3. Precipitation Gradient Across the UK
M Hulme

(Top) Scotland winter precipitation (dated by January).
(Middle) SE England summer precipitation
(Bottom) The ratio of Scotland winter to SE England summer precipitation

Specific indicators
Any shift towards drier summers and wetter winters will be most easily seen in this indicator, which contrasts winter precipitation in Scotland with summer precipitation in SE England and gives the ratio of Scotland winter to SE England summer precipitation.
[Source: Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ]

Relevance
The England and Wales Precipitation series described in the previous Indicator consists of five regions of roughly equal area: SE England, CE England, SW England, NE England and NW England. Within each region, seven well-distributed gauges contribute to the regional series. These regional series exist back to 1873. A similar regional series has been compiled for Scotland, a series that extends back to 1757. The ratio of the winter precipitation for Scotland and the summer precipitation for SE England creates an index measuring the strength of the regional and seasonal gradient in precipitation across the UK. Scotland is wetter in winter than SE England in summer by a factor of about three over the 1961-90 period. Trends in the ratio of these regional precipitation series reveal whether Scotland has been getting wetter in winter and the southeast of England drier in summer.

Sensitivity to climatic and other factors
This index of the precipitation gradient across the UK reflects the relative contribution of different precipitation-bearing synoptic systems to total precipitation. It is therefore related to circulation changes affecting UK climate. This index has also been selected because climate modelling studies suggest that over the UK global warming might induce stronger precipitation gradients from NW to SE. The ratio of Scotland's winter and SE England's summer precipitation is therefore a potential anthropogenic climate change detection variable for UK climate.

Change over time
The ratio of winter precipitation in Scotland to summer precipitation in SE England has increased in recent decades. Whereas in the first 20 years of record the ratio only exceeded 3.0 on five occasions, in the last 20 years of record ten values were in excess of 3.0. Unusually high ratios occurred in 1921, 1949 and then in a series of years in the 1970s and 1980s. In 1995 the highest ratio, close to 12.0, was recorded, Scotland having a very wet 1994/95 winter and SE England a very dry 1995 summer. This coincided with the highest value of the North Atlantic Oscillation index (see next Indicator) recorded this century.

The last four years have recorded ratios close to the long-term average of 2.9, with the exception of 2000 which recorded a ratio of 5.0 due mainly to above average precipitation in the Scottish winter. Climate modelling studies suggest that global warming might induce precipitation increases in northern UK, especially in winter, and decreases in the south, especially in summer. If so, then we would expect to see higher values of this index in the future. Regional precipitation exhibits large natural variations from year-to-year and from decade-to-decade, however, and identifying such contrasting trends in regional precipitation may require many more years of data.