楼主#更多 发布于：2014-02-17 10:25
A Global Perspective on the Recent Storms and Floods in the UK
帖内置顶 – wusifeng – 2014-02-17 18:38
This winter the UK has been affected very severely by an exceptional run of winter storms, culminating in serious coastal damage and widespread, persistent flooding.
This period of weather has been part of major perturbations to the Pacific and North Atlantic jet streams driven, in part, by persistent rainfall over Indonesia and the tropical West Pacific.
The North Atlantic jet stream has also been unusually strong; this can be linked to exceptional wind patterns in the stratosphere with a very intense polar vortex.
This paper documents the record-breaking weather and flooding, considers the potential drivers and discusses whether climate change contributed to the severity of the weather and its impacts.
Although no individual storm can be regarded as exceptional, the clustering and persistence of the storms is highly unusual. December and January were exceptionally wet. For England and Wales this was one of, if not the most, exceptional periods of winter rainfall in at least 248 years. The two-month total (December + January) of 372.2mm for the southeast and central southern England region is the wettest any 2-month period in the series from 1910.
During January and into February the tracks of the storms fell at a relatively low latitude, giving severe gales along the south and west coasts and pushing the bulk of the ocean wave energy toward the southwest of Ireland and England. Peak wave periods were exceptionally long; each wave carried a lot of energy and was able to inflict significant damage on coastal infrastructure.
In a series from 1883, flow rates on the River Thames remained exceptionally high for longer than in any previous flood episode. Correspondingly, floodplain inundations were extensive and protracted.
The severe weather in the UK coincided with exceptionally cold weather in Canada and the USA. These extreme weather events on both sides of the Atlantic were linked to a persistent pattern of perturbations to the jet stream over the Pacific Ocean and North America. There is a strong association with the stormy weather experienced in the UK during December and January and the up-stream perturbations to the jet stream over North America and the North Pacific.
The major changes in the Pacific jet stream were driven by a persistent pattern of enhanced rainfall over Indonesia and the tropical West Pacific associated with higher than normal ocean temperatures in that region.
The North Atlantic jet stream has also been unusually strong; this can be linked to an unusually strong westerly phase of the stratospheric Quasi-Biennial Oscillation (QBO), which in turn has driven a very deep polar vortex and strong polar night jet.
As yet, there is no definitive answer on the possible contribution of climate change to the recent storminess, rainfall amounts and the consequent flooding. This is in part due to the highly variable nature of UK weather and climate.
从1883年以来的资料看，泰晤士河流量维持异常高位，且汛期比以前维持异常长时间，涝原泛滥情况广泛而持久。英国的极端天气巧合地域美加地区的极端严寒天气同时发生。这些大西洋两侧的极端气候事件与太平洋与大西洋喷流的持续扰动模式有关。英国12-1月所经历的强风暴与北美和北太平洋的高层喷流扰动密切关联。太平洋喷流主要改变乃由印尼和热带西太的持续较强降雨 - 正距平水温 - 有密切关系。北大西洋喷流也异常的强劲，可能与平流层准两年振荡 Quasi-Biennial Oscillation (QBO) 异常强劲的西风位相有关，这造成非常深邃的极涡和强极地冬季喷流。不过，究竟气候变化造成多少上述风暴、雨量和洪水，目前并未有的确切答案。这是因为英国天气气候本身就是多变的。
Sea level along the English Channel has already risen during the 20th century due to ocean warming and melting of glaciers. With the warming we are already committed to over the next few decades, a further overall 11-16cm of sea level rise is likely by 2030, relative to 1990, of which at least two-thirds will be due to the effects of climate change.
Recent studies suggest an increase in the intensity of Atlantic storms that take a more southerly track, typical of this winter’s extreme weather. Also the long-term warming of the
注 准两年振荡 Quasi-Biennial Oscillation (QBO) ：指的是位于热带平流层底部， 赤道区域性的东风和西风之间的准周期性振荡，平均周期约28-29个月。
P.3sub-tropical Atlantic will also act to enhance the amount of moisture being carried by storms that take this more southerly track.
There is an increasing body of evidence that extreme daily rainfall rates are becoming more intense, and that the rate of increase is consistent with what is expected from fundamental physics. There is no evidence to counter the basic premise that a warmer world will lead to more intense daily and hourly heavy rain events.
More research is urgently needed to deliver robust detection of changes in storminess and daily/hourly rain rates. The attribution of these changes to anthropogenic global warming requires climate models of sufficient resolution to capture storms and their associated rainfall. Such models are now becoming available and should be deployed as soon as possible to provide a solid evidence base for future investments in flood and coastal defences.
Throughout December, January and February 2013/14, the UK has been affected by an exceptional run of severe winter storms, culminating in the coastal damage and widespread flooding from January onwards. The impacts on individuals, businesses and infrastructure have been substantial. This paper documents the statistics of these storms, how unusual they were in the terms of past records, and considers the global context in which these storms formed. Finally the question of whether the intensity of these storms, and their impacts on flooding, has been influenced by climate change will be considered.
During December widespread high wind speeds were recorded across UK, as a sequence of deep lows tracked across or to the north of the country. The storm on 4th-5th December generated a major North Sea storm surge event, which coincided with one of the highest tides of the year and threatened much of the east coast in a similar manner to the 1953 event. With improved coastal defences built by the environment agency and accurate early warnings several days in advance major damage was avoided. The Environment Agency Thames Barrier was raised to protect London from the largest tide recorded at Southend since it became operational.
