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Circulation and dispersion in the Gulf of Trieste as measured by surface drifters

Pierre-Marie Poulain, Riccardo Gerin, Vassilis Zervakis, Zoi Kokkini
OGS
(Abstract received 05/15/2012 for session A)
ABSTRACT

As part of the EU MED TOSCA project, more than 40 surface drifters were operated in the Gulf of Trieste (northern Adriatic) between 23 and 27 April 2012 to study the Gulf surface circulation and the relative dispersion by surface currents. The majority of the drifters were CODE instruments designed to measure the currents in the first meter under the sea surface. Drifter GPS positions measured every 15 minutes were transmitted via GSM and Iridium telephone networks. Other drifters designed to follow the very surface currents and/or oil slicks were also used. Two drifters were equipped with ADCPs to assess the drifter slip and measure current profiles in the water column.

The above Lagrangian measurements were complemented by HF radar measurements of surface currents, meteorological observations with a moored buoy and surface gravity wave measurements with a waverider buoy and temperature/salinity profile measurements (CTD). A local regional numerical model was also run in nowcast mode.

Preliminary results obtained from the above-mentioned Lagrangian drifters will be presented. The Gulf circulation during the survey period was forced by southeasterly Scirocco winds the first few days, followed by calm conditions. It essentially consisted of a basin-wide anticyclonic eddy with period of about 18-21 hours, close to the inertial period but also the Adriatic seiche period. Due to this circulation pattern, an significant Isonzo River plume originating from the northern coast expanded south and southeastward in most of the Gulf in the top meter of water. This stratification corresponded to significant shear of the upper horizontal currents measured by the ADCPs, thus complicating the assessment of drifter slippage. Some drifters were deployed in tight clusters to study relative dispersion at scales as small as 50 m. The mean squared separation of particle pairs was calculated.