European Drought in November 2011
This Climate Indicator Bulletin describes Autumn 2011, a very dry period in central Europe. The observations presented put this event in its historical context.
Last update: 19 March 2014.
Autumn 2011 was observed to be dry over most of Europe, with the most severe drought in November 2011 when several European countries reported the lowest precipitation amount ever recorded for November. During this period a high pressure system persisted over central Europe suppressing the formation of clouds and precipitation.
Several news papers and organisations reported about the impacts of the droughts in autumn 2011. A few of them are mentioned here:
The Independent Newspaper, UK, reported "During October and November 2011, the lack of rain over much of Europe triggered the worst drought in decades for this time of year, dropping river levels to record lows and creating concern in parts of central and eastern Europe. Power supply reservoirs were running low in Serbia, drinking water shortages hit Bosnia, and the 2012 crop production is in jeopardy in Romania, Bulgaria, Hungary and the Ukraine (farmers say that the Ukraine, the major Black Sea grain producer, could lose up to 30 percent of its winter grain). Germany, The Netherlands, Slovakia and the Czech Republic experienced the driest November since records began."
According to the Washington Post, in November 2011 the "waters of the Danube river were so low, that dozens of cargo ships were stuck, stranded in ghostly fog or wedged into sand banks on what is normally one of eastern Europe's busiest transport routes". Also the water levels of the river Rhine were well below average making it difficult for ships and barges to move. The fall in water level of the river Rhine also uncovered a large unexploded World War II bomb near Koblenz and 45,000 residents had to be evacuated whilst the bomb was defused, which caused huge disruption right across the city of Koblenz.
Fire Danger Forecast, 29 November 2011: "The extreme dry weather in Central and Eastern Europe not only affected river levels and agriculture, but also triggered unusual (forest) fires in several countries. On 20 November 2011, a forest fire started in Upper Bavaria on the south side of the Schwarzberg mountain, near Lenggries. On 28 November 2011, several fires broke out in the Ukraine, Moldova and Slovakia, when a cold front with strong winds and no precipitation particularly enhanced the risk of fire. Any existing fire (which might originally have been man-made) quickly fanned out by gusty winds and became uncontrolled."
Figure 1 shows the E-OBS precipitation anomaly in percentage of the normal period (1961-1990) for autumn and November 2011. Although September and October experienced below average precipitation for the central zonal part of Europe, the drought was most pronounced in November. Large areas of Europe have a precipitation anomaly of -100%, which means no rain at all. This is also seen in Figure 2 where the consecutive number of dry days (CDD) for stations throughout Europe in the November 2011 is shown as well as the number averaged over the normal period (1961-1990). Several stations reported 30 consecutive dry days in 2011, which means that all days were dry at that location. This very low amount of precipitation resulted in relatively low amounts of soil moisture in November 2011 as observed by satellites (Figure 3).
Figure 1: Precipitation anomaly in percentage of the 1961-1990 climatology. The Danube catchment is included for illustrative purposes. Left: Autumn 2011, right: November 2011.
Figure 2: Consecutive number of dry days for stations throughout central Europe. Left: November climatology 1961-1990, right: November 2011.
Figure 3: ESA soil moisture map for November 2011 in m3/m3. Note that the areas in the Netherlands, northern Germany and Denmark with high soil moisture content are areas where groundwater levels are regulated. The satellite registers the soil moisture content of the upper ~2 cm of the soils. With a value of 0.10 m3/m3 soil moisture, this corresponds to a storage of approximately 100 mm water in 1 m3 soil or 1 mm in the top cm.
Autumn and especially November 2011 were very dry compared to the normal period. From ECA&D, series were selected for those stations that have precipitation from 1920 or earlier until at least 30 November 2011 to show how exceptional the situation of autumn 2011 actually was. The 200 stations are located in the area with the most pronounced drought around central Europe (44 - 55°N and -12 - 30°E). For each season the precipitation anomaly w.r.t. the normal period (1961-1990) was determined. In Figure 4, drought is shown in red (negative anomaly) and wet seasons are shown in blue (positive anomaly). The top 10 driest years are given in Table 1. For September and October, the year 2011 was not in the top 10 of driest years. From this table, it is seen that November 2011 was the driest November since 1920 and autumn 2011 was the sixth driest autumn since 1920.
Figure 4: Precipation anomaly per year w.r.t. the 1961-1990 climatology averaged over 200 stations within 44 - 55°N and -12 - 30°E. Left: autumn, right: November.
Table 1: Ranking of driest autumns and Novembers together with the precipitation anomaly w.r.t 1961-1990 averaged over 200 stations in Europe [mm].
An alternative way of assessing the drought of November 2011 is by using a standard drought metric. The Standardized Precipitation Index (SPI) is one of the metrics often used and is based on precipitation standardized to local conditions. The SPI is the drought metric endorsed by the World Meteorological Organization. This metric is uniquely related to probability with positive values indicating anomalously wet conditions and negative values indicating anomalously dry conditions. Using the notion that the most extreme droughts (or pluvials) are also the rarest, drought intensity can be related to SPI values. SPI values between -1.5 and -2 are usually related to "severe" drought; values lower or equal than -2 relate to "extreme" drought (Table 2).
By accumulating precipitation over one or more months, droughts can be assessed over various time scales, which is relevant because the most persistent droughts are often the most devastating ones. Figure 5 shows the SPI-1, with precipitation accumulated over a 1-month period, for November 2011. It shows a large area in Europe under extreme drought, with its emphasis in an arc stretching from northeastern Germany to Belarus.