A measure of the extent and severity of the December storms can be seen in the number of stations from the observational network that recorded maximum gust speeds greater than 50, 60, or 70 Knots (excluding stations with an altitude >=250m and four exposed offshore sites). For each of the thresholds the number of high wind gusts in December 2013 is higher than for any other December back to 1969 (Figure 1), and is one of the windiest calendar months for the UK since January 1993.
Figure 1: Record of the number of stations reporting wind gusts in excess of 60 kts during December.
The stormy weather continued into January with a major storm on 5th and 6th January, which caused widespread coastal damage and flooding in southern England. With a brief respite in January the stormy weather returned with the first week of February seeing a sequence of very deep depressions running into the UK with very high winds and storm surges that caused substantial damage along the south coast.
The exceptional duration of the stormy weather and the clustering of deep depressions has been a notable feature of this winter. Rainfall records were broken in both December and January (Figure 2). Scotland had its wettest December since records began in 1910. In southern England, January was the wettest recorded since 1910 (Figure 3), and the statistics suggest that this was one of, if not the most, exceptional periods for winter rainfall across
England and Wales in at least 248 years. The two-month total (December + January) of 372.2mm for the southeast and central southern England region is the wettest of any 2-month period in the series from 1910.
Figure 2: Rainfall for December 2013 and January 2014 from the observational network, showing the distribution of rainfall anomalies as a % of the long-term average from 1981-2010.
A particularly exceptional aspect of January 2014 has been the number of days with rain across southern England (Figure 3, lower panel), which far exceeded anything previously recorded for January. Overall there have been very few dry days since 12th December. This continuous sequence of rain events led to increasing saturation of the ground so that widespread flooding became inevitable when the major storm of 5th and 6th January arrived over the UK. The Thames, in particular, recorded some of the highest flow rates ever measured at this time of year. In January 2014, the Environment Agency Thames Barrier was raised on 13 consecutive times to protect people and property as high fluvial flows and high spring tides coincided. Rainfall continued to be well above average through January, giving little respite for areas already affected by flooding especially in southern England, and notably Somerset.
As already noted this winter has been exceptionally stormy. A particularly intense depression passed to the north of UK on 24th December with Stornoway recording a mean sea level pressure of 936mb (Figure 4). Pressures below 950mb for UK land stations are relatively rare, and this is the lowest such value at a UK land station for many years. Based on an analysis by Burt (2007) it is potentially the lowest land station pressure record since 1886. This storm led to widespread disruption to travel and the loss of power to hundreds of thousands of homes over the Christmas period.
Figure 3: Time series of total rainfall (mm) for January in southern England from records going back to 1910 (top panel), and the number of days with rain (>1mm) in southern England from records going back to 1961 (bottom panel).
Figure 4: Surface pressure chart for 12z, 24th December showing the formation of an intense depression to the north of Scotland.
This sequence of storms continued into January 2014, with a deep depression forming over the North Atlantic on 5th January. A notable feature of this storm was the size of the depression (Figure 5), which affected the whole North Atlantic. Storms of such size and intensity are rare. This meant that the fetch and strength of the winds built up a huge swell with some of highest recorded wave heights reaching the shores of Western Europe. The west coasts of the UK were severely affected by the storm surge and the exceptionally high waves resulting in extensive damage to sea defences. Rainfall associated with this system also caused extensive flooding in areas already saturated by the wet weather in the preceding months.
Figure 5: Surface pressure charts for 12z, 5th January and 00z, 6th January showing the strength and size of the
depression and its associated frontal systems.
Throughout the development of the storm that affected the UK on 5th, 6th and 7th January, the Met Office ocean and wave forecast models were giving very useful guidance. On the global scale, significant wave heights in excess of 16m were predicted to the south west of the UK, consistent with other estimates of wave heights exceeding 15m (50ft) (Figure 6, left panel). Higher resolution forecasts using the UK 4km model showed that these waves would reach UK shores as a strong, very long period swell (Figure 6, right panel). Consequently, each wave carried a lot of energy and was able to inflict significant damage on coastal infrastructure.
Storms generating waves of this height are not particularly unusual for the northeast Atlantic, but several factors mark out the event on 6th and 7th January. The track of the storm fell at a relatively low latitude for an event of this type, pushing the bulk of the wave energy towards the southwest of Ireland and England. Peak wave periods were exceptionally long (even compared with storms of similar wave height occurring in December), and enhanced the impact of the waves at the coastline. The combination of significant wave height and peak period is likely to mark out the storm as a one in 5-10 year event in the southwest of the UK, based on experience of waves over the last 30 years. In terms of the coastal system as a whole, pre-existing river and groundwater levels plus impact on coastal sediment levels of a sequence of highly energetic wave events during December may make this is a far rarer event.
Figure 6: Significant wave heights (m) from the Met Office global wave model valid for 06z on 6th January (left panel)
and peak period (seconds) between waves from the UK 4km model for 18z on 6th January.
With sea defences already weakened the storms that affected southern England on 4th and 5th February, and
expected for 7th and 8th February, (Figure 7) have caused serious localised damage to infrastructure. The strength
the waves, driven onshore by the very strong winds, and the consequent height of the storm surge produced very
dangerous conditions along southern coasts. The heavy rainfall that accompanied these systems also led to
worsening conditions in areas already affected by prolonged flooding.
Figure 7: Surface pressure chart for 00z, 5th February showing the intense depression to the south west of the UK
(left panel) and the expected chart for 00z, 8th February showing the potential for an equally severe storm (right