References
Introduction
The United Kingdom floods of October and November 2000 occurred during the wettest autumn in England & Wales since records began in 1766, with a total of 503mm of rain, exceeding the previous maximum by almost 50mm (Marsh and Dale, 2002), and resulting in almost double the average (1961-1990) seasonal precipitation (Alexander and Jones, 2001).
Just under 10,000 properties, 58% of which had no flood defences, were flooded at over 700 locations and there was widespread disruption to road and rail services. Train services were cancelled, major motorways closed, power supplies disrupted and 11,000 people were requested to evacuate their homes. The total costs are of the order £ 1.0 bn (EA, 2001). Figure 1 illustrates the nationwide scale of the events, in terms of properties flooded. The most heavily affected areas were the South East, North and South Wales and Yorkshire, and Figure 2 shows these regions received well above the climatalogical (1961-90) September-December rainfall average, as did most others.
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| Fig 1. Flood events by number of properties affected for the Autumn 2000 floods in England and Wales.Taken from EA (2001) |
Fig 2. September-December 2000 regional rainfall (in mm), as a percentage of the 1961-1900 average.Taken from Marsh and Dale (2002) |
The national picture was of persistent, widespread heavy rain, rather than very strong localized events, although these did also occur. Synoptic data for the period showed that the October-November period was dominated by persistent and repeated frontal depressions, with broad bands of rain enclosing notable high-intensity rain cells across the British Isles. The resultant flooding was hydrologically complex with large local and regional variations in its severity and was the cumulative effect of the series of exceptional rainfalls affecting different parts of the country at different times and in many cases more than once. Some areas flooded two or three times in the autumn and some even five times during 2000 (DEFRA, 2001).
Antecedent conditions and main flood events
Rainfall is most hydrologically effective over the November-April period when evaporation losses are very modest and soils generally moist. The rainfall for this period in the three preceding years had been above the 1961-1990 average (and largely explains the transformation in water resources since the drought conditions in the mid-1990s), but the 1999-2000 recovery was by far the greatest and most rapid. In 2000, particularly unsettled weather patterns in April and May ensured that soils did not begin to dry out until the later spring, prolonged the aquifer recharge season, and meant that flows in rivers draining the most permeable catchments remained healthy throughout the summer, even withstanding the below average summer rainfall (CEH, 2001). Then during the Autumn, the main periods of prolonged rainfall were the 29-30th October and 2nd, 5th, 6th November for Wales, South West, Central and Northern England; 9th-12th, 15th-18th October, 5th and 6th November for Southern England (EA, 2001). There were distinct phases to the flooding associated with these periods, summarized here from Marsh and Dale (2002):
The second half of September and early October saw localized and short-lived events with heavy frontal rainfall resulting in early spate conditions in Northern Ireland and western Scotland, and urban flooding in parts of southern England in response to intense convective rainfall. This heralded severe flooding in the South East in the second week of October associated with thunderstorms forming along a near-stationary front, causing flooding in some impermeable South East catchments. Storms around the 19th October substantially increased the number of saturated or near-saturated catchments and extended the area which was at high risk of flooding. River flows significantly increased at the end of the month following widespread and persistent rainfall, and a low-pressure system produced rainfall totals of 20-40 mm across most of southern Britain. This was followed by the passage of an extremely active system which tracked north-east from the Bristol Channel to the North sea. Flood plain inundations were especially extensive in Yorkshire, and by the second week of November, severe flood warnings (issued by the Environment Agency) covered rivers across most of England.
Associated meteorology
Blackburn and Hoskins (2001) find that the Autumn 2000 season was characterized on average by a displacement of the Atlantic Jet-stream eastward from its climatological position, with the region in which the air exits from the jet being more marked than usual over western Europe and displaced south of the UK from its normal elongated position near 55oN (Figure 3). This brought intense systems into the area, where they slowed, repeatedly leading to prolonged precipitation events.
Fig 3. See original caption for details.Taken from Blackburn and Hoskins (2001)
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Furthermore, it was argued that the anomalous jet-stream was associated with a train of anomalies in geopotential height extending from the subtropical Atlantic across Eurasia, with a cyclone over the UK and strong anticyclone over Scandinavia (Figure 4, left). The wave-like pattern was indicative of quasi-stationary Rossby waves, and the persistence and coincidence of this pattern with the mid-September to mid-December wet period in the UK were notable. Moreover, a correlation of autumn England & Wales precipitation with global geopotential height, for the preceding 42 years produced a remarkably similar pattern to the anomalous pattern in Autumn 2000 (Figure 4, right), and very similar to the Scandinavia or Eurasia-1 pattern. Correlations in excess of 0.4, significant at the 99% level, were found in the four peaks of the pattern over Eurasia.
Fig 4. See original caption for details.Taken from Blackburn and Hoskins (2001)
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Blackburn and Hoskins (2001) concluded that wet UK Autumns have not only been associated (unsurprisingly) with low pressure in situ, but also with the above pattern of anomalies stretching over a wide area. In proposing a tropical catalyst for this pattern, in the form of anomalous deep convection overlying tropical sea surface temperature (SST) anomalies, they then identify idealized modelling studies showing upper-troposphere horizontal divergence above a region of converging tropical convection driving an eastward propagating wave-packet-like response along great circles. With their own idealized global barotropic model, with a suitable forcing to represent the observed convective anomalies in Autumn 2000, the main result is the generation of a Rossby wave-train propagating into the extra-tropics remarkably similar to those in Figure 4.
References
- Alexander, L. and Jones, P. (2001). Updated precipitation series for the U.K. and discussion of recent extremes. Atmos. Sci. Lett., 1, 142\u2013150.
- Blackburn, M. and Hoskins, B. (2001). Atmospheric variability and extreme Autumn rainfall in the UK. Available from http://www.met.rdg.ac.uk/ mike/autumn2000.html.
- CEH (2001). Hydrological review of 2000. Technical report, Centre for Ecology and Hydrology, Wallingford/British Geological Survey.
- DEFRA (2001). To what degree can the October/November flood events be attributed to climate change? Technical Report FD2304 (Final Report), Centre for Ecology and Hydrology, Wallingford/UK Met Office report to DEFRA.
- EA (2001). Lessons Learned: Autumn 2000 Floods. Technical report, Environment Agency.
- Marsh, T. and Dale, M. (2002). The UK floods of 2000-2001: A hydrometeorological appraisal. J. Chartered Inst. of Water and Env. Man., 16, 180\u2013188.