Figure 5: SPI with 1-month accumulation time scale for November 2011.
|SPI values||drought category|
|0 to -0.99||mild drought|
|-1.0 to -1.49||moderate drought|
|-1.5 to -1.99||severe drought|
|<= -2.00||extreme drought|
Table 2: Relation between SPI values and drought category as proposed by McKee et al. (1993).
How much of Europe was affected by this November 2011 drought? This question can be addressed using the SPI. The percentage area of Europe under severely dry conditions (or worse) using monthly values of SPI-1 was for the November 2011 drought 28.1%. Here we define Europe as the land area between between 35°-75°N and 25°W-45°E. The percentage land area under severe drought is calculated for all months since January 1950 and these values are presented in the probability density plots of Figure 6, with horizontally the percentage land area under severely dry conditions and vertically the corresponding fraction of months. The top left figure shows the probability density plot using SPI-1 and November 2011, indicated by the vertical red line, stands out. November 2011 ranks fifth driest month since January 1950.
However, the impact of this dry month on droughts characterized by longer time scales is small. The top right of Figure 6 shows a similar probability density plot, but now based on 3-month accumulated precipitation, the SPI-3. To calculate this index, precipitation accumulated over the autumn (Sept. Oct. Nov.) is used to calculate the SPI value for November 2011. This month now ranks at place 16 and November 2011 gets even more common when precipitation over a 6-month period (accumulating the preceding summer and autumn precipitation) is used in the SPI-6 (bottom left).
These figures show that November 2011 was an extreme event, but its impact in terms of long-term drought was low.
Figure 6: Probability density plots for the area fraction of Europe under severe drought (or drought), using the SPI-1 (top left), the SPI-3 (top right) and the SPI-6 (bottom left). The vertical lines show the November 2011 values.
The sea level pressure in Europe is shown in Figure 7 for autumn 2011 as well as for November. It is clearly seen that in November the area was dominated by a blocking high pressure system which already developed in October 2011. A high pressure system over central Europe steered away the depressions, and the associated precipitation. The blocking high pressure system that presisted during large parts of autumn 2011 let to subsidence and a high positive sunshine duration anomaly in November (Figure 8) has therefore let to the dry conditions that have been observed.
Figure 7: Sea level pressure in ERA interim. Left: autumn 2011, Right: November 2011.
Figure 8: Sunshine duration anomaly in November 2011 w.r.t. the 1961-1990 climatology. Note that the network of stations providing sunshine duration to ECA&D does not fully cover Europe.
The trends in three indices of extremes are determined to analyse if the observed conditions of autumn 2011 are in line with the long-term perspective. The indices are total precipitation amount, the number of consecutive dry days and the maximum 5-day precipitation amount (RX5Day, Klein Tank et al. 2009). The trends are determined for the whole of autumn and for the month November 2011 only, and the results are shown in Figure 9. Only stations for which the trend is significant at the 25% level are coloured, other stations are not shown. It is seen that the total precipitation shows an overall increasing trend for autumn precipitation, while large parts of Europe show a decrease in November precipitation. The number of consecutive dry days is decreasing in autumn while a general increase, especially in eastern Europe, is observed in November. The maximum 5-day precipitation amount is increasing in autumn while a much more noisy pattern is evident for November. All these results indicate a trend towards wetter conditions for autumn. In November, eastern Europe shows a drying trend while western Europe is observed to get wetter. The extensive drought of November 2011 and the dry conditions during the whole autumn season, are not consistent with the long-term trends as observed in a large number of stations.
Figure 9: Trend over the period 1951-2012 when the trend is significant at 25% or higher. Stations for which the trend is not significant at 25% are not shown. Top left: Precipitation in autumn, top right: precipitation in November, middle left: consecutive dry days in autumn, middle right: consecutive dry days in November, bottom left: Maximum 5-day precipitation amount in autumn, bottom right: Maximum 5-day precipitation amount in November. Note that the erratic nature of precipitation adds to the noisy character of the trends in precipitation amount and maximum 5-day precipitation amount.
The 20-year return values for consecutive dry days in November determined over the period 1991-2010 is shown in Figure 10. It is seen that the observed number of 30 consecutive dry days in November 2011 is even more rare than the number which is, on average, observed about once per 20 years.
Figure 10: 20-year return values of consecutive dry days in November determined over the period 1991-2010.
McKee, T.B., N.J. Doesken and J. Kleist, (1993) The relationship of drought frequency and duration to time scales. In: Proc. 8th Conf. Applied Climatology, January 17-22, American Meteorological Society, Boston, Massachusetts, 179-184.
Klein Tank, A.M.G., F. Zwiers and X. Zhang (2009) Guidelines on Analysis of extremes in a changing climate in support of informed decisions for adaptation. WMO/TD-1500, Climate Data and Monitoring WCDMP-No. 72. World Meteorological Organization, Geneva, Switzerland.
Authors: Else van den Besselaar, Gerard van der Schrier, Gé Verver, Albert Klein Tank, Phil Jones, Roxana Bojariu.
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The development of this Climate Indicator Bulletin is part of the EURO4M, an EU-FP7 project. The research leading to these results has received funding from the European Union, Seventh Framework Programme (FP7/2007-2013) under grant agreement nÂ° 242093.
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