Widespread tsunami-like waves of 23-27 June in the Mediterranean and Black Seas generated by high-altitude atmospheric forcing

Abstract

A series of tsunami-like waves of non-seismic origin struck several southern European countries during the period of 23 to 27 June 2014. The event caused considerable damage from Spain to Ukraine. Here, we show that these waves were long-period ocean oscillations known as meteorological tsunamis which are generated by intense small-scale air pressure disturbances. An unique atmospheric synoptic pattern was tracked propagating eastward over the Mediterranean and the Black seas in synchrony with onset times of observed tsunami waves. This pattern favoured generation and propagation of atmospheric gravity waves that induced pronounced tsunami-like waves through the Proudman resonance mechanism. This is the first documented case of a chain of destructive meteorological tsunamis occurring over a distance of thousands of kilometres. Our findings further demonstrate that these events represent potentially dangerous regional phenomena and should be included in tsunami warning systems.

Figure 1. Locations of meteotsunami events observed in the Mediterranean and Black seas during late June 2014 superimposed on meteotsunami-favourable coastal areas for which 0.9 < Fr < 1.1, where Fr is the Froude number.

Also shown are the sea level series measured at Odessa and air pressure series measured at two Romanian stations. Small stars mark the positions of air pressure observations; circles denote sea level stations used in Fig. 2. Black dotted lines indicate the approximate onset times of high-frequency sea level oscillations over the Mediterranean and Black seas. The Froude number is defined as the ratio of the wind speed at a height of 500 hPa and the phase speed of long ocean waves. Figure was created using MATLAB software and ECMWF and GEBCO data.

Figure 2. Major atmospheric pressure (AP) and sea level (SL) oscillations measured in various regions of the Mediterranean during 22–27 June 2014.

(a) AP and SL records after high-pass filtering using a 3-hour Kaiser-Bessel window; and (b) frequency-time (f-t) diagrams of the records in (a).

Figure 3. Propagation of the meteotsunamigenic synoptic pattern of June 2014 together with the maximum heights of corresponding sea level oscillations at the times of the meteotsunami events.

Left panel: temperature at 850 hPa; middle panel: wind speed and direction at 500 hPa: and right panel: the dynamically instable atmospheric layers (collared denotes Ri < 0.25) overlaid by circles showing the maximum height of high-frequency (periods < 3 h) sea level oscillations; the red circles denote measured wave heights, the green circles denote wave heights estimated from videos and eyewitness reports. Figure was created using MATLAB software and ECMWF data.

Figure 4. Illustration of the meteotsunami generation processes.

Numerous atmospheric gravity waves (represented by bubbles) are generated at the interface of unstable and stable atmospheric layer at places of strong wind shear. Atmospheric gravity wave propagating with speed U (represented by the red wave), which is equal to wind speed (u) of unstable layer, becomes trapped and propagates in the stable layer as a “duct wave”. Vertical energy loss and dissipation of the duct wave is prevented by an unstable layer. Air pressure change (a surface manifestation of atmospheric gravity waves) generates long-ocean waves which can be amplified through several processes: (1) Proudman resonance (due to matching of long-ocean waves speed and speed of atmospheric gravity wave); (2) shelf amplification (due to shoaling); and (3) harbour resonance (due to matching of frequency of incoming long-ocean waves and harbour eigenperiods). Incoming ocean waves can be amplified more than 100 times before hitting the coast as a destructive meteotsunami. Numbers shown in figure are for illustration only and are highly dependent on the properties of the atmospheric gravity waves, bathymetry and topography of the impacted area.