Has there been a recent increase in UK weather records?

Transcription

1 Has there been a recent increase in UK weather records? Number of records National Climate Information Centre, Met Office, Exeter Our starting point is monthly gridded datasets of mean temperature, rainfall amount and sunshine duration at 5km resolution covering the UK. These are calculated by interpolation of observations from individual weather stations with standard climatological exposure (Perry and Hollis, 25). The gridding process generates complete and consistent datasets which minimize the effect of a changing weather station network with time. The temperature and rainfall gridded data extend back to 191, those for sunshine to Rainfall includes all precipitation including that from snow. These gridded data are used to calculate areaaverage statistics (referred to as areal statistics ) for the UK, constituent countries and nine climate districts (Figure 1 and Table 1). More detail about the origin of these areas is given in Box 1. We also consider recent records set in the longer-running Central England Temperature (CET) series from Introduction The Met Office National Climate Information Centre (NCIC) routinely produces UK climate statistics, for example area-average values of mean temperature, rainfall amount and sunshine duration. These are published on the Met Office website, appear in Weather (in the Weather Log and seasonal weather summaries), and often feature prominently in the media (e.g. BBC News) when new records are set. Recent record-breaking periods have included the wet winter of 213/214, the wet April, June and year of 212, the warm spring and autumn of 211 and the exceptionally cold December of. An apparent clustering of records in recent years has naturally led to the question of whether the UK is experiencing more record-breaking weather than in the past. In this analysis, we investigate the occurrence of series records of area-average temperature, rainfall and sunshine statistics, by decade. These series records cover monthly, seasonal and annual timescales only, we do not consider shorter durations (e.g. daily). The series record is simply the highest (or lowest) value in the entire series; in a stationary climate it might be expected to occur anywhere in the series with equal probability. It s worth noting that the records people actually experience if they monitor the weather over the course of their lives will be broken and re-broken with a naturally reducing frequency as the time-series increases in length. There may be several such records: as an example, the hottest July in the UK series based on the first 5 years was that of This record was then broken in 1976, re-broken in 1983 and again in 26. While July 26 is the current series record, these earlier records (quite possibly within the memory of many readers) are not included in this analysis. So the analysis of trends in frequency of series records may fundamentally differ from people s perception of how often records have been broken. (a) 1659 (Parker et al., 1992) and England and Wales precipitation (EWP) series from 1766 (Alexander and Jones, 21). We thus obtain a homogeneous timeseries for each area and each month, season and year overall, from 191 to date (temperature and rainfall) and 1929 to date (sunshine), for which record high and low values exist. To select three recent examples of records for rainfall: The wettest August on record for western Scotland, in a series from 191, is August 29 (297.5mm) The driest spring on record for southern England, in a series from 191, is spring 211 (54.2mm). The wettest year on record for England, in a series from 191, is 212 (1126.1mm). Weather December 214, Vol. 69, No. 12 Mike Kendon It is immediately apparent that there is a very large number of records. Seventeen areas (UK, countries, and districts), 17 periods (month/season/year) and two types (high and low) give combinations (b) Figure 1. The UK, constituent countries (a) and Met Office climate districts (b). 327

2 Weather December 214, Vol. 69, No. 12 UK weather records 328 Table 1 Met Office areas which are used to produce areal statistics. Area Name Area Type Approximate size (km 2 ) UK UK 244 England National 13 4 Wales National 2 8 Scotland National 78 9 Northern Ireland National 13 9 England and Wales Multi-national England North Sub-national 45 England South Sub-national 85 3 Scotland North District 31 5 Scotland East District 24 7 Scotland West District 22 9 England East and Northeast District 23 3 England Northwest and North Wales District 22 Midlands District 37 3 East Anglia District 19 2 England Southwest and South Wales District 29 3 England Southeast and Central Southern District 2 2 Rainfall (mm) Box 1 The first appearance of the Met Office climate districts (shown in Figure 1(b)) would appear to be in a Met Office UK Quarterly Weather Report for 1876 (the