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Andrea Bergamasco
and
Miroslav Gačić

Abstract

The response of the Adriatic Sea to bora wind forcing in stratified conditions is analyzed using a baroclinic multilayer model. The model is linearized in the momentum equations and fully nonlinear in the thermodynamic equations. The model was forced with an idealized wind field having horizontal shear and also convergence. The alongshore shear is approximated by a harmonic function of the alongshore distance with an 80-km space scale, and a maximum corresponding to a moderate wind of 3 dyn cm−2. Idealized initial temperature and salinity vertical profiles similar to an autumn situation are assumed. The model was integrated for five days, presumably a duration of a more persistent bora event. Due to strong shear in the bora wind field the sea level is characterized by a series of highs and lows distributed along the longitudinal axis of the Adriatic. The highest positive sea level departures are observed at the northernmost corner of the Adriatic near Venice associated with the wind setup. The strongest upwelling occurs along the northern portion of the Albanian coast where the bora wind has a nonzero alongshore component. The average current field on the fifth day shows a vertical distribution suggesting Ekman dynamics. The current component perpendicular to the coast in the surface layer is oriented in an offshore direction, which then results in a coastal upwelling, especially along the Albanian coast. The alongshore surface velocity component along that portion of the Adriatic coast is in the direction opposite to that associated with the coastal upwelling. Current reversal takes place at a depth of about 30 m, which coincides with the estimated depth of the surface Ekman layer. The model results thus show that at intermediate depths (below the Ekman layer depth) the bora wind weakens the inflowing branch of the Adriatic residual cyclonic circulation (along the eastern shore) and intensifies the return flow along the western shelf break. Therefore, the bora reduces the Levantine Intermediate Water inflow and probably causes its occasional blocking or even complete current reversals. The period of the geostrophic adjustment is characterized by strong inertial oscillations that die down quickly in the coastal boundary layer and persist for the entire period of simulation outside of it. The mean kinetic energy density is higher at the upwelling than at the downwelling coast of the Adriatic. For illustration of numerical results, a satellite, infrared image is presented of one situation in the Adriatic Sea characterized by a strong bora wind forcing.

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Paola Malanotte Rizzoli
and
Andrea Bergamasco

Abstract

The northern half of the Adriatic Sea is constituted by the continental shelf with very shallow depths (20 m) in the northernmost extremity. In particular, the new-coastal region adjacent to the Italian coastline forms a shallow strip, with isobaths running parallel to the coast and a topography gently increasing towards the interior of the basin. In the region immediately south of the Po River delta—the major source of fresh water input into the Adriatic—important eutrophication phenomena have recently occurred in summer. The controversial question thus arises whether these eutrophication phenomena are to be ascribed to nutrient inputs from local sources or from the Po River waters carried southward parallel to the Italian coastline in the general cyclonic gyre characterizing the Adriatic yearly average circulation. The dynamically important question is, then, whether and how a localized source of freshwater drives the nearcoastal shelf circulation.

To answer this question a multi-level hydrodynamic model coupled with equations for temperature and salinity was constructed to study the northern Adriatic circulation, which in the summer season can be approximated by a two-level system. The model was run in a basic numerical experiment, with real input data, from 15 September to 16 October 1978, taken as a typical summer test case. The general conclusion of the investigation is that the “signal” of the Po River water, represented by the salinity field, is lost when progressing towards the coastline, even during intense episodes of northeast wind, when significant advective effects are present. In the new-coastal strip, moreover, the total transport in alongshore direction is most often directed northward contrary to what occurs in winter. Dynamical considerations suggest that the near-coastal circulation is driven by the bottom torque, which dominates the dynamical bounce of forces as soon as an alongshore density gradient is present. The direction of the vertically integrated alongshore flow can be ascribed to this alongshore density gradient, which is significantly influenced by the Po freshwater outflow. Current records and preliminary experimental results seem to confirm the above numerical and dynamical considerations.

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Alvise Benetazzo
,
Francesco Barbariol
,
Filippo Bergamasco
,
Andrea Torsello
,
Sandro Carniel
, and
Mauro Sclavo

Abstract

In this paper, an observational space–time ensemble of sea surface elevations is investigated in search of the highest waves of the sea state. Wave data were gathered by means of a stereo camera system, which was installed on top of a fixed oceanographic platform located in the Adriatic Sea (Italy). Waves were measured during a mature sea state with an average wind speed of 11 m s−1. By examining the space–time ensemble, the 3D wave groups have been isolated while evolving in the 2D space and grabbed “when and where” they have been close to the apex of their development, thus exhibiting large surface displacements. The authors have selected the groups displaying maximal crest height exceeding the threshold adopted to define rogue waves in a time record, that is, 1.25 times the significant wave height (H s ). The records at the spatial positions where such large crests occurred have been analyzed to derive the empirical distributions of crest and wave heights, which have been compared against standard statistical linear and nonlinear models. Here, the maximal observed wave crests have resulted to be outliers of the standard statistics, behaving as isolated members of the sample, apparently uncorrelated with other waves of the record. However, this study has found that these unexpectedly large wave crests are better approximated by a space–time model for extreme crest heights. The space–time model performance has been improved, deriving a second-order approximation of the linear model, which has provided a fair agreement with the empirical maxima. The present investigation suggests that very large waves may be more numerous than generally expected.

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