Abstract
Abstract. The Bora wind field is characterised by strong vorticity and divergence. Several numerical experiments, in which an oceanographic model was forced with northeasterly winds having climatological alongshore variability, were performed in order to study the influence of spatial variability in the bora wind field on the surface currents in the northern Adriatic. Numerical model results showed that during bora episodes with lower speeds and fast offshore decay surface currents along transect Rovinj - Po River are predominantly in the downwind direction. On the other hand, during bora episodes with strong intensity and slow offshore decay, a cyclonic gyre due to the pronounced bora alongshore variability is formed in the northernmost part of the Adriatic Sea and the studied transect is influenced by the counter currents. Moreover, bora having a high speed and a short offshore range produces the same effect in the eastern part of the Rovinj - Po River transect as low-speed bora characterised by slow offshore decay. Eulerian current measurements performed in the northern Adriatic during bora episodes characterised by different synoptic conditions supported the numerical model findings. Surface currents during the bora episode of 8-11 February 1984 were directed downwind, whereas during the episode of 12-19 February 1984 they were directed upwind. The first episode was characterised by a deep bora layer with cyclonic activity over the western Mediterranean and Genoa Bay, whereas the second one was accompanied by temperature inversion and a southwesterly tropospheric wind above a shallow bora layer. According to the hydraulic theory developed by Smith (1985), an observed descent of isentropes during the second bora episode led to the stronger acceleration in the bora layer and its larger offshore extent. Different offshore bora decays during studied events were confirmed by a comparison of the wind data originating from the meteorological stations positioned on the opposite Adriatic coasts.
Highlights
Bora is a strong and cold katabatic wind that blows along the eastern Adriatic coast from the northeastern quadrant
In the experiments with a smaller decrease, the cyclonic gyre in the northernmost part of the Adriatic is generated and transect Rovinj – Po River is in the area of the counter current (Figs. 3d,e,f)
In order to support numerical model results we present Eulerian current measurements from the gas field Ivana in the northern Adriatic (Fig. 1, 44◦ 47’ N, 13◦ 25’ E), obtained during bora episodes related to different synoptic situations
Summary
Bora (in Croatian this wind is known by the name bura) is a strong and cold katabatic wind that blows along the eastern Adriatic coast from the northeastern quadrant. Influence of vorticity and divergence of the bora wind field on the surface currents is studied in this paper using an oceanographic model forced by winds having a climatological alongshore profile with various intensities and offshore ranges. Several numerical experiments were performed in order to examine the influence of the bora wind field spatial variability on the surface currents of the northern Adriatic. In order to support numerical model results we present Eulerian current measurements from the gas field Ivana in the northern Adriatic (Fig. 1, 44◦ 47’ N, 13◦ 25’ E), obtained during bora episodes related to different synoptic situations. Detailed analysis of synoptic situations, with a possible explanation of the bora-controlling mechanisms, is given
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