Baroclinic Response of the Adriatic Sea to an Episode of Bora
Wind
Journal of Physical Oceanography: Vol. 26, No. 7, pp. 1354–1369.
Andrea Bergamasco, Istituto per lo Studio della Dinamica delle Grandi Masse,
Venice, ltaly
Miroslav Gacic, Osservatorio Geofisico Sperimentale, Trieste, Italy
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.
|
