Meteotsunamis

We all know what a tsunami is, but what on earth is a meteotsunami? William Thomson sheds some light on this unusual and alarming phenomenon

- William Thomson

On the morning of June 27 2011, a mysterious wave rumbled up the River Yealm in Devon. It looked just like one of the tidal bores we explored last month, but the Yealm doesn’t get bores. Headlines of ‘Tsunami’ echoed through the media, despite geologists finding no signs of an earthquake that would have triggered it. After hearing reports of people’s hair standing on end in St Michaels Mount, Cornwall (whose causeway flooded during the event], the experts started to investigate meteorological origins. Sure enough, a storm brewing in the Bay of Biscay the previous night had travelled into the English Channel around the same time as the wave in the Yealm, and further analysis proved that the event was in fact a little-known phenomenon called a ‘meteotsunami’.

Meteorological tsunamis behave similarly to regular tsunamis, but the processes that create them are very different. While the typical tsunami is powered by a sudden displacement of water, meteotsunamis are made by air pressure. It is a fascinating quirk of nature that for every 1mb drop in atmospheric pressure, the sea rises by 1cm; as hot air rises, it reduces the weight of air at sea level and the water rises. This is why tides are often higher when there’s wet and windy weather, because it is generally low pressure at the time. This effect is usually ‘static’, meaning the sea-level rise sticks to a single area. But in exceptional circumstances, a rapid pressure drop can create a wave that moves with the weather, sometimes for hundreds of miles. This wave is the meteotsunami, and as it approaches shallow water it can grow from three centimetres to three metres.