predecessor to the Weekly and Monthly Weather Reports before 1881). At that time, the boundaries were fairly approximate, with perhaps no need for them to be defined more precisely. By 191, the boundaries were more precisely drawn out this presumably becoming necessary as the number of stations in these publications increased. It is probable that the boundaries were based on historical counties, grouped together into climatologically homogeneous districts. Remarkably, the boundaries have remained unchanged to the present day. All values are final Rainfall - July - UK Data Average Smoothed Kernel Filter Highest Lowest Most recent value 1925 i.e. 578 records for each variable. In addition to this, duplicate records may occur. As an example, Figure 2 shows UK July rainfall. Whereas there is a single distinct low record ( mm), there are four comparable high values ( mm, mm, mm, mm). Areal statistics are estimates, Figure 2. UK rainfall for July from 191 to 213. The black dotted link shows a smoothing trend, removing short-term variability. Year with associated uncertainty values (Legg, 214). Set against this uncertainty, the July 29 total is nominally the highest but is not statistically distinguishable from the other wet months listed. Therefore these four July UK rainfall totals may be considered as duplicate records. In this analysis, we have applied rules to define duplicates, given in Table 2. These rules result in well over 7 records for each variable. Dependence between records Given the length of the series, there are therefore around seven records per year, on average. However, many of the records are spatially and/or temporally dependent. Record rainfall often spans an area comprising several districts so the records for these districts (and potentially larger areas) would be spatially dependent. Similarly, a record warm month might also be associated with a record warm season or year overall, so these records would be temporally dependent. To illustrate the spatial dependence, in practice we find 233 individual recordbreaking periods for rainfall, but only half that number 115 for temperature. So when a new temperature record is set, it is often set simultaneously across many different areas (almost seven on average), and this is also true but to a lesser extent for rainfall which has greater spatial variability (three areas on average). For temperature, there are several periods where a record was simultaneously set across all areas examples include April 211 (warmth) and December (cold), whereas there are no such examples for rainfall. For temperature, there are a few examples where a record was set for only one area, whereas this is a more common phenomenon for rainfall (especially for wet records). There are no examples where simultaneous hot/ cold records were set for different parts of the UK, whereas spring was simultaneously the wettest on record for Scotland North and the driest on record for England and Wales (Figure 3). The observed spatial patterns of UK rainfall are beyond the scope of this article but have been quantified elsewhere, for example in Wigley et al. (1984). Temporal dependence of records is perhaps more complex than spatial dependence, but good illustrative examples are wet records for where records were set for April, June, summer and the year overall. The June records would also be linked to the wet summer, and the wet April, June and summer would all be linked to the wet year. Thus, in the same way that areas may overlap (for example, East Anglia falling within England, falling within the UK), periods overlap (month falling within season falling within year) and so any individual event may be associated with several records, depending on its characteristics (extent, duration and severity). In this analysis, the fact that we are considering relatively large areas (district or larger) and long durations (month or longer) will impact on the types of events associated with these records. Spatial and temporal dependence will be determined

3 Table 2 Rules for duplicate records. In practice, uncertainty in areal statistics will be greater for smaller areas (e.g. district compared with UK) and shorter timescales (e.g. monthly compared with annual) but for simplicity the same rules have been applied throughout. However, a higher % threshold has been applied for sunshine duration, reflecting a greater uncertainty in the areal statistics compared to rainfall. This is because the sunshine observing network is very much sparser. Variable Rules applied for duplicate Mean temperature Duplicate counted if this falls within.1 degc of the record value Rainfall amount Duplicate counted if this falls within either 2mm, or 2%, of the record value Sunshine duration Duplicate counted if this falls within either 2 hours, or 3%, of the record value Figure 3. UK rainfall for spring. This was simultaneously the wettest spring