Summer 2008 in Europe - post-mortem analysis
Temperature
During the summer of 2008, much of Europe, from Scandinavia to the Black Sea, was warmer than average (Fig. 1). Particularly warm was in Ukraine, where mean summer surface air temperature (SAT) anomalies exceeded 2°C. Negative SAT anomalies reaching -1°C were observed in Finland and northern Russia. In the westernmost parts of Europe, from Ireland to Portugal, temperatures were near or slightly below average. The mean summer SAT averaged over Europe was 1.57 standard deviations (std) above the 1971-2000 average. It was the seventh warmest summer on record since 1948.
Although the signs of SAT anomalies over most of Europe coincide with those on the forecast map, there are two major areas where the forecast and observed SAT anomalies differ. First, it was expected that negative SAT anomalies would cover much of eastern Europe, but in fact they were limited to northern Russia. Second, negative SAT anomalies in the westernmost part of Europe, although small, were not predicted correctly.
Rerunning the model (this time it included both forecasting and diagnostic rules) shed some light on colder than expected summer in the westernmost part of Europe. The cooling in this can partly be explained by a strongly negative phase of the North Atlantic Oscillation (NAO). This summer the NAO index dropped to -2.21, which was the second lowest value on record since 1950 (Fig. 2). Although the influence of the North Atlantic on Europe appears to be much weaker in summer than in winter, negative NAO indices tend to be associated with colder summer conditions in the British Isles.
In addition, the East Atlantic index was positive, which promoted colder temperatures in the British Isles and warmer temperatures in the Balkans (rule 187). Rule 267 links temperature and precipitation fields over Europe. It shows that this summer both fields closely resemble the corresponding fields in the summer of 1994. During that summer the temperature field also featured negative SAT anomalies in Ireland and European Russia and positive anomalies in central Europe.
It is also worth mentioning rule 490 that worked well this summer. It describes a teleconnection between the Indian Ocean Dipole (IOD) and European temperature anomalies based on a recent work of Asok (2007). The rule correctly suggested warmer than normal temperatures in central Europe and the Mediterranean.
Precipitation
The distribution of precipitation anomalies (percent of normal) in the summer of 2008 looks very patchy (Fig. 3). Still, predominantly dryer than normal conditions were observed in Scandinavia and the Mediterranean, and wetter than normal conditions in the British Isles and parts of central and eastern Europe.
The forecast correctly predicted dryer than normal conditions in the Mediterranean. It also suggested generally wetter than normal conditions in eastern Europe. However, the severe flooding in parts of central and eastern Europe was not predicted. In late July 2008, heavy rains and storms in the Carpathian Mountains led to flooding in parts of Ukraine, Moldova, Romania, Slovakia and Hungary. Areas of Ukraine, Moldova and Romania located near the rivers of Prut and Dnestr were reported to be the most seriously affected. A total of over 35,000 people have been evacuated across the three countries, and 36 people are reported killed. It was the worst flooding in decades in this part of Europe.
In the British Isles, it was the second summer in a row with heavy rains. In both cases the NAO index was sharply negative, exceeding one standard deviation (Fig. 2). When the NAO index is negative, the jet stream tends to be shifted south, directing storms toward the British Isles. At the sea level, it is seen as a band of anomalously low pressure, with a center over the British Isles (Figs. 4 and 5). Fig. 2 shows that the NAO index exhibits a negative trend in recent decades. If the trend continues, the British Isles may see more wet summers.