The principal hydrographic structures and dynamic conditions of the northern Tyrrhenian Sea are studied through the comparison of in situ field data and a modeling simulation. The field observations are long‐term Eulerian and Lagrangian current measurements associated with two hydrographic campaigns conducted in late summer and in winter. The data indicate that a well‐marked cyclonic gyre affecting the water column from the surface to the bottom is present in the northern Tyrrhenian Sea in both periods. The gyre undergoes significant seasonal changes in both shape and size. Following the indications of past studies, a numerical quasi‐geostrophic model driven by the wind jet blowing from the Strait of Bonifacio was able to reproduce the principal gyre characteristics. However, the wind jet alone does not explain all the gyre features, especially its seasonal variation. A study of the model results and field data indicates that these variations are linked to the seasonal variability of the northward surface current flowing along the eastern boundary of the Tyrrhenian Sea.

Abstract. In this paper we investigate the dispersion of a tracer in a semi-enclosed basin characterized by a steady flow with recirculations. In particular, we focus on the macroscopic behavior of the system, characterized by the total concentration of the tracer in the basin, C'(t), and by its residence time T. As a case study, the circulation in an idealized 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 the northern part of it. Numerical simulations are performed from several release points in the basin and for two different values of the diffusivity coefficient K. Two independent models for dispersion are used, an Eulerian and a Lagrangian one, allowing us to validate both the basic formalism and the numerical results. The experiments show that the macroscopic properties of dispersion are largely influenced by the presence 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 initial 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 constant probability of tracer escaping the basin. The e-folding timescale of the decay is the inverse of the principal eigenvalue of the advection-diffusion operator, and it can be computed a priori knowing the flow 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 outflow, is the initial decay of C'(t) distinctly different from an exponential, and a more detailed analysis is necessary. The basic results appear generalizable to a number of other similar systems with recirculations.

This paper addresses the study of the thermohaline circulation and convective processes induced by the presence of a coastal polynya in Terra Nova Bay (Ross Sea, Antarctica). These processes are analysed by means of oceanographic observations and numerical simulations. Time series of hydrological parameters and currents show the High Salinity Shelf Water (HSSW) formation with the mixing of the water column down to great depths, and a seasonal cycle of kinetic energy. A two-dimensional time dependent model for the numerical simulation of the effects of polynya on the circulation has been implemented. This model allows estimation of the contribution of thermohaline forcing on the circulation pattern, the amount of ice production consistent with the observed salinity distribution and annual cycle of currents and the amount of HSSW exported. The thermohaline forcing due to the polynya is able to induce strong horizontal currents up to 40 cm s−1 near the polynya area, as well as vertical movements of the order of 1 cm s−1. An annual ice production, ranging from 45 m to 90 m, mixes the water column at great depths, up to 750 m, and increases the salinity of the surface incoming waters up to 34.82. The estimated net north-eastward transport of HSSW ranges from 0.34 Sv to 1.23 Sv, depending on ice production.

A nonlinear stochastic model for the dynamics of a population with either a continuous size structure or a discontinuous stage structure is formulated in the Eulerian formalism. It takes into account dispersion effects due to stochastic variability of the development process of the individuals. The discrete equations of the numerical approximation are derived, and an analysis of the existence and stability of the equilibrium states is performed. An application to a copepod population is illustrated; numerical results of Eulerian and Lagrangian models are compared.

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