Dynamical characteristics of the Adriatic Sea using Lagrangian methods

Abstract Lagrangian drifters are especially suitable to study transport processes since they move with the currents following closely the motion of water parcels, at least at the large and mesoscale,considering that transport by ocean currents plays a crucial role in many scientific and applied marine issues. In the coastal zone and semi-enclosed seas, transport by surface currents are even more important due to the variety of flotsam and jetsam, like debris, pollutants, oil spills, persons lost at sea, etc., encountered in the vicinity of populated coasts and highly navigated areas. The Adriatic Sea is an interesting and challenging place for such kind of problems because of strong boundary currents and interior gyres. The objective of this thesis is to simulate and study passive tracer dispersion by statistical properties of the currents in the Adriatic Sea from velocity data produced by numerical model outputs and real drifter data set. Each chapter of thesis consists of the content of manuscript which is ready to be submitted to peer reviewed journals. In chapter 2, Statistics of transit and residence times in the surface Adriatic Sea, a semi- enclosed basin of the Mediterranean, are estimated from drifter data and Lagrangian numerical simulations. The results obtained from the drifters are generally underestimated given their short operating lifetimes (half life of ? 40 days) compared to the transit and residence times. This bias can be removed by considering a large amount of numerical particles whose trajectories are integrated over a long time (750 days) with a statistical advection-diffusion model of the Adriatic surface circulation. Numerical particles indicate that the maximum transit time to exit the basin is about 216-260 days for objects released near the northern tip of the Adriatic, and that a particle entering on the eastern Otranto Channel will typically exit on the other side of the Channel after 170-185 days. A value of 150-168 days is estimated for the residence time in the Adriatic basin. In chapter 3, we examine the dispersion characteristics of surface drifter pairs in the Adriatic Sea. Relative dispersion of all surface drifters deployed in the Adriatic Sea between 25 August 1990 and 1 January 2007 has been calculated for different initial separation distance, using both chance and original pairs. Relative dispersion ( D 2 ) is explained in two different regimes. First non local regime which changes as a exponential function of time, the second one is local regime and it can be divided in three separated parts, Richardson ( D 2 ? t 3 ), ballistic ( D 2 ? t 2 ) and diffusive ( D 2 ? t ) regimes. The distribution of finite-scale Lyapunov exponent (FLSE) fields in the Adriatic Sea and Lagrangian Structure Function (LSF) have also been calculated from drifter trajectories, which both describe intrinsic physical properties at a given scale, also values of mean separation angle, displacement kurtosis and absolute dispersion have been found which can help us to know more about the behavior of pair trajectories. The aim of this work in chapter 4 is to quantify the near-surface transport properties and residence times in the Adriatic semi-enclosed basin by using synthetic drifters. We analyzed the simulated trajectories computed from the daily averaged velocity fields obtained by the MIT general circulation model implemented in the Adriatic Sea and integrated for the period from October 2006 till the end of 2008. Each numerical particle trajectory was obtained by integrating and interpolating velocity field between grid points using a fourth-order Runge†"Kutta scheme and bilinear interpolation. In particular the surface circulation properties in two contrasting years (2007 with mild winter and cold autumn, 2008 with normal winter and hot summer) are here compared. A comparison between the transport statistics for numerical particles crossing three selected sections located along the Italian coast (Conero and Gargano Promontories and Strait of Otranto) and the similar statistics driven by an existing climatology of the Adriatic surface velocity field (obtained by drifters measurements) has been carried out in order to corroborate the model results. Results indicate that the numerical particles are slower in this simulation when comparing them with the particles simulated by the flow field obtained by real drifters. This is because of the less energetic flow field generated by the MIT general circulation model during the selected years. Lagrangian statistics for the entire Adriatic basin after removing the mean Eulerian circulation for numerical particles have also been calculated and it can be found that the values of mean angular momentum, diffusivity and Lagrangian velocity covariance are less than the real drifter observations, but maximum Lagrangian integral time scale is the same. Because of the weather condition observed in 2007 and 2008, and the different kinetic energy of the mean flow (the yearly averages of MKE are 3.1408e-009 and 3.7907e-009 ( km 2 s ?2 ) in 2007 and 2008 respectively ) and the mean eddy kinetic energy (2.5638e-009 (2.8961e-009) ( km 2 s ?2 ) in 2007(2008)) during these years, the comparison between transport properties and Lagrangian statistics has been done considering these two different periods. The obtained results showed that the effects of wind driven recirculation in north of the Po River (which would be as a sea response to the Bora wind field) and Po River discharge on surface circulation induce the value of residence time to be similar during two years (182 (185) days in 2007 (2008)). Keywords: Adriatic Sea, Lagrangian Statistics, Transit and Residence Times, Relative Dispersion, Finite-Scale Lyapunov Exponent.