Monitoring the record-breaking wave event in Melilla harbour (SW Mediterranean Sea)

Abstract

Abstract. During 4–5 April 2022, a record-breaking wave storm hit Melilla harbour (SW Mediterranean Sea) with the violent overtopping of breakwaters. This unprecedented episode was compared against the six most extreme events previously registered by the Melilla coastal buoy during 2011–2022 to disentangle their common atmospheric driving mechanisms. A dipole-like sea level pressure (SLP) pattern, characterised by two adjacent (northwestern) high- and (southeastern) low-pressure systems, induced intense easterly winds and high waves over the entire SW Mediterranean Sea. The record-breaking 2022 event differed from the rest in the much stronger SLP gradient (2 Pa km−1) and northeasterly winds (above 20 m s−1), which concurrently gave rise to a maximum significant wave height (SWHo) and mean period (Tm) of 7.32 m and 9.42 s, respectively, beating previous historical records. The associated return period decreased from 53 to 25 years, which must be considered for updated security protocols and the sound design of future port facilities. Hourly observations from the Melilla tide gauge covering the 2011–2022 period were used to investigate the relationship between offshore energetic waves penetrating into the harbour and the sea state inside. The harbour agitation, which also reached a record-breaking value (1.41 m) during the storm, was proved to be modulated by both the offshore SWHo (correlation coefficient of 0.87) and Tm. The highest values of agitation (above 1 m) were registered for incident high waves coming from the angular sector between 50 and 70° (clockwise from true north) with Tm and peak period (Tp) values above 7 and 10 s, respectively. By contrast, the astronomical tide and the storm surge had negligible effects on harbour agitation during the seven extreme wave events. Infragravity waves, with periods between 30 and 300 s and maximum values up to 0.58 m during the 2022 storm, were also detected within the harbour basins and exceeded previously reported peaks. The energy in the infragravity band (IGE) was significantly correlated (0.96) with an offshore forcing parameter proportional to SWHo2 ⋅ Tp, evidencing that energetic swell was responsible for the highest IGE values (above 2000 m2 s). Furthermore, a 30-year (1993–2022) regional wave reanalysis was used to characterise the intra-annual variability in the 99th percentile of SWHm over the Alborán Sea on a monthly timescale and identify the existence of trends. Results revealed that the intensity of extreme wave events impacting Melilla harbour and surrounding areas increased for April, while observed trends indicate a significant decrease of the 99th percentile of SWHm for June and October. Finally, outcomes from this work could be useful for implementing a multi-hazard early warning system and ad hoc mitigation plans within the harbour territory.