Cold winters and the relation to atmospheric blocking

This report discusses the relation between cold temperatures over the Netherlands and large-scale anomalous circulation and temperature patterns over Europe and the North Atlantic. The report has been split in two parts: ‘Cold Winters’ and ‘Cold Spells’. Part I, ‘Cold Winters’ addresses the local and large-scale structure of heating degree days of entire winters, thereby focussing on the climatological aspects (Chapter 3). Part II, ‘Cold Spells’, describes more the ‘weather’-related aspects of shorter cold periods (Chapters 4-6).

Chapter 3 demonstrates the geographical extent of cold European winters and shows that winters that are anomalously cold in the Netherlands are also cold in a large part of Europe. Chapters 4 and 5 investigate the large-scale pressure patterns that go along with cold spells. Two different patterns are found, characterized by anomalously high pressures over the Greenland Iceland region, and the Scandinavia Russia region. Particularly the latter pattern is associated with very high pressures and a large horizontal extent. Both patterns have in common that the prevailing westerly winds are blocked. Chapter 6 discusses the subject of persistence of cold spells. It is more likely to stay cold if it is already cold for some days. The optimum probability is found for events that last already between 10 and 20 days. Under the A1B emission scenario and using climate model output (ESSENCE data), there is a significant drop in the probability for a cold-spell event to last longer for the period 2051-2100.

Since persistent cold spells are associated with atmospheric blocking this observation calls for further investigation of in particular the persistence of atmospheric blocking under future climate scenarios. Note that the statements about the future climate state were inferred from climate model output from a one particular model (ESSENCE) and one particular emission scenario (A1B). Other data sets obtained with different climate models should be examined in order to investigate uncertainties related to model error.

The following few pages give a more detailed summary of the main undertakings (in bold) and conclusions (in bullets) of the two parts (‘Cold Winters’ and ‘Cold Spells’) of the report.