A study on the sea level variations and the 'Milghuba' phenomenon in the coastal waters of the Maltese Islands

Abstract

The phenomenology, generation and associated dynamics of short period seiche oscillations observed along the northern coast of the Maltese Islands are studied from a set of densely sampled, long term hydro-metrological observations made at a permanent sea level gauge, together with simultaneous observations of bottom pressure recordings at offshore positions and across the Malta Channel. This coastal seiche, known locally as the 'milghiibd, manifests itself in conjunction with the occurrence of mesoscale atmospheric gravity waves travelling in the lower troposphere. The associated open sea waves excite the water bodies of the various inlets, bays and harbours into resonant osculations which reach a range of up to lmin Melfeha Bay. Numerical experiments based on the free surface, non-linear Princeton Ocean Model (POM) in 2D mode explain the response characteristics of two adjacent wide-mouthed open embayments. The seiche-induced barotropic circulation and impact on the flushing of Mellieha Bay are studied by means of an advection-diffusion scheme implemented within POM. Sea level signals on the synoptic, planetary wave and seasonal scales dominate the residual spectrum A multiple regression model and a novel analytic technique based on the wavelet transform provide in combination a very effective means of studying the composition of the sea level signal and the dependence of its variability in time on one or more correlated parameters. Atmospheric pressure is the predominant factor determining the sea level variability at frequencies lower than 0.75cpd. In the upper synoptic frequency (0.3 < f < 0.5cpd) the response is very close to isostatic, with an average gain of 0.7. At other frequencies the overall response is non-isostatic implying that other factors besides mesoscale atmospheric pressure forcing contribute to the sea level variability. On account of the station's position close to the latitudinal axis of the Strait of Sicily, these signals are important in understanding the control of the Strait on intra-basin exchanges. Seasonal changes in the mean sea level show a major minimumin March and a major maximum towards the last months of the year. Besides the usual steric and direct meteorological effects, this variability is attributed to adjustments in the mass balance of the whole Mediterranean basin.