Winter 2008 forecast for Europe

Summary: Due to a positive phase of the North Atlantic Oscillation (NAO), which is expected in the winter of 2008, much of Europe (except the Mediterranean region) will likely be warmer than normal. The certainty factor for a mild winter is highest for Scandinavia. Overall, the atmospheric circulation will be unstable, and periods of strong westerly winds will be intertwined with periods of meridional circulation. As a result, cold air outbreaks may be expected, particularly for central and southern Europe.

Fig. 1. The forecast of surface air temperature anomalies in Europe in the winter (DJF) of 2008.  The blue (red) contours and the letter C (W) indicate colder (warmer) than normal temperatures. CL means near normal. The base period for calculating anomalies is 1971-2000. The numbers at the contours are confidence factors of the forecast.

Discussion

The forecast for Europe is largely determined by the positive phase of the NAO, which is expected in the winter of 2008 (see the forecast for the North Atlantic). The correlation map between the NAO index and SAT anomalies (it can be calculated at the ESRL/PSD website) shows that most of Europe tends to be warmer than normal when the NAO index is positive. Negative SAT anomalies can be expected only in the Mediterranean region.

When the NAO index is positive (and particularly when the Icelandic low is shifted eastward), the atmospheric flow over Europe is more zonal, which means an enhanced influence of warm air masses from the North Atlantic. However, when the positive NAO is accompanied by a La Niña event (see the forecast for ENSO), the periods of a zonal flow tend to be intertwined with periods of enhanced meridionality of atmospheric circulation. A phenomenological analysis of large-scale circulation pattern over Europe (“Grosswetter”) by Fraedrich and Muller (1992) has shown a tendency toward meridional type of circulation, with reduced number of cyclonic days during La Niña events. This may translate in colder temperatures in Spain (particularly in late winter) and eastern Mediterranean countries.

The European response to ENSO events is more apparent in Scandinavia, where SAT anomalies tend to be positive during La Niña events. When combined with the effect of the positive NAO phase, it increases the certainty of a mild winter in this region. 

At the same time, the results for central Europe remain quite controversial. On the one hand, the forecast based on solar activity (Rule 50) suggests positive SAT anomalies, which is consistent with the effect of the positive NAO phase and the forecast map produced by the CFS. On the other hand, the extrapolation rules indicate a decrease in SAT from the last winter, possibly to the point below the average value for the past 10 years. The latter rule is correct in 67% cases for the period 1885-2007. Given those opposed influences, the combined CF is only slightly in favor of a mild winter in central Europe.