The meteorology of the exceptional winter of 2015/2016 across the UK and Ireland

The meteorological winter of 2015/2016 will be remembered as another exceptional winter across the UK and Ireland, with numerous climate records broken and high impact weather events causing considerable disruption from flooding and high winds. A succession of winter storms tracked across the region, bringing persistent and in places record-breaking rainfall, including the highest 24 and 48 h rainfall accumulations on record, from storm Desmond on 4–6 December. Persistent rain, particularly through the first half of the winter, resulted in new records for both monthly and seasonal rainfall accumulations widely across Ireland, Scotland, Wales and northern England. Temperatures were also exceptionally high through much of December and in late January. In this paper we document the main meteorological and climate features that defined this exceptional winter season, and consider its wider historical context. The rain in early winter fell onto already saturated ground following a notably wet November, so we extend our analysis to include some consideration of November. Barker et al. (2016, this issue) discuss the hydrological situation. In September 2015, the Met Office and Met Éireann announced a pilot project inviting the public to provide names for wind storms that were forecast to potentially cause substantial impacts to the UK or Ireland. The purpose of this was to communicate in unison the approach of severe weather through media partners and other government agencies to the public, and to help raise awareness of severe weather and improve public safety. It is reasonable to expect several named storms in a typical winter, and the naming of storms does not imply that any individual storm was climatologically exceptional. It should also be noted that the storm naming was based on the forecast of the likelihood of disruptive impact from high winds, for which the wind thresholds vary with place and time of day. For example, wind gusts occurring during the rush hour of major cities are more likely to cause significant impacts than the same strength winds across sparsely inhabited areas. It should be noted that storm naming during winter 2015/16 was not based on flood impacts, and some of the significant flood impacts discussed were caused by unnamed systems. We begin with a summary of the winter overall, then consider the constituent months and briefly describe all named storms (Abigail to Imogen) that affected the UK and Ireland over the period November 2015 to February 2016. Of the final eleven named storms, six occurred during meteorological winter 2015/2016 (the months December to February), that is from Storm Desmond to Storm Imogen. We will conclude with brief consideration of the large-scale circulation and potential climatic drivers. Figure 1(a) shows that the meteorological winter season spanning December to February was the wettest on record for the Island of Ireland in a rainfall series from 1850, at 189% of normal (602 mm), followed by 2013/2014 at 173% (550 mm). It was the second wettest for the UK at 159% of normal (519 mm), behind 2013/2014, which saw 167% (545 mm; Kendon and McCarthy, 2015), and for both Ireland and the UK the winters of 2013/2014 and 2015/2016 are the only winters in the series that have recorded winter rainfall totals in excess of 500 mm. It was the eighth wettest in the England and Wales Precipitation Series that extends back to 1766 (Figure 1 restricted to 1851 for clarity). It was also the wettest winter on record for each of Scotland, Wales, Northern Ireland and northern England in series from 1910 (not shown). Southern and central England were wetter than average, primarily due to rainfall during January and February, but at 127% of average it was not an extreme season here. Rainfall in excess of twice normal was observed across southern Ireland, north Wales, north England and southern and eastern Scotland (Figure 2). Across Ireland, mean air temperatures were above their Long-Term Average (LTA), with half of Irish stations reporting anomalies over 1 degC from their seasonal means. A time series of five long-term stations in Ireland, shown at the top of Figure 1(b), recorded an average winter temperature of 7.0 °C, a value which has been exceeded only five times since 1901. It was the third mildest winter overall for the UK at 5.5 °C, behind only 2007 (5.6 °C) and 1989 (5.8 °C). Temperatures were widely in excess of 2 degC above the 1981–2010 climatology across central and southern England and Wales, shown in Figure 2. In the long running Central England Temperature (CET) series from 1659, it was nominally the second warmest winter at 6.7 °C, behind 1868/1869 (6.8 °C); however, it should be noted that the uncertainty on monthly mean CET is estimated at ±0.2 degC (Parker and Horton, 2005), making these 2 years comparably warm given the uncertainty. For clarity the middle panel of Figure 1 only presents the CET from 1851 onwards. There are only 13 winters in the 357-year CET series that have mean winter temperatures that exceed 6.0 °C; seven of these winters have occurred since 1975 and five have occurred since 1990. As mentioned above the storm naming initiative provides a focus on wind storms over the winter, but we do not have climate records equivalent to named storms. Therefore, in order to evaluate the historical context for the storminess of the winter season, we have used the gale index of the Jenkinson–Collison indices derived from mean sea level pressure patterns from reanalyses centred over the UK and Ireland (Jones et al., 2013). From these data a gale index has been derived based on the flow and shear vorticity, as described in Jones et al. (1993). Jenkinson and Collison (1977) defined a gale day over the UK as one where this gale index exceeded 30, and this has been extended by the Climatic Research Unit (CRU, 2016), with thresholds of 40 and 50 defined as days of severe gale and very severe gale respectively. Figure 1(c) shows the count of gale days for each standard winter season (DJF) since 1871/1872. With 42 gale days and 15 severe gale days, 2015/2016 ranks as the eighth highest for gale days and joint third for severe gales. Ten of the severe gale days, and the only very severe gale day (1 February 2016) were associated with named storms. 