Regional weather and climates of the British Isles - Part 1: Introduction

Transcription

1 Regional weather and climates of the British Isles - Part 1: Introduction Julian Mayes 1, Dennis Wheeler 2 1 MeteoGroup UK, London 2 University of Sunderland The aim of this series Climate and weather are experienced as a combination whose character is something more than the sum of its parts. With these words, the current authors introduced a book entitled Regional Climates of the British Isles (Wheeler and Mayes, 1997: p. 1). Fifteen years have elapsed since that publication and, in a new series for Weather, the opportunity is taken to re-examine the nature of British regional weather and climate in the light of the climatic averages for The series of articles will explore the variation of both weather and climate within eight regions and present an overview of the distinctiveness of each. This first article provides an introduction, and the final (tenth) one will provide an assessment of regional climatic change. The regions are as follows:- 1. South East England and East Anglia 2. The Midlands 3. North East England and Yorkshire 4. South West England and the Channel Islands 5. Wales 6. North West England and the Isle of Man 7. Ireland 8. Scotland The series follows a tradition of interest in the climate of the British Isles of which there have been several comprehensive analyses. Notable amongst these are Bilham s Climate of the British Isles (1938), which took a systematic approach and was notable for highlighting the role of local climates. Gordon Manley s Climate and the British Scene (1952) has been revered for decades for its lucidity and engaging style. Lamb s The English Climate (1964) considered additional topics such as climate and health, while Chandler and Gregory s The Climate of the British Isles (1976) and Hulme and Barrow s Climates of the British Isles (1997) both provided comprehensive research reviews with the material again arranged thematically, though the latter study also examined past and future climate change. Two books written for a wider audience contain a wealth of information on past weather events and extremes: Stirling s The Weather of Britain (1997) and Eden s Change in the Weather (2005). Wheeler and Mayes (1997) adopted a regional approach, a style that is continued in this series of articles and which emphasises the rich diversity of climate and weather across the relatively limited area embraced by the British Isles. The origins of synoptic-scale and regional contrasts The weather and climate of the British Isles are products of the conflict between polar and tropical air within the turbulent zone of the mid-latitudes and the regional contrasts can be best understood by reference to its behaviour and guiding forces. Not only does the presence of the contrasting air masses contribute to temperature fluctuations and help to generate rainfall but the direction taken by airstreams at the surface and in the middle and upper troposphere influences the regional distribution of weather, particularly rainfall. The key variable in the mid-latitudes is the degree to which the large-scale airflow comes directly from the west, a facet of our climate best summarised by the North Atlantic Oscillation (NAO) index. This is a measure of the latitudinal surface air pressure gradient across the North Atlantic, often represented as the pressure difference between Iceland and either the Azores or Iberia (Jones et al., 1997; Hurrell et al., 2003). This is the principal influence on the synoptic meteorology of western Europe and is an indicator of the status of the atmospheric circulation as a whole, notably the latitude of the polar jet stream. The average route of the polar jet stream and hence of mid-latitude depression centres lies to the north of the British Isles, placing us in the path of the mid-latitude westerlies. As such, northwestern areas lie closer to the influence of low pressure systems and their attendant fronts. The consequence is an overall northwest to southeast climatic gradient that is a primary feature of our weather and, by implication, of our climate. It is reinforced by the distribution of high ground which determines the location of orographic enhancement and of rainshadow areas in different airstreams. This south to southeastward gradient of decreasing rainfall with increasing warmth and sunshine was quantified by Hatch (1973). It is however, and importantly, modulated by the relatively rapid changes in the nature of the NAO that provide day-to-day changes in our weather that may depart significantly from the general picture stated above. It is these changes that provide the context for understanding variations in the weather and climate of the British Isles and its constituent regions. Figure 1 shows the