Weather with Malcolm RileyStrange Sea Level Fluctuations

From the Advocate Newspaper 23rd of November 2009

“The tide came in really close to shore about 11.45am ... it came right into the rocks at the caravan park. I went inside and noticed it was going back out again. We sat in the front room and monitored it for a while. It came in really quickly four times in about an hour and a half and went about three quarters (about 400-500 metres) of the way out. The full tide was not due until about 3.30pm.”
What is described above has all the attributes of a small tsunami. However, these sea level fluctuations were not believed to be caused by the usual tsunami generating process; earthquake, undersea landslip or volcanic activity. It is most likely that this was a “meteotsunami” or seiche.
To understand the motion of seiche in the ocean fill a large serving tray with water and then walk a few paces. Invariably you will spill the water as the motion of walking sets up a gravity wave (seiche) in the water. Annoyingly, as you try and correct the motion of the tray, the gravity wave in the tray increases and often you spill more water.
Seiche waves generally form in areas of relatively enclosed and shallow water such as lakes, bays and shallow seas.
Small seiches are common in enclosed bays and lakes where a resonance wave in the water fluctuates the water level back and forth. This resonance can be initially caused by the tide coming in and out of an area. For instance Spring Bay in Tasmania has a fluctuation of one hour as the water in the bay sloshes back and forth.
Larger seiche can be generated by varying weather conditions, usually changes in air pressure or an onshore wind massing water against the shore … when wind blows onshore for many hours water collects or masses against the shore raising the sea level in that area.
When the wind stops or relaxes, the water sloshes away from the area and may build up an oscillation back and forth.
Similar effects can occur with air pressure differences. High air pressure pushes the sea down and low pressure takes weight off the water allowing it to rise. This effect is one centimetre per hectopascal in southern Australian latitudes. If there is a rapid change in pressure a seiche can be generated.
For instance, if a high pressure system pushing down on the water rapidly reduces in pressure; this takes weight of the water allowing the water level to rise. This imbalance starts the seiche ‘sloshing’ motion as the water level regains its natural level. Moving squall lines and thunderstorms may also generate seiche.

More on the Bass Strait Event

A resident of East Sandy Point (just north of Bridport, Tasmania) reported that the sea level ran out like a low tide for five minutes and then came in very fast roaring swirling and bubbling and came up to the bottom of the sand dunes.
On 15 June 2006 in the port of Ciutadella on the island of Menorca in Spain, the water level suddenly dropped four metres. The mooring lines of almost all the boats snapped. The normal tide difference in the port is around 20cm. Some minutes later the vessels fell prey to the returning water, which came in three metres higher than before. More than forty boats were sunk or seriously damaged.The water rose above the high water mark covering the ‘beach road’. This sea level rose approximately two metres from the lowest point from when the sea receded.
The sea went in and out several more times but not as severe. The outgoing flow took old trees and beach rubbish out to sea and they could be seen swirling around in the bay. Other reports of strange fluctuations were reported from Stanley and Port Sorell in Tasmania.
The fluctuation in sea level was recorded at the Bureau of Meteorology’s tide gauge at Burnie. The gauge recorded a fluctuation of 60 centimetres. The larger fluctuation reported anecdotally may have been influenced by shoaling as most were on parts of the coast that are relatively shallow.
Meteotsunamis are well known in the many parts of the world and, in the main, seem to be in relatively shallow seas. In Australia these include Bass Strait and the South Australian Gulfs.
There are some good photos of a Meteotsunami at www.keesfloor.nl/artikelen/zenit/meteotsunami/index.htm
While the site is not in English, it has photos of the port of Ciutadella on an island off the Spanish coast. The pictures above show the trough arriving and the sea level receding.
The water level dropped by three to four metres snapping mooring ropes on vessels. The crest of the wave then arrived at three metres higher than normal, smashing the now floating free boats into each other. Damage was estimated at tens of millions of Euros.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.