Sebastià Monserrat

Pressure-Forced Seiches of Large Amplitude in Inlets of the Balearic Islands

Large-amplitude harbor seiches usually occur in summer in the Balearic Islands. A significant correlation between sea level oscillations and atmospheric pressure disturbances has been found, though a proved physical mechanism to account for this atmosphere-ocean interaction is still missing. Using a flat bottom, shallow water model, we show that a direct coupling between atmospheric pressure and the free mode of an inlet is unlikely but that an oceanic wave of atmospheric origin can act as an intermediate mechanism and adequately force the inlet by resonance. The phase relationship derived from this mechanism is in good agreement with observations, provided the whole spectrum of oceanic waves is in opposite phase to the generating atmospheric disturbance. We also show that the very large oscillations observed at Ciutadella (an elongated, shallow inlet in the west coast of Menorca, Balearic Islands) can be explained in terms of the particular shape of this inlet.

On the reconstruction of the transfer function for atmospherically generated seiches

A method recently proposed to separate source and topographic effects in observed tsunami spectra is revised and applied to atmospherically generated seiches. The method is used to investigate the origin of abnormal seiche events (‘rissaga waves’) recorded in the region of Ciutadella, Balearic Islands. The reconstructed open-sea source spectra and the observed atmospheric pressure spectra are combined to estimate the ‘transfer function’ between the atmosphere and the sea surface. This function is used to predict sea level spectra for given locations near to the coast. Theoretically computed spectra for two ‘rissaga’ events are in good agreement with observations.

Model intercomparison in the Mediterranean: MEDMEX simulations of the seasonal cycle

The simulation of the seasonal cycle in the Mediterranean by several primitive equation models is presented. All models were forced with the same atmospheric data, which consists in either a monthly averaged wind-stress with sea surface relaxation towards monthly mean sea surface temperature and salinity fields, or by daily variable European Centre for Medium Range Weather Forecast (ECMWF) reanalysed wind-stress and heat fluxes. In both situations models used the same grid resolution. Results of the modelling show that the model behaviour is similar when the most sensitive parameter, vertical diffusion, is calibrated properly. It is shown that an unrealistic climatic drift must be expected when using monthly averaged forcing functions. When using daily forcings, drifts are modified and more variability observed, but when performing an EOF analysis of the sea surface temperature, it is shown that the basic cycle, represented similarly by the models, consists of the seasonal cycle which accounts for more than 90% of its variability.

Analytical simulation of edge waves observed around the Balearic Islands

The research is also partly supported by National Science Foundation Grants to Cornell University. CYTMAR MAR95-1863 project provided the necessary funding for the acquisition of the data

Mediterranean Sea Level Variability and trends

Lionello, P. (ed.). The Climate of the Mediterranean Region : from the past to the future. [S.l.]: Elsevier, 2012, p.257-299

Comparison and Combination of Coastal and Off-Shore Tide Gauge Measurements from Eivissa Island, Western Mediterranean

Two sea-level records, spanning eight months of simultaneous data in 2002, have been derived from pressure tide gauge measurements at Sant Antoni harbor (Eivissa Island, Western Mediterranean) and off Es Vedrà island, 15 km SW from Sant Antoni. The two time series are compared and used to characterize the sea-level variability in the Balearic Sea. The main forcings for the observed variations, astronomic tides, and meteorologic forcing by the inverse barometer effect are quantified. In a further step, an amplitude transfer function is determined, reflecting the frequency dependent amplification or damping of the nontidal sea-level residuals between both locations due to their local topography. The interpretation of the obtained transfer function indicates sea-level modulating topographic specifics not only for the coastal record at Sant Antoni, but also at the Es Vedrà site. In general, the application of such a transfer function allows the prediction of off-shore sea-level variations from coastal tide gauge measurements. In the present study, ellipsoidal sea-surface heights for Es Vedrà are inferred from Sant Antoni tide gauge data with an accuracy of about 1.5 cm by applying the determined transfer function and a geoid model.

Influence of the Atmospheric Wave Velocity in the Coastal Amplification of Meteotsunamis

Extremely large seiche oscillations are regularly observed in some specific areas around the world even in the absence of any seismic forcing. These seiches have been successfully associated with strong atmospheric pressure perturbations inducing sea level oscillations at the open ocean, before entering the inlet, which are in turn resonantly amplified by the geometric characteristics of the inlet. The coastal behaviour of such waves, although of different origin, is similar to tsunami waves behaviour, and is sometimes referred to as meteotsunamis. In some specific places, such as Ciutadella inlet, Balearic Islands, western Mediterranean, these seiche oscillations are stronger than expected, even taking into account the large amplification factor by resonance of the inlet. For these cases, some external amplification, before entering the inlet, is necessary to explain the phenomenon. In this paper, it is numerically shown how the phase speed of the atmospheric pressure disturbance generating the surface waves is a critical factor in the energy transfer between the atmosphere, and the ocean.