distribution of air pressure associated with the two different modes of the NAO. The positive mode is one in which the pressure and temperature gradients over the NE Atlantic are above average and is thus associated with more dominant westerly winds. The negative mode is, on the other hand, associated with lower-than-average pressure and temperature gradients (or sometimes a reversed pressure gradient), in which the influence of westerly winds is reduced and the airflow is more likely to be influenced by blocking activity. Such large-scale pressure disturbances result from changes in the route of the polar jet stream and are witnessed in contrasting local weather over the regions of the British Isles. The positive mode of the NAO is associated with depressions and fronts sweeping eastwards across the North Atlantic: the resulting weather in the British Isles is typically mild and changeable in winter with frequent rain in most areas and an enhanced risk of strong winds. As surface winds tend to come from the west or southwest, highest rainfall as at any time of year is normally experienced on western uplands, though most of the British Isles is wet to differing degrees depending upon exposure to these winds: the climatic gradients of temperature, rainfall and sunshine across the British Isles are often above average with an enhanced rain-shadow being cast Weather January 2013, Vol. 68, No. 1 3

2 4 Weather January 2013, Vol. 68, No. 1 Regional weather and climates of the British Isles (a) Figure 1. Sketch maps showing the typical distribution of air pressure and temperature anomalies, surface winds and the mid-latitude jet-stream in (a) positive and (b) negative modes of the North Atlantic Oscillation Index. over eastern regions, accompanied often by clearer skies and longer hours of sunshine. The NAO negative mode, and the consequent latitudinal meanderings of the upper westerlies, is linked with blocking anticyclones developing across northwest Europe or the northeast Atlantic. Depending on the location of the blocking high, the British Isles is subject to winds from different directions. By such behaviour regions sheltered from the prevailing winds in one situation may find themselves exposed in another. This means that the weather associated with blocking can vary between different events and even within a single blocked period. On average, as the likelihood of surface winds having an easterly component increases, eastern districts may become wetter and western districts much drier than average. If either upper or surface lows are situated close to South East England, this area can, in contrast to the overall tendency, have the wettest, dullest and coolest weather of the British Isles. The synoptic origin of rainfall can also be expressed in terms of the Lamb airflow types (Sweeney and O Hare, 1992). The swings of the NAO therefore influence the strength of the climatic gradient across the British Isles: the positive mode can enhance the gradient (as in 1990 and 2011) and the negative mode weaken or reverse it (as in the summer of 1968, in 2001 and in the winters of 2009/2010 and 2010/2011). Hence regional expressions of climate can be viewed in two ways: on the one hand there is the traditional approach in which long-term averages for precipitation, temperatures and sunshine (to name but a few phenomena) have contrasting regional expressions but, and on the other hand, it needs to be recognised that regional weather differs in response to the different states of the NAO and may present conditions quite at variance with the long-term regional averages. (b) Furthermore, a few months do not fit this pattern so neatly. The NAO index only measures the mid-latitude airflow over a small sector of the Northern Hemisphere, and on occasions blocking over a major continental area may not be detected over the North Atlantic: a recent example was in early February 2012 when blocking became well-developed to the east of the British Isles. Geographical factors also play an important role in accentuating these regional contrasts. A principal consideration is the concentration of high ground towards the west and north of the British Isles. Indeed, the north-south alignment of the major mountain ranges (approximately at right angles to the dominant westerly airflow) might be said to be the governing factor in creating precipitation gradients. This configuration creates large areas of orographic enhancement on windward slopes and rainshadow areas to the lee, the cumulative consequences of which are evident in the distribution of mean annual rainfall (Figure 2). This however is the average, the Figure 2. Distribution of average annual rainfall across the United Kingdom, (Crown Copyright, Met Office, supplied courtesy of the National Climate Information Centre.)