1989/1990 and 2013/2014 stand out as the stormiest winters. We can say that overall the winter 2015/2016 was notably stormy, but otherwise the comparison of years is very sensitive to the wind or storminess metric that is used, as well as the region of interest. The context for the extremes of the winter season can be further drawn out from consideration of the monthly data. Figures 3 and 4 show maps of monthly rainfall and temperature anomalies; the month of November is included to provide the antecedent situation. Both November and December were exceptionally wet and mild months across the region as a consequence of a persistent moist southwesterly flow. During November, England and Wales saw monthly mean temperatures widely 2–3 degC above the 1981–2010 average, and much of Ireland and Scotland were 1–2 degC above. Trawsgoed (Ceredigion) recorded 22.4 °C on 1 November, with Dooks (County Kerry) reaching 20.1 °C on the same day, which are the highest November daily maximum temperatures on record for UK and Ireland respectively. For the UK (series from 1910), Central England Temperature (series from 1659), and Ireland (series from 1900), it was the third mildest November on record. It was also very wet for some, with much of southwest Scotland, northwest England and parts of central Ireland recording over twice the average November rainfall. For northwest England, north Wales and west Scotland, it was the second wettest November in a series from 1910, behind only 2009. In Ireland it was the tenth wettest November in the series since 1850. December was the wettest calendar month on record for the UK in a series from 1910, with Wales, Scotland and northern England widely recording 2–4 times the 1981–2010 average. Fourteen sites across Cumbria and Snowdonia recorded monthly accumulations in excess of 1 m, with 1396 mm at Crib Goch (Snowdonia) and 1361 mm at Birkside (Cumbria) both exceeding the previous monthly record of 1349 mm at Styhead (Cumbria) from November 2009. These remarkable records are discussed in more detail by Burt (2016) in this issue. It was the wettest December over Ireland in a time series from 1850, with the greatest anomalies in the south and southwest. Five stations in counties Cork and Kerry broke the previous Irish record monthly accumulation of 790 mm, with a new record of 943.5 mm occurring at Gernapeka, County Cork. Impacts, especially from severe flooding, were of national significance following a number of the weather events that contributed to these remarkable rainfall accumulations. The impact of this rainfall was exacerbated by the already saturated ground in many areas following the high rainfall during November. December was exceptionally mild in most parts of the region, with mean temperatures 5–6 degC above the 1981–2010 climatology across southern England (Figure 4). At Exeter airport the mean temperature for December was 11.2 °C, 5.5 degC above average and higher than the months of January to April, October and November of that year. At 3.6 degC above average, Dublin (Phoenix Park) recorded its mildest December in a series back to 1855. It was the mildest December on record for England and Wales by a considerable margin, the fifth mildest for Scotland and the mildest for Ireland. In the long running Central England Temperature series, December 2015 is the highest monthly anomaly for any calendar month on record. More detail on the remarkable temperatures in December is provided by Burt and Kendon (2016) elsewhere in this issue. The first week of January saw continued persistent rain affecting most of the British Isles, but particularly eastern Scotland and northeast England. Parts of Aberdeenshire exceeded their monthly average January rainfall within the first 3 days of the month, and had received 2–3 times the normal monthly January rainfall by the 7th. Widespread flooding and associated disruption consequently affected parts of eastern Scotland, especially from rivers draining the eastern Grampians. A number of sites across Ireland, including Cork and Dublin, also reported their wettest January in 20–40 years, with nearly half the total for the month falling in just 2 days on the 5th and 9th. A spell of colder and generally drier weather did manage to take hold as the flow became northerly for a time in the middle of the month. Milder conditions returned towards the end of the month when there was a further flow of exceptionally mild tropical maritime air reminiscent of December. Once again there was heavy rain across the north and west, while on 24 January temperatures reached 15 °C in the London area and around Chester, Carlisle, Edinburgh and Inverness, and 16 °C in the northwest Highlands. Oxford recorded 15.9 °C, the highest January temperature there in a 163-year record. Overall January temperatures were closer to average than the preceding months, but they remained widely 1–2 degC above average. The first half of February remained unsettled and in a west to southwesterly flow. Mid-month it turned generally colder, drier and sunnier. Southwest Ireland recorded some of the highest rainfall anomalies of