engleski

Baroclinic coastal waves resonantly driven by diurnal-period winds around the island of Lastovo (Middle Adriatic)

A project supported by the US Office of Naval Research and the Croatian Ministry of Science, Education and Sports was carried out in the Middle Adriatic from February to September 2006. As implied by its title (Internal Tidal Hydrodynamics and Ambient Characteristics of the Adriatic – ITHACA) the project aimed at documenting baroclinic tides and background against which they develop. It resulted in an unexpected finding: subsurface temperature oscillations, characterized by the 24, 12 and 8 h periods, were exceptionally strong at the Lastovo island thermistor station and the corresponding baroclinic current variability was largest at a nearby ADCP station. In order to distinguish between the tidal and wind forcing of the variability, wavelet spectral analysis was performed on the meteorological and oceanographic time series. It showed that the sea/land breezes were responsible for the periodic interchange of upwelling and downwelling in mid-July 2006, with the relatively weak morning breeze corresponding to thermocline reaching the deepest point, the relatively strong afternoon breeze coinciding with thermocline being close to the sea surface. The range of thermocline variability surpassed 20 m. A simple straight-coast, reduced-gravity model reproduced the observed phases but underestimated the corresponding amplitudes, probably because it did not allow for the resonant excitation of baroclinic coastal waves traveling around the island in a clockwise sense nor did it take the nonlinear interaction of various wind-driven constituents into account. These issues were addressed by process-oriented numerical modeling, thus also enabling to detect the other spots at which diurnal upwelling/downwelling occurs and from which semidiurnal inertia-gravity waves emanate.

Adriatic; Lastovo; baroclinic costal wave; resonance

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