By Prof. Aldo Drago, Physical Oceanography Research Group
The sea level changes observed last Tuesday early morning at St Paul’s bay could have appeared like a mini tsunami, but in reality have no relation at all to seismic activity. Such sea level fluctuations occur as a series of lows and highs repeating cycles every few minutes. As the water retreats it uncovers the seabed underneath the boats. Upon returning back the water rose higher than the adjacent side quay. These dramatic sea level changes occurred over a matter of a few minutes which gave the appearance much like that of a tsunami. They are a most remarkable feature in the sea level signals observed in the Maltese Islands occurring as an expression of a coastal seiche, known by local fishermen as the ‘milgħuba’ (from the verb in Maltese ‘lagħab’, as a connotation to the play of the anomalous sea level movement). This phenomenon is triggered from the atmosphere and is commonly referred by scientists as a meteo-tsunami.
In the early hours of the day a train of atmospheric gravity waves hit the Maltese Islands from the East; their impact on the sea is like a big sea hammer hitting repeatedly the sea surface and producing long period waves in the open sea areas. When these waves reach the coast they cause the water bodies in embayments like St. Paul’s Bay to resonate, leading to amplified sea level excursions closer to the coast.
Real-time monitoring stations forming part of the national observing system run by the Physical Oceanography Research Group at the Department of Geosciences of the University of Malta have captured this phenomenon. The atmospheric waves were particularly coherent and intense, starting at around 2:30 am local time, and protruding for a stretch of around three hours.
Sea level measured in PortoMaso showed a slight seiching response, but the rubberstamp inside the embayments is definitely much more amplified. The same phenomenon must have also occurred in the sister embayment inside Mellieha Bay which is known to oscillate in antiphase with St. Paul’s Bay as two connected water bodies.
It is also worthy to note that sea level excursions in Mellieħa Bay exceeding 1m have been measured in the past.
Owners are advised to avoid leaving vehicles on the quay in St. Paul’s Bay as the oscillating water was repeatedly brimming over the quay. Just a few more centimeters higher and the high sea level could have well penetrated the vehicles.
Graph: Atmospheric gravity waves propagating over the Maltese Islands in the early hours of Tuesday 18 June.
For further information or if you would like to send any feedback, you may contact Prof. Aldo Drago by email.