the month, with some locations receiving twice their long-term average: Carron (Clare) recorded 265 mm (223% of average), and Ballymacoda (Cork) recorded 187 mm (253% of average). Rainfall was also above average across western areas of the UK, with Capel Curig (Gwynedd) recording a further 345 mm (154% of average), and Keswick (Cumbria), 192 mm (160% of average). Rainfall was, however, below normal in eastern areas. Temperatures remained above average across England and Wales, but were up to 1 degC below average across Scotland and Ireland. The season therefore concluded somewhat closer to seasonal normals, but still wet. The accumulation of rainfall through the whole period is shown in Figure 5 for a selection of rain gauges representing some of the worst affected areas. The significant rainfall events such as Desmond in early December and the rain in early January across Aberdeenshire, and late January across northern England and north Wales are particularly notable. At Aboyne (Aberdeenshire) 314 mm fell in the period spanning the last 10 days of December and first 10 days of January, amounting to 60% of the 517 mm that fell through the whole period. At Capel Curig 2801 mm was recorded in these 4 months, exceeding the average annual accumulation at this location of 2612 mm. At Shap in Cumbria, 1843 mm fell, also exceeding its annual average rainfall. Typically the November to February period accounts for around 40–45% of annual rainfall, but both the sites had exceeded their long-term November to February average by 9 December. Daily minimum sea level pressures at Belmullet (County Mayo), Malin Head (County Donegal), Stornoway (Western Isles) and Lerwick (Shetland) are shown in Figure 6. The passages of the significant storm events are marked, and Table 1 provides the maximum wind gusts associated with each storm. The season was stormy, but in comparison the 2013/2014 winter season saw the storms track slightly further south, and sea level pressure dipped close to or below 950 hPa at Stornoway on several days that winter (Kendon and McCarthy, 2015) leading to considerable issues along exposed coastlines from storm surges (Sibley et al., 2015). The first of the named storms, Abigail, passed to the north of Ireland and Scotland through the 12th and 13th with a maximum wind gust of 73kn (38 ms−1) at South Uist. Fronts associated with the low pressure system following directly behind Abigail through the 14th and 15th resulted in rainfall accumulations of 50–100 mm or more on the 14th leading to flooding incidents across north Wales and northwest England as well as parts of Northern Ireland and southwest Scotland. Seathwaite in Cumbria recorded 207.8 mm between 0900 utc on the 14th and 0900 utc on the which for nearly a of the rainfall for the month and over half of the monthly long-term average. Storm high winds, continued rainfall and further disruption through November. Storm at the end of the month saw further heavy rain and to ms−1) at A wind gust of ms−1) at (County on the associated with Storm was the highest November gust recorded in the Storm Desmond was a significant storm system that affected Ireland and the UK from 4–6 December. The storm tracked to the north of Scotland, but an exceptionally mild and moist air was associated with a very persistent rainfall across southern Scotland, northwest England and parts of Ireland. the received than mm of rain, resulted in mm across the In the 24 h to utc on the mm fell at (Cumbria), a new UK record for any 24 h period. The at recorded mm in the 48 h at 0900 utc on which is also a new UK record. The previous records for both 24 and 48 h accumulations were in Cumbria during November 2009. further on the remarkable rainfall over Cumbria during December 2015 is also provided by Burt et al. The highest daily rainfall accumulation in Ireland was mm at in County the at recorded the highest 48 h total of mm, which is the highest on record in the Irish which from The features associated with Desmond with the extreme of November an which also saw record-breaking rainfall accumulations and to significant flooding in the south of Ireland and Figure the analysis for utc on 5 December with that of November 2009. Both these events were associated with with a very warm and to the temperatures recorded at Keswick (Cumbria) exceeded °C for both and rain for h as shown in Figure For the period from utc on 4 December to utc on 5 rain at in Cumbria were in excess of with mm. The of the system on the of December is then with a to air at Keswick and is with the of the The rainfall was exceptional as a of its than during any individual of a also occurred on January (not which also to severe flooding in western Ireland, north Wales and parts of northern England. the exceptional rainfall from Desmond much of the of the there were also impacts from the high winds, with of and across northern England. of the overall from storm Desmond were estimated at to 2015). The Irish were to the with flood in the counties of Cork and Kerry on 5 December 2015, with further to and between December and 9 December Ireland, 2015). In the again for the first time in making this highest above ground 2015). and 24 December storm further disruption from high winds, but more was the continued and persistent rain through the and in flooding across parts of north and west with of and mm of rain fell across the high ground of the south across north and between half and of the December average rainfall with this falling on saturated ground and with rivers already in severe flooding Storm on the then further flooding to Scotland, in from of with including particularly flooding also occurred in the south and of Ireland. The winds of the named storms affecting Ireland