Resonant coupling of oceanic gravity waves forced by travelling atmospheric disturbances with destructive effects in harbors: meteo-tsunamis in the Balearic Sea

Renault L., Vizoso G., Wilkin J., Tintore J.

Meteo-tsunamis are long-period oceanic waves that possess tsunami like characteristics but that are meteorological in origin: in the western Mediterranean, travelling atmospheric gravity waves and/or convective pressure jumps generate long surface waves in the ocean that are amplified and produce strong ‘seiche’ oscillations in some inlets and harbors, resulting in sudden drop of sea level inside the harbor that can be repeated cyclically from few hours to several days. Theses high amplitude oscillations can produce significant hazards with social and economic consequences, in particular in the local pleasure and fishing fleet of Ciutadella harbor, southwest of Menorca Island.
In this presentation we focus on this is phenomenon locally known as Rissaga. Ciutadella is a natural elongated inlet about 1 km long, 100 m wide and approximately 5 m deep, with a free period of oscillation (Helmholtz mode) of 10 minutes. This specific geometry confers to the harbor a large Q-factor, which results in significant resonant amplification of long waves from the open sea. Rissaga events typically occur in summer, last from a few hours to several days, with sea level oscillations of approximately 0.5 m in average. Extreme oscillations have been also observed: on June 15, 2006, wave heights about 4–5 m were reported inside the harbor, with very strong and destructive associated currents.
We have used ROMS to study the propagation and coupling between the atmospheric and the oceanic waves and the amplifications occurring in different regions travelled by the atmospheric forcing disturbance (open ocean, shelf, channel and harbor).

First we analyze the problem using a simple academic basin from which the major physical processes can be identified. . The parent domain is a 1 km resolution, 100 m depth rectangular basin sloping up to 10 meters depth. The child domain is an idealized harbor with similar dimensions to Ciutadella with 20 m spatial resolution. Results show that ROMS can adequately reproduce the main features of the “Rissaga” events described in the literature. Sensitivity analysis to different atmospheric forcing conditions (intensity, travelling direction, etc.) and bathymetries is also carried out.

Second, we study two real cases, using ROMS forced by WRF regional atmospheric model (three embedded domains with 30km, 6km, 1.5 km spatial resolution) and using realistic atmospheric conditions from July 2006 (extreme event) and May 2008 (normal oscillations). A sea level travelling atmospheric wave train of 5 hPa in good agreement with observations is adequately reproduced by WRF. The associated inverse barometer wave front in the open ocean is progressively amplified in the different regions of the domain reaching almost 50 cm at the mouth of Ciutadella and more than 3 m inside in the extreme case, in reasonable agreement with observations.
We analyze theses findings and the feasibility of establishing an operational forecasting system that can result in the near future in the establishment of an early warning system of meteo-tsunamis in the Mediterranean.