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Baroclinic Response of the Adriatic Sea to an Episode of Bora Wind

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  • 1 Istituto per lo Studio della Dinamica delle Grandi Masse, Venice, ltaly
  • | 2 Osservatorio Geofisco Sperimentale, Trieste, Italy
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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.

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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