3 (a) (b) Regional weather and climates of the British Isles Weather January 2013, Vol. 68, No. 1 Figure 3. Monthly mean temperatures for (a) January and (b) July over the United Kingdom, (Crown Copyright, Met Office, supplied courtesy of the National Climate Information Centre.) long-term climatic expression of day-by-day variations in the weather, and this average can change dramatically if, for example, depressions track over southern regions. In these situations Scotland may experience easterly winds, and thus a reversal in the distribution of windward and lee slopes. In Great Britain the rain-shadow areas to the lee of high ground in easterlies are smaller than in westerlies due to the proximity of high ground to western coasts. By contrast in Ireland 1, rather larger rain-shadow areas may develop in easterlies due to the presence of high ground in some eastern districts. The concentration of maritime-atlantic influences in the west and north, together with more continental effects in the south and east, dominates the seasonal climates and this is nowhere more clearly marked than in respect of temperatures. Figure 3 shows mean January and July temperatures, illustrating the well-known shift in the highest temperatures from southwestern coasts in winter to the southeastern half of England in summer. The geography of temperature variations is not only different from that of precipitation but also shows marked differences between the seasons - again in con- 1 Maps showing the rainfall and temperature averages for Ireland can be found in Walsh (2012). trast to precipitation where such regional variations are all but absent, though an exception is the greater contribution of thunderstorms to total rainfall in the southeast in the summer. W Scot wet / SE Eng dry Regional variability Such is the diversity of climates across the British Isles that it is appropriate to consider how far disparate regions vary or, perhaps, W Scot dry / SE Eng dry W Scot wet / SE Eng wet W Scot dry / SE Eng wet Figure 4. The annual rainfall anomaly (percentage difference from normal) for Western Scotland (WS, vertical scale) and South East England (SEE, horizontal scale) for The extremely dry year of 1921 is an outlier due to the persistence of anticyclonic conditions in SEE. It is noteworthy that 2011, the wettest year in the series in WS, was much drier than average not only over SEE but also over much of central England. 5

4 Weather January 2013, Vol. 68, No. 1 Regional weather and climates of the British Isles are similar over time periods from a year down to days. If we consider the cases of pre - cipitation over South East England and Western Scotland the results are informative. Annual variability Figure 4 shows that there is almost no association between the rainfall anomalies over these two regions in any particular year, confirming the work of Glasspoole (1923). In other words, wet years (dry years) in the northwest are not associated with dry years (wet years) in the southeast. The figure also shows that even over a period as long as a year, regional rainfall totals can vary from a little over half the average to nearly oneand-a-half times the average. There is lower percentage variability over Western Scotland (WS) compared with South East England (SEE). This is because rainfall in the northwest occurs more frequently and tends to comprise a large number of moderate- rainfall events. Further south and east, a larger proportion of rain falls on occasional very wet days, sometimes (but by no means always) associated with convective activity. Although this introduction is not concerned with climate change per se, rainfall over WS has increased over the period shown in Figure 4, an increase that partly coincides with the prevalence of the positive mode of the NAO between about 1980 and the late 1990s. This was accompanied by an increased climatic gradient of sunshine and temperature at this time (Mayes, 2000). 200 W Scot wet / SE Eng dry May Aug Feb 2010 W Scot wet / SE Eng wet Nov 2009 Jul 2007 W Scot dry / SE Eng dry W Scot dry / SE Eng wet -100 Figure 5. Variations in monthly rainfall anomalies in WS and SEE for the period September 2003 to December Monthly variability Rainfall Figure 5 shows the same relationship but for monthly rainfall. The sample is the 100 months to December The greater tendency for large percentage anomalies in SEE is shown even more clearly (the anomalies may be smaller in absolute terms, of course). Again, there is hardly any relationship between the two rainfall series; the two wettest months in WS were actually drier-than-average in SEE (they were August 2009 and May 2011). Both months were characterised by westerly airflows over northern parts of the UK and an anticyclonic influence close to SEE. The rainfall distribution on 20 August 2009 (Figure 6) shows the way in which rainfall can become concentrated in northwestern areas, with particularly large totals being recorded over west-facing hills where orographic enhancement of the prevailing southwesterly airflow takes place. The positive state of the NAO, bringing vigorous westerly winds and high rainfall to the northwest of the British Isles, can persist for several months. A notable example was Figure 6. Cumulative rainfall radar for the 24 hours to 0600 UTC on 20 August (Courtesy of MeteoGroup UK.) the first three months of 1990 when WS recorded exceptionally heavy rainfall. In March, over 1000mm of rain fell in the Western Highlands whereas only 10mm was recorded at Montrose on the east coast. The north-south pressure gradient across the UK that month was 22mbar: the average March pressure gradient is 9mbar. High rainfall in central and southeastern England can, in contrast, have a variety of origins but a southward displacement of the jet stream is usually involved. For example,

