Such is the nature of the UK weather that meteorological records seem to be broken fairly regularly. However, last weekend (5 & 6 December 2015) set some unprecedented new rainfall records in Northwest England, including 341.4 millimetres in 24 hours at Honister Pass, Cumbria, which the Met Office reports as being a new 24-hour UK rainfall record (“Did climate change have an impact on Storm Desmond?”).
This comes hot on the heels of the previous such record set at nearby Seathwaite in November 2009 – already noted as being a 1 in 200 year event or rarer – when there were similarly devastating floods in Keswick and Cockermouth. There were also notable serious floods in Carlisle in January 2005. The recent (2015) event also set a new 48-hour rainfall record of 405 mm at Thirlmere, Cumbria, and other (4-6 December) 48-hour totals included a still very high 178.4 mm at Keswick and 174.8 mm at Blencathra – the latter being of course the long-standing former location of our second-year undergraduate BSc Geography fieldclass.
The extreme rains were accompanied by very high winds of up to 99 mph at Great Dun Fell, and the Department of Geography’s High Bradfield (highly exposed, nearby Peak District) field site is reported by the Met Office as having recorded a gust of 77 mph during the same storm. However, as is often the case with mountain-induced (orographic) enhancement of rainfall, Sheffield missed the worst excesses, with only 4.9 mm recorded by the Department of Geography’s rooftop weather station during 4-6 December. My home weather station in Newark, Nottinghamshire, about 60 km southeast of Sheffield, lies even more in the rain shadow that is the East of England, and on 4-6 December we had just 0.6 mm of rainfall. The Rover Ouse, draining from the Yorkshire Dales, flooded at York (Figure 1).
The meteorological conditions that gave rise to Storm Desmond do not appear, on first sight, to be very unusual for late autumn/early winter. Desmond was a typically vigorous – but not exceptionally so – low-pressure system that raced north-eastwards across the Atlantic, passing just south of Iceland. It dragged up relatively warm subtropical airmasses (temperatures reached 14.0°C at Newark on 6 December and 15.0°C on 7th December, which are very high values for December). This air held relatively large amounts of water vapour due to a basic physical law that says the warmer the airmass, the more water vapour it is able to hold. Finally, the Met Office noted – and US National Oceanographic and Atmospheric Administration (NOAA) charts confirm – that sea-surface temperatures in the Caribbean and off the eastern coast of the USA were unusually high. At the same time SSTs further north in the Atlantic were a couple of degrees lower than normal. This combination of factors might be reasonably expected to increase the energy source for driving the “atmospheric river” or jet stream (Figure 1). Jet-stream activity can be partly gauged using the North Atlantic Oscillation (NAO) index, which was highly positive – indicating strong south-westerly airflow across the UK – on the days in question. The daily NAO values peaked on 5 December when they were high but, again, not exceptional.
Dame Julia Slingo, Met Office Chief Scientist, is on record as saying that “all the evidence…suggests there may be a link between climate change and record-breaking winter rainfall”. Certainly the last decade has seen an increase in the frequency and intensity of certain types of extreme weather over the UK. One idea is that we are liable to see more intense rainfall under similarly unsettled weather patterns as before, due to global warming. Just two years ago, winter 2013/14 was one of the wettest and stormiest on record, and we also saw record-breaking summer rainfall in both 2007 and 2012, with each of these events seeing many serious impacts across the country. Moreover we have also seen quite recently, in 2009/10 and 2010/11, some surprisingly cold and snowy UK winter spells. The increasingly capricious year-to-year UK winter weather conditions have previously been commented on in connection with a recent University of Sheffield research project carried out in collaboration with the Met Office*.
Are the more variable and severe UK winter weather conditions really indicative of human-induced global warming, and will they therefore carry on into the future? Certainly this is a distinct possibility for which we must be prepared. However, since most of the above-mentioned noteworthy weather extremes occurred only within the last decade, and we need a more sustained and collective body of such events to be able to link them more reliably to climate change, considerably greater confidence will come from having 5-10 years’ more of rainfall data (and from ongoing improvements in computer-model simulations of climate change).
Professor Edward Hanna