DISPERSION PROCESSES AND RESIDENCE TIMES IN A SEMIENCLOSED BASIN WITHRECIRCULATING GYRES - AN APPLICATION TO THE TYRRHENIAN SEA

In this paper we investigate the dispersion of a tracer in a semi-encl osed basin characterized by a steady flow with recirculations. In part icular, we focus on the macroscopic behavior of the system, characteri zed by the total concentration of the tracer in the basin, C(t), and b y its residence time T. As a case study, the circulation in an idealiz ed basin mirroring some of the major characteristics of the Tyrrhenian Sea is considered, with a northward current connecting the inflow and the outflow regions of the basin, and with a main cyclonic gyre in th e northern part of it. Numerical simulations are performed from severa l release points in the basin and for two different values of the diff usivity coefficient K. Two independent models for dispersion are used, an Eulerian and a Lagrangian one, allowing us to validate both the ba sic formalism and the numerical results. The experiments show that the macroscopic properties of dispersion are largely influenced by the pr esence of the main gyre, while they do not depend strongly on K in the considered range. Namely, after a first phase which depends on the in itial conditions, the tracer tends to be concentrated in the region of the northern gyre, and this influences the trend of C(t) versus time and the value T. For almost all the simulations, the decrease of C(t) in time can be approximated by an exponential decay, indicating a cons tant probability of tracer escaping the basin. The e-folding timescale of the decay is the inverse of the principal eigenvalue of the advect ion-diffusion operator, and it can be computed a priori knowing the fl ow field; this allows us to compute also an a priori estimate for the residence time T. Only when the initial release is very close to the o utflow, is the initial decay of C(t) distinctly different from an expo nential, and a more detailed analysis is necessary. The basic results appear generalizable to a number of other similar systems with recircu lations.