Long-Term Changes in English Channel Storms

In this post we outline our work to construct a storm series for the English Channel back to the mid-eighteenth century and describe some of the difficulties in using early-instrumental period pressure data.
Published in Earth & Environment
Long-Term Changes in English Channel Storms
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Context

Windstorms are one of the most damaging natural hazards across Europe and result in estimated annual losses across the EU of €5 billion[1]. However, despite the importance of this hazard, our knowledge about the long-term changes in storms across the region – and hence the potential forcing mechanisms – is significantly lacking. An increase in storminess has been experienced across northwest Europe since the 1990s but it is uncertain if this indicates a long-term trend or is a result of decadal-scale variability. This uncertainty is principally due to a lack of long-term, homogeneous data, with which to place recent storm events in a suitable context.

Extending the Storm Series

In this paper we have developed a storm series back to the mid-eighteenth century using newly developed, homogeneous sea-level pressure series from sites around the English Channel. This is the longest instrumental-based storm series so far developed and extends back to the final stages of the Little Ice Age.

It has long been suspected, based on documentary accounts and proxy evidence, that the 1790-1820s were particularly stormy.  We demonstrate for the first time that the increased winter storminess experienced since the 1990s was of a similar magnitude on average to that experienced in the early nineteenth century. A key finding of the paper is that the increases in storminess since the 1990s have been confined to the winter season, and have coincided with a strengthening and northeastward extension of the north Atlantic jet stream. This has also resulted in an increasing tendency for winds to occur from a south-westerly direction. This contrasts with conditions during the early nineteenth century where increased storminess extended into the spring and autumn seasons. Notable stormy summer seasons also occurred during that time - such as the notorious year-without-a-summer of 1816 - which have become much less likely to occur in modern times.

Quality Control

Wind speeds have been recorded for several centuries, and provide the most direct measure of storminess. However, these readings are particularly sensitive to local environmental conditions and the construction of long, homogeneous series is particularly challenging. Sea-level pressure data are generally easier to homogenize, and thus the estimation of long-term changes in storminess using pressure gradients constructed from such data tend to be more reliable. As such three sea-level pressure series were used to calculate geostrophic wind speed estimates. Nonthetheless, there were several difficulties that needed to be addressed before the data could be used in this way. The first of these was the different times at which the observations were taken at the three sites.

From around the mid-nineteenth century, with the establishment of the National Meteorological Agencies, observation times for meteorological data became increasingly standardized. Before that time, and particularly during the eighteenth century, observations were generally recorded relative to local (solar) time. These times have been converted to Greenwich Mean Time (or equivalently coordinated universal time, UTC). Given the relative proximity of Paris and De Bilt to the Greenwich Meridian, these adjustments amount to GMT-9 minutes and GMT-20 minutes respectively. There remains the possibility that the readings were not actually taken at the time stated. If this occurred then an artificial increase in the geostrophic wind calculation would occur.

An additional problem with the construction of the geostrophic wind series is the daily frequency of pressure observations. Since the 1950s most countries have adopted an hourly reporting schedule. Before that time different schedules were used, and in the earliest period observations may only have been taken once or twice a day. In the case of the Paris Observatory series in the late eighteenth/early nineteenth century, pressure observations were only taken at (local) noon. As such the geostrophic wind series used in this paper has been constructed throughout as a noon value. The series have been interpolated to a noon-equivalent value, where a direct measurement was not made.

A further challenge in the construction of the geostrophic wind series using these data is the changing location of the observation sites. The three pressure series have been constructed by joining segments of data, of varying length, recorded a sites in the vicinity of the modern-day locations. These series have then been homogenized to the final segment. For example, in the case of the London series this is at Heathrow Airport. As such the coordinates of the final respective segment have been used to construct the geostrophic wind series. While this is considered to be the most appropriate way to approach this problem, there remains the possibility that this approach could introduce biases into the results.

All of the quality-control issues described in this section would have the effect of artificially increasing the geostrophic wind speed. To mitigate the potential effect of such occurrences on the results, the data were closely examined for unusual changes in values, in combination with any supporting contemporary evidence, such as notes on the general state of the weather in the weather journals. The results in the paper were also compared against other wind storm series, notably the documentary-based series from the Armagh Observatory in Northern Ireland, as well as paleo-modelling results. The increases seen during the early eighteenth century were also observed in the results from those analyses, and this adds support for the conclusions in our paper that the results at that time were not purely the result of data artefacts.  

Monitoring Changes

A key feature of the geostrophic wind series developed in this paper is that current storm events can be placed in a 200+ year context. The series are regularly updated and this allows for the continued monitoring of potential changes in storminess across the English Channel region.

References

[1] Spinoni J., Formetta G.(a), Mentaschi L., Forzieri G., and Feyen L., Global warming and windstorm impacts in the EU, EUR 29960 EN, Publications Office of the European Union, Luxembourg, 2020, ISBN 978-92-76-12955-4, doi:10.2760/039014. JRC118595.

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