Sea level variability and the ‘Milghuba’ seiche oscillations in the northern coast of Malta, Central Mediterranean

Abstract

The characteristics of the water level variations in the northern coastal area of Malta are studied by a set of densely sampled data collected in the period 1993–1996 (43 months) at a permanent coastal sea level installation in Mellieha Bay. These measurements constitute the first digitised sea level recordings in the Maltese Islands and were collected as part of a research programme that produced a long time series of simultaneous water level and meteorological parameters in the Central Mediterranean. Tidal oscillations reach a maximum range of only 20.6 cm on average and are predominantly semi- diurnal. Important non-tidal signals are however found to span the whole spectral range of frequencies. Seiche oscillations in the form of large amplitude sea level fluctuations, known locally as the ‘milghuba’, carry substantial energy in the range of long wave frequencies (0.2–2 cph) and often mask completely the tidal signal. These coastal seiches are believed to be the expression of shelf scale resonances; in the numerous embayments on the northern coastline of the Maltese Islands, these seiches are greatly ampli- fied and have associated swift alternating currents that are useful for the mixing and exchange of the water body in the embayments with the adjoining open sea areas, but can constitute a nuisance to nav- igation especially at harbour entrances. In the synoptic and sub-synoptic time scales, variations in atmo- spheric pressure associated with mesoscale meteorological phenomena produce a predominant effect on the sea level, but the response of the sea is non-isostatic and carries the signature of oceanographic conditions in the region as well as that of non-local forcing resulting from intra-basin differences. Strong seasonal non-eustatic fluctuations in the mean sea level are characterised by a high sea level in December and is typically followed by a sharp fall to a minimum in February/March. This seasonal variability is a manifestation of the adjustments in the mass balance of the whole Mediterranean basin.