Simulated Global Climatic Influences on the Record Wet UK Winter of2013-14

The winter of 2013-14 was an extreme one both in Europe and North America. In the UK, for instance, it was by far the wettest winter records dating back to 1910 and was also the wettest winter in the England and Wales precipitation record dating back to 1766. Furthermore, it was exceptionally stormy, with the highst numbers of stations recording gusts of over 60 knots since 1969. In the US, in contrast, winter 2013-14 was one of the coldest on record in parts of the Midwest with several states recording conditions in the coldest 10% since records began in 1895. The immediate cause of these highly anomalous conditions was the regional atmospheric circulation, with intense and persistent negative mean sea level pressure (mslp) anomalies over the North East Atlantic. The pattern of these anomalies resembles the positive phase of the North Atlantic Oscillation (NAO - with negative mslp over the mid- to high- latitude North Atlantic and positive mslp over the Sub-Tropics), with the addition of greater-than-usual troughing over Western Europe.

Here, we will investigate the potential global atmospheric drivers of these highly anomalous conditions. Two putative mechanisms will be examined: firstly, the strong westerly phase of the quasi-biennial oscillation (QBO) that occurred in winter 2013-14, and secondly, an up-stream influence from the tropical West Pacific via the anomalous circulation patterns that caused the intensely cold winter conditions observed over North America. To test these hypotheses we utilise sets of seasonal-length reforecasts of the 14 winters in the period 1996-2009, with 3 members each year initialised on 1st November from ERAI reanalyses. In these experiments, we perform a linear relaxation of atmospheric winds and temperatures in (i) different parts of the stratosphere, (ii) over the Aleutian region, and (iii) over the tropical West Pacific, towards conditions observed in December-February 2013-14. In addition, we perform a control set without nudging to allow us to remove the influence of the November initial conditions.

Analysis from these sets of experiments will be presented to diagnose the effect of the identified remote anomalies on the atmospheric circulation of the North Atlantic-European region, in particular the NAO, with a view to investigating mechanisms for this extreme winter.