27th June 2011: The English Channel meteorological tsunami

By Ozgun Ozsoy

Conditions associated with the UK’s extreme coastal sea levels and flood events (surges, waves and high tides) are not usually observed during the summer. However, a rare sea level event did occur in the English Channel four years ago today [27th June 2011], caused by a weather-related phenomenon known as a ‘meteorological tsunami’. Although a surge was observed at several sites along the south coast, this event does not appear in the SurgeWatch because sea levels did not exceed our 1 in 5 year return period threshold.

Amateur footage (see YouTube video) shows a bore-like wave propagating up the Yealm Estuary near Plymouth. This attracted media interest, and many early reports speculated upon a geological origin (conventionally, tsunamis result from earthquakes, landslides, volcanic eruptions or even asteroid impacts). The peculiarity of this event and absence of any seismic anomaly, attracted interest from researchers who proposed that a storm in the North Atlantic was to blame. Professor David Tappin, a marine geologist based at the British Geological Survey, Nottingham, together with a team of other researchers published a paper (Tappin et al., 2013) presenting convincing evidence to suggest a meteorological cause. In southern Britain, similar past events may have been wrongly interpreted as of geological origin. David Tappin’s paper was the UK’s first proven example of a meteorological tsunami.

Meteorological tsunamis can be simply described as shallow water waves which are usually generated by an air pressure disturbance travelling over the sea. When these waves propagate into shallower water they can be amplified by resonance, and in some cases reach several metres in amplitude. Furthermore, in certain situations these waves can result in seiching – standing waves observed in an enclosed or semi-enclosed basin. The 27th June 2011 event affected approximately 200 miles of coastline, and sea level anomalies of up to 40 cm were reported along southern Britain. At St Michael’s Mount in Cornwall, there were reports of the causeway being flooded unexpectedly, while in the Yealm Estuary the wave was resonantly amplified to a height of 40-50cm (Tappin et al., 2013). Haslett & Bryant (2009) document that meteorological tsunamis have previously caused coastal damages and even loss of life.

While meteorological tsunamis alone may not be as notorious as storm surges, these phenomena can form an important component of sea level. In locations where there is a very small difference in extreme sea levels, even a contribution of a few centimetres can be significant. The potentially destructive impacts of meteorological tsunamis are known in several locations around the world. For example, in Ciutadella harbour (Menorca, Spain) these waves are known locally as ‘Rissaga’, can reach amplitudes of over 4 metres, and have in the past been very damaging for coastal infrastructure (Jansa et al., 2007). Elsewhere, they are known locally as ‘Abiki’ in Nagasaki Bay, Japan; ‘Milghuba’ in northern Malta; and, ‘Death Waves’ in western Ireland.

We are currently undertaking an assessment of high-frequency sea level fluctuations for the Solent, which we hope will provide an insight into the occurrence and significance of meteorological tsunamis for UK’s south coast. Details of historical events such as 27th June 2011 provide a useful background for this work – as does a previous study of seiching in this region (Wells et al., 2001).

 

References

Haslett S. K., Bryant, E. A. (2009). Meteorological Tsunamis in Southern Britain: An historical review. The Geographical Review. Vol 99, No. (2), pp. 146-163. Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1931-0846.2009.tb00424.x/abstract

Jansa, A., Monserrat, S., & Gomis, D. (2007). The rissaga of June 2006 in Ciutadella (Menorca), a meteorological tsunami. Advances in Geosciences. Vol. 12, pp. 1–4, 2007, Available at: http://www.adv-geosci.net/12/1/2007/

Tappin D., Sibley A., Horsburgh K., Daubord C., Cox D., Long D. (2013). The English Channel ‘tsunami’of 27 June 2011 – a probable meteorological source. Weather. Vol. 68, No. 6, pp 144-152. Available at: http://nora.nerc.ac.uk/502944/1/wea_2061_rev_drt.pdf

Wells, N.C., Baldwin, D.J., Wang, J.Y. & Collins, M.B. (2001). Modelling of extreme storm surge events in the English Channel for the period 14-18 December 1989. The Global Atmosphere and Ocean System, Vol. 7, No. 4, pp. 275-294. Available at: http://eprints.soton.ac.uk/6078/