5 the two wettest months here in the period shown in Figure 5 were July 2007 and November The former was a cyclonic, blocked month in England (with exceptional rainfall on the 20th leading to widespread flooding) while the latter was a very unsettled month when strong southwesterly winds brought frequent active fronts across the country. One of the clearest examples of a reverse-gradient month was May 1984 when Eskdalemuir (Dumfries and Galloway) and Nairn (Inverness) were the driest places in the UK each having just 4mm. By contrast, in Hampshire over 150mm was recorded. This was a highly blocked month with upper and surface lows slow-moving close to South East England (similar to Figure 1(b)). A more recent example was June 2012 when the far north of Scotland was very dry, whilst England and Wales had their wettest June since at least Temperature and sunshine Monthly variations in temperature and sunshine are also indirectly related to the NAO (Mayes, 2000). For sunshine related closely, of course, to cloud cover the principles are similar to those governing rainfall distribution, in which areas sheltered from prevailing winds tend to be sunnier (relative to average) than exposed areas around windward coasts and hills. This is most noticeable when airflows tend to be mostly stable, a situation in which orographic effects are transmitted through the boundary layer. An example of this feature is the high sunshine totals in tropical maritime air masses over eastern Scotland and North East England, both regions where the orographic effect on westerlies may be expected to be at its greatest for the UK. Indeed there are often occasions in winter, at times when westerly circulations dominate the picture, when these regions, despite their latitude, are the sunniest in the UK. Conversely, in unstable air, windward coasts can be very sunny in the spring and early summer when seasurface temperatures are low and induce greater stability, in contrast to areas downwind where convective cloud develops. Monthly temperature variations reflect more than anything else the relative frequencies of the different air masses. As such air masses embrace often wide areas, their spatial patterns of temperature response tend to be less marked than those for precipitation and sunshine and the regional distinctions are therefore less obvious. Nonetheless, such differences can exist in certain situations. For example, cyclonic months in which depressions track persistently across central parts of the British Isles may result in largely polar air masses over Scotland and tropical air over southern England giving rise to notable south-tonorth temperature gradients. Modifications can occur within these schemes but these tend to be local rather than regional in nature. Sheltered areas downwind of high ground, for example, may be warmed in any season, especially if stable airflows predominate; examples of föhn-effect warming will be referred to in the relevant regional articles. Such genuinely regional contrasts that do appear in the temperature regime tend to be a consequence of cloud cover: because of the contrasting influence of cloud amount and the radiation balance on temperature between night and day, the regional distributions of maximum and minimum temperatures in a given month may differ considerably. Daily variability influence of air masses The weather of an individual day can also be accounted for by reference to the prevailing air mass affecting the British Isles at the time. Each air mass has typical characteristics of temperature, humidity and stability according to the source area and the track taken by the air on its approach to the British Isles. Table 1 shows the main characteristics of air masses that reach these islands. Those that are warmed on their route from cold source areas tend to Regional weather and climates of the British Isles Weather January 2013, Vol. 68, No. 1 Table 1 The main characteristics of air masses that reach the British Isles. Air mass Typical wind direction Stability Source region Temperature description in winter Temperature description in summer Weather Tropical maritime SW Stable Polar maritime NW or W Unstable Sub-tropical North Atlantic Very mild, especially in east Average or rather cool in west, warm and humid in east Orographic enhancement of pre cipi tation over western uplands; föhn-effect warming to lee of high ground North Atlantic Near average or rather cold cold in upland areas. High lapse rates Cool or very cool Showers over windward coasts in winter, further inland in spring/summer as land-based instability develops Returning polar maritime SW Unstable Arctic maritime N Very unstable North Atlantic track via southwest approaches Mild or close to average Rather cool Heavy showers over windward southwestern coasts and hills in winter; showers reach east in summer Arctic Very cold, especially in Scotland. Rare in the south Very cool in the north, less cool by day in the south Showers, frequently wintry in winter, particularly on windward north-facing coasts of Scotland and eastern England. Sunny in sheltered areas. Arctic continental NE to E Mostly stable, can destabilise over North Sea Polar continental NE to SE Stable in spring/summer, rather unstable in winter Tropical continental SE Usually stable Siberia, northern Russia, Baltic states Bitterly cold Indistinguishable from polar continental The coldest weather of winter for much of England and Wales, heavy snow around troughs. Central northern and eastern Europe, southern Russia Very cold, especially in South East England Cool near North Sea coasts, warm in west East coast counties: snow showers in winter, stratus cloud (haar, fret) in spring/ summer. Western areas mainly fine. Central/southern Europe Rare in winter (but cold in east, mild in west) Very warm or hot (but may not reach Scotland or Ireland) Mainly fine, sunny and warm or hot in summer. Cooler near windward coasts. Can be destabilised by Spanish plume events 7

