Why Was It so Wet in Winter 2013–14 over North West Europe?

A central aim for predictions on seasonal and sub-seasonal timescales is to highlight the risk of extreme events and seasons. Such a season occurred in the winter of 2013-2014, which brought exceptionally cold conditions to many parts of North America and record precipitation and storminess to parts of Western Europe including the UK. Although predictability of winter climate is now being realised in seasonal prediction systems, indications in advance of this winter suggested only a modest increase in risk of extreme rainfall. It is a high priority, therefore, to understand the global climate dynamical influences, including climate change, that gave rise to the record precipitation that was actually observed in this case. In this way, it may be possible to identify necessary improvements to prediction systems, or at least better understand their limitations. Here, we attempt to analyse the causes of the very wet winter in Western Europe using observations and model experiments. We investigate the possible influence of forcing of the extratropical atmospheric circulation from different parts of the tropics, including the Tropical West Pacific and Tropical Atlantic. We further show that conditions in the stratosphere, including the strong westerly phase of the Quasi-Biennial Oscillation, were likely to have contributed to the observed circulation patterns. Finally we present observational evidence which indicates that the warming climate likely increased the winter's rainfall totals. Daily England and Wales winter rainfall from 1900 to present is partitioned using an analysis of atmospheric circulation types derived from North Atlantic-European mean sea level pressure data. While the frequency of occurrence of different circulation types can explain much of the year-to-year variability of seasonal rainfall totals, there is also a long-term trend towards wetter winters that is not explained by changes in circulation.