engleski

Why the inverted barometer effect may be surpassed by two orders of magnitude in the Middle Adriatic archipelago?

On the morning of 21 June 1978 exceptional sea-level oscillations, having a trough-to-crest height of 6 m and a period of 10-20 min, occurred in Vela Luka Bay. Slightly less pronounced variability was observed in a wider Middle Adriatic archipelago and, with some delay, along the west Adriatic coast. In the presentation one of the hypotheses put forward to interpret the event, the one relating it to a mesoscale air-pressure disturbance, is reconsidered, by using all the available data as well as the state-of-the-art meteorological and oceanographic models. A fresh look at the meteorological data confirms that the atmospheric disturbance propagated at about 22 m/s in a northeastward direction, and additionally suggests that it had the shape of the boxcar function characterized by an air-pressure offset of 3 hPa and duration of 10 min. The meteorological model employed (WRF-ARW) proves unable to reproduce the mesoscale disturbance coinciding with the flood, but shows that the background atmospheric conditions were favorable for the development of such disturbances. The oceanographic model ADCIRC-2DDI, forced by the described air-pressure disturbance, successfully reproduces sea-level variability in Vela Luka Bay reaching a few meters and thus surpassing the inverted barometer response by two orders of magnitude. The enhancement appears to result from a four-phase process: (1) the air-pressure disturbance travels from the west Adriatic coast to the mouth of Vela Luka Bay and generates a pronounced wave packet in the sea due to the speed of the forcing disturbance being close to the speed of shallow-water waves, (2) leading waves in the packet enter the bay and are amplified due to the funnel-shaped form of the basin, (3) the waves are reflected at the bay head, and (4) the waves return to the bay mouth, are partially reflected there, and are constructively superimposed on the waves entering the bay from the open sea. The model also suggests that elsewhere in the archipelago the conditions were not so close to the resonant ones as in Vela Luka Bay and that the scattering due to the variable bathymetry and the reflection from the east Adriatic coast resulted in waves that returned towards the west coast and generated considerable sea-level activity there.

Middle Adriatic archipelago; inverted barometer effect

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