6 Weather January 2013, Vol. 68, No. 1 Regional weather and climates of the British Isles (a) Figure 7. Cumulative rainfall radar for the 24 hours to 0900 UTC on (a) 19 November 2009 and (b) 20 November 2009 showing the persistence of the heavy orographic rain that fell over the Cumbrian Mountains. (Courtesy of MeteoGroup UK.) become unstable because the warming is from below; this is why polar maritime and arctic maritime air masses are usually associated with showers and cumuliform cloud. Conversely, those that are cooled on their journeys from warm sources tend to become more stable and are associated with layer cloud and non-convective precipitation. These characteristics will vary from season to season but, and importantly, the seasonal frequency of all air masses (with the notable exceptions of arctic and tropical continental) differs relatively little. Once the air mass has reached the British Isles regional weather is influenced by the degree of exposure to, or shelter from, the resulting wind direction. Galvin (2010) provided a comprehensive description of each of the air masses, illustrated by satellite imagery and tephigrams. Stable air masses tend to be modified also by the topography of the land areas over which they travel. For example, uplift of tropical maritime air over western hills induces strong orographic enhancement of precipitation. As more rain is induced by the lifting process of the (thermally) stable air, strong rain-shadows form downwind of the mountain barriers. An obvious recent example occurred over northern England and southern Scotland in November The detailed rainfall distribution shown in Figure 7 reveals the extent to which even in Cumbria some areas received far less rain than the western slopes and hills. Additionally, the low rainfall totals in North (b) East England downwind of the Cumbrian mountains are evidence of the effective removal of moisture from the airstream as it rose over the high ground further west. Such examples of regional contrasts are abundant but need not be rehearsed here where enough has been offered to describe the fundamental background to regional climates in the British Isles. Conclusion The aim of this introduction has been to convey a sense of the significance and, at the same time, the subtlety of regional climate variation across the British Isles. It can be expressed at different scales of temporal resolution from the decadal to the annual, monthly and daily, and it differs yet again depending upon the state of the atmospheric circulation and the NAO in particular. Yet these seemingly contrasting atmospheric expressions can be brought together to provide climatic definition to the regions of the British Isles. Acknowledgement We are very grateful to the Climatological Observers Link and to the Met Office for allowing us to quote the new climatic averages from the amateur and official observing networks respectively. In particular, we thank John Prior of the National Climate Information Centre, Met Office, for supplying Figures 2 and 3. In doing so we acknowledge also the efforts of all weather observers who have contributed to the national archive of weather observations. Thanks are also due to the two reviewers who commented on the first draft of the article. References Bilham EG The Climate of the British Isles. Macmillan: London. Chandler TJ, Gregory S The Climate of the British Isles. Longman: London. Eden GP Change in the Weather; Weather Extremes and the British Climate. Continuum Books: London. Galvin J From Observations to Forecasts Part 4. Air masses a view from space. Weather 65: Glasspoole J A comparison of the fluctuations of annual rainfall over the British Isles. Br. Rainf. 62: Hatch DJ British climate in generalised maps. Weather 28: Hulme M, Barrow E Climates of the British Isles: Present, Past and Future. Routledge: London. Hurrell JW, Kushnir Y, Ottersen G, Visbeck M The North Atlantic Oscillation: Climate Significance and Environmental Impact Geophysical Monograph Series. American Geophysical Union: Washington, DC. Jones PD, Johnsson T, Wheeler D Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and south-west Iceland. Int. J. Clim. 17: Lamb HH The English Climate. English Universities Press: London. Manley G Climate and the British Scene Collins: London. Mayes JC Changing regional climatic gradients in the United Kingdom. Geog. J. 166: Stirling R The Weather of Britain. Giles de la Mare: London. Sweeney J, O Hare G Geographical variations in precipitation yields and circulation types in Britain and Ireland. Trans. Inst. Br. Geogr 17: Walsh S A Summary of Climatological Averages for Ireland , Climatological Note 14. Met Eireann: Dublin, Ireland. ie/climate-ireland/summaryclimavgs.pdf [accessed 27 August 2012]. Wheeler DA, Mayes JC Regional Climates of the British Isles. Routledge: London. Correspondence to: Julian Mayes Julian.mayes@meteogroup.com 2013 Royal Meteorological Society DOI: /wea