occurred during on December with 10 mean winds of ms−1) and during with a gust of 73kn (38 ms−1) at Belmullet (County The Office of remained in a in the rainfall for much of December. It was also a notably month in the south and southwest of Ireland, with the climate at Island (Cork) recording a total of just h a record low In early 16 were during December 2015 in England The of British estimated the associated with flooding and from the winter 2015/2016 storms be around exceeding for winter 2013/2014 2015). A succession of resulted in persistent rain causing flooding for parts of the UK at the of January, and further flooding occurred in several counties in Ireland. the flooding affected eastern Scotland, especially Aboyne causing and and there was heavy on the of the Northern Ireland was also and on the and on the of were as reached a The storm of the month was Storm on January, with 10 mean winds of ms−1) and maximum gust of Table at Malin Head (County In Scotland storm caused of and around saw the winds of the winter during with a maximum gust of but the impacts were the strength of the winds. the of storm the north of Scotland of February, with winds affecting most areas. 1 February is also the only day to have passed the very severe gale for the gale index in Figure it was on the that was the most significant storm of the month, with 10 mean winds of ms−1) and maximum gust of 73kn (38 ms−1) at Island (County a more track to some other storms of the season, and consequently the stations in Figure saw their pressures recorded in the season, to hPa at rain and high winds affected southwest England and Wales in with the wind gust recorded on the of The of southwest England was affected by reminiscent of the winter 2013/2014 storms. the weather was generally much during the of the The exceptional weather across the UK and Ireland in winter 2015/2016 was, as be directly to wider circulation patterns in early These resulted in extremes and record weather across parts of and the The northern circulation anomaly during both November and December 2015 was by a of over the eastern and and with a particularly over the north as of a spanning the as shown in Figure The (Figure was north over the eastern and the associated moist air with high from the of shown in Figure in high temperatures and their warmest December on record, and it was overall the wettest December on record for the 2015). At the very end of December the of a low pressure over the and a of high pressure over eastern resulted in moist air the temperatures briefly above the with °C recorded by a the on December The UK and Ireland of the low with the and southwest to northeast (Figure and the of storm close to the British and Figure shows the low level winds. The mean sea level pressure in Figure shows a low anomaly over the north and high over the The flow resulted in of warm and moist air (Figure over the British and Ireland as discussed across their mildest December on record, with the highest temperature anomalies above 4 degC in the north For it was a with central and southern a significant rainfall Parts of the received than of average rainfall in December. Northern by was exceptionally particularly in a from southern Ireland to western During January and February this broke The further south, and rainfall to southern of northern for example, received over of average rainfall during January The low level circulation as by mean sea level pressure for and are shown in Figure and During January and February pressure over the to in a low pressure anomaly centred over the British Isles, with the unsettled conditions through the not as exceptional as during November and December. of extreme are particularly to given the location and of the UK and Ireland, along with of rainfall patterns to in the and storm The very annual in the climate of the UK and Ireland is in Figure During 2015 one of the most significant events in the record in the et al. have a but significant between patterns of sea temperatures in the tropical with the early winter of circulation that the of the and as was observed in November and December of More et al. (2016) that the of the 2015/2016 winter was up by the Met Office seasonal forecast and in of the significant tropical it many to the winter of 2015 was also the warmest on record the northern was degC above the average 2016), and it is likely that has been the cause of the observed since the 2013). The of of climate on extreme weather events is a area of et al., and provide of of extreme winter (DJF) for the UK circulation patterns to 2013/2014, and a to of rainfall extremes by a of as a consequence of climate is on the with the 500 hPa over the British and west region from the 2013/2014 winter, for which there are between 2013/2014 and the low anomaly over the north in 2015/2016 was not as as analysis of the index used by not 2015/2016 as the highest in a UK series from with the accumulation on December 2015 representing the wettest period of the We expect the of to slightly with 2015/2016 and there are in such but it is reasonable to consider the of as that we expect some of climate to an of rainfall extremes in winters of and The winter 2015/2016 was exceptional and has occurred during a of and wet winters since the Figure et al. used data from and the Jenkinson–Collison indices discussed above to show a in storminess over western and northern Kendon the more monthly to seasonal UK rainfall records than any other in the record, only half a and the period since accounts for of all Rainfall records and severe impacts from events such as in November winter 2013/2014 and winter 2015/2016 are of these raise the of extremes from both and climate that are for UK climate to Burt for