineteen percent decrease in summer rainfall since 1914. Precipitation has increased in North and West Scotland in all other seasons although the only trend that is statistically significant is the more than twenty percent increase in West Scotland in spring. Annual mean precipitation has increased across most of Scotland since 1914, and, with the exception of the summer season, rainfall has increased in all regions. The exception to this is the slight reduction of both annual mean and winter precipitation in East Scotland. This is the opposite of the trend calculated over more recent decades. These changes are not however significant, with the exception of the summer drying. Therefore, it is not possible to say definitively at this time whether there is a trend of long term drying in East Scotland and wetting over the rest of Scotland. The apparent contradiction between the two periods of data can be explained by looking at a time-series of annual mean total precipitation since 1914 (Figure 23). The full sequence of data from 1914 is shown. It can be seen that during the 1960s and 1970s the inter-annual variation in rainfall (particularly in the North and West regions) is quite low and the totals are also lower than in the earlier record. This is then followed by an increasingly wet period. This is why the analysis over the shorter period produces such large percentage changes in annual, and possibly winter, averages. The wet period in the 1980s may also account for why the drying trend in East Scotland over the 1914 to 2004 period is not seen in the shorter time period from 1961. It is apparent from the time-series data that East Scotland has a much drier climate than the other two regions. The high correlation between the North and West Scotland time-series indicate the two regions are likely to have a very similar rainfall climatology. This can be seen in Figure 24 which shows the 1961 to 1990 rainfall annual rainfall climatology for Scotland 12 . North and West Scotland have a wetter climate than eastern regions. This is to be expected given the presence of the Highlands, which provide a rain shadow effect for eastern areas, due to the prevailing westerly winds over Scotland. 12 The 1971 to 2000 climatology is very similar and can be viewed on the Met Office website (http://www.metoffice.gov.uk/climate/uk/averages/index.html). 25
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Patterns of climate change across Scotland: technical report - Sniffer

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