in the series from 191 for Scotland North, and also the driest (or equal driest) spring for England and Wales, England, the Midlands and England southwest and south Wales illustrating spatial dependence between these dry records. Table 3 Number of possible records by area type and period type. The table sums to = 289 i.e. hot, cold, wet, dry, sunny and dull records each have 289 records, with more if duplicates are included. Area type Month (12) Season (4) Year (1) District (9) Sub-national (2) National (4) Multi-national (1) UK (1) by the event meteorology; for example, a blocking high pressure system might be expected to span a wide area, whereas intense convective rainfall might be much more localised in space and time. The analysis here is based on fixed areas and standard periods, rather than being event-based (i.e. varying areas and non-standard periods). This is because these records are within selfconsistent (homogeneous) time-series used by NCIC for routine UK climate monitoring. An inevitable consequence is that many of the records will not be independent of each other. Record hierarchy We have counted the number of records which have fallen into each decade through the series, for each variable. However, some records are more noteworthy than others; consider a UK record compared to a national or district record, or an annual record compared to a seasonal or monthly record. This record hierarchy partly reflects the possible number of record occurrences (Table 3) and would be a major factor in media interest: a monthly-district record might pass unremarked, a UK annual record certainly would not. Simply counting the number of records fails to take into account this hierarchy, and gives too much prominence to the 18 monthly district records. A scoring system has therefore been devised to give more weight to the more significant records by applying a period score and an areal score to each record, so that: Record score = period score areal score The break-down of period scores and areal scores is given in green and orange in Table 4. Taking an example, there are 16 seasonal-national records (Table 3). Each of these records scores 3 3 = 9. The total available score from these records is thus 9 16 = 144 (Table 4). The score therefore gives a much greater weight to UK and national records, compared with district records, and similarly a greater weight to annual records, compared with monthly records. We then sum the scores, by decade, to give totals that reflect not just the number of records, but also their relative importance. The results present both the counts and scores by decade, so that the influence of the scoring system may be seen. Duplicate records are each given the same score as single distinct records, rather than dividing the score between them. So the occurrence of duplicates increases the total available score to be more than that given in Table 4. Overall, the scoring system therefore does include duplicates, but does not take into account the dependency between different records. Neither does it give any greater weight to records that are the wettest/driest/hottest/coldest of any calendar month, and areas with the same areal score may differ in size (Table 1). So the scoring is not perfect, but it is adopted as a compromise to take reasonable account of record hierarchy while maintaining simplicity. It also ensures that the analysis includes all available areas and periods for which UK statistics are made available on the Met Office website. 1 1 The data on which this analysis is based are available at climate/uk/summaries/datasets. These are updated on a rolling monthly basis. UK weather records Weather December 214, Vol. 69, No

4 Weather December 214, Vol. 69, No. 12 UK weather records Table 4 Area scores (orange), period scores (green), and overall scores for records by area type and period type. The table sums to 15 i.e. hot, cold, wet, dry, sunny and dull records each have an available 15 points, with more if duplicates are included. Period type Month Season Year Area type Score District Sub-national National Multi-national UK (a) Temperature record counts by decade (b) Temperature record scores by decade 8 7 hot cold hot cold 192 Rainfall records Figure 5 shows a similar analysis repeated for rainfall records by decade. Were we to assume no trends, we might expect a roughly steady number of records by decade both wet and dry, but this is far from the case. Since, there have been 1 times as many wet records (16) as dry records (11). Examples of recent periods where many wet records were set include 212 (April, June, summer and the year overall) and winter 213/214, whereas the only prominent dry period based on this record scoring system was spring 211. The period since accounts for 45% of all wet-record scores, with the highest record score for the s which is only half a complete decade. The 191s also saw a clustering of wet records. By contrast, the dry-record scores remain fairly steady throughout until a pronounced reduction from onwards, the period since accounting for only 2% of dry record scores. When considering seasonal records only, the period since accounts for 38% of wet-record scores but only 5% of dry-record scores. In a similar way, the period since accounts for 88% of annual wet-record scores but % of annual dry-record scores (i.e. no annual dry-records). Since, there have been no UK-wide dry-records set for any standard period (monthly, seasonal or annual). As with temperature, these wet/dry results are reflected in the long-running EWP series. Since, the EWP series has seen the wettest (or near-wettest) January (214), April (212), June (212), autumn (), winter (213/214) and year (212) 6 out of a possible 17 records within the last 15 years of a 248-year series but with no record dry periods. 33 Figure 4. Temperature record counts (a) and scores (b) by decade hot and cold. Decades are broken down into the period , , etc. The s decade includes data up to February 214. Temperature records Figure 4 shows temperature record counts and scores by decade, for both hot and cold records. Since, there have been 1 times as many hot records (24) as cold records at all (2). No cold records at all were set during the period 29, or during 1939, although there have been several since, most prominently the exceptionally cold December of. By contrast, a large number of hot records were set for example in 211 (April, spring, autumn, November and the year overall), and in 26 (July, summer, September, autumn and the year overall). The period since accounts for two-thirds of all hotrecord scores, but only 3% of the cold-record scores. 7% of the cold record scores are in three decades, the 191s, 192s and s. When considering seasonal records only, the period since accounts for over 7% of hot-record scores but only 2% of coldrecord scores. In a similar way, it accounts for 98% of annual hot-record scores (all but one record) but no annual cold-record scores. So the striking contrast between hot and cold records since applies just as much at seasonal and annual time-scales as for individual months. The prevalence of recent hot records is replicated in the long-running CET series. Since, the CET series has seen the warmest April (211), July (26), September (26), October (21), Spring (211) Autumn (26) and year (26) 7 out of a possible 17 records within the last 15 years of a 355-year series with only December coming close to setting a cold record. Sunshine records Figure 6 shows the analysis repeated for sunshine. In contrast with temperature and rainfall, the s have seen relatively few sunshine records overall (unsurprising with this being only a half-decade) and there are no clear trends apparent in the figures. There is a notable contrast between the sunny and dull record scores in the s with numerous sunny periods for example April 27, spring 27, spring 28, winter 28/9, autumn 23, year 23) but August 28 the only dull period of note. Discussion The question of whether the UK has seen more recent weather records than might be expected, and whether this can be linked to an underlying cause, is far from new. The same question was posed, for example, a century ago following the UK s wet winter of In discussions following a paper describing this winter,

5 (a) rainfall record counts by decade 8 wet 7 dry (b) rainfall record scores by decade 35 wet 3 dry Figure 5. Rainfall record counts (a) and scores (b) by decade wet and dry. Decades are broken down as for Figure 4. (a) sunshine record counts by decade sunny dull Figure 6. Sunshine record counts (a) and scores (b) by decade sunny and dull. The series starts in 1929 but this is included in the s i.e , 1949, etc. The s decade includes data up to February 214. Mr C Salter 2 remarked: All these occurrences [of unprecedented wetness or dryness] in so short a time were surely very remarkable, and considering the fact that meteorological observations were during that period more (b) sunshine record scores by decade 7 sunny 6 dull In 1915 Carle Salter was the then assistant director of the British Rainfall Organization, which produced annual publications of British Rainfall from 186 to Scanned copies of British Rainfall are available at metoffice.gov.uk/archive/british-rainfall numerous and accurate then ever before, it seemed as if it ought not to pass the wit of man to connect these strongly-marked rainfall fluctuations with other meteorological phenomena in such a way as to explain their cause (Mill and Carter, 1915). It is still apparent today that the 191s experienced a clustering of extreme events shown in Figures 4 and 5, including the wet summer of 1912 (Kendon and Prior, 211) and winter of 1914/1915. These wet records then switched to a cluster of dry records in the 192s and s, with prominent droughts in and (Marsh et al., 27). Often the question of cause will be asked by the media, this being a fairly natural response to extreme weather, and perhaps reflecting as much the various impacts experienced as the weather itself. There has certainly been a very large number of weather impacts since, most notably from various flood episodes. This link to weather impacts adds complexity due to the dependence on other factors vulnerability of an area to extreme weather, population density, infrastructure at risk and so on. Exceptionally high rainfall or drought would attract much greater attention in southeast England reflecting the much greater concentration of developments in the floodplain and pressure on water resources compared to northwest Scotland. Extensive media coverage during the exceptionally wet April and June of 212 across England and Wales contrasted with relatively little media interest in the record wet May of 211 in western Scotland. Another possible factor driving media interest is the sheer number of possible records at least 7 for each variable. Modern computing provides the capability to produce and monitor record statistics for all these periods and areas more comprehensively than perhaps might have been done in the past. Temporal and spatial dependencies means that records are often grouped together, but it is pertinent that the longest period without any records in this analysis is only 7 years for temperature (July 1927 to November 1934) and 4 years for rainfall (September 23 to March 27). This implies that we would expect records at frequent intervals even in a stationary climate and with no trends. These intervals would be even shorter if highly ranked values (second, third hottest, etc.) were also taken into account, and also if smaller county-sized areas were considered. Unfortunately, however, the occurrence of records may not provide the most reliable measure of how extremes may be changing which is most likely the real question being asked. More information might, for example, be obtained from a sample of high-ranked values - and there are many pitfalls associated with the analysis of records (Beran, 22). Beran makes the key points UK weather records Weather December 214, Vol. 69, No

6 Weather December 214, Vol. 69, No. 12 UK weather records 332 that: records will always be broken and the popular perception [is] that an unprecedented event requires an extraordinary explanation whereas the occurrence of a record breaker is not of itself an indication of any change in causative mechanisms. Nevertheless, the records in this analysis do reveal some interesting patterns. Rather than evenly distributed records, some of the clustering will be associated with non-independence, and annual, decadal and multi-decadal variability will all be factors. These features can be seen in the time-series in Figure 2: for example, consider the sequence of wet Julys in the late s and late s, and dry Julys in the early s. Conversely, a record wet month may be preceded or followed by a very dry one: consider 1935/1936 and 1988/1989. Figures 4 and 5 show some interesting decadal clustering of records earlier in the series: the 191s and s saw many cold records and the 191s many wet records, for example. However, the longer-term climate change signal is also apparent; the contrast between the hot and cold records is striking but not necessarily surprising. Seven of the ten warmest years in the UK series have occurred since so we would expect more hot records to be broken in the most recent decades. In contrast, any recent cold records that have been set (December being the key example, but also March and spring 213) are arguably all the more remarkable bearing in mind the underlying warming trend in the data. The contrast between wet and dry records is more surprising. The cluster of wet records is associated with recent prominent flood episodes, and may be indicative of trends in underlying rainfall patterns (the subject of ongoing Met Office research). Indeed, we would expect an increase in heavy rainfall with climate change (Trenberth et al., 23), although in contrast to temperature change, for rainfall extremes any such change might not be detectable above natural variability until later this century (for example Fowler and Wilby, ). Interestingly, when the results are broken down further to consider months and districts only, there is a very much smaller increase in wet records since. This suggests that this increase has been mostly due to records for larger areas (e.g. national) and/or longer durations (seasonal and annual). This is supported by the contrast between Figure 5(a) and (b); the higher wet bars in Figure 5(b) since being associated with higher scoring records. Such records covering larger areas and longer durations might tend to be associated with more significant weather impacts for example, a wet winter (December to February) seeing more flooding than an individual wet month. However, the notable absence of dry records since is unexpected. It is possible that this might relate to a reduction in prolonged blocking high pressure systems affecting the UK, potentially an area of further research. A paradox here is that an absence of records in a period is un-newsworthy (Beran, 22), perhaps explaining why this has not been previously noted. Nevertheless, the absence of dry records does still seem surprising given the prominent England and Wales drought of 212 (Kendon et al., 213). This may partly be because the drought was over a longterm multi-season period, and therefore not exceptionally dry for a standard month, season or year overall so monthly, seasonal or even annual dry records may not necessarily capture a severe longer-term drought. This also highlights another trap in the analysis of records the danger of pre-selecting a period of interest after scrutiny of the data i.e. Cherry-Picking statistics (Beran, 22). Extreme weather events may not necessarily fit neatly into the standard calendar. The analysis covers climate extremes at monthly, seasonal and annual scale, but not shorter duration synoptic events to which they may or may not necessarily be linked. As examples: the record-breaking heat of 1 August 23 formed part of a longer lasting 1-day heat-wave, but despite this, August 23 overall was not record-breaking; the extreme rainfall which led to devastating flooding in Boscastle, Cornwall on 16 August 24 was highly localised in nature, whereas August 24 rainfall for the region of England Southwest and South Wales was unexceptional. In contrast, the record breaking heat of 19 July 26 formed part of a longer heat-wave in the second half of the month, and July 26 was the warmest July on record for the UK. Finally, there were no individual exceptional daily rainfall events during the wet winter of 213/214, rather the exceptional wet winter was associated with persistently wet weather. These examples demonstrate that a record-breaking month may either be associated with a short-duration extreme, or a more prolonged event. This analysis does not distinguish between these scenarios. In conclusion, the recent absence of dry records may not have attracted much media interest, but overall, bearing in mind the prominence of hot and wet records across the UK, combined with CET and EWP records, the patterns shown since in this analysis are striking. It is small wonder that the multitude of recent record values has led to such widespread interest in the media, public, and climate researchers. Caution is needed when interpreting these record statistics, but nevertheless to the question posed by the title has there been a recent increase in UK weather records? the answer suggested, by this analysis at least, is yes. Information on the climate of the UK, including monthly, seasonal and annual summaries and statistics, is available at: Acknowledgements Thanks are due to Dan Hollis, Lizzie Kendon and Mark McCarthy (all Met Office) for helpful comments and advice. References Alexander LV, Jones PD. 21. Updated precipitation series for the U.K. and discussion of recent extremes. Atmos. Sci. Lett. 1: doi:1.6/asle Beran M. 22. On breaking weather records: part 1: the past. Weather 57: doi:1.1256/ Fowler HJ, Wilby RL.. Detecting changes in seasonal precipitation extremes using regional climate model projections: implications for managing fluvial flood risk. Water Resour. Res. 46: W3525. doi:1.129/28wr7636. Kendon M, Prior J Two remarkable British summers perfect 1911 and calamitous Weather 66: doi:1.2/wea.818. Kendon M, Marsh T, Parry S The 212 drought in England and Wales. Weather 68: doi:1.2/wea.211. Legg T Uncertainties in gridded and area-average monthly temperature, precipitation and sunshine for the United Kingdom. Int. J. Climatol. doi:1.2/ joc.462. Marsh T, Cole G, Wilby R. 27. Major droughts in England and Wales, Weather 62: doi:1.2/wea.67. Mill HR, Carter HE The wet winter of Q. J. R. Meteorol. Soc. 41: doi:1.2/qj Parker DE, Legg TP, Folland CK A new daily Central England Temperature Series, Int. J. Climatol. 12: doi:1.2/joc Perry MC, Hollis DM. 25. The generation of monthly gridded datasets for a range of climatic variables over the UK. Int. J. Climatol. 25: doi:1.2/ joc Trenberth KE, Dai A, Rasmussen RM et al. 23. The changing character of precipitation. Bull. Am. Meteorol. Soc. 84: doi:1.1175/bams Wigley TML, Lough JM, Jones PD Spatial patterns of precipitation in England and Wales and a revised, homogeneous England and Wales precipitation series. J. Climatol. 4: doi:1.2/ joc Correspondence to: Mike Kendon michael.kendon@metoffice.gov.uk 214 Crown copyright, Met Office. Weather 214 Royal Meteorological Society This article is published with the permission of the Controller of HMSO and the Queen s Printer for Scotland. doi:1.2/wea.2439