Trapped Low Frequency Waves On The Northern Israeli Continental Shelf

Time series of sea level measurements from different instruments deployed around the Haifa Bay area are analyzed. All observations indicate the existence of persistent low frequency oscillations with periods corresponding to approximately 1hour and to approximately 30minutes. These oscillations are persistent throughout most of the year and reached maximal wave heights of around 30cm, a comparable value to tidal variations, making these oscillations an important factor in determining the average sea level elevation. The work identifies the phenomenon as resonant trapped waves, which are normal modes of the continental shelf extending from Haifa promontory to Achziv Canyon. Their temporal and spatial characteristics are learned through field measurement data analysis, wavelet analysis, numerical simulations, and exploration of an analytic solution. Finally, possible mechanisms for the generation of these waves are discussed. Such waves, which are commonly not surveyed, can be an important factor affecting coastal dynamics such as breaker line and coast line locations. Furthermore, the resonance behavior found has the capability of increasing the deep-to-shallow amplitudes in up to two orders of magnitude depending on incident angle. Noting that seismic or atmospheric events can agitate oscillation with similar wave periods, the intensification poses a hazard to Haifa Bay and coastal area.Plain Language Summary Different instruments for observing surface sea level variability had been deployed in Haifa Bay, North of Israel. In addition to expected changes caused by tidal forces and atmospheric influence, the observations revealed significant and persistent oscillations with wave periods of 1hour and 30minutes. This means that the average sea level in this area has been changing much more frequently and is less predictable than previously thought. By using analytic explanations and numerical simulations we show that these oscillations are observable due to natural amplification that happens on the continental shelf, the relatively shallow region between Haifa promontory and Achziv Canyon. This intensification reaction could constitute a threat to the coastal area in an event of tsunami event or extreme weather conditions, which are known to generate waves in the same frequency range.Key PointsSea level oscillations with periods of about 1hour and 30minutes are measured in Haifa Bay Numerical simulations are utilized to investigate the response of the coastal region to incident low-frequency waves Resonance on the continental shelf between Haifa promontory and Achziv Canyon explains the observations in terms of